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Tectonics and Seismic Interpretation

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DOI: 10.1007/978-3-319-26491-2_4

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Chapter 5
Seismic Stratigraphy and Seismo-Tectonics
in Petroleum Exploration

Abstract Mutually integrated, the seismic stratigraphy and seismo-tectonics stud-

ies provide a highly effective technique, which may be considered indispensable in
petroleum exploration. The conjoined analysis enables the building up of the basic
tectono-stratigraphic framework for petroleum system modeling. Modeling helps
evaluate potential of source generation, reservoir facies, and migration, trapping
mechanism, accumulation and preservation for estimating hydrocarbon resource
base for the basin. It also leads to the generation of hydrocarbon prospects that are
evaluated for technical and financial risks before being drilled.
The synergistic analysis of seismic stratigraphy and seismo-tectonics for
hydrocarbon accumulation, complimenting each other, is highlighted. As faults
play major roles as conduits, seals, and leaks in hydrocarbon migration and
accumulation, fault attributes analysis and its trap integrity is stressed.

Seismic stratigraphy and seismo-tectonics are highly effectual interpretation tech-

niques, indispensible in the quest of discovering hydrocarbons. Mutually integrated,
the techniques support understanding the evolution of geologic basins. They help in
reliable evaluation of hydrocarbon potentials of plays and prospects for exploratory
ventures. The studies are especially useful in virgin or least-explored areas, where
well data do not exist or are fragmentary and inadequate. The degree of success,
however, depends on the experience and skill of the interpreter in comprehensive
understanding of fundamentals of tectonic styles and depositional systems. The
entwined link played by seismic stratigraphy and seismo-tectonics in analysing the
process of hydrocarbon accumulation is outlined.

Basin Evaluation

The seismic investigation through synergy of seismic stratigraphy and seismo-

tectonics reveals the depositional and tectonic history of the sedimentary fill of the
basin, allowing recognition and mapping of potential geological plays (leads) and
their assessment for hydrocarbon exploration. Evaluation of a basin for hydrocar-
bon potential requires identification of its petroleum systems – a term used for the
geologic elements and processes responsible for hydrocarbon accumulation. Four

© Springer International Publishing Switzerland 2016 91

N.C. Nanda, Seismic Data Interpretation and Evaluation for Hydrocarbon
Exploration and Production, DOI 10.1007/978-3-319-26491-2_5
92 5 Seismic Stratigraphy and Seismo-Tectonics in Petroleum Exploration

crucial elements define the petroleum system: (1) source and generation; (2) reser-
voir; (3) migration (pathways); and (4) entrapment and preservation. Each element
is briefly discussed examining the role of seismic stratigraphy and seismo-tectonics
in application.

Source and Generation Potential

The potential for hydrocarbon generation in a basin is assessed based on knowledge

of amount of organic-rich fine clastics (shale), its organic content, kerogen type, burial
history and thermal maturity. The geological environments linked to deposition of
source rocks are known to include fluvial/lacustrine, deltaic, shallow marine and
deep-marine settings. Seismic stratigraphy studies can identify the distribution and
extent of these varied depositional environments and associated lithofacies, which
indicate not only the presence of source rocks but also the type of organic matter
(kerogen). The source potential can be assessed as oil or gas prone, depending on
kerogen type linked to depositional environment as fluvial, deltaic or marine. Kerogen
which comprises most of organic matter in sediments, characteristically exhibits low
velocity and density and high porosity and kerogen- rich shales may be detected
directly from seismic reflections like that of coal under favourable situations.
Tectonic analysis unravels clues to burial history of source rocks, useful in assess-
ing thermal maturity to produce oil and gas at depths and also its expulsion. Consider,
for instance, the geological implications of a syn-sedimentary normal gravity fault in
the shelf margin, episodically active and for a long time. The seismo-tectonic analy-
sis of the fault not only provides information on the increasing thickness of source
rock (shale) on the basin side under anaerobic conditions, but also on its maturation
history. The seismic can point out the details of the individual episodes of recurring
subsidence with time that helps infer rate of subsidence and the depth attained during
a geologic age. These are generally considered as factors favourable for potential
source generation and maturation. Inferences of severely tectonised zones and volca-
nic activities from seismo-tectonic analysis can also offer valuable clues to geother-
mal gradients in the area in analysis for source maturation.
However, it may be noted that it is not the generation but the quantity of expul-
sion that is important as it decides the amount of hydrocarbon eventually captured
in traps. The expulsion from source rock known as primary migration is believed by
many to be a process triggered by high pore fluid pressure within it. As hydrocar-
bons are generated by thermal cracking of kerogen, the generated fluid tries to
occupy larger volume than that of the original rocks. Rise in temperature with sub-
sidence further increases the fluid volumes resulting in high pore pressures within
the source and at some point of time the continuing increase in pressure causes
micro fractures that facilitate expulsion of hydrocarbon. Each episode of subsid-
ence, linked to seismic evidence, thus can be a clue to a new expulsion phase of
hydrocarbons as the source continues to get buried under more and more layers of
strata.
Basin Evaluation 93

Reservoir Facies

Reservoir rocks with good primary porosity and permeability are commonly depos-
ited on shelf and slopes associated with high energy fluvial/lacustrine, deltaic and
shallow marine depositional environments. Reefal mounds also offer excellent
porosities. Seismic sequence and seismic facies analysis of external forms and inter-
nal reflection configurations can identify reefs and potential high energy clastic res-
ervoir facies as in delta lobes, fans, channel cut and fills. Tectonic stresses leading
to uplifts, faults and unconformities often induce secondary porosities in the form
of leaching, channelling, vugs and fractures, which enhance permeability and facili-
tate production. Seismic stratigraphy and seismo-tectonic analysis help identify the
depositional and tectonic elements to evaluate reservoir facies and types and more
importantly their distribution in the basin.

Migration

Hydrocarbon migration process deals with generation and expulsion from source
rock to transmission through carriers and eventually to traps for accumulation.
Thus, source-reservoir connectivity is required for hydrocarbons to migrate from
source to trap. After expulsion (primary migration), the hydrocarbon continues to
move up-dip in the porous and permeable rocks until it reaches the structurally
highest part of a reservoir, where it gets accumulated in the trap (secondary migra-
tion), formed by surrounding nonpermeable rocks, called the seal. Expulsions from
source may be upward or downward which determine the migration pathways and
charging of traps located in the corridor (England and Fleet 1991).
The charging of hydrocarbon into a reservoir is facilitated by unconformities,
faults and permeable beds, which are considered critical elements for migration.
Comprehending migration pathways for charging of traps requires knowledge of
carrier bed geometries with vertical and lateral permeability characters and paleo-
dips in the subsurface at the time of hydrocarbon expulsion. Migration paths are not
simple two dimensional paths as is often assumed by interpreters such as a fault
connecting the source to reservoir above. Migration is a more intricate three dimen-
sional process which is challenging to envisage precisely and can be dealt better by
‘3D basin modelling’ (discussed later in the chapter).
Nonetheless, broad pathways can be conveniently predicted from seismic
sequence and tectonic studies and paleostructural analysis. In simple systems, such
as in a delta sequence where the delta-front sands are in contact with the pro-delta
marine source rocks, migration and charging process is rather simple and straight
forward as hydrocarbon expulsion charges the juxtaposed trap directly. Vertically
stacked delta sequences, often with growth faults and roll-over structures, are con-
sidered highly potential exploration plays that can be recognized and evaluated
from seismic stratigraphy and seismo-tectonics studies.
94 5 Seismic Stratigraphy and Seismo-Tectonics in Petroleum Exploration

Timing of migration with respect to the presence of a trap is a key factor in the
accumulation process. Hydrocarbon migration misses accumulation if the mapped
trap was not present at the time of migration but formed later. Similarly a mapped
paleo structure (trap) may have no accumulation if it has undergone distortions
before time of migration. Since hydrocarbon moves up dip, it is also important to
analyse the paleodips at the time of migration, together with presence of traps, to
analyse migration pathways and accumulation. Seismo-tectonic studies compre-
hend chronological history and make it convenient to evaluate the ‘timing’ factor.

Entrapment and Preservation

Reservoirs need seals to trap hydrocarbons for accumulation. Impervious rocks

such as shale or evaporites (salt), as well as some faults, commonly act as seals to
form ‘traps’. An assessment of the trapping mechanism includes evaluation of lat-
eral and up dip seals for reservoirs, to act as effective traps for accumulation of
hydrocarbons. While structural closures are generally considered as safe traps,
stratigraphic and combination traps can be of varied types and need careful analysis
of the trapping processes for their integrity to hold hydrocarbons. In this context,
fault related and associated traps which are usually common can be risky and may
demand rigorous attention. Faults play a very significant role in migration, sealing,
accumulation and distribution of oil and gas and are briefly discussed later.
Preservation includes remigration, biological degradations and possible escape
of accumulated hydrocarbons from the trap that are caused by diagenesis, faults,
basinal tilts, and erosions. Particularly, post-accumulation rejuvenated and new
young faults can play major roles in hydrocarbon redistribution and obscure under-
standing the mode of occurrence in the prospect with different hydrocarbon phases
and fluid contacts. Hydrocarbon escaping to younger and shallower reservoirs from
the main accumulation can sometimes be misleading as the explorationists may
consider it as new found geologic target for pursuing future exploration and devel-
opment. Seismo-tectonic studies provide the useful clues to evaluate properly the
accumulation and preservation phase in a petroleum system.

Basin and Petroleum System Modeling

Basin evolution and evaluation for hydrocarbon potential is commonly carried out
in an exploration stage to understand the hydrocarbon locales in a basin. With the
advent of sophisticated softwares and fast computers, a unified geological modeling
of petroleum systems and the tectono-sedimentary framework of a basin, known as
basin and petroleum system modelling (BPSM) is in practice as the comprehensive
model helps understand and predict hydrocarbon habitats better. Essentially, the
model reconstructs the dynamic process of the hydrocarbon sequence
Fault Attributes Analysis and Trap Integrity 95

chronologically – from generation to preservation. The geological and thermal evo-

lution of elements like trap evolution, temperature and pressure history, and timings
of generation, migration, accumulation and preservation of hydrocarbon in the
basin/prospect are recreated through geologic time to reveal the hydrocarbon accu-
mulation episode. The integrated basin model makes use of geological, geophysical
and geochemical data and can be prepared in 2D or 3D mode. However, the migra-
tion process is a complex three dimensional problem and cannot be handled effi-
ciently in 2D basin modelling.
The key geological parameters include crucial input from interpretation of stra-
tigraphy and tectonics from seismic data- the tectono-stratigraphic frame work.
Highly sophisticated recent 3D basin modeling softwares are capable of addressing
the complicacies of modelling petroleum systems including simulation of hydrocar-
bon expulsion and accumulation. Nevertheless, the model predictions can some-
times be flawed as it depends on the exactness of data input to the model and is
expected to be as good as the geological and seismic input- the basic tectono-strati-
graphic framework, which to a large extent is the interpreted version with subjectiv-
ity. For instance, 3D modelling of an area may suggest a significant quantity of
hydrocarbon generation and expulsion with abundance of reservoir and seal rocks,
yet subsequent drilling of several wells may establish no sizable accumulation. The
drilling results may prove the elements of the petroleum system that went into mod-
elling as legitimate except for the key one, the timing; that is, the lack of harmoniza-
tion between timing of the peak expulsion and that of the deposition of an effective
cap rock to form traps in place, standing by, and awaiting accumulation.

Fault Attributes Analysis and Trap Integrity

Faults are known to play important roles as conduits, seals, and leaks in the migra-
tion, accumulation and (re)distribution of hydrocarbons and become an integral part
of basin petroleum system modeling work flow. Critical analysis of fault attributes
like type (stress genesis), throws, age and history from seismo-tectonic studies,
allows their proper definition and mapping. This assists in investigating the likely
role(s) played by faults in hydrocarbon accumulation, which can have significant
influence on exploration and development of prospects. Fault properties have been
exhaustively studied and modelled by several authors in different types of geologic
settings to understand their role in hydrocarbon accumulation.
A fault, per-se, has no sealing properties, is neither an open conduit nor a leaky
one. It is emphasized that the fault shown in a map may be considered as just a mere
line of discontinuity indicating probable changes in the rock properties and in struc-
tural dips of the strata across it (Downey 1990). The migration and trapping at a
fault depends upon the type of strata juxtaposed at the fault which needs analysis.
For example, impermeable beds juxtaposed against permeable beds makes the fault
act as lateral seal, whereas permeable beds coming in contact by juxtaposition, let
the fault leak hydrocarbons (Allan 1989).
96 5 Seismic Stratigraphy and Seismo-Tectonics in Petroleum Exploration

Tensional fault planes are often considered as pathways for hydrocarbon migra-
tion. The growth fault planes are mostly regarded as conduits at shallow depths,
where they behave as open fractures and at greater depths, where buoyancy driven
fluid enhances up-dip hydrocarbon movement. The co-joined permeable zones,
present along the fault planes, facilitate migration from source to reservoirs (Downey
1990) as shown in Fig. 5.1. Buoyancy driven fluid in overpressures also enhances
hydrocarbon movement along a fault. On the other hand, for a reverse fault in com-
pressional regime, the fault plane may hardly act as open fracture and facilitate
migration.
A fault acts as an effective up-dip seal of a trap where the throw is more than
reservoir thickness so that nonpermeable rocks rest laterally against the reservoir
(Fig. 5.2). However, faults commonly change throws along their length which can
complicate the effective sealing process, especially in thin and multi-pay reservoirs.
It is for this reason that the throws across faults need to be correctly estimated and
represented by contours in the structure map (see Chap. 3). An elaborate approach
to understand hydrocarbon migration and accumulation in fault-associated traps is
by using an analytical technique known as ‘fault-plane-mapping’ analysis (Allan
1989). The analysis consists of mapping of strata juxtaposed across the faults
three-dimensionally with appropriate structural dips to bring out the contact zones
of permeable and nonpermeable rocks to judge the role of the fault.
Faults may be considered as seals where the fault planes comprise of imperme-
able rocks such as clays or cemented materials at the walls in the throw interval.
Fault zones, especially of growth faults developed in clastic sequences of predomi-
nantly shales can cause smearing of the fault walls, isolating the permeable strata on

Migration and growth fault

conduit at shallow depths,

Fault as an open fracture

Migration takes place along

co-joined permeable beds

Conduit at deeper depths,

along fault plane in geopressures

Fig. 5.1 A sketch illustrating hydrocarbon migration pathway along growth- fault. At shallow
depths the fault plane behaves as open fractures and acts as conduit. At greater depths, growth fault
augments upward movement of hydrocarbon due to buoyancy driven fluid under geopressures.
Elsewhere, the co-joined permeable zones present along the fault plane facilitates migration (After
Downey 1990)
Fault Attributes Analysis and Trap Integrity 97

Fig. 5.2 Sketches illustrating fault properties. (a) Faults act as seal when the impermeable rock is
juxtaposed to the reservoir, (b) faults occurring post entrapment may cause separate contacts in the
faulted blocks when the fault plane gouge acts as lateral seal and (c) faults may cause hydrocarbons
to leak partly or completely depending on occurrence of partial or no trapping due to permeable
rocks set against reservoir (Modified after Harding and Tuminas 1989)

either side and termed “clay smear potential” of a fault (Doughty 2003). The behav-
iour of clay smears during the growth of a fault has important implications for fault
seal analysis as its discontinuity over the prospect area can cause leaks. This may be
hard to comprehend from seismic and quantitative fault seal prediction in the sub-
surface becomes a difficult task (Doughty 2003).
Similarly, granulations created along the fault surface during the tectonic stress
can be cemented during diagenesis over a period of time to act as a nonpermeable
plane. Big granules along a fault plane are known as ‘fault breccias’ where as the
smaller ones are known as gouges. Quantitative analysis to estimate net shale con-
tent in a fault plane is useful in determining the efficacy of fault seal and is termed
“Shale Gouge Ratio” (Freeman et al. 2008). Softwares are available to analyse
sealing efficacy of clay smear potential (CSP) and shale gouge ratio (SGR) of faults
but the results need calibration with subsurface data such as formation pressures and
fluid contacts for successful predictions (Doughty 2003).
Occurrence of new faults or reactivation of old faults post oil and gas accumula-
tions, may cause redistribution of trapped hydrocarbons differently in the reservoir,
and in worst case scenarios, cause leaking of the entire volume of hydrocarbon, as
in ‘breached structure’ (Figs. 5.3 and 5.4). Fault attributes revealed from seismo-
tectonic evaluation further assist in proper reconstruction of the episodic stress and
deformation history to assess precisely the timing of hydrocarbon charge and accu-
mulation. For instance, a cursory interpretation of a growth fault affecting the entire
sequence, on close examination of seismic stratal reflections, may unravel its epi-
sodic history as that of a cyclic growth punctuated by intermittent reactivations. It is
vital to understand the genesis and age of faults affecting a prospect, with respect to
the moments of expulsion, migration and accumulation of hydrocarbon, and as well
to its status in the trap as pre-existing, concurrent or post-charge. This understanding
is vital. Without it, one may be led to flawed evaluations of hydrocarbon prospectiv-
ity ending in an exploration debacle.
Fault analysis can also be important during development phase as its presence
may impede flow continuity within the reservoir, leading to compartmentalisation
of the field. Separate fault blocks may have separate fluid contacts that require more
98 5 Seismic Stratigraphy and Seismo-Tectonics in Petroleum Exploration

Fig. 5.3 Sketches illustrating role of faults occurring after hydrocarbon accumulation. Reactivation
or generation of new faults (a) redistribute the trapped oil/gas in the shallower reservoirs or (b)
facilitate escape of accumulated hydrocarbon (Modified after Harding and Tuminas 1989)

Fig. 5.4 A seismic segment showing an example of a ‘breached’ structure. Young faults occurring
after accumulation facilitate escape of trapped hydrocarbon (Image: courtesy ONGC, India)

production wells to be drilled, increasing the operational cost. Stresses causing

faults sometimes also induce fractures, which can augment porosity and permeabil-
ity, and introduce elements of anisotropy in the reservoir, affecting production and
water injection strategies.

Prospect Generation and Evaluation, Techno-Economics

After basin evaluation, detailed interpretation upgrades the identified geological

leads and plays to firm up prospects for drilling. This is sometimes achieved through
fresh evaluation of the existing data in the light of new geological findings from near
Prospect Generation and Evaluation, Techno-Economics 99

by well in the area. Frequently, new and better close-grid seismic data in the area are
acquired to support improved interpretation for prospects that have definitive hydro-
carbon potential. The prospects are invariably assessed first for their probable com-
mercial values before making a decision to drill. Prospect evaluation deals with
technical and financial risk analysis – an exercise which proffers an estimate of
profitability of the exploration venture in case of a hydrocarbon find. Clearly, the
results depend to a large extent on reliability and probability of the geological
parameters input from seismic analyses, as well as on other important factors like
commercial, political, logistical and environmental. We shall, however, restrict dis-
cussions to technical (geological) risk assessment.
Vital evaluation parameters for a technical evaluation of a prospect include
essentially aerial extent and amplitude (thickness) of the structure, the source type
and reservoir facies, the seals and the entrapment mechanism to form an effective
trap. However, a more significant point in the exercise, as discussed earlier, may be
the assessment of the moment of trap formation with respect to the timing of hydro-
carbon charge as well as post migration tectonics affecting accumulation. It may be
stressed that comprehending the tectonics and its chronological history from seis-
mic is extremely crucial for justifiable assessment of all prospective traps, like four-
way anticlinal closures, fault-closures, wedges, primary stratigraphic and
unconformity traps, for their disposition to charging, effectiveness of entrapment
mechanism, and redistribution of hydrocarbon to establish the ultimate habitat.
Some of the critical geological parameters which play major part in risk evalua-
tion linked intimately to financial consequences and drilling decisions are outlined.
A few of these may be pertinent to later phases like appraisal (described briefly at
the end) and production and may not be of immediate concern at this stage of explo-
ration. Yet the evaluation of these factors to assess attendent risks may be made as
it helps management to be aware of likely financial implications. Nonetheless, these
points are touched to underscore technical risk evaluation in a wider perspective that
is relevant to decision making process on future commercial and strategic plans in
case of a discovery achieved. This is particularly applicable in situations where
acreages and assets, offered on contract for exploration and development license,
are to be geologically evaluated before taking important decisions to bid for enter-
ing into exploration agreements.

Type of Source

Forecasting the expected type of hydrocarbon find in the prospect is important as

economics for gas and oil differ greatly depending on market, price and mode of
transport etc. For a given prospect size, generally oil fields may need drilling of
more production wells than that needed for a gas field. The capital and operational
costs on engineering and infrastructures for developing a field varies with type of
hydrocarbon and the future strategy may be guided by decisions depending on
resources of a company.
100 5 Seismic Stratigraphy and Seismo-Tectonics in Petroleum Exploration

Migration-Timing and Pathways

Source-reservoir connectivity and existence of trap at the time of migration is a

prerequisite for accumulation which needs careful evaluation. In this context, pres-
ence of carrier beds, unconformity surfaces and faults acting as migration pathways,
need to be carefully assessed for their uncertainty.

Reservoir Lithology (Clastic/Carbonate)

Clastic and carbonate reservoirs may have different development and production
plans. The productivity and recovery varies with the type of reservoir, e.g. sand
reservoirs generally have higher primary recovery of reserves in-place. Carbonate
reservoirs, on the other hand, are more complex and heterogeneous in nature, often
fractured and offer relatively lower primary recovery. Enhanced oil recovery (EOR)
processes and their efficiencies vary greatly for the two types of reservoirs with sand
reservoirs being generally more amenable to enhanced secondary recovery through
water injections. Enhanced recovery in carbonates may also have the added risk of
producing the dissolved H2S gas in hydrocarbon (sour oil/gas) and being corrosive
may need special processing that increase expenditures considerably.

Type of Traps

Traps can be structural, stratigraphic or a combination of the two and implicitly may
be risked accordingly. For example, in a typical single-pay structural prospect, the
hydrocarbon rock volume estimate may be relatively straightforward but may not be
so easy in a multi-pay stratigraphic prospect due to uncertainty in disposition of
number of reservoirs and varying thicknesses. Stratigraphic traps may also require
stricter assessment of trapping mechanism and may be considered more risk prone.
Structural prospect with several criss-cross faults may also need to be appropriately
risked. As stated earlier, faults may compartmentalize the field needing more pro-
duction wells and may also exhibit anomalous pattern of fluid flow during produc-
tion, seeking new solutions that involve more expenditure.

Estimate of Hydrocarbon Volume

Estimate of hydrocarbon volume is the most important outcome, eagerly awaited, to

be followed by financial calculations to assess cash flow for the company.
Hydrocarbon pore volume is estimated by multiplying the likely hydrocarbon
 References 101

bearing area of the prospect with thickness, the porosity, and the hydrocarbon satu-
ration. In structural traps, the hydrocarbon accrual depends on the spill point, con-
trolled by top, bottom and vertical closure of the structure and on the hydrocarbon
water contact. The volume of hydrocarbon-in-place (volumetric) gives estimate of
hydrocarbon at surface by multiplying it with an appropriate factor. For oil it is
known as the shrinkage factor as oil volume is reduced due to gas coming out of the
solution when produced. For gas, it is the reverse; it expands in volume at surface
and the factor is known as gas formation volume factor.
Since the estimates are based on reservoir parameters assessed from interpreted
seismic maps and geologic prediction of petrophysical parameters and all of these
have inherent uncertainties, it is likely to be highly influenced by technical subjec-
tivity and the analyst’s bias. Skill and experience of an interpreter only helps come
out with a judicious realistic estimate of hydrocarbon volume in place.
After a discovery well, the work enters the appraisal phase, in which additional
data are acquired – usually including drilling more wells – and analyses are con-
ducted to manage and reduce the risks before making an investment decision to
develop the prospect as a field. Synergistic study of seismic stratigraphy and seismo-
tectonics, however, provides information at all phases, which is the key to assess
hydrocarbon prospectivity sensibly. Seismic studies reduce uncertainties and help
minimise exploration and production risk.

References

Allan SU (1989) Model for hydrocarbon migration and entrapment within faulted structures.
AAPG Bull 73:803–811
Doughty PT (2003) Clay smear seals and fault sealing potential of an exhumed growth fault, Rio
Grande rift, New Mexico. AAHE Bull 87:427–444
Downey MW (1990) Faulting and hydrocarbon entrapment. Lead Edge 9:20–22
England WA, Fleet AJ (1991) Introduction. Geol Soc Lond Spec Publ 59:1–6
Freeman B, Yielding G, Needham DT, Badley ME (2008) Fault seal prediction: the gouge ratio
method. Geol Soc Spec Pub 127:19–25
Harding TP, Tuminas AC (1989) Structural interpretation of hydrocarbon traps sealed by basement
normal faults at stable flanks of fore deep basins. AAPG Bull 73:812–840

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