Fault Seal Potential Research Articles

A prospect analysis of the Upper Oligocene sandstone reservoirs of the Wenchang 9 block, Pearl River Mouth Basin, northern South China Sea margin

Faults can act as a migration pathway and/or a lateral seal in structural traps in the Wenchang 9 block (WC9B) oil field, affecting the potential for enhanced hydrocarbon recovery in the field. In this study, three-dimensional (3D) seismic survey and well data are used to characterize Upper Oligocene fan delta - tidal flat heterogeneous reservoirs and investigate fault seal potential in two new structural traps interpreted in the WC9B oil field. Five lithofacies are identified in the Upper Oligocene Zhuhai Formation based on the well log analysis and core description. The fan delta front and sand-rich tidal flat sandstone of facies F1 in reservoir unit A of the middle Zhuhai member is characterized by the best porosity and permeability. The RMS amplitude attribute analysis and facies modeling help to predict gradual upward progression of the F1 fan delta deposits from the proximity of the Shenhu Uplift to the NW into the basin for ∼ 7–8 km during accumulation of the middle member. The NE-dipping conjugate normal faults of lower throw in the hanging wall of the NW-dipping main normal basement faults contributed to the formation of hydrocarbon-bearing structural traps and side seals. Two new structural traps in the middle member of Zhuhai Formation interpreted in the WC9B oil field. The traps are 3-way dip closure bounded by the NE-dipping normal faults of the conjugate set that are characterized by good sealing potential in the interval of the middle member. The results of this study contribute to the optimization of the enhanced hydrocarbon recovery of the WC9B oil field.

Quantifying and modelling the effects of pre-existing basement faults on folding of overlying strata in the Surat Basin, Australia: Implications for fault seal potential

Ancient basement fault geometry has a systematic effect on the folding of overlying sedimentary successions. Using aeromagnetic, Bouguer gravity, as well as 2D and 3D seismic datasets, the characteristics of the basement structure underlying the Jurassic-Cretaceous Surat Basin were examined and used to contextualise the nature of its folded strata. Using Fault-Forward modelling software, the deformation of the basement structural layers was modelled and analysed for its influence on the reactivation of faulting. The results show that the major fault trends in the eastern Surat and Bowen basins consist of five key features with N- and NE-trending orientations: the Goondiwindi, Moonie, Undulla Transition Zone, Burunga-Leichhardt, and Cockatoo fault systems. Most of the major fault deformation ceased in the Late Triassic. However, some faults (i.e., the Moonie and Goondiwindi fault systems) were variably reactivated along their lengths during the Jurassic. Three models of pre-existing basement reactivation are proposed to explