Onderzoek Geothermie Noord Holland Zuid

8/7/2019 Onderzoek Geothermie Noord Holland Zuid

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Consultant IF WEP

Velperweg 37

PO Box 605

6800 AP ARNHEM

T +31 (0)26 35 35 555

F +31 (0)26 35 35 599

E [email protected]

Contact: Mr Nick Buik

Geothermal energy Noord-Holland

Geological study of the Slochteren Formation in the southern

part of the province of Noord-Holland

22.039/109140/NB

June 8, 2010


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Index

Samenvatting ...................................................................................................................... 41 Introduction ................................................................................................................ 112 Geology ..................................................................................................................... 13

2.1 Research area ............................................................................................... 132.2 Structural framework ..................................................................................... 132.3 Geothermal gradient ..................................................................................... 18

3 Sedimentological analysis ......................................................................................... 213.1 Regional depositional environment ............................................................... 213.2 Well data ....................................................................................................... 223.3 Mineral content .............................................................................................. 253.4 Conclusions ................................................................................................... 26

4 Petrophysical analysis ............................................................................................... 274.1 Introduction .................................................................................................... 274.2 Core measurements ...................................................................................... 274.3 Log data ........................................................................................................ 314.4 Transmissivity of the wells ............................................................................ 354.5 Water quality ................................................................................................. 364.6 Well correlations ............................................................................................ 364.7 Conclusions ................................................................................................... 37

5 Basin analysis ............................................................................................................ 395.1 Maximum burial depth determination ............................................................ 395.2 Burial history reconstruction method ............................................................. 415.2 Burial depth estimation .................................................................................. 435.3 Determination of the porosity ........................................................................ 445.4 Determination of the mean transmissivity ..................................................... 475.5 Conclusions ................................................................................................... 49

6 Fault systems ............................................................................................................ 506.1 General .......................................................................................................... 506.2 Analysis ......................................................................................................... 516.3 Location specific ............................................................................................ 546.4 Conclusion ..................................................................................................... 55

7 Uncertainty analysis .................................................................................................. 567.1 Methodology .................................................................................................. 567.2 Zandvoort Ridge northwest ........................................................................... 587.3 Zandvoort Ridge southeast ........................................................................... 587.4 Conclusions ................................................................................................... 59


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8 Reservoir stimulation ................................................................................................. 608.1 Zandvoort Ridge northwest ........................................................................... 608.2 Zandvoort Ridge southeast ........................................................................... 618.3 Expected flow rates ....................................................................................... 648.5 Conclusions ................................................................................................... 65

9 Conclusions and recommendations .......................................................................... 6610 References ........................................................................................................ 68

Appendices:

1 Geological timescale

2 Geological maps Slochteren Formation3 Petrophysics

4 Complete workflow basin modelling

5 Abbreviations geological formations


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Samenvatting

In het zuidelijke deel van Noord-Holland lopen veel initiatieven voor geothermie en zijn

reeds enkele opsporingsvergunningen aangevraagd. Uit eerdere studies blijkt dat de

Slochteren Formatie (Slochteren Fm) het meest geschikt is in dit deel van de provincie.

Het probleem is echter dat weinig gegevens van deze laag beschikbaar zijn waardoor de

mate van onzekerheid van het realiseerbare debiet, en dus vermogen, groot is. Een vol-gende stap naar het realiseren van een geothermisch systeem is het uitvoeren van een

onderzoek naar de ruimtelijke verspreiding van de eigenschappen van de Slochteren

Formatie. Op basis van de hieronder beschreven resultaten kan besloten worden of het

nog steeds interessant is om verder te gaan.

In deze studie zijn de volgende partijen betrokken:

Provincie Noord-Holland; Jamuflor; PrimAviera; Schiphol Group Real Estate; Floricultura;

Gemeente Amsterdam; Gemeente Haarlem; Gemeente Nieuwkoop.

Het onderzoeksgebied en de gebruikte boringen zijn weergegeven in figuur 1.

Figuur 1 Onderzoeksgebied en de putten die de Slochteren Formatie bereiken.


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Uit de verschillende putten zijn temperatuur gegevens gebruikt om de geothermische

regionale gradint te bepalen voor het gebied. Deze kan het best worden weergegevendoor onderstaande formule.

0,0267 18,254 (1)

De metingen in de put SPL-01-S1 wijken echter af van de metingen in de overige putten.

Hierdoor is de verwachte temperatuur op bijvoorbeeld 2.000 m diepte, bijna 20 C hoger

dan op basis van de regionale gradint (1) kan worden verwacht. Deze hogere tempera-

tuur wordt veroorzaakt door het breuksysteem in de directe omgeving van de put. In dit

systeem wordt dan warmer water van grotere diepte naar boven getransporteerd. Indien

dit het geval is, is in de breuk een hogere temperatuur te verwachten. Het is nog onduide-

lijk hoe ver van de breuk vandaan de temperatuur hierdoor benvloed wordt. Hier wordt in

de aanbevelingen verder op ingegaan.

De reservoireigenschappen die bepalend zijn voor het debiet wat onttrokken kan worden,

worden deels bepaald door het milieu ten tijde van de afzetting. Kaarten van het regiona-

le afzettingsmilieu van de Slochteren Fm laten zien dat het grootste deel door de wind is

afgezet (eolisch) en een deel door rivieren (fluviatiel). De boorgatmetingen laten zien dat

het onderste deel een wat grilliger karakter heeft wat correspondeert met een fluviatiel

milieu, terwijl het overgrote deel een homogeen (eolisch) pakket is. De karakteristieken

van het reservoir zijn vrij constant over het gehele gebied. In verschillende putten is do-

lomiet en/of anhydriet aangetroffen in de porin waardoor de porositeit en daarmee de

permeabiliteit verlaagd wordt. Elk mineraal heeft een andere mate van invloed. Uit de

logmetingen kan niet worden bepaald welk mineraal zich in de porin bevind.

In dit onderzoek is ook een petrofysische analyse uitgevoerd waarbij de eigenschappen

van de formatie ter plekke van de boringen kwantitatief zijn bepaald. Deze gegevens zijn

in combinatie met de resultaten van de bekkenanalyse gebruikt om de eigenschappen in

de gebieden buiten de putlocaties te bepalen.

Ook deze analyse laat een vrij homogene Slochteren Fm zien waarbij het onderste deel

meer klei bevat. Aan de hand van boorgatmetingen is de porositeit van de formatie op de

locatie van de put bepaald. Aan de hand van kernmetingen is een relatie vastgesteld

tussen de porositeit en de permeabiliteit. Deze relatie is zichtbaar in figuur 2.


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Figuur 2 De relatie tussen de porositeit en de permeabiliteit van de Slochteren Fm

(eolisch).

De put LSM-01 laat een andere relatie zien dan de andere putten in de omgeving. Er zijn

geen duidelijke aanwijzingen waarom deze waardes lager zijn. Verwacht wordt dat een

dichtbij liggende breuk heeft gezorgd voor een plaatselijke verandering van het gesteente

met de mineralen in de porin in het bijzonder. De metingen in deze put zijn meegeno-

men in de onzekerheid van de relatie tussen de porositeit en de permeabiliteit. Hetzelfde

geld voor de metingen uit Q07-01 die een beduidend hogere permeabiliteit hebben dan

zou worden verwacht.

De eigenschappen van de formatie worden ook bepaald door de diepte waarop de forma-

tie zich bevindt of de maximale diepte die het gesteente ooit heeft bereikt. Door de druk

van het bovenliggend gesteente nemen de porositeit en permeabiliteit van het gesteente

af. Dit is een onomkeerbaar proces waardoor de slechtere eigenschappen behouden

blijven als het gesteente weer opgeheven wordt.

In het gebied zijn breuken aanwezig die in de geologische tijd verschillende keren actief

zijn geweest. Ze hebben ervoor gezorgd dat de Slochteren Fm in sommige delen van het

gebied vroeger dieper heeft gelegen dan de tegenwoordige diepte. Aan de hand van de

inkolingsgraad van de koollagen kan worden bepaald wat de maximale diepte is waarop

het gesteente heeft gelegen. Van 6 putten in de omgeving waren deze gegevens be-

schikbaar. Ter plekke van de putten is bepaald hoeveel gesteente verdwenen is door

0. 0.06 0.12 0.18 0.24 0.3core porosity [-]

0.01

0.1

1.

10.

100.

1000.

corepermeability[mD]

21 points plotted out of 6596

RMA: Log(CORE:KHds_is_Juh) = -3.7689 + 27.4040 * CORE:PHIT_DECds R2= 0.8375

Well Zone Depths

(1) Q07-01 (2) Slochteren For 2 375.M - 2568.M

(14) LANDSMEER-01 (3) Slochteren For 1454.M - 1650.M

(2 ) Q11-01 -s2 (4 ) Slochteren For 2878 .M - 3043 .M

(6) Q14-02 (4) Slochteren For 2 888.89M - 3004.M


3 OZN-01 3 Slochteren For 1310.M - 1472.M


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erosie. Aan de hand van geologische kaarten van het gebied is dit ook tussen de putten

in geschat.Vervolgens is aan de hand van deze diepte de porositeit bepaald welke is omgezet naar

een permeabiliteit. In combinatie met de dikte kaarten van de Slochteren Fm is de trans-

missiviteitskaart van de regio gemaakt die is weergegeven in figuur 3. Deze kaart is ook

te vinden in bijlage 2. Hierin is te zien dat de formatie in het zuiden een stuk dieper heeft

gelegen dan nu het geval is. Deze gebieden hebben een lagere porositeit en dus een

lagere permeabiliteit, wat wordt bevestigd door de putten.

Figuur 3 De transmissiviteit in de Slochteren Fm aan de hand van de maximale

begravingsdiepte van de Slochteren Fm.

In het gebied zijn ook breuksystemen aanwezig die verschillende keren in de geologische

tijd actief zijn geweest. Natuurlijke actieve breuksystemen hebben vaak een hogere per-

meabiliteit dan het gesteente en zijn daardoor ook interessant voor de winning van aard-

warmte. In het gebied komen verschillende actieve breuksystemen voor die in aanmer-king komen hiervoor. In figuur 4 is een kaartje weergegeven met de breuken aan de ba-

sis van de Boven Noord Zee Groep. Deze breuken zijn allen afschuivingsbreuken, maar

op enkele locaties hebben ze ook een zijschuiving component. De gebieden met een

afschuiving n zijschuiving component hebben veelal een hogere permeabiliteit. Deze

zijn aangegeven in figuur 4 voor twee breuksystemen binnen het gebied.

Berekeningen van de neiging tot het schuiven langs en verwijding van het breukvlak zijn

uitgevoerd voor de breuk ten zuiden van Haarlem-Schiphol en voor de breuk ten noorden

van Haarlem. Deze berekeningen laten zien dat het zeer waarschijnlijk is dat nog steeds

bewegingen langs de breukvlakken plaatsvinden. Dit wordt bevestigd door de aardbeving


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in Beverwijk in 1997 waarvan de locatie op de breuk ten noorden van Haarlem valt. Het

debiet wat onttrokken kan worden uit de breukzone is vaak een factor 2 tot 3 hoger danhet debiet wat uit de formatie zelf onttrokken kan worden.

Figuur 4 Breuken aan de basis van de Boven Noord Zee Groep in rood. Gear-

ceerde gedeelten: vermoedelijk hogere permeabele zone.

Voor verschillende gebieden op het Zandvoort Hoog, het noordwesten en het zuidoosten,

is een onzekerheidsanalyse gemaakt. Deze zijn aangegeven in figuur 5. In de onzeker-

heidsanalyse is de onzekerheid van het haalbare debiet bepaald aan de hand van de

onzekerheden in de diepte, temperatuur, porositeit, permeabiliteit en dikte. In tabel 1 zijn

de resultaten weergegeven. Aan de hand van het debiet en de productie- en injectietem-peratuur kan het thermisch vermogen worden bepaald.

Figuur 5 Locaties gebieden onzekerheidsanalyse.


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Tabel 1 Resultaten van de onzekerheidsanalyse exclusief reservoir stimulatie

[m/h].p90 p50

Regio COP1

10 COP 15 COP 20 COP 10 COP 15 COP 20

zuidoost 30 20 15 75 50 40

noordwest 200 150 120 350 220 175

De resultaten laten zien dat de eigenschappen van de Slochteren Fm niet overal vol-

doende zijn voor een economisch rendabel project. Om deze reden is gekeken naar de

mogelijkheden van reservoir stimulatie. Aan de hand van state-of-the-art modellen van de

olie- en gasindustrie is gekeken naar de verbeteringsfactor die kan worden gehaald als

het reservoir gestimuleerd wordt. Uit de resultaten blijkt dat hoe lager de permeabiliteit,

hoe groter het effect van de reservoirstimulatie is. De resultaten zijn weergegeven in ta-

bel 2. Hieruit blijkt dat door reservoirstimulatie toe te passen, het debiet met een factor1,3 tot 3,6 verbeterd, afhankelijk van de oorspronkelijke permeabiliteit.

Tabel 2 Resultaten van de onzekerheidsanalyse inclusief reservoir stimulatie

[m/h].

Regiop90 p50

COP 10 COP 15 COP 20 COP 10 COP 15 COP 20

zuidoost 100 70 55 260 180 150

noordwest 275 200 155 350 280 220

De onzekerheidsanalyse is tevens uitgevoerd voor de situatie waarbij de waardes van

LSM-01 en de hoge waardes van Q07-01 niet mee zijn genomen. De resultaten van de

p90 zijn weergegeven in tabel 3. Hierin is te zien dat het debiet hoger is als de kernme-tingen uit LSM-01 en de hoge waardes uit Q07-01 buiten beschouwing kunnen worden

gelaten.

Tabel 3 Resultaten van de onzekerheidsanalyse (p90) exclusief de extreme poro-

siteit-permeabiliteit waardes [m/h].

Regioexcl. reservoir stimulation incl. reservoir stimulation

COP 10 COP 15 COP 20 COP 10 COP 15 COP 20

zuidoost 40 30 22 120 90 65

noordwest 250 200 160 300 240 190

Vervolgonderzoek

Om de onzekerheid in het haalbare debiet te verkleinen is het belangrijk om te bepalenwaarom LSM-01 een afwijkende relatie laat zien. Aangeraden wordt om hiervoor slijp-

plaatjes van de kern op te vragen bij de NAM of, indien niet beschikbaar, te laten maken.

Op deze manier kan worden bepaald welke mineralen zich in de porin bevinden en of

deze ook op andere plekken in het onderzoeksgebied voor kunnen komen.

De onzekerheid in het haalbare vermogen in het zuidoosten kan ook worden verkleind

door de onzekerheid in de temperatuur te verkleinen. Aangeraden wordt een tempera-

1

COP:een COP geeft aan hoeveel delen energie opgewekt kunnen worden bij het gebruik van 1deel (pomp)energie


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tuurmodel op te stellen. Hierin kan worden bepaald hoe ver van het breuksysteem van-

daan een verhoogde temperatuur kan worden verwacht.

De zekerheid in het gebied zuidoosten kan ook worden vergroot door het uitvoeren van

een (gezamenlijke) exploratieboring. Met deze boring kunnen dan de eigenschappen ter

plekke worden vastgesteld. Tevens kan op deze manier meer zekerheid verkregen wor-

den over de temperatuurgradint in het gebied.

Aangeraden wordt ook de aanwezige 2D en 3D seismiek te herinterpreteren voor het in

kaart brengen van de diepte en dikte van de Slochteren Formatie en de aanwezige geo-

logische structuren. De ligging van de verschillende breuken, en daarmee de locatie van

de genverteerde delen, kunnen hierdoor beter worden vastgesteld.


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1 Introduction

In the province of Noord Holland, several initiatives for the application of geothermal en-

ergy are running. Multiple exploration licenses have been applied for, some of them have

already been granted by the Ministry of Economical Affairs. The studies executed in this

region all concluded the Slochteren Formation (Slochteren Fm) is the most promising

reservoir for geothermal exploration. However, the amount of data is relatively scarce andas a consequence the uncertainty of the achievable flow rate and thus the thermal power

is large. Furthermore, the properties of the formation change in a short distance between

wells located in the area. Therefore a regional study of the properties of the Slochteren

Formation is a next step towards realisation of a geothermal project in the area. A re-

gional study gives better insight in why and how the properties change geographically.

The regional study includes a petrophysical analysis of the wells which reach the forma-

tion in the area. In this analysis, the properties of the reservoir are determined within the

wells. A sedimentological analysis is also included. This gives information of the deposi-

tional environment as well as the mineral composition at the well locations.

These analyses are used as input for the basin analysis. Within this analysis the maxi-

mum burial depth of the Slochteren Formation will be determined for the whole area asthe maximum burial depth influences the reservoir properties. The analysis is performed

by first constructing burial graphs for the wells and followed by using geological maps to

extent the maximum burial depth geographically.

The next step in the research is the uncertainty analysis to get more insight in the feasibil-

ity of a geothermal system at a specific location. It is expected that the properties of the

Slochteren Formation will not always be sufficient for the economical extraction of geo-

thermal water. Therefore, the influence of reservoir stimulation on the flow rate is also

investigated. These last steps are location specific and will therefore be analysed for spe-

cific areas.

Several parties are joined in this research project. These include:

Province of Noord Holland P. van Herk

Jamuflor J. Mulder

PrimAviera R. Brown

Schiphol Group P. Rouendal

Floricultura K. Schoone

Municipality of Amsterdam J. van Zanten

Municipality of Haarlem S. Andela

Municipality of Nieuwkoop J. Horstman


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Their locations are shown in figure 1.1. The different locations have different focuses of

the research. Therefore, the report is subdivided in a general part for the whole area anda part is location specific.

The outcome of this study can be used for a more detailed design in the next stage. Fur-

thermore, this report may be used as input for the Dutch Geothermal Guarantee Fund or

for a private insurance company.

Figure 1.1 The locations of the parties involved in this study.


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2 Geology

2.1 Research area

Within the area 14 wells have reached the Slochteren Formation of the Upper-Rotliegend

Group. These wells are shown in figure 2.1. The research area is also indicated. As can

be seen in figure 2.1, the wells are located at the boarders of the research area. To ob-tain more certainty of the middle area, a basin analysis is needed.

Figure 2.1 Research area and wells reaching the Slochteren Formation.

2.2 Structural framework

The area focused on in this research lies in an area with three different structural ele-

ments. It is located mostly on the Zandvoort Ridge, but is also partly in the West-

Netherlands Basin and the Central Netherlands Basin. This is shown in figure 2.2. In this

chapter, the structural framework of the area will be described using geological time peri-

ods. A geological timescale can be found in appendix 1.


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Figure 2.2 Map of Late Jurassic to Early Cretaceous structural elements with the

area in red (After H.A. van Adrichem Boogaert and Kouwe, 1993). Dark

brown: structural high, partly sub-aerial landmass; light brown: platform

intermittently flooded; white: basin.

In the late Visean the collision between Gondwana and Laurussia took place. As a result,

a foreland basin was formed partly located in the Netherlands. Large amounts of clastic

material were deposited from the south. The facies changed from deepwater deltaic to

coal bearing shallow deltaic as the basin was filled. From the Westphalian C, the deposits

of the foreland basin were folded and locally uplifting occurred. As a result of the dextral

movement between Laurussia and Gondwana during the Stephanian, a regional tran-

stensional fault system developed. This fault system has a NW-SE orientation and mayalready have been formed during the Caledonian orogenic phase. During the Stephanian

and Early Permian, the deposits in the foreland basin were folded into broad anticlinal

structures. As a result, a large part of the Westphalian deposits has been eroded.

Sedimentation was resumed during the Late-Permian after a long period of non-

deposition and erosion. As a result of thermal subsidence and transtensional movements

north of the Variscan orogen, the Southern Permian Basin developed. The area focused

on in this study is located at the former southern edge of this basin. As a result, the thick-

ness reduces towards the south and increases towards the north. Large differences in

thicknesses and the absence of evidence of erosion where thin layers of the Upper-


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Rotliegend Group are present, indicate a paleo-surface with small hills and broad valleys.

The thickness of the Upper-Rotliegend Group is plotted in figure 2.3 and in appendix 2.

Figure 2.3 Thickness of the Upper-Rotliegend Group.

A transgression at the end of the Saxonian was the beginning of the sedimentation of the

Zechstein Group. The thermal subsidence continued. As a result of the rising sea level,

the supply of clastic sediments ceased and the deposition of the carbonate-evaporite

cycles started. In the area of investigation, only the first three cycles are well developed

as the location at the southern rim of the basin resulted in less influence of marine proc-

esses.

Thermal subsidence continued during the Triassic. The Zandvoort Ridge is characterized

by smaller subsidence rate then the surrounding basins. The Central Netherlands Basin

on the northern side of the Zandvoort Ridge had a higher subsidence rate than the West

Netherlands Basin on the southern side. The sedimentation environment changed from

marine to fluvial and playa.

The Hardegsen tectonic phase occurred during the Late Scythian and resulted in local

uplift and erosion of the Main Buntsandstein Subgroup. In the investigated area, the ero-sion increases from north to the south with the thinnest Main Buntsandstein present at

the Zandvoort Ridge. This indicates an uplift of the northern rim of the West Netherlands

Basin probably as a result of regional transpression.

In the Anisian a regional transgression occurred resulting in the marls and carbonates of

the Muschelkalk and a connection with the Tethys was established. This connection was

not always maintained which resulted in evaporitic deposits on the southern side of the

Zandvoort Ridge.


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The first Cimmerian phase eroded parts of the Muschelkalk and Keuper deposits. This

phase was a result of regional transverse faulting accompanied by uplift of the basin rims.The breakup of Pangea occurred during the Jurassic and Early Cretaceous. This largely

influenced the subsidence of the West and Central Netherlands Basins. The Mid Cimme-

rian phase has had little effect in the area of interest. The Late Cimmerian phase how-

ever, resulted in uplift of the basin rims and highs. This uplift resulted in erosion of mainly

Jurassic sediments in the southern part of the area of interest.

As a result of a regional sea level rise and a thermal subsiding after the Late Cimmerian

phase, a transgression occurred. During the Albian the Austurian tectonic phase took

place. Combined with a long term lowstand in sea level, this resulted in the development

of the Late Cimmerian unconformity.

At the end of the Albian a period of tectonic rest started. As a result of the fast sea levelrising in the Late-Albian, the sources of clastic sediment supply were submerged and the

deposition of the Chalk started.

The subhercynian phase at the end of the Conacian until the end of the Campanian took

place in the form of compression in the north-south direction. This resulted in the inver-

sion of the Mesozoic sequences in the West- and Central Netherlands basins. During this

inversion, the subsidence of the platforms and formal basin rims continued. The inverted

basins where therefore acting as local sediment source for the continuing sedimentation

of the Chalk in the subsiding areas. The gradual decrease and ending of the inversion

during the Campanian resulted in a decrease of the relief.

The Cenozoic was a period of shallow and deep marine sedimentation influenced by the

Alpine tectonic activity. The West Netherlands Basin was uplifted during the Laramide

phase, resulting in the erosion of the Chalk deposits of the Upper Cretaceous. During the

Eocene a gradual deepening of the basin occurred. The uplift during the Pyrenean phase

resulted in erosion of the deposits of the Early Tertiary. During the Early Oligocene the

subsidence was resumed and a shallow sea occupied the area of interest. The Savian

tectonic phase, a last inversion during the Oligocene-Miocene boundary, resulted in the

Mid North Sea Unconformity. The last phase of uplift in the Pliocene resulted in new

sediment supply in the West Netherlands Basin.

In figure 2.4 and in appendix 2, the present day depth map of the base of the Slochteren

Formation can be seen. Several cross sections have been made to illustrate the struc-tures in the area of interest, see figure 2.5 till 2.7.


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Figure 2.4 Present day depth of the base of the Slochteren Formation, with the loca-

tions of the cross sections in figure 2.5-2.7.

Figure 2.5 Profile 1 from NNE-SSW over Floricultura and Haarlem.


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Figure 2.6 Profile 2 from NNE to SSW over Zaandam and Schiphol.

Figure 2.7 Profile 3 from NNE to SSW over the southeastern part of Amsterdam and

Waverveen.

2.3 Geothermal gradient

With the help of bottom hole temperatures (BHT), the geothermal gradient has been de-

termined. Due to the presence of mud in the borehole, the measured temperature will be

lower than the original formation temperature. To determine the formation temperature,the Horner method has been used. For this correction, more than one temperature

measurement is needed at different times after the mud circulation has stopped in the

borehole. The data is not sufficient to be able to correct all the measurements that were

taken. The resulting plot is shown in figure 2.8.


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Figure 2.8 The determined regional geothermal gradient (blue).

The measured temperatures, both corrected and uncorrected, measured in SPL-01-S1

are higher than expected at that depth. The difference is about 20 C. Therefore, the un-

corrected values have been used to determine another gradient for this well only. This

one is shown in figure 2.8 as a grey dotted line. The measurements of SPL-01-S1 can be

seen in figure 2.9.

The reason why this gradient is higher is not clear. The nearby fault may transport water

of higher temperature to shallower depth via convection and warm the water in SPL-01-

S1 which is located near a fault. The higher temperature may also be caused by wrong

calibration of the tool which measured the temperature during the different log runs.

As the difference is sufficiently large, the gradient as determined by the measurements inSPL-01-S1 has been applied to the area directly around the location of the well, near the

fault zone. It is not clear how large the area is in which the temperature has been af-

fected. This can be determined by building and applying a detailed temperature model.

