fault and their potential

8122019 Fault and Their Potential

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Faults and their potential influence on fluid flow

Nicol A1 Seebeck H1 Hemmings-Sykes S12 Ilg B3 Childs C4Walsh J4

1 GNS Science Lower Hutt New Zealand2 Victoria University of Wellington Wellington New Zealand3 New Zealand Petroleum amp Minerals Wellington New Zealand4 Fault Analysis Group School of Geological Sciences University College Dublin Dublin 4Ireland

Email anicolgnscrinz

AbstractFaults have the potential to be both barriers to lateral flow and conduits for vertical

movement of fluids We examine where and why faults locally enhance the flow of fluids

using mainly outcrops tunnel excavations and seismic reflection lines Outcrop studies of

normal faults within the Mount Messenger Formation reveal that the thickness of fault zones

(containing clay-rich fault rock and fractures) generally increases with displacement and

varies by several orders of magnitude on individual faults Elevated densities of small-scale

faults and the greatest fault zone widths typically occur at irregularities on fault surfaces

(eg relays bends and fault intersections) Data from ground water flow in tunnels and gas

chimneys imaged in seismic reflection lines show that for low permeability rocks (eg seals)

fluid flow is primarily achieved via fault zones that flow rates are greatest on larger faults

and that migration of f luids is locally enhanced along fault irregularities (ie the sites where

the highest densities of small-scale faults occur) Geomechanical analysis of faults does notappear to be a reliable predictor of gas chimney locations and migration pathways on

individual faults perhaps because stress conditions near faults can depart significantly from

the regional stress field used in these analyses

Keywords fault zones fluid flow gas chimneys geomechanical modelling

IntroductionIn many sedimentary basins it has long been postulated that clay-rich fault rock and

associated small-faultsfractures strongly influence the sub-surface migration and

accumulation of hydrocarbons (eg Wade 1913 Illing 1942 Neglia 1979 Faulkner et al2010) Individual faults can be both barriers to lateral flow and conduits for vertical movement

of fluids The potential for faults to impede lateral flow of fluids within reservoir units has been

extensively studied (eg Yielding et al 1997 Manzocchi et al 1999 2010) Faults can also

act as conduits to fluid migration enhancing their up-sequence flow and influencing the ability

of traps to retain oil or gas columns over geological timescales Up-sequence fluid flow along

faults is thought to occur in the Taranaki Basin where petroleum generated from Late

Cretaceous to Eocene source rocks have the ability to migrate through a thick (1-3 km)

Oligocene to Miocene mudstone-rich sequence which has implications for the locations of oil

and gas fields It is therefore important to determine where and under what circumstances

faults (and fractures) enhance the movement of petroleum fluids



In this paper we utilise outcrop tunnel and seismic reflection data (2D amp 3D) from the

Taranaki Basin and the North Island of New Zealand to describe the structure of normal fault

zones examine where and why fluid flow is focused within these fault zones and test the

predictive power of geomechanical techniques for identifying faults or parts of a fault that

may locally enhance up-sequence flow Our analysis supports the view that channelized flow

of gas and water often occurs within fault zones Geomechanical techniques (ie SlipTendency Dilation Tendency and Fracture Stability) proved an unreliable means of

predicting the locations of channelized flow Qualitative analysis of the data suggest that in

many cases fluid flow is observed close to geometric complexities on fault surfaces (eg

fault relays bends and intersections) where the densities of small-scale faults is likely to be

highest

Fault Zone StructureFaults typically comprise zones that accommodate heterogeneous shear strains (eg Caine

et al 1996 Childs et al 2009 Faulkner et al 2010) The highest shear displacements are

focused within (or bounding) fault rock that typically contains fine-grained fault gouge and

breccia (products of crushing and intense fracturing) Fault rock in the Mount Messenger

Formation varies in content and dimensions depending on the host rock fault-zone structure

and total displacement (Childs et al 2007 2009 Nicol et al 2013)(Fig1a) Fault rock is

generally (but not always) accompanied by fracturing and small-scale faulting and

collectively these structures form the fault zone (Fig 1a) Fault zones typically comprise an

anastomosing system of intersecting fault segments which bound lenses of variably

fractured host rock The thickness of fault zones shows a broad positive relationship with

displacement however there is significant heterogeneity of fault zone thickness for a given

displacement The thicknesses of both fault zones and fault rock can change on individual

structures by an order of magnitude or more over distances of metres (eg Childs et al2009 Nicol et al 2013) For example a recently studied normal fault with ~6-8 m of

displacement displayed a factor of five change in fault-zone thickness over 5 m horizontal

distance (10 cm to 50 cm) and a factor of 25 change over 40 m (10 cm to 25 m) These

changes indicate that the spread in fault-zone thickness for a given displacement on all

sampled faults may be approximately replicated by the variability on individual faults (Fig

1b) The 1-25 orders of magnitude range of fault zone thickness for a given displacement on

all faults in Fig 1b reflects the fact that fault zones comprise a combination of single planar

fault surfaces and multiple slip surfaces with variable amounts of associated secondary

fracturing The widest fault zones are generally associated with fault complexities including

segmentation and segment boundaries (eg relays) fault bending fault intersections andfault terminations (Childs et al 2009 Nicol et al 2013) In many cases these zones of fault

complexity are sites with increased numbers of fractures and are considered by many to be

zones of likely elevated fault-zone permeability (eg Gartrell et al 2004 Ilg et al 2012)

The role of these zones of fracturing on fluid flow and petroleum fluid migration are

discussed in the following sections

Fluid Flow along FaultsMany quantitative and qualitative models have been proposed to explain where and why

fluid flow occurs along faults Historically one of the most significant impediments to testing

these models has been a lack of empirical data on fluid flow in areas where faulting is wellunderstood Here we present the results of three studies in which data are available for both



983137 983138

faults and fluid flow (Hemmings-Sykes 2012 Ilg et al 2012 Seebeck et al 2014) These

studies confirm that fluid flow (water or gas) occurs within fault zones and is highly

channelized and heterogeneous They also permit testing of geomechanical methods for

predicting the locations of elevated fluid flow on fault surfaces

Fig 1 a) Schematic diagram showing a fault with definitions of fault rock (also referred to as fault

core) and fault zone illustrated (from Childs et al 2009) b) Plot of fault zone width versus

displacement for normal faults in the Mount Messenger Formation on the north Taranaki coast

Vertical bars show variations in fault zone thickness for individual faults

Water flow into tunnels through fault zones has been analysed to determine the relationships

between faulting and fluid flow (locations and rates) (Seebeck et al 2014) The datasetincludes information on fault geometries and their spatial relationships to water flowing into

tunnels located along the margins of the Taupo Rift New Zealand (Fig 2) Faults and water

flow data from engineering geological logs and reports have been used to examine the

factors influencing the rates and localisation of groundwater flows in relation to fault zone

architecture and connectivity of the fault-fracture network As faults locally promote fluid

migration through seal and reservoir rocks our analysis may have implications for the

exploration and production of petroleum fluids (Manzocchi et al 2010 Ilg et al 2012) In

the tunnels localised ground water inflows occur almost exclusively (ge~90) within and

immediately adjacent to fault zones Fault zones in contrasting lithologies comprise fault

rock small-scale faults and fractures with thicknesses of 001 to ~110 m approximating

power-law distributions and bulk permeabilities of 10-9-10-12 m2 (Fig 2a) Variability in fault-

zone structure results in highly heterogeneous flow rates and channelised flow Within

basement rocks ~80 of the flow rate occurs from fault zones ge10 m wide with ~30 of the

total localised flow rate originating from a single fault zone (ie consistent with the golden

fracture concept) (Fig 2a) No simple relationships are found between flow rates and either

fault strike or hydraulic head (Fig 2b) with le50 of fault zones in any given orientation

flowing A general positive relationship does however exist between fault zone thickness and

maximum flow rate Higher flow rates from larger fault zones may arise because these

structures have greater dimensions and are more likely (than smaller faults) to be connected

to other faults in the system and the ground surface These results suggest that faults

passing through the reservoir and seal in hydrocarbon systems are most likely to promote oil

andor gas migration through the seal However it is clear that not all faults or all parts of



faults connecting the reservoir to strata above the seal will compromise petroleum traps

