bense & van balen2004 - the effect of fault relayand clay smearing on gw flow pattern

8/10/2019 Bense & Van Balen2004 - The Effect of Fault Relayand Clay Smearing on GW Flow Pattern

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The effect of fault relay and clay smearing ongroundwater flow patterns in the Lower RhineEmbaymentVictor F. Bensen and Ronald Van Balenw

nDepartmentof HydrologyandGeo-Environmental Sciences,Vrije Universiteit Amsterdam, Amsterdam,TheNetherlands

wDepartment of QuaternaryGeologyandGeomorphology,Vrije Universiteit Amsterdam, Amsterdam,

TheNetherlands

ABSTRACT

Faults strongly impact groundwater ow in the unconsolidated sediments of the Lower Rhine

Embayment. Hydraulic head maps show that many individual faults form a barrier to uid ow

whereas relay structures in these faults are sites of hydraulic contact between otherwise separated

aquifers.The uid ow patterns around the Rurrand Fault close to the largest open- pit mine in the

Lower Rhine Embayment is one of the rst well-documented examples of uid ow around a fault

relay zone.The eect of clay smearing could be quantied using the Shale Gouge Ratio (SGR) methodthat is co mmon in hydrocarbon-related studies but has not been applied to groundwater ow data so

far.The eect of fault relay zones on groundwater ow is analysed using numerical simulations. It is

concluded that fault relay needs special consideration in the evaluation of the sealing capacities of

faults in sedimentary basins. Moreover, it is demonstrated that the SGR methodology is a promising

tool for the estimation of fault zone hydraulic properties in hydrogeological modelling.

INTRODUCTION

Faults have an important role in the distribution of uids,

hydrocarbons and groundwater in sedimentary basins(Smith, 1980; Knipe, 1993; Aydin, 2000). Recent studies

show the impact of faults to groundwater ow patterns at

shallow depth (o500 m) in the Roer Valley Rift System

(Bense & Van Balen, 2003). The hydraulic behaviour of

faults cutting through unconsolidated sediments at shal-

low depth is likely to be dierent from that of faults at

depths where rocks are lithied (41000 m) as fault perme-

ability is a function of burial depth and the rheological

properties (e.g. strength) of the faulted rock (Rawling

et al., 2001; Benseet al., 2003b). Moreover, the heteroge-

neous and anisotropic distribution of permeability in fault

zones is dicult to characterize and needs extensive eld

studies. Therefore, the development of generic models offault zone hydrology remains elusive. Recent empirical re-

presentations of fault zone properties to predict fault zone

hydraulic properties in petroleum reservoir models has

shown promise (Fristad et al., 1997; Yielding et al., 1999;

Harriset al., 2002). The Shale Gouge Ratio (SGR) is one

of the most widely applied of these algorithms relating

the clay or shale content of the faulted lithology and fault

throw (Yieldinget al., 1997; Freeman et al., 1998). Doughty

(2003) calculated fault zone parameters for an exhumed

fault in the Rio Grande Rift, New Mexico, and argued that

of all available algorithms, the SGR algorithm is best cap -

able of predicting fault zone hydraulic properties on theoutcrop scale, although this was not rigorously tested.

The area of the Lower Rhine Embayment (LRE) is the

southward continuation of the Roer Valley Rift System

(RVRS). These rift systems are part of the Neogene Rift

System in western Europe that also comprises the Upper

Rhine Graben (Ziegler, 1994; Michon et al., 2003). Figure

1 shows an overview map of the LRE. On many places in

the RVRS and the LRE faults form strong barriers to uid

ow (Wallbraun, 1992; Bense & Van Balen, 2003; Bense

etal., 2003a). Field studies (Ernst & De Ridder, 1960; Leh-

ner & Pilaar, 1997; Bense etal., 2003b) show that clays mear-

ing along the fault plane is the most important mechanismto account for the observed low permeabilities along faults

in the RVRS and LRE. Despite the sealing properties of

faults in these areas, there are locations where there exists

lateral hydraulic contact between otherwise compartmen-

talized aquifers. These locations are found to coincide

with relay zones that exist where fault zones have an en

echelonstructure of overstepping segments.

In this paper, the SGR approach is used to predict fault

zone hydraulic properties, based upon geological and

groundwater owdata around the Rheindahlen Fault situ-

ated in the LRE.Values of SGR are calculated at four loca-

tions along the Rheindahlen fault in the LRE that are

compared with observed hydraulic head dierences across

Correspondence: Victor F. Bense, Department of GeologicalSciences, Indiana University, 1001 East 10

thStreet, Bloomington,

IN 47405, USA. E- mail: vi [email protected]

BasinResearch (2004)16, 397411, doi: 10.1111/j.1365-2117.2004.00238.x

r 2004 Blackwell Publishing Ltd 397


2/15

the fault zone. In addition to this analysis, groundwater

ow simulations were carried out to better understandthe eect of fault relay structures on groundwater ow.

The results of these simulations are compared with the

observed groundwater ow eld around the Hambach

open-pit mine that is located close to a regionally impor-

tant relay zone in the Rurrand Fault. Around this relay

zone the groundwater ow is exceptionally well con-

strained as a result of a very dense network of groundwater

ow observation wells.

In the next sections, the geological setting of the LRE is

described and a background on the hydrological impact of

fault zone processes (e.g. clay smearing) and fault segmen-

tation, in unconsolidated sediments, is provided. Subse-

quently, the groundwater ow patterns in the LRE are

discussed in relation to faults. Based upon this discussion,

the two case studies will be introduced which are furtheranalysed in this paper.

GEOLOGICAL SETTING OF THE LRE

The LRE was formed at the northwestern margin of the

Schiefergebirge (Fig. 1). Subsidence took place along

NW^SE trending faults. During theTertiary a thick sedi -

ment pile was deposited consisting of shallow-marine

sand (Schferet al., 1996). During the Miocene, a coastal

plain with swamps and forests developed. The peat layers

that formed during this period are today mined as lignite

in large open-pits. During the Pliocene, the North Sea

53o

4o 9o

100 km

N

E

0 20 km

Rhine

Schiefer Gebirge

Maas

RheindahlenFault

RurrandFault

PeelB

ound

aryFau

lt

Ruran

dFau

lt

ErftFault

ViersenFault

TegelenFault

NL

GB

North

Sea

PeelBlock

RoerValleyGraben

ErftBlo

ck

VenloBlock Kr

efeldBlock

Fig.1. Tectonic overview map of the

Lower Rhine Embayment with the

locations of the tectonic blocks and

faults as mentioned in the text.

The grey area indicates the extent

of the Schiefer Gebirge. NL, the

Netherlands; G, Germany; B,

Belgium; R, Rhine river; M, Maas

River.The dashed line indicates the

extent of Fig. 5.

r 2004 Blackwell Publishing Ltd, Basin Research, 16, 397^411398

V. F. Bense andR.VanBalen


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retreated further and uvial systems developed that were

dominated by the Rhine River. Basin subsidence ceased,

tectonic uplift took place and uvial terrace gravels spread

widely to cover the area. Figure2 shows the lithologic units

that comprise the Venlo Block (for location see Fig. 1).

The normal faults that developed in this extensional re-

gime are synsedimentary (i.e. fault throw increases with

depth). The Rurrand Fault, which is known as the Peel

Boundary Fault in the Dutch part of the system, separates

the Roer Valley Graben from the Peel Block in the north

and the Erft Block in the south. The Venlo Graben north

of the Peel Block is delineated by the Wegberg Fault in the

south (in the Netherlands known as theTegelen Fault) and

the Viersen Fault in the northeast.

THE IMPACT OF FAULTS ON FLUID FLOW

The most direct impact of faults on uid ow is that, as a

result of fault throw, conning units can be juxtaposed

against aquifers resulting in a blocking of uid ow. This

is illustrated in an interpreted photograph of a trenched

outcrop across the Geleen Fault zone that is located at the

southern edge of the Roer Valley Graben in the Dutch part

of the Roer Valley Rift System (Fig. 3). Here, juxtaposition

of well-permeable sands against low-permeable silt isobserved.

Two additional aspects of faults that can have a strong

impact on uid ow are discussed in the following para-

graphs. These are (1) tectonic processes (deformation)

along the fault plane that impact the fault zone hydraulic

properties and (2) the impact of horizontal and vertical

segmentation of faults on uid ow around the fault zone.

Fault zone processes

In sedimentary successions where clay or shale layers are

present, dragging or ductile ow of these layers along thefault plane between the up- and down-thrown source

beds, can result in a clay smear along the fault plane

(Smith, 1980; Knipe, 1993; Lindsay etal., 1993; Fulljameset

al., 1997). Clay smearing is a process that can strongly re-

duce the eective hydraulic conductivity of a fault zone

(Takahashi, 2003). Field studies in the open- pit lignite

mines (Lehner & Pilaar, 1997) and a trenched outcrop over

the Geleen Fault show that clay smearing is occurring

along faults in the Roer Valley Rift System as well (Hout-

gast et al., 2002; Bense & Van Balen, 2003). However, the

latter studies show that that next to clay smearing, drag

or injection of sand and gravel along the fault plane also

occurs (Fig. 3).

Vclay[%]Name

&Description

ID

Q

TR

6D

6C

6B

6A

Gravels and

coarse sands

Frimmersdorf

Lignite

Mrken

Lignite

Tegelen and

Reuver Clay

5

8

MaximumThickness

[m]

Age

Late-Pleistocene

&

Holocene

Late Pliocene -

Early Pleistocene

180

20

40

80

20

50

50

> 100

Hauptkies

Middle Miocene

Cologne

Sand

Late Miocene -

Early Pliocene

Neurather

Sand

Early Miocene

Sand

5

80

5 60

5 60

30

30

5 60

5 60

Fig. 2. General stratigraphical column of geological and

corresponding units discerned on the Venlo Block.Vclayis

the volume percentage of clay (0^100).

Fig. 3. Clay smear in unconsolidated sediments in a trenched

outcrop over a secondary fault in the Roer Valley Rift System.

The coin has a diameter of 2cm.

r 2004 Blackwell Publishing Ltd, Basin Research, 16, 397^411 399

Effect of fault relayand claysmearing ongroundwater flowpatterns


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Because both clay and sand can be dragged into the nar-

row fault core,the permeability of the fault core is expected

to be strongly anisotropic so that it is enhanced along the

fault plane whereas perpendicular to the fault plane per-

meability can be very low. A tectonically deformed zone

(i.e. damage zone) anking the fault core can potentially

form a preferential ow path in unconsolidated sediments

because the deformation bands developing in the damagezone through particulate ow can have a slightly enhanced

permeability (Bense etal., 2003b). Cataclasis (i.e.the break-

ing and crushing of grains) that strongly reduces pore

space and permeability along deformation bands is be-

lieved to be the dominant deformation mechanism only

at depths larger than 500^1000 m (e.g. Antonellini &

Aydin, 1994; Fulljames etal., 1997; Bense etal., 2003b).

Fault zone structure

Normal faults that develop in extensional tectonic regimes

are segmented. They are generally composed of arrays of

overstepping segments (Peacock & Sanderson,1994; Walshetal., 1999, 2003). In the RVRS the overstepping nature of

faults is the result of structural inheritance combined with

a present day stress eld that is oblique to the fault struc-

ture( Houtgastetal., 2002). Fault relay or transfer zones oc-

cur on places where fault displacement is transferred from

one fault segment to another. Figure 4 shows,schematized,

the structure of such a zone.

Although a fault zone process such as clay smearing re-

duces the eective fault zone permeability, in contrast, re-

lay structures can be the locus of hydraulic contact in an

otherwise compartmentalized reservoir (Larsen, 1988).

Peacock & Sanderson (1994) argue that fault relay zones intheir rst stages of evolution will be the places where foot-

wall and hangingwall will be connected while in later

stages, when the dierent fault segments start to interact,

secondary fractureswill develop and progressively connect

both segments. In the North Sea Basin (Morley et al.,

1990), relay ramps in this later stage of their evolution are

a trap for hydrocarbons.

Faults can also be vertically segmented (Childs et al.,

1996), which is observed in both outcrop (Burhannudin-

nur & Morley, 1997) and on seismic sections (Morley &

Burhannudinnur, 1997). As a result of vertical segmenta-

tion it can be expected that preferential ow paths existin the fault zone that would otherwise be absent.

GROUNDWATER FLOW PATTERNS INTHE LRE

On a regional scale, groundwater ow in the study area is

from south to north, from the Schiefergebirge towards

the North Sea. At a local scale groundwater ow is driven

by smaller scale topographic features and directed towards

the Rhine and Maas Rivers. As a result of the regional-

scale lowering of the groundwater table in the lignite

mining area that started at regional scale in the mid-

1950s, the impact of the faults on groundwater ow in the

area has become increasingly important (Spelter &

Schneider, 1989; Peukert & Helmbold, 2000). The impact

of faults on groundwater ow patterns in the area is as-

sessed by inspection of the hydraulic head maps. Hydrau-

lic head is dened as the uid potential for ow through

porous media comprised of both pressure head and eleva-

tion head.These maps are based upon a dense network of

groundwater observation wells (typically one for each

square kilometre) in which hydraulic head is monitored.

The hydraulic head distribution in one of the deeper

aquifers in the lignite mining area and the location of thelargest open-pit lignite mines (e.g.Hambach and Garz-

weiler) are shown in map-view in Fig. 5. This map also

shows the location of the most important groundwater ex-

traction points at the depth of these aquifers. In the more

shallow aquifers groundwater extraction is concentrated at

the location of the open-pit mines while in the deeper

aquifers, groundwater extraction is concentrated at centra-

lized points in each tectonic block (Fig. 5). Groundwater

lowering in the deeper aquifers causes a regionally extend-

ing cone-shaped depression in hydraulic head on the Erft

Block and the adjacent tectonic blocks. Regional- scale

groundwater lowering in the area started in the mid-1950s. Exploitation of lignite in the area is ongoing with

abandoned mines closing and new ones opening. As a re-

sult, in combination with the complex regional scale

water-management measures, the groundwater ow sys-

tem is strongly transient. Complex numerical models are

used to evaluate the eect on groundwater ow of future

extension and deepening of the largest mines (e.g Ham-

bach and Garzweiler) in the area (Peukert & Helmbold,

2000).

Figure 5 shows that the ow system in the LRE is

strongly compartmentalized. Most faults are separating

areaswith strongly contrasting mean hydraulic headvalues

and in many places groundwater ow (approximately

Mapview

Relay ramp

Fig.4. Overstepping fault segments are separated by a relay zone

(re-drawn after Larsen, 1988).The dierent fault traces are

overstepping in map view.




5/15

perpendicular to contours of hydraulic head) is parallel to

faults. Consequently, faults are zones of very high hydrau-

lic gradients and must have a strongly reduced permeabil-

ity. Because groundwater ux increases towards the centre

of the groundwater extraction, the hydraulic head dier-

ence over the fault (Dhfault) increases in the direction of

the area where the main well elds are situated. For exam-

ple,Dhfault over the Rurrand Fault along the southern mar-

gin of the Erft Block increases from 20 m in the north to

almost 120m close to Hambach open- pit mine. However,

high hydraulic gradients can also be caused by other aqui-

fer properties, for example, south of the Inden mine high

hydraulic gradients occur as a result of the thinning of the

aquifer towards the edge of the Schiefergebirge.

Groundwater flow around the RheindahlenFault

In the Venlo Block mining activities are concentrated at

the Garzweiler open-pit mines. Figure 6 shows the

RVG

EB

VB

Rh

Rr

Figure 6/8

Figure 7

80

25

20

20

20

10

0

120

160

18

0

40

40

20

0

40

40

120 100

120

40

20

40

60

80

80100

0

100

60

-140

20

4080100

20

60

40

Hamb

ach

Inden

10 km

40

20

0

Lignite mine

Major groundwaterextraction

Fault line

60Isoline of hydraulic head(meters above mean sealevel)

Garz-weiler

Garz-

weiler

Fortuna

100

Fig. 5. Map (extent indicated in Fig.1) of the hydraulic head distribution (contours in m) in one the deeper aquifers (no. 5) in the Lower

Rhine Embayment (situation as of October 1987). Groundwater levels are lowered articially to make open- cast mining possible.The

distribution of hydraulic heads clearlydemonstrates the strong impact of faults on the groundwater ow system in the area.This map is

based on a dense network of groundwater observation wells that is operated to monitor the regional eects of groundwater lowering inthe area (Rh, R heindahlen Fault; Rr, Rurrand Fault; RVG, Roer Valley Graben; EB, Erft Block; VB,Venlo Block).




6/15

hydraulic head mapfor aquifer no. 5 aroundthe Rheindah-

len Fault in the Venlo Block and, additionally, the hydraulic

head distribution in more shallow aquifers (6B and 6D).

Groundwater ow in the Venlo Block is mainly towards

the mining area in the south and the lower-lying areas

in the direction of the Rhine river in the north. In the

hydraulic head pattern in the shallow aquifer (6D) the

impact of topography is recognized; however, at larger

depth (aquifers 6B and 5) the impact of the topography-

driven ow system is dampened (Fig. 6). Dhfault varies

laterally along the Rheindahlen Fault: it increases to-

wards the south in the direction of the Garzweiler open-pit mine. Another important observation is that with

increasing depth the hydraulic head dierence over the

Rheindahlen Fault increases. This may be the result of

vertical and lateral variations in fault hydraulic proper-

ties that are coupled to variations in clay content of

the fault zone, which, in turn, are linked to lateral and

vertical variation in fault throw. Summarizing, ground-

water ow patterns in this area, and hence Dhfault, are

determined by fault permeability as well as topogra-

phy-driven ow and induced groundwater ow as a

result of dewatering measures around the Garzweiler

open-pit mine.

Fault relay in the Rurrand Fault

Figure 5 shows that the cone of depression in the Erft

Block is strongly hampered by the Rurrand Fault to propa-

gate into the Roer Valley Graben. However, the sealing ba-

haviour of the Rurrand Fault is interrupted at a relay zone

close to the Hambach mine. Figure 7 shows this lo cation

in detail. A dense network of observation wells monitors

how groundwater ow is forced through the relay zone

that is only around 1km wide. The direction of ow is

strongly deected into the relay zone. As a result of the

small width of the relay zone, groundwater ux must behigh resulting in a steep hydraulic gradient within the re-

lay zone. Moreover, in the Roer Valley Graben the cone-

shaped depression is centred around the relay zone.

From these observations it is concluded that although

the areal extent of the relay structure in the Rurrand

Fault is quite limited, its hydraulic properties and struc-

ture are major controlling factors on the amount of

groundwater lowering in the Roer Valley Graben as a

result of the dewatering measures around Hambach

open-pit mine.

ANALYSIS

In the following sections two scenarios, sealing faults and

leaky relay zones, are quantitatively analysed. First, the

SGR method is applied in order to estimate the variation

of clay content along the Rheindahlen Fault. The calcu-

lated SGR values are compared with observed hydraulic

head dierences over the fault. Second, to explore the dy-

namics of the ow system around relay zones as a function

of fault properties and the structure of the relay zone,

groundwater ow simulations were carried out. These

modelling results are compared with groundwater ow

patterns around the Hambach open- pit mine.

6B

6D

5

5

5

5060

45

75

70

65

60

55

50

45

40

Increasingdepth

G

G

G

1 km

Fig. 6. Hydraulic head distribution for aquifers at dierent

depths around the Rheindahlen fault (situation as of November

1996; the extent of this map is shown in Fig. 5). Aquifer IDs refer

to gure,G indicates the location of the Garzweiler mine.The

dashed lines indicate fault lines, the arrows indicate roughly the

direction of groundwater ow. Contours are in m above mean sea

level.The maps show how the impact of the topography-driven

ow system towards the Rhine and Maas Rivers is decreasing

with increasing depth, whereas the impact of the groundwater

lowering measures around the Garzweiler mine increases.




7/15

Clay smear modelling along the RheindahlenFault

Figure 8 shows a map of the area around the Rheindahlen

Fault that indicates the locations of the groundwater ob-servation wells and cross- sections that are used in the fol-

lowing analysis. Figure 2 summarizes the hydrogeological

build-up around the Rheindahlen Fault. Bore-hole logs,

available for each groundwater observation well, had to be

used to estimate the volumetric clay content (Vclay) of the

lithologies anking the R heindahlen Fault because no

samples of the sedimentswere available for laboratory ana-

lysis. These data are combined with estimates of the

amount of throw (y) that are based upon interpreted geo-

logical cross-sections. Throw and clay content are com-

bined to calculate the distribution of the SGR at the

locations where cross- section nos. 413, 433, 453 and 477 in-

tersect the Rheindahlen Fault (Fig. 8).

Application of the SGR algorithm

The SGR serves as an estimate for the proportion of clay

in the fault gouge (Yielding etal.,1997; Freeman etal.,1998;

Yielding, 2002). The SGR algorithm (Fig. 9) requires a

stratigraphical column that is discretized in intervals (Dz)

each with an assigned value forVclay. For a given distribu-

tion of throw (y) with depth, the SGR along the fault is

calculated via:

SGR

P Vclay Dz

y 100% 1

The data that are used in this study are non-standard

compared with other studies where the SGR methodology

has been applied. In such studies, typically, seismic data

are used to map the fault structure in combination with

g-ray logs from deep wells to estimate the shale content of

the lithology as a function ofdepth (e.g. Harrisetal., 2002).

40

35

30

25

160

150

11

0

140

120

130

160

150

170

200 2

10

220

25

0

240

230

210200

190

180

170

70

80

90

100

120

110

50

60

140

130

RurrandFault

Werhah

n

Fau

lt

1 km

groundwater observation well

Hambach

ErftBlock

RoerValleyGraben

Fig.7. Relay structure in the Rurrand Fault close to the Hambach mine (situation as of January 1996; the extent of this map is

shown in Fig. 5).




8/15

In this study, cross - sections nos. 413, 433, 453 and 477

(Fig.10) are used as if they were seismic images and are be-

lieved to provide a reliable model of fault structure.Throw

is determined for key horizons. Subsequently, fault throw

is linearly interpolated between these key horizons and ex-

trapolated above and below the upper- and lower-most key

horizons.

The bore-hole logs yield a detailed,verbose description

of the local lithological stratigraphy. From these logs aver-

age values ofVclay were estimated in intervals of 5 m. As

the descriptions in the logs are concise and systematic,

these could be converted in a consistent way to an estimate

for Vclay. Therefore, each lithological description in the

log was assigned a specic value of Vclay. For example,

clayey sand represents aVclay value of 0.3 (30%) whereas

clay is assumed to consist of 80% of clay. Lignite has

comparable hydraulic properties as clay. Also, it is known

from eld exposures in the Garzweiler open-pit mineand from other studies (Lindsayetal.,1993) that at shallow

depth, lignite can be smeared along the fault plane.

For that reason Sperrevik et al. (2002) suggests that the

eect of lignite layers should be taken into account in the

calculation of SGR. In order to simulate the expected

smearing behaviour of lignite, this lithology has been

assigned an arbitrary Vclay value of 0.3 in the present

analysis. In case the clay content and the thickness of the

units vary considerably over the fault, the Vclay estimates

and thickness of the unit on both sides of the fault have

been averaged.

Figure 11 shows the inferred throw and calculated SGR

variations as a function of depth at each location and on

each section. The displacement pattern along the Rhein-

dahlen Fault compares well with those found generally in

synsedimentary fault systems (Walsh & Watterson, 1989;

Childsetal., 2003). Fault throw increases northwards from

line 413 (average y5 77 m). Mean fault throw is maximal

along line 453 (average y5139m) and fault throw decreases

towards the north to line 477 where the average value ofyis

84 m. Figure 1 shows that further towards the north the

Rheindahlen Fault is not continued. Figure 11 shows how

throw is increasing with depth at each section. For exam-

ple, in section 413 vertical throw at the base of unit 6A is

90 m, at the top of 6C y5 80m.The inferredVclay logs show thatthe lithologycut bythe

Rheindahlen Fault is dominated by clean sand with low

clay content. The maximum calculated SGR values

( 24%) occur along section 453 where, also on the aver-age, y and Vclay are largest.The average SGR values for sec-

tion 413, 433, 453 and 477 are 15.6%, 17.3%, 17.0% and

11.0%, respectively. Figure 11 shows that at section 477

and413,the verticalvariation in SGRvalues is greater than

at the other two sections. The larger throw along 453

and 433 results in a smoothing of the inferred clay content

of the fault zone relative to the other two sections with a

lesser throw.In order to test whether the calculatedvariation of SGR

is related to observed hydrological data around the Rhein-

dahlen Fault, hydraulic head observations are used from

the same wells as the lithological logs were available. A

complex pattern of juxtaposition of the dierent aquifers

exists at the fault. For example, along the fault there can

be a trajectory where aquifer 5 is juxtaposed to aquifer 6D

whereas along another trajectory the same aquifer (no. 5) is

in hydraulic contact with aquifer 6B. The hydraulic head

dierence (Dhfault) for each occuring juxtaposition is calcu-

lated based upon the piezometeric observations within

each aquifer where it is assumed that the vertical hydraulic

gradient within each aquifer is neglible (i.e. no vertical ow

80671

80688

477

453

433

413

81603

81602

81432

80683

80691

90771

ViersenFault

DulkenFault

Rheindahlen

Fault

Garzweiler

1 kmWegbergFault

Fig. 8. Map (extent indicated in Fig. 5) showing the locations

of the cross - section s 413, 433, 453 and 477 (dashed lines) and

bore-holes (open circles) used for the modelling of clay smear

along the Rheindahlen Fault.

Throw

Clay/Shale

Sand SGR =Throw

100 %

Fig.9. The concept of the calculation of the Shale Gouge Ratio

(SGR) along a fault plane that is anked by alternating units of

sand and clay with varying values ofVclay.




9/15

accurs within the aquifer). Finally, the SGR values for each

depth interval along which a juxtaposition occurs are then

plotted against Dhfault(Fig. 12). Hydraulic head data from

1990 to 1997 have been used for comparison.

Apart from fault permeability, additional factors, as dis-

cussed above, will determine the hydraulic head contrast

over the fault. However, it appears that the hydraulic head

dierence over the Rheindahlen Fault (Dhfault) and the cal-

culated fault properties (SGR) are still strongly related

(Fig.12)suggesting that fault permeability is the dominant

factor controlling the head dierence over the fault. Sensi-

tivity tests were carried out by varying the estimatedVclayof randomly selected units with 10%.This did not re-sult in signicantly dierent trends between Dhfault and

SGR. However, the relation between Dhfaultand SGR is

non-unique and, therefore, cannot be directly coupled to

fault permeability. With increasing exploitation of the lig-

nite mines over time, groundwater levels are further low-

ered and groundwater uxes increase. A small increase in

Dhfault between 1990 and 1997 is seen for most analysed

wells (Fig.12), but SGR variations have a dominant eect.

Similar trends are reported by Harris et al. (2002) and

Yielding et al. (1999), both of which describe data from

deep hydrocarbon reservoirs. These obse rvations suggest

that there is a direct relation between fault permeability

and SGR, but the exact physical meaning of this relation

remains unresolved.

Fault permeability

In order to be able to incorporate a given fault zone into a

groundwater ow model, fault zone hydraulic conductivity

(Kfault) and fault zone width (w) are required.We have tested

whether the SGR approach leads to reasonable estimates

of fault zone hydraulic properties by comparing the resultsfrom the SGR approach to fault zone hydraulic properties

that have been inferred independently from numerical

groundwater ow models. First we will show how Kfaultand wcan be estimated from SGR and fault throw, respec-

tively, using empirical relations.