Therefore the regional gradient has been applied to the other areas. This means there

might be an underestimation of the temperature at Schiphol, PrimAviera and Jamuflor. A

map of the temperature based on the regional gradient can be found in appendix 2.

The regional geothermal gradient is expressed in formula 1. The gradient of SPL-01-S1 is

expressed in formula 2.

0

20

40

60

80

100

120

140

160

180

0 500 1,000 1,500 2,000 2,500 3,000 3,500 4,000

Temperature[C]

TVDSS[m]

SPL01S1 WRV01

ARV01 WOB01S1

Q0701 Q1101S2

LSM01 T=0.0267 xz+18.254


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0.0267 18.254 (1)

0.0363 16.17 (2)

Figure 2.9 Geothermal gradient as determined in SPL-01-S1.

0

20

40

60

80

100

120

140

160

180

0 500 1,000 1,500 2,000 2,500 3,000 3,500 4,000 4,500

Temperature[C]

TVDSS[m]

SPL01S1uncorrected

SPL01S1corrected

schipholgeothermal

gradient

regionalgeothermalgradient


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3 Sedimentological analysis

Core measurements and well logs supply information about the petrophysical properties

of the reservoir. They also give other information of the reservoir such as the depositional

environment and the direction of the sediment supply. Using this information, the reser-

voir can be better understood. Core measurements or descriptions give information about

the pore content. Are the pores clean or do they contain minerals from diagenetic proc-esses which lower the permeability?

3.1 Regional depositional environment

The Slochteren Formation is one of the formations which contain gas in several parts of

the Netherlands. As a consequence this formation has been studied extensively. Re-

gional studies of the depositional environment of the Slochteren Formation have been

made. A map of the depositional environment as a result of these studies is shown in

figure 3.1.

Figure 3.1 Map of the regional depositional environment of the Slochteren Forma-

tion. TIJH: Texel-IJsselmeer High (Wong et al., 2007).


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The region of interest is for the most part located in the eolian environment. Towards the

south, this transfers to a fluvial environment. An eolian environment consists of dunedeposits which result in a homogeneous clean sandstone with a relative high porosity.

Fluvial deposits, in contrast, are highly heterogeneous as the type of deposit depends on

the location and type of river system. The corresponding log responses will show more

variety.

3.2 Well data

Each of the wells has been analysed for the depositional environment. The reservoir

characteristics such as shape of the log response, colour, grain size distribution and grain

shape give information of the depositional environment.

Gamma ray log response

The gamma ray log is a primarily indicator of the amount of clay content within the forma-

tion. The gamma ray of well LSM-01 is plotted in figure 3.2. The gamma ray has a more

or less block shape which means the clay content is relatively constant. This corresponds

to the eolian deposits. The lower part of the Slochteren Formation is more irregular which

means more varying clay content. This could be caused by a change in depositional envi-

ronment from fluvial at the bottom to eolian at the top of the formation. The shape of the

gamma ray is more or less constant in the different wells as can be seen in the multi-well

plots in appendix 2. A homogeneous upper part of the formation is visible with a varying

gamma ray in the lowest part.


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Figure 3.2 Gamma ray log response of the Slochteren Formation in LSM-01.

Eolian

Eolian/Fluvial


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Reservoir characteristics

A short description of the reservoir characteristics determined from the different logs andcores is given in table 3.1.

Table 3.1 Overview of the reservoir characteristics derived from the logs and core

measurements.Well colour grain size sorting grain shape remarks

Q07-01 grey-brown-

grey

fine-med well rounded-

angular

breccia & conglomerate

in lower part

Q10-03 grey with red

iron stains

fine-med poor - no visible porosity

Q11-01-S2 red, grey,

white

- - - conglomerate in lower

part

Q11-03 brown-grey fine-med poor-moderately

- poor visible porosity,mud losses at base

Q14-02 dark grey-

brown

fine-med well subrounded good visible porosity

ARV-01 light brown fine-med - - -

HEK-01 light grey-light

brown

med-

coarse

- subrounded the base contains con-

glomerate

LSM-01 white to pink fine well - friable sands at bottom

OZN-01 grey fine-med - - towards base more

calcareous

SPL-01-S1 light grey-

brown grey

very fine-

fine

poorly rounded-

subrounded

fair visible porosity,

friable sandsWRV-01 grey fine-med fairly Subrouned sidewall coring porosity

fair-none

WOB-01-S1 - - - - circulation losses

The table shows the reservoir characteristics are more or less constant over the area.

There is some variation in sorting as well as the grain shape which is caused by a slight

change in depositional environment towards the base of the formation. Furthermore in

several wells conglomerate are present at the base of the formation which indicates a

fluvial source.

Dipmeter

Several of the wells contain dipmeter measurements. A dipmeter log provides a continu-ous record of formation dip and direction. The dipmeter can provide facies information,

bedform orientation and paleocurrent directions. The dip has been measured in wells

Q10-03, Q11-01-S1, Q10-03, Q14-02, HEK-01, LSM-01, WOB-01-S1 and WRV-01.

All of the wells have visible bedding, except for WRV-01 which is located on a structural

high at the time of deposition. This well contains a thin Slochteren Formation and is there-

fore considered to be non representative for the research area. Wells Q10-03 and LSM-

01 show foresets in the middle of the formations which are interpreted as dune deposits.


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The wells all show the source of the sediments is situated within the SSE in the lower part

of the formation. This changes to NNW upward in the formation. The lower parts of theformation have more sudden variation in dip direction which corresponds to a fluvial envi-

ronment.

3.3 Mineral content

Some of the cores have been analysed for their pore space filling. A thin section has

been made of a core in well Q07-01. Microscopic analyses showed patches of authigenic

dolomite as pore space filling. In some places the grains have a coating of clay minerals.

These clay minerals consist of kaolinite, secricite and a very low amount of illite. In some

places of the thick sections the rock consists of 30% kaolinite. Kaolinite minerals have the

shape of vermicular (wormlike) aggregates. The kaolinite is formed as a result of acid

fluid flow from the underlying Carboniferous Coal Measures source rocks (Lanson et al.,1996).

Illite is a clay mineral which reduces the permeability significantly due to the hair-like or

lath-shaped particles while the high porosity of the formation is retained. According to

Lanson et al. (1996), illitization occurred during the Cimmerian phases when rapidly illiti-

zation of the kaolin-group minerals was favoured both by increased heat flow in the sedi-

mentary pile and by widespread presence of faults which permitted significant fluid flow

probably from the Zechstein formation. As the majority of the present clay minerals in the

wells were they were analysed are kaolinite and secricite, which belong to the kaolin-

group, the illitization has had little to no influence in the research area.

The wells Q07-01, Q10-03, Q11-03, OZN-01, SPL-01-S1 and WRV-01 all show the pres-

ence of anhydrite and dolomite as cement. The amount is not quantified. Traces of pyrite

and mica are also present. The presence of dolomite and/or anhydrite also reduces the

permeability significantly. The presence is likely caused by fluid flow from the Zechstein

Formation shortly after deposition.

The low permeability caused by the presence of clay minerals, dolomite and/or anhydrite

can be increased by applying reservoir stimulation. This can be done either chemically or

hydraulically. A mixture of HCl and HF have been used for years within the oil and gas

industry for acidizing sandstones to increase the flow rate. HF is used to dissolve clay

particles and silicates. HCL is used for the treatment of carbonates such as calcite or

dolomite. The treatment is temperature dependent as the solubility of dolomite stronglyincreases above a temperature of a 100 C (Lund et al., 1972). The downside of chemical

stimulation is the relative small distance from the borehole at which the acid is still affect-

ing the formation. With hydraulic stimulation, a fluid is used to generate fractures which

enhance the permeability of the formation. The fractures are generated by pumping water

with a viscosity enhancer at a high pressure into the formation which break the rock and

create fractures. Proppants can be added to keep the fractures open. A combination with

a chemical treatment is possible.


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3.4 Conclusions

The sedimentary analysis show the Slochteren Formation consists of a bottom part which

has been deposited in a fluvial environment. The majority of the formation is of eolian

nature and is therefore considered to be homogeneous. This is consisted with the re-

gional interpretation and the observations in the boreholes as well as the log responses.

Thinsection analyses show the presence of clay minerals such as kaolinite which are

present as pore filling. Furthermore dolomite and anhydrite have been encountered as

cement. All three of them reduce the permeability of the formation while the higher poros-

ity might be retained. If the permeability of the reservoir is too low to deliver the required

thermal power at the surface, reservoir stimulation can be applied. This can be done ei-

ther chemically, hydraulically or a combination of them.


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4 Petrophysical analysis

4.1 Introduction

To be able to quantify the reservoir properties of the Slochteren Formation, a petrophysi-

cal analysis has carried out for 12 of the 13 wells. Well OZN-01 is excluded as the avail-

able log data was not sufficient for an analysis.

Sandstones not only consist of sand, but often also contain silt and clay fractions. The

important properties which should be determined include clay volume, porosity, perme-

ability and water quality. In this analysis, the pore-filling is also determined. This may

consist of water, oil, gas or salt. In the end, the permeability and the net thickness of the

reservoir determine the transmissivity. The transmissivity is a measure of the quality of

the reservoir and is directly related to the yield.

The data used are from wells drilled between 1970 and 1992. These wells contain

enough log- and core-data for the determination of the reservoir properties. Besides core

and log data, knowledge is used of the petrophysical properties within the region. The

used data, results and parameters are listed in appendix 3.

According to the well log of SPL-01-S1, cores of the Slochteren Formation have been

taken. However, de data does not exist in the databases. The cores are also absent in

the core depot in Zeist. The data is not available by original concession holder of the

area, Amoco. The chances of getting the cores are low and it still needs to be seen if

analyses have been performed on them.

4.2 Core measurements

Of the different wells in the research area reaching the Slochteren Formation, 381 core

measurements were available. In appendix 3 the dataset for each well is given.

Porosity

Porosity measurements have been carried out under atmospheric pressure. To determine

the porosity at depth, a correction factor needs to be applied. This factor is determined by

the pressure within the reservoir. A correction factor of 0.95 has been applied.

The distribution of the corrected porosity is given in figure 4.1 and 4.2. The porosity for

each well is represented by a normal distribution.


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Figure 4.1 Corrected porosity of the Slochteren Fm. in the wells Q07-01, HEK-01,

OZN-01 and Q11-01.

Figure 4.2 Corrected porosity of the Slochteren Fm. in the well Q14-02.

Permeability

The measurements of permeability have been done under atmospheric pressure and

using air instead of water. The air permeability differs from the permeability for water.

Furthermore, the permeability generally decreases by increasing pressure from overbur-

den rock. Therefore, the data need to be corrected. The measurements of the permeabil-

ity have been corrected using the Juhasz compaction correction (Juhasz, 1986). The

equations and parameters for this correction are listed in appendix 3.

0

5

10

15

20

25

30

frequency

[#]

porosity[%]

Q0701

Q1101s2

OZN01

LSM01

HEK01

0

20

40

60

80

100

120

frequency[#]

porosity [%]

Q1402


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The distribution of the corrected permeability is given in figure 4.3. The permeability for

each well is represented by a log normal distribution.

Figure 4.3 Corrected permeability of the Slochteren Formation of several wells.

Porosity-permeability relationship

The porosity and permeability measured in the core analyses can be used to determine

direct relationships between the two. Figure 4.4 shows the corrected porosity and perme-

ability values of the Slochteren Formation. RMA2-regression has been applied to the data

within the area defined by the blue curve. The resulting relationship represents the opti-

mal relation between the porosity and permeability:

10.. (1)

where Kis the permeability in mD and the porosity in fraction. The correlation coeffi-

cient (R) of this relation is 0.8375.

2RMA: Reduced Major Axis.

0

2

4

6

8

10

12

14

16

18

frequency[#]

Permeability[mD]

Q0701

Q1101S2

OZN01

LSM01


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Figure 4.4 Porosity-permeability relationship for the Slochteren Formation based on

core data.

As can be seen in figure 4.4, the core measurements of LSM-01 are significantly lower

than the other measurements. The measurements of the nearby well OZN-01 have been

corrected for the permeability using the relationship between the corrected and uncor-rected permeability of well LSM-01, see figure 4.5. The corrected measurements are

situated in between the other measurement. As no structural features are present which

may explain the different relationship, it is assumed that this measurements in LSM-01

has been influenced by hydrothermal alteration through a fault. LSM-01 shows this may

occur locally. Therefore, the measurements in this well have been used to define the

lower boundary of the uncertainty range of the porosity-permeability relationship.

Likewise, measurements in well Q07-01 are locally much higher than other measure-

ments in the same well. These have also been used to determine the upper boundary of

the uncertainty range of the porosity-permeability relationship.

0. 0.06 0.12 0.18 0.24 0.3core porosity [-]

0.01

0.1

1.

10.

100.

1000.

core

permeability[mD]

21 points plotted out of 6596

RMA: Log(CORE:KHds_is_Juh) = -3.7689 + 27.4040 * CORE:PHIT_DECds R2= 0.8375

Well Zone Depths


(14) LANDSMEER-01 (3) Slochteren For 1454.M - 1650.M

(2) Q11-01-s2 (4) Slochteren For 2878.M - 3043.M

(6) Q14-02 (4) Slochteren For 2 888.89M - 3004.M


3 OZN-01 3 Slochteren For 1310.M - 1472.M


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Figure 4.6 Results of the log analysis of the Slochteren Formation in well LSM-01.

LANDSMEER-01Scale : 1 : 1000DEPTH (1420.06M - 1675.03M) 3-2-2010 09:42DB : IP_59479 (14)

LITHOSTRATIGRAPHY

Gamma ray (GAPI)0. 150.

DEPTH

(M)

TVDSS

(M)

Deep resistivity (OHMM)0.2 2000.

Shallow res istivity (OHMM)0.2 2000.

Sonic (US/F)140. 40.

Matrix density (gm/cc)2.5 3.

Matrix core density (G/C3)2.5 3.

Eff ective porosity0.25 0.

Core porosity0.25 0.

Water s aturation1. 0.

VWCL (Dec)0. 1.

VSILT (Dec)0. 1.

Effective porosity1. 0.

Clay

Silt

Sandstone

Porosity

Z1FringeCarbon

Z1Lower

ClaystoneMember

CoppershaleMember

SlochterenFormation

RuurloFormation

1500

1600

-1339

-1412


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22.039/109140/NB June 8, 2010 33

Porosity

For each well, the porosity has been calculated using the log measurements. De distribu-tion of the log-derived porosity is given in figure 4.7. All wells approach a normal distribu-

tion.

Figure 4.7 The log-derived porosity of several wells.

The quality of the log data is checked by the correlation of the log-derived porosity and

the porosity measured in the cores. Figure 4.8 gives the correlation between the log-

derived and the core porosity. The closer the data to the line, the better the log-derived

data represent the true values within the formation. The data points are located above

and below the perfect correlation-line which means there is no systematic over- or under-

estimation of the log-derived porosity.

0

50

100

150

200

250

300

350

400

frequency[]

porosity[%]

Q0701

Q1101S2

LSM01

Q1003

SPL01S1

ARV01

ALE01


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Figure 4.8 Correlation between the log-derived effective porosity and the corrected

core porosity.

PermeabilityPermeability cannot be measured directly by well log measurements. Neither can it quali-

tative be derived from log measurements. To calculate the permeability, the relationship

between the porosity and the permeability derived from core measurements is applied to

the calculated porosity logs. As the correlation between the measured and log derived

porosity is considerably, the porosity permeability relationship can be safely applied to

determine the permeability.

The distribution of the calculated permeability is given in figure 4.9. All wells show a rea-

sonable log-normal distribution of the permeability.

0. 0.05 0.1 0.15 0.2 0.25corrected core porosity [-]

0.

0.05

0.1

0.15

0.2

0.25

logeffectiveporosity[-]

39 points plotted out of 56001Well Depths

(1) Q07-01 0.M - 2999.99M

(2) Q11-01-s2 2840.M - 3068.88M

(5) Q11-03 0.M - 3199.94M

(6) Q14-02 2800.M - 3200.M

14 LANDSMEER-01 0.M - 1705.M


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Figure 4.9 The calculated permeability of the different wells.

4.4 Transmissivity of the wells

The transmissivity is a measure of how much water can be transmitted horizontally. It is

calculated in a well by the summation of all the permeabilities. However, this is for one

point only. For upscaling, the product of the mean permeability times the net thickness of

the reservoir is taken. The mean permeability can be calculated using different methods.

The most representative mean depends on the permeability distribution in the reservoir.The distribution can be uniform in a certain direction (e.g. marine or eolian deposits) or

more heterogeneous (e.g. a braided river system). The Slochteren Formation consists

mainly of eolian sediments. Only the lower part has been deposited by river systems but

still contains a more or less uniform permeability in horizontal direction. Therefore, the

mean permeability is calculated by using the following formula:

(2)

Where Keff is the effective permeability, Kthe permeability and a constant. Fora

value of 1/3 has been used, which belongs to a 3D situation (Desbarats, 1992; Herweijer,

1997).

For the wells containing permeability core measurements, it is assumed that the meas-

ured permeability in the cores is representative for the whole of the formation. The wells

that do not contain any permeability core measurements, the calculated permeability from

the logs is used for the calculation of the mean permeability.

The transmissivity of the wells is calculated by multiplying the net thickness with the cal-

culated mean permeability. The net thickness of the reservoir is determined by excluding

the thickness of the intercalated claystone from the total thickness. Assumed are a mini-

mal clay volume of 40% and a maximum porosity of 6% for the claystone.

The results are shown in table 3 in appendix 3.

0

50

100

150

200

250

300

350

frequency[#]

Permeability[mD]

Q0701

Q1101S2

LSM01

Q1003

SPL01S1

ARV01

ALE01


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4.5 Water quality

The quality of the water is directly related to the total dissolved solids (TDS) in the water.

The TDS can be determined by the electrical resistivity of the formation water. This resis-

tivity can be derived from the resistivity measurements of the formation measured in the

logs and the temperature of the formation where the resistivity measurements took place.

The TDS can also be determined directly in a production test. These tests are more reli-

able as these direct measurements instead of derived from logs.

In the well OZN-01, drill stem tests have been carried out. In these tests, a TDS of 52-58

g/l has been measured. Production tests in WOB-01-S1 measured a TDS of 145 g/l.

The different wells show a range of 100 to 250 g/l for the total dissolved solids. Excep-

tions are WRV-01, which is considered to be non representative for the area, and LSM-01, which has a very low resistivity. The production test in well Q07-01 measured a TDS

of 255 g/l. This is below the saturation point of natrium chloride at that temperature. The

risk of well blockage due to salt precipitation within the wells is therefore considered to be

low.

4.6 Well correlations

The wells have been compared, using several multi-well plots, to see if the formation is

lateral homogeneous. In figure 4.10, the plot of the wells LSM-01, SPL-01-S1 and WOB-

01-S1 can be seen. This plot shows the thickness is reducing from the north to the south.

Furthermore, the gamma ray values of LSM-01 and SPL-01-S1 are showing a clean for-

mation from the top until the lower part where more clay is present according to the

gamma ray and other observations. WOB-01-S1 shows a clean upper half of the forma-

tion while the middle and lower part have a different response in the gamma ray. These

observations show that the Slochteren Formation is fairly homogeneous on the Zand-

voort-Ridge, but slightly changes within the West Netherlands Basin. This was expected

as WOB-01-S1 is located closer to the sediment source and will therefore have slightly

different depositional facies from the wells more to the centre of the basin where large

dunes where present.

The other multi-well plots are shown in appendix 3. The plot of HEK-01, SPL-01-S1 and

ARV-01 confirms the observations of figure 4.10. The plot of Q07-01, Q11-01-S2, Q11-

03, SPL-01-S1 and WRV-01 also show a fairly homogeneous Slochteren Formationwhich is less clean at the base of the formation. In this plot, it is also shown the thickness

does not vary much from west to east, except in WRV-01. This reduced thickness is due

to deposition due to a high in the paleo-landscape at the time of deposition.


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Figure 4.10 Multi-well plot of LSM-01, SPL-01-S1 and WOB-01-S1.

4.7 Conclusions

The petrophysical analysis shows a fairly homogeneous Slochteren Formation which is

less clean at the base of the formation. The core measurement show permeabilities

which are lower than expected based on the porosity of the wells. This is likely due to

cementation of dolomite and anhydrite of the grains as discussed in chapter 3. Well LSM-

01 has a very low porosity-permeability relationship compared to the other wells which all

follow the same trend. The permeability of the well has most likely been reduced due to

hydrothermal alteration at the influence of a nearby fault. As this may occur throughout

LITHOS

GR(GA PI)0. 150.

TVDSS

(M)

Middle Claystone Memb

1 Middle Claystone Memb

1 LowerClaystone Membe

CoppershaleMember

SlochterenFormation

Limburg Groep

-1266

-1339

-1412

-1485

LANDSMEER-01

0. - 1718. (DEPTH)lithostra

GR(GA PI)0. 150.

TVDSS

(M)

Mainclayst.Member

ZEUpper Claystone

Z3CarbonateMember

GreySaltClaystone

ZeMiddle Claystone

Z1Middle Claystone

1 LowerClaystone Membe

SlochterenFormation

Limburg Groep

-2004

-2046

-2090

-2134

-2178

SCHIPHOL 1 SIDETRACK 1

2600. - 2890. (DEPTH)Lithostr

GRS ( API)0. 150.

TVDSS

(M)

Mainclayst. Member

ZEUPPCLAYST

Z3 CARBONATE

GREY SALTCLAY

Z2 MIDDLE CLST

Z1 MIDDLE CLST

1 Fringe Carbonate Memb

SlochterenFormation

Limburg Groep

-2527

-2576

-2625

-2674

-2722

-2771

-2819

WOUBRUGGE 01 SIDETRACK 1

1394. - 3795.1 (DEPTH)

Z1LowerClaystoneMember

SlochterenFormation

Lim

burgGro

ep

Limb

urgGro

ep


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22.039/109140/NB June 8, 2010 38

the area, the measurements in this well have been used as the lower uncertainty range of

the permeability.

Overall the values show a formation which can at some places be used as a reservoir for

geothermal energy, depending on the amount of cementation. The area around HEK-01,

LSM-01 and SPL-01-S1 seems suitable, but the large uncertainty in the permeability

needs to be taken into account.


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5 Basin analysis

The porosity evaluates with time essentially due to the compaction resulting from the

increasingpressure coming from the deposition of the sediments. In the case of inverted

basins, the acquired measurements could be higher than expected because they have

been buried deeper than the present situation. Mechanical and chemical processes influ-

ence the properties of the formation. In this chapter, the changes in reservoir propertiesdue to mechanical compaction are determined. In order to have a good determination of

the porosity in a given reservoir the estimation of the maximum burial depth is required.

The exact workflow of the basin analysis is given in appendix 4.

5.1 Maximum burial depth determination

To determine the maximal burial depth, the original thicknesses of the reservoir and over-

lying formations need to be estimated. This can be determined in two steps. The first

step, in 1D, recreates the evolution of the basin (e.g. sedimentation, tectonic inversion,

erosion phases ...) at the location of the wells in order to estimate the thickness of the

eroded layers. In the second step, the 3D geological model is used to estimates the areas

of each layer where no erosion has taken place.

For the first step, vitrinite reflectance data is used. From these measurements the degree

of coalification can be determined. The degree of coalification is a measurement for the

change that the organic material has undergone. The chemical signature of sedimentary

layers change through time due to the influence of temperature, pressure and time itself.

If the coalification is higher than expected based on the present day depth, the layer has

been located at greater depth in the past. Of the wells WRV-01, OZN-01, ARV-01, Q11-

01-S1, SPL-01-S1 and WOB-01-S1 vitrinite data was available.

The studied area covers a part of three tectonic structures. As shown on figure 5.1, it

covers the northern part of the West Netherlands Basin, the Zandvoort Ridge and the

southern part of the Central Netherlands Basin. In profile (figure 5.2), it is clear that the

evolution of the areas have been different. Within the Zandvoort Ridge, a first erosion

phase took place in the late Jurassic eroding the Jurassic deposits. In the West Nether-

lands Basin and Central Netherlands Basin, the deposition is continuous and conformably

from the Carboniferous to the end of the Cretaceous. At the end the Cretaceous a major

tectonic inversion appears, creating an erosion phase which has eroded the Mesozoic

depositions.


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Figure 5.1 Location of the area of interest on the structural map of the Rijnland

Group (modified after TNO, 2004)

Figure 5.2 Location of the area on a cross section. Dashed lines shows the erosion

interface (modified after TNO, 2004)


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5.2 Burial history reconstruction method

Burial history reconstruction in 1D

The modelling imitates the evolution of the tectonic and sedimentary events during thegeological history starting from the oldest layers in the considered well. The wells arecorrected for their deviation to obtain vertical thicknesses. The process takes in accountthe thickness of the layers, the timing of the deposition and the lithology. The boundaryconditions are the Paleo Water Depth (or PWD), the base sediment heat flow, and theSurface Water Interface Temperature (or SWI-Temperature). The base sediment heatflow is assumed to be equal to the surface heat flow. The PWD values have been inter-preted from the stratigraphic nomenclature of the Netherlands. The surface heat flowshows an evolution from the Central Netherlands Basin and the West Netherlands Basin.In the Central Netherlands Basin and the Zandvoort Ridge, the heat flow is 70 mWm

-

from 350 My to 225 My and 75 mWm- from 200My to present day. In the northern part of

the West Netherlands Basin, the heat flow decreases to 65 mWm- from 350 My to 225My and 70 mWm

- from 200 My to present day. And in the central part of the West Neth-

erland Basin, the heat flow is 50 mWm- from 350 My to 225 My and 55 mWm

- from 200

My to present day. The SWI-temperature has been calculated at 52 of latitude north.The example of the resulting burial history of well WOB-01-S1 is shown in figure 5.3.