(see next section for further discussion)

Fig 2 Fluid flow and fault data from tunnels in the Taupo Rift (Seebeck et al 2014) a) Proportion of

localised flow carried by basement fault zones Illustrates ~80 of total cumulative flow rate occur on

lt3 of basement faults which typically have zones gt10 m wide Note one fault accounts for ~30 of

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The role of normal faults in up-sequence flow of fluid has also been examined using 2D and

3D seismic-reflection data from the southern Taranaki Basin New Zealand (Hemmings-Sykes 2012 Ilg et al 2012) The spatial distributions of late-stage normal faults gas

chimneys thickness of the Oligocene mudstone-rich seal (Otaraoa Formation) and modelled

petroleum expulsion volumes have been compared in our studies Gas chimneys are most

common above Cretaceous source rocks modelled to have expelled hydrocarbons Most

(~70) of the observed gas chimneys follow andor are rooted in late-stage normal faults

These faults are the primary seal bypass mechanism for hydrocarbons where they displace

the seal (or intersect faults that displace the seal) and the seal is thick (eg gt~340 m) Gas

flow up along faults in low permeability mudstones (lt1 mD) is channelized with steep

chimneys often occurring close to fault tips and relay ramps In these cases gas flow may be

focused by the presence of high densities of open fractures (as is observed in outcrop)locally elevating up-sequence bulk permeabilities in the seal

Gas chimneys and normal faults imaged in a 3D seismic reflection volume provide a means

of testing the ability of geomechanical models to predict the locations of up-fault hydrocarbon

migration The use of geomechanical methods for predicting leakage risk is only applicable

as a first-order estimate of which fault sets present the highest risk of up-dip migration

(Hemmings-Sykes 2012) Slip Tendency and Dilation Tendency were able to differentiate

fault orientations in the Kupe Area most at risk of leakage and both indicated higher risk of

leakage for the fault set striking parallel to the maximum horizontal stress (approximately

northeast-southwest) In contrast Fracture Stability was not able to differentiate fault setsmost at risk of leakage in the Kupe Area The ability of geomechanical modelling methods to

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locate zones on fault surfaces with a high risk of up-sequence gas flow appears to be

limited There was no statistical difference in leakage risk between chimney and non-

chimney locations on fault surfaces when applying Slip Tendency Dilation Tendency and

Fracture Stability (Fig 3)

Fig 3 Oblique view of fault surfaces from the Kupe region with colours showing contours of slip

tendency on the fault surfaces (Hemmings-Sykes 2012) White line polygons indicate the outline of

gas chimneys mapped on the fault surfaces The chimneys are of limited extent and their locations do

not seem to be related to zones of higher Slip Tendency Refer to Hemmings-Sykes (2012) for further

details

The inability of geomechanical analyses to predict the locations of chimneys might arise

because i) these techniques do not include small-scale information on the local stress

tensor andor on the locations of open interconnected fractures and ii) they do not takeaccount of whether individual faults breach the seal and are likely to intersect a petroleum

source The poor relationship between fault orientation and fluid flow rates in the tunnel

dataset supports the view that in some cases geomechanical methods may not provide a

robust means of assigning risk of up-fault fluid flow for fault sets with widely varying strike

(Seebeck et al 2014) In our experience geomechanical modelling should be used with

caution and does not seem to be a reliable predictor of fluid flow in the New Zealand

examples studied

Conclusions

Analysis of outcrop and seismic-reflection studies of normal faults and fluid flow in NewZealand support a number of conclusions These are

1) The thickness of fault zones (containing clay-rich fault rock and fractures) generally

increases with displacement and varies by several orders of magnitude on individual

faults Elevated densities of small-scale faults (and greatest fault zone widths) typically

occur at irregularities on fault surfaces (eg relays bends and fault intersections)

2) Ground water flow in tunnels and gas chimneys imaged on seismic reflection lines show

that in low permeability rocks (eg seals) fluid flow is primarily achieved via fault zones

and that flow rates are greatest on larger faults

3) Geomechanical analysis does not appear to be a reliable predictor of where channelized

flow will occur on individual faults



4) Up-fault migration is locally enhanced at fault irregularities (ie the sites where the

highest densities of small-scale faults occur)

References

Caine JS Evans JP Forster CB 1996 Fault zone architecture and permeability

structure Geology 24(11) 1025-1028Childs C Walsh J J Manzocchi T Strand J Nicol A Tomasso M Schopfer M

Aplin A 2007 Definition of a fault permeability predictor from outcrop studies of a

faulted turbidite sequence Taranaki New Zealand In Structurally Complex

Reservoirs Geological Society of London Special Publication 292 235-258

Childs C Walsh JJ Manzocchi T Bonson C Nicol A Schoumlpfer MPJ 2009 A

geometric model of fault zone and fault rock thickness variations Journal of

Structural Geology 31 117-127

Faulkner DR Jackson CAL Lunn RJ Schlische RW Shipton ZK Wibberley

CAJ Withjack MO 2010 A review of recent developments concerning the

structure mechanics and fluid flow properties of fault zones Journal of StructuralGeology 32 1557-1575

Hemmings-Sykes S 2012 The influence of faulting on hydrocarbon migration in the Kupe

area south Taranaki Basin New Zealand Unpublished MSc Victoria University of

Wellington p 224

Gartrell A Zhang Y Lisk M Dewhurst D 2004 Fault intersections as crictical

hydrocarbon leakage zones integrated field study and numerical modelling of an

example from the Timor Sea Australia Marine amp Petroleum Geology 23 1165-1179

Ilg BR Hemmings-Sykes S Nicol A Baur J Fohrmann M Funnell R Milner M

2012 Normal faults and gas migration in an active plate boundary southern Taranaki

Basin offshore New Zealand Amercian Association of Petroleum Geology Bulletin 96(9) 1733-1756

Illing VC 1942 Geology applied to petroleum Proceedings of the Geologists Association

53(3-4) 156-187

Manzocchi T Walsh JJ Nell P Yielding G 1999 Fault transmissibility multipliers for

flow simulation models Petroleum Geoscience 5(1) 53-63

Manzocchi T Childs C Walsh JJ 2010 Faults and fault properties in hydrocarbon flow

models Geofluids 10(1-2) 94-113

Nicol A Childs C Walsh J Schafer K 2013 A geometric model for the formation of

deformation bands Journal of Structural Geology 55 21-33

Neglia S 1979 Migration of f luids in sedimentary basins American Association ofPetroleum Geology Bulletin 63(4) 573-597

Seebeck H Nicol A Walsh JJ Childs C Beetham RD Pettinga J 2014 Fluid flow

in fault zones from an active rift Journal of Structural Geology 62 52-64

Wade A 1913 The natural history of petroleum Proceedings of the Geologistsrsquo

Association 24(1) 1-13 IN1-IN3

Yielding G Freeman B Needham DT 1997 Quantitative fault seal prediction American

Association of Petroleum Geology Bulletin 81(6) 897-917



In this paper we utilise outcrop tunnel and seismic reflection data (2D amp 3D) from the