Empirical data show that the permeability of a given

fault gouge is strongly related to its clay content (Sperrevik

et al., 2002; Yielding, 2002). As shown in Fig. 11, the in-

ferred SGR along the fault plane, an estimate of the clay

content of the fault, varies considerably with depth. It is

expected that the permeability will vary in a similar way.

Several empirical relations have been proposed to calcu-

late fault zone permeability from estimated clay contents

Fig. 10. Four cross- sections (schematized) over the Rheindahlen Fault showing the variation of vertical throw along the fault

and the resulting varying geometries of aquitards and aquifers at the fault.The Rheindahlen Fault is indicated by an arrow.




10/15

of the fault rock (Manzocchi et al., 1999; Sperrevik

et al., 2002). Among these, only the relation proposed by

Sperreviket al. (2002) seems valid for shallow fault zones(e.g. o250 m depth) because it takes into account both

the depth at the time of deformation (zD) and the maxi-

mum burialdepth(zmax).The depth at time of deformation

is of considerable importance because the type of defor-

mation mechanism is controlled by the burial depth. Dis-

sagregation zones will form at relatively shallow depth

(o500 m depth) via particulate ow whereas deeper than

500 m deformation bands will form through cataclasis

(Fulljameset al., 1997; Sperreviket al., 2002). Dissagrega-

tion zones have a higher permeability than the unfaulted

sediment (Bense et al., 2003b) whereas cataclastic defor-

mation bands have very low permeabilities (Antonellini &

Aydin, 1994).

Figure 13 shows the proposed relation (from Sperrevik

et al., 2002) between the estimated clay content and the

bulk hydraulic conductivity (K) for faulted and unde-formed rocks. According to these curves, with decreasing

clay content of a given fault, the predictability of its hy-

draulic properties decreases because the values ofzDand

zmax are more critical. These curves also predict an

increase in per meability (via particulate ow) of clean sand

when deformed at shallow depth as compared with the

undeformed host material. However, when the clay con-

tent is larger than about 15%, permeability is lower in the

deformation band (via clay smearing) compared with the

host material.

Additionally, empirical relations are available to esti-

mate w from fault throw (Knott et al., 1996; Manzocchi

et al., 1999; Sperrevik et al., 2002). Based upon these, a

40 60 80 100 120 140 160 180

0 5 10 15 20 25 30

250

200

150

100

50

0

50

100

477

Throw [m]

SGR [%]

Depth[m]

40 60 80 100 120 140 160 180

0 5 10 15 20 25 30

433

Throw [m]

SGR [%]

250

200

150

100

50

0

50

100

Depth[m]

Throw [m]

SGR [%]

40 60 80 100 120 140 160 180

0 5 10 15 20 25 30

413

250

200

150

100

50

0

50

100

Depth[m]

Throw [m]

SGR [%]

Depth[m]

453

40 60 80 100 120 140 160 180

0 5 10 15 20 25 30

250

200

150

100

50

0

50

100

Fig. 11. The variation of fault throw

(dashed line) and Shale Gouge Ratio

(SGR) (solid line) with depth along

the Rheindahlen Fault at the

location of sections 413, 433, 453

and 477.




11/15

fault with a throw of, for example, 100 m is estimated to

have a width of1^5 m (Eq. (3) in Sperrevik etal., 2002).In groundwater ow studies, the hydraulic properties of

a fault zone can be quantied using the fault resistance (c),

which is dened as (Bense & Van Balen, 2003):

c5w

Kfault: 2

Wallbraun (1992) and Spelter & Schneider (1989) show

that, based on groundwater ow modelling, many faults

in the lignite mining area have hydraulic resistances ran-

ging from 100 to 750 days.

We compare the results from groundwater ow studies

in the following way to those from the SGR approach.The hydraulic conductivity of the aquifers around the

Rheindahlen Fault is 10^25 m day1 (Wallbraun, 1992).

These aquifers have a Vclay 10%. This sediment isdragged to the fault together with anking clay layers

(Vclay 70%), resulting in values of fault clay content(esti-mated via the SGR algorithm; Fig. 9) varying between 10

and 25%. Based on the curves proposed by Sperrevik et

al. (2002) (Fig. 13) and assuming that the burial depth of

these sediments never exceeded 1000 m, the hydraulic

conductivity of the fault gouge along the Rheindahlen

Fault ranges between 10 and 0.03 m day1. A fault is ex-

pected to have a continuous clay smear at SGR420%

(Sperrevik etal., 2000; Yielding 2002). Consequently, appli-

cation of Eq. (2) shows that a sealing fault (SGR5 25%,

Kfault5 0.03m day

1, w5 5 m) will have a resistance ofaround 170 days. This is in reasonable agreementwith the values for fault resistance that result from the

SGR [%]

hfault[m]

SGR [%] SGR [%]

413 433 453 477

Hanging wall/Footwall

6D/56D/6B 8/6D6D/5

6B/5 6D/6D

5/5 6D/56B/5

5/5

0

5

10

15

20

25

30

0 5 10 15 20 25 30

0

5

10

15

20

25

30

0 5 10 15 20 25 30

1

10

100

0 5 10 15 20 25 30

1997

1990

1997 1990

hfault[m]

hfault[m

]

Fig.12. Calculated Shale

Gouge Ratio (SGR) values vs.

hydraulic head dierences

between dierent

hydrogeological units (see

Fig. 2) over the Rheindahlen

Fault along lines 413, 433, 453

and 477.