Figure 5.3 Burial history of the well WOB-01. For abbreviations see appendix 5.

The erosion phases can now be modelled using the burial history of the wells. The addi-

tional inputs are the thicknesses which have been eroded and the timing of this erosion

phase. Within the model, a layer can be created if it has been completely eroded. The

initial assessment of the amount of erosion for each layer can be estimated in first ap-

proximation from the thicknesses left in the surrounding area. The values are then cali-

brated using the vitrinite reflectance values (Middleton, 1990). Figure 5.4 shows the re-

sults of the modelled vitrinite reflectance obtained for the six wells. The crosses represent


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the vitrinite reflectance measurements (the extension of the horizontal segment gives the

uncertainty on the measurements). Horizontal parts of the modelled vitrinite reflectancecurve are the representation of the erosional phases.

Figure 5.4 Vitrinite reflectance calibrations of the erosion phases using 1D model-

ling.

Estimation of the un-eroded section of a present day layer

For each layer in the area, the areas which had no erosion have been determined.

This has two purposes. First of all, it gives the initial thickness reference for the 1D model

previously discussed. Given the low number of available wells for the 1D modelling, it

gives a better coverage of initial thickness for the maximum burial reconstruction. This is

done by determining the areas on the map which still have the younger formation on top.

If this condition is not respected, the area is not selected for the determination of the ini-

tial thickness. Figure 5.5 shows a profile were the striped section are the zones where


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this condition is not met for that specific layer. Figure 5.6 shows the results after the par-

tially eroded areas have been removed.

Figure 5.5 Representation of the concept to select the non-eroded area for each

layers

Figure 5.6 Areas where the initial thickness of the deposits of the Lower Cretaceous

is preserved.

At this stage the initial layer thickness is still stored into two different datasets. In order to

perform an interpolation between the available data, the two separate dataset have been

regrouped. The interpolation in the empty area has been performed with a natural

neighbours technique.

5.2 Burial depth estimation

The maximal burial of the Slochteren Formation could have appeared at three different

moments in geological history. For each of these, a separate scenario has been created.

The maximum burial is the present depth burial;

The maximum burial appears before the late Jurassic erosion phase;

The maximum burial appears before the inversion of the Late Cretaceous.


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The second scenario is only pertinent for the Zandvoort Ridge because it is the only area

affected by this erosion phase in the research area. The un-eroded layers considered arePermian, Triassic and Jurassic. In the third scenario, the layers considered are the un-

eroded Permian and Mesozoic Layers. Within the Zandvoort Ridge, the following layers

are already eroded at the end of the Cretaceous: Schieland and Niedersachsen Groups

(Upper Jurassic), Altena Group (Lower Jurassic) and Germanic Trias Group (Triassic).

For each of the scenarios the maximum burial depth is calculated. As it is not known at

which moment the maximum burial occurred, the highest burial value of the three scenar-

ios is used for the map. The maximum burial depth of the top of the Slochteren Formation

is shown in figure 5.7. An uncertainty of 10% on the results of the maximum burial depth

calculations is assumed.

Figure 5.7 Maximum burial of the top ROSL (top of the Slochteren Fm)

5.3 Determination of the porosity

In order to model the porosity within the Slochteren Formation, the available average

porosity and depth data of the wells were used to estimate the relationship between

depth and porosity.


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Porosity-depth relationship

In order to model the porosity within the Slochteren Fm the available values has beenstudied to estimate the relation between depth and porosity. The wells and their data areshown in table 5.1. The maximum burial of the top of the Slochteren Fm. has been mod-elled for each well (TVD Maximum burial).

Table 5.1 Available porosity values in the Slochteren Fm in the working area.

WellTrue vertical

depth [m]

TVD Maximum burial [m] average effectiveporosity [%]

1D modelled estimated

Q07-01 2,375 2,374 14.7

Q10-03 2,941 2,939 15.1

Q11-01-S2 2,878 2,973 13.5Q11-03 2,824 2,939 12.6

Q14-02 2,889 2,885 10.6

ARV-01 2,400 2,631 13.0

SPL-01-S1 2,747 4,089 17.7

WOB-01-S1 2,605 3,051 12.2

WRV-01 2,340 2,335 6.7

LSM-01 1,454 2,888 17.4

ALE-01 1,733 2,479 13.4

OZN-01 1,355 2,986 13.0

It can be noticed that with the exception of Q11-01-S2, the Slochteren Formation is pres-

ently at its maximum burial depth in the offshore wells. This also holds for WRV-01, the

only well representing the Zandvoort Ridge. The Slochteren Formation in the other wells

all has been buried deeper in the past.

Using the Athys Law (Athy, 1930) a porosity-depth curve has been determined (equation

1).

kzez 101 (1)

Where z is the porosity at depth in %, 1 the minimal porosity in %, 0 the initial po-rosity in %, kthe Athys factor in km

-1and zthe depth in km.

The initial average porosity of the formation is assumed to be 41 %. The minimum aver-

age porosity is assumed to be 1% and Athys factor is assumed to be 0.43 km-1

.

The result of the Athys law is the curve calibrated by the average porosity measurements

at their maximum burial depth shown in figure 5.8. Two measurements are noticeably out

of the main trend which are both located close to the border of the Zandvoort Ridge. The

average porosity of SPL-01-S1 is too high for the calculated maximum burial depth. This

is probably due to the closeness of the wells to the border of the Zandvoort Ridge. Well

WRV-01 has an average porosity which is too low for the maximum burial depth at this

location. This can be explained by the difference in sedimentation as this well was loca-


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tion on a paleo-high in the landscape and was considered to be non-representative for

the research area.

Figure 5.8 Athys law curve fitted to the average porosity - TVD maximum burial

measurements.

The maximum burial depth has been used as a reference to calculate the porosity, within

each cell the average porosity has been calculated using the Athys Law (see equation

1). The results are shown in figure 5.9. The map shows average porosities ranging from

8% to 20%. The highest porosity on top of the Zandvoort Ridge gives values from 13% to

20%. The Central Netherlands Basin has intermediary values from 12% to 17%, and the

West Netherland Basin indicates the lowest values with a minimum of 7% in the south-

west of the research area.

The uncertainty of the porosity results is depending on the uncertainty on the maximum

burial (see figure 5.7) and the uncertainty on the porosity (see figure 5.8). The results of

the sum of these uncertainties is shown in the figure 5.10. Figure 5.9 can also be found in

appendix 2.

The comparison between the calculated porosity and the measured values is coherent,

with the exception of SPL-01-S1 and WRV-01. From a porosity point of view the geo-

thermal potential of the Slochteren Fm is the highest in the Zandvoort Ridge (SPL-01-S1),

acceptable in the Central Netherland Basin and very variable in the in the West Nether-

lands Basin.

2000

2500

3000

3500

4000

5.00 7.00 9.00 11.00 13.00 15.00 17.00 19.00

Porosity (%)

Depth(m)

Arty's Law

Porosity measurments - 1D

modelled depth calibrated

Arty's Law +10%

Arty's Law -10%

Porosity measurments - 1D

modelled depth uncalibrated


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permeability of the wells which is shown in figure 5.10. The relationship is best described

by the following formula;

0.00007.

Where is the average permeability in mD and the average porosity in fraction. The

correlation coefficient (R) of this relation is 0.8563 which means the relationship is reli-

able.

Figure 5.10 Average porosity versus average permeability of the wells.

The average permeability in the area is used, in combination with the thickness of the

Slochteren Formation to obtain the transmissivity in the area. The resulting map is shown

in figure 5.11. The map can also be found in appendix 2.

0.01

0.1

1

10

100

1000

0.00 0.05 0.10 0.15 0.20

Averagepermeability[mD]

Averageporosity[]


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Figure 5.11 Transmissivity of the Slochteren Formation in the area.

5.5 ConclusionsThe results of the basin analysis show the offshore wells are presently located at their

maximum burial depth. The same holds for the Zandvoort Ridge. The Slochteren Forma-

tion in the other wells all has been buried deeper in the past. The porosity and permeabil-

ity in these areas were influenced by the greater depth and are lower in these areas.

From this, it can be concluded that the location of the structures determine the areas

which have lower reservoir properties. It is therefore advised to reinterpret the available

seismic data for the exact location of the faults.


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6 Fault systems

6.1 General

Natural fractured rock may have a higher permeability and is therefore interesting as a

source for geothermal energy. Fracture zones along actively deforming normal or strike-

slip faults are well-known for their high permeabilities which make them particularly inter-esting for hydrogeothermal projects. Not all fractures will have a higher permeability; this

depends on a couple of factors.