Taranaki Basin and the North Island of New Zealand to describe the structure of normal fault

zones examine where and why fluid flow is focused within these fault zones and test the

predictive power of geomechanical techniques for identifying faults or parts of a fault that

may locally enhance up-sequence flow Our analysis supports the view that channelized flow

of gas and water often occurs within fault zones Geomechanical techniques (ie SlipTendency Dilation Tendency and Fracture Stability) proved an unreliable means of

predicting the locations of channelized flow Qualitative analysis of the data suggest that in

many cases fluid flow is observed close to geometric complexities on fault surfaces (eg

fault relays bends and intersections) where the densities of small-scale faults is likely to be

highest

Fault Zone StructureFaults typically comprise zones that accommodate heterogeneous shear strains (eg Caine

et al 1996 Childs et al 2009 Faulkner et al 2010) The highest shear displacements are

focused within (or bounding) fault rock that typically contains fine-grained fault gouge and

breccia (products of crushing and intense fracturing) Fault rock in the Mount Messenger

Formation varies in content and dimensions depending on the host rock fault-zone structure

and total displacement (Childs et al 2007 2009 Nicol et al 2013)(Fig1a) Fault rock is

generally (but not always) accompanied by fracturing and small-scale faulting and

collectively these structures form the fault zone (Fig 1a) Fault zones typically comprise an

anastomosing system of intersecting fault segments which bound lenses of variably

fractured host rock The thickness of fault zones shows a broad positive relationship with

displacement however there is significant heterogeneity of fault zone thickness for a given

displacement The thicknesses of both fault zones and fault rock can change on individual

structures by an order of magnitude or more over distances of metres (eg Childs et al2009 Nicol et al 2013) For example a recently studied normal fault with ~6-8 m of

displacement displayed a factor of five change in fault-zone thickness over 5 m horizontal

distance (10 cm to 50 cm) and a factor of 25 change over 40 m (10 cm to 25 m) These

changes indicate that the spread in fault-zone thickness for a given displacement on all

sampled faults may be approximately replicated by the variability on individual faults (Fig

1b) The 1-25 orders of magnitude range of fault zone thickness for a given displacement on

all faults in Fig 1b reflects the fact that fault zones comprise a combination of single planar

fault surfaces and multiple slip surfaces with variable amounts of associated secondary

fracturing The widest fault zones are generally associated with fault complexities including

segmentation and segment boundaries (eg relays) fault bending fault intersections andfault terminations (Childs et al 2009 Nicol et al 2013) In many cases these zones of fault

complexity are sites with increased numbers of fractures and are considered by many to be

zones of likely elevated fault-zone permeability (eg Gartrell et al 2004 Ilg et al 2012)

The role of these zones of fracturing on fluid flow and petroleum fluid migration are

discussed in the following sections

Fluid Flow along FaultsMany quantitative and qualitative models have been proposed to explain where and why

fluid flow occurs along faults Historically one of the most significant impediments to testing

these models has been a lack of empirical data on fluid flow in areas where faulting is wellunderstood Here we present the results of three studies in which data are available for both



983137 983138








































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details









examples studied

Conclusions















References














Wellington p 224








53(3-4) 156-187
















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983110 983154 983141 983153 983157 983141 983150 983139 983161

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details









examples studied

Conclusions















References














Wellington p 224








53(3-4) 156-187






















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983142983151983154 983137 983149983137983160983145983149983157983149 983144983151983154983145983162983151983150983156983137983148 983155983144983151983154983156983141983150983145983150983143 983156983154983141983150983140983145983150983143 983137983156 983166983088983094983088ordm 983080983154983141983140 983148983145983150983141983081983086




















983120983154983151983152983151983154983156983145983151983150 983156983151983156983137983148 983142983148983151983159

983110 983137 983157 983148 983156 983162 983151 983150 983141 983159 983145 983140 983156 983144

983149

983110 983154 983141 983153 983157 983141 983150 983139 983161

983138983137

983105983162983145983149983157983156983144











details









examples studied

Conclusions















References














Wellington p 224








53(3-4) 156-187
























details









examples studied

Conclusions















References














Wellington p 224








53(3-4) 156-187


















References














Wellington p 224








53(3-4) 156-187














fault and their potential

Documents