K

[m/day]

Clay Content[%]

zmax[m]

106

0.0001

0.01

1

100

104

0.001

0.1

10

1000

105

107

0 5 10 15 20 25 30

Perm

eability[mD]

1000

2000

3000

zD[m] : 0 ; 250 ; 500

Fig.13. The relation between the bulk hydraulic conductivity (K)

of the faulted material (curved lines) and undeformed host rock

(straight lines), and its clay content, for dierent scenarios of the

depth at the time of deformation (zD) and maximum burial depth

(zmax).These curves are based on Sperrevik et al.(2002).




12/15


13/15

DISCUSSION AND CONCLUSIONS

For a correct estimation of SGR values, the quality of the

estimated Vclay of the lithologies anking the fault is

essential. In the current study, this was dicult as directmeasurements of Vclay of the sediments anking the

Rheindahlen Fault were not available. Especially the prop -

erties of the lignite are largely unknown. TheVclayvalues

we used here are best estimates. The estimates of the hy-

draulic properties of the R heindahlen Fault, following the

approach that we applied here,seem to be reasonable when

compared with those inferred from groundwater ow

modelling. The advantage of the method applied in this

paper is that it can be used to make an assesment of the

heterogeneity of the fault zone whereas the fault resistance

following from groundwater ow modelling is a vertically

averaged value. The eect of anisotropy is not captured inthis approach,but even faultswith expected highvalues for

SGR can act as vertical conduits (Langguth & Ullrich,

2000). This should also be considered in the assessment

of the sealing capcity of faults.

Apart from the example presented in this paper, relay

strucutures are observed on other locations in the rift sys-

tem. Houtgast et al. (2002) show that the Feldbiss Fault

Zone (at the southern boundary of the Roer Valley Graben)

consists of several segments close to the German^Dutch

national border. For the same fault zone, in an area further

towards the north-west, an aerial photograph (Bense &

Van Balen, 2003) and geo-electrical soundings (Vanden-

berghe, 1982) show fault segmentation. At this eld site

Fig.14. Results of numerical

simulations of the hydraulic head

distribution around an area of major

groundwater lowering inuenced by a

segmented fault zone. As fault zone

transmissivity (m2 day1) is increasing

the impact of the fault relay zone is

decreasing. Contour interval is 10m.

Tf[m2/day]

5

0

5

10

15

20

25

0.001 0.01 0.1 1 10 100 1000

q[m2/day]

Relay zone

Upper fault segment

Lower fault segment

Fig.15. Plot of the amount of groundwater ow per unit length

over the upper and lower fault segment, and through the relay

zone as a function of fault transmissivity.




14/15

hydrochemical data indicate that in this setting, the Feld-

biss Fault does not form a barrier to uid ow.

We conclude that the structural setting of a fault zone is

of considerable importance to the net impact of the fault to

uid ow patterns, additional to the hydraulic properties

of the fault itself (e.g. Fig. 14). A general implication for

uid ow within a sedimentary basin is that, although it

might be possible to make a sensible estimate of the per-

meability of a given fault zone (for example via the SGRapproach), in case this fault is strongly segmented, a thor-

ough assessment of this segmentation is needed to be able

to ass ign bulk hydraulic propertie s to the fault. Fault seg-

mentation can be of crucial impact to regional ground-

water ow patterns. One of the rst well-documented

examples of uid ow patterns around a fault relay struc-

ture is presented in this study (Fig. 7).

The insights and techniques summarized above can be

used to improve the way faults are implemented in existing

hydrogeological modelling studies of sedimentary basins

(Garven et al., 1999; Mailloux et al., 1999). So far, fault

permeabilities are mostly very uncertain in this kind ofmodelling studies.

ACKNOWLEDGEMENTS

All data that are integrated in this analysis are made

available by the hydrogeological service of RWE Power

(formerly Rheinbraun AG), which is greatly acknowledged.

Bryne Ngwenya, Mark Person and Graham Yielding

are thanked for providing thoughtfull review comments

that helpedimprove the quality of the original manuscript.

This is publication no. 20040602 of the Netherlands

Research School for Sedimentary Geology.

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Manuscript received 12 January 2004; Manuscript accepted 25

June 2004



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