1. The fault system needs to be active;Minerals in the water flowing through the fractures will precipitate and seal the frac-tures. If a system is active, there has not been enough time to fill the fractures withminerals. In this study, a fault system is considered to be active if it crosscuts thebase of the Upper North Sea Group. This means the fracture has been active in thelast 24 million years.

2. The stress direction;The direction of the present day stress determines whether a fault system is poten-

tially open or closed.

3. The amount of offset;The amount of offset determines also the width of the fractured zone. The wider thezone, the larger the fractured area and the greater the drainage area of a well. If theamount of offset is less than the reservoir thickness, there will still be reservoir-reservoir contact along the fault zone. This will increase the drainage area as waterfrom the reservoir rock will also reach the well.

4. Fault stepping;In the area where the movement is transferred from one fault to the next, the frac-tured zones around the faults overlap. In this area, the amount of fractures is higherand the flow rate which can be achieved is higher. This factor is not necessary, butincreases the probability.

The results of the petrophysical analyses show a large uncertainty in the permeability.

Therefore, fault systems present within the Slochteren Formation who full fill the factors

mentioned above are interesting as the permeability is expected to be higher around that

area. Generally speaking, the flow rate from a fracture zone is higher by a factor 2-3

compared to non-fractured areas in the same formation.


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6.2 Analysis

Several of the faults in the area cut through the base of the Upper North Sea Group. Thiscould be seen in the geological cross sections in figure 2.5 to 2.7. In figure 6.1, the faultsat the base of the Upper North Sea Group are shown. The faults in figure 6.1 are thosewhich have been mapped in the regional mapping. Detailed analyses of the seismic linesare therefore needed to determine the exact location of the faults. Examination of some2D lines present in the area showed more faults are present than those mapped in theregional mapping. This means that in the areas where no faults are indicated, faults mightstill be present.

Figure 6.1 The faults at the base of the Upper North Sea Group. The shaded areas

probably contain higher permeabilities due to fractures.

As the fault is bended at several places, there are areas where the direction of maximum

stress does not align with the fault. At these places, another component besides the nor-

mal component is present. In these areas transtensional faulting is taking place. It is be-

lieved these areas have higher permeabilities than normal faults. These areas are indi-

cated in figure 6.1 as well.

Principal stress orientation

The knowledge about the principal stress orientation in the Netherlands is based on

borehole breakout (BO) and Drilling Induced Fracture (DIF) data. Figure 6.2 shows the

maximum horizontal stress orientations for the Netherlands and neighbouring countries

(Heidbach et al., 2008)


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Figure 6.2 Maximum horizontal stress orientation in The Netherlands and surround-

ing regions(Heidbach et al., 2008).

Figure 6.2 indicates a large data gap for the central and southern part of the Netherlands.

However, based on the data from the NE and offshore it can be stated that the maximum

horizontal stress orientation in the area of interest should be about NNW-SSE. For the

calculation of the slip and dilation tendency a value of N155E has been taken.

Stress regime

The available stress data do not give direct information about the stress regime. DIFs in

offshore Netherlands may point to a strike-slip stress regime (Zoback, 2008). However,

active tectonics and fault-guided subsidence in large areas of the central and southern

Netherlands point to a normal faulting stress regime. Thus, for the calculation of the slip

and dilation tendency it has been decided to take a normal faulting stress regime into

consideration, which means that the maximum principal stress 1 equals the vertical

stress Sv, medium principal stress 2 equals maximum horizontal stress SH, and minimum

principal stress 3 equals minimum horizontal stress Sh (Anderson, 1951).

Stress magnitudes

Stress magnitudes are not available for BO and DIF data (Heidbach et al., 2008). There-

fore, for the calculation of the slip and dilation tendency magnitudes for the three principal

stresses have to be estimated. The vertical stress magnitude can be calculated as fol-

lows:

gzSV

Where is the rock density in kg/m, g is the gravity acceleration in m/s and z is the

depth in m.


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For the determination of the minimum horizontal stress the following equation can be

used in case of a normal faulting regime (Zoback, 2008):

22

3

1 ])1([

pS

pS

h

V

Wherep is the pore pressure in N/m2

and is the coefficient of friction.

In a normal faulting stress regime pore pressure is predominantly hydrostatic, and a

good guess for 0.6. Assuming a critical stressed crust, the equation gives a lower

limit for the value of the minimum horizontal stress Sh, (Zoback, 2008)The magnitude of

maximum horizontal stress SH will not exceed Sv. and will not fall below Sh. A more pre-

cise estimate of the SH-magnitude without any further data is not possible.

For the calculation of the vertical principal stress SV an average rock density of 2.500

kg/m3

and for the calculation of the minimum horizontal stress Sh a coefficient of friction

= 0.6 has been taken.

Calculation method of the Slip and Dilation tendency

Slip is likely to occur on the fault surface when the shear stress on the fault plane equals

or exceeds the frictional resistance to sliding which is proportional to the normal stress

acting across the fault plane. The slip tendency TS is therefore the ratio of shear stress

to normal stress n:

nS

T

Faults with values TS 0.6 are ideally oriented for slip. The slip tendency therefore pro-

vides information about which faults are near the ideal orientation for slip and are most

likely to be associated with zones of increased fracture density and enhanced fracture

permeability (Morris et al., 1996).

Dilation of fractures is controlled by normal stresses as a function of lithostatic and tec-

tonic stresses as well as pore pressure. The ability of a fracture to transmit fluids is di-

rectly related to its aperture, which in turn is a function of the effective normal stress n

acting on the fracture surface. Given the 3D stress tensor (1,2,3) the effective normal

stress n can be calculated for all fracture plane orientations. The normal stress can be

normalized with respect to differential stress. The dilation tendency TD for a fault surfaceis then defined as (Morris et al., 1996):

31

1

n

DT

The range of possible TD values is within 0 and 1, whereby 1 indicates faults with highest

possible aperture.


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6.3 Location specific

The pore pressure is given with 4430 psi or 30.5 MPa at a depth of 2.925 m and 4020 psi

or 27.7 MPa at a depth of 2680 m. In addition, information about the effective uniaxial

compressional strength is given with 4860 psi or 33.5 MPa.

North of Haarlem

The fault north of Haarlem is considered to be active. This is confirmed by the earthquakeof tectonic origin which occurred at Beverwijk. Its location is shown in figure 6.3. Historicalsources also indicate the occurrence of two seismic events in the 19

thcentury which were

felt in Haarlem.

Figure 6.3 The location of the Beverwijk-earthquake in blue on January 30, 1997with the faults at the base of the Upper North Sea Group.

The amount of offset is enough to create a wide fracture zone. Therefore, the dilation andslip tendency have been calculated for this fault. The trace of a fault at the base of theRotliegend reservoir has been taken from the geological survey map. The fault dip is

given with 58.6N.

The principal stresses were calculated for a depth of 2,300 m and a pore pressure of 23.4

MPa, which is hydrostatic. The magnitude for SH has to be estimated. The following re-

sults were obtained: SV = 56.4 MPa, SH 41.5 MPa, Sh = 34 MPa.

The slip tendency for the investigated fault supply high values between 0.36 and 0.60

with slightly higher values in the southeast. They indicate a favourable orientation of the

fault with respect to the recent tectonic stress field and point to ongoing deformations

along the fault. The values for the dilation tendency TD are in the range between 0.53 and

0.73. The highest values are indicated for the small bend near the south-eastern termina-

tion of the fault, which is almost parallel to the SH-direction. Based on the calculated val-

ues for the slip and dilation tendency ongoing displacements along the fault with a signifi-cant dilation component is likely to occur.

Haarlem-Schiphol

Based on the regional mapping information, the fault underneath Haarlem and the airportSchiphol is not really interesting as the offset of the formation along the fault is in therange of tens to hundred meters. This produced a smaller fractured zone. Seismic inter-pretation can be used to further qualify the offset.


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South of Haarlem-Schiphol

The fault just north of SPL-01-S1 is considered to be active. The amount of offset is alsoenough to produce a wide fracture zone; near SPL-01-S1 the amount of offset is 300 m.

The principal stresses have been calculated for a depth of 1,900 m using a hydrostatic

pore pressure. The resulting magnitudes are as follows: SV = 46.6 MPa, SH 34.2 MPa,

Sh = 27.9 MPa.

The situation for this fault is very similar to the situation of the fault north of Haarlem be-

cause of the similar fault trend. The difference is given by the slightly lower dip angle for

the fault south of Haarlem-Schiphol. The slip tendency is very high all along the fault (Ts

between 0.52 and 0.68), w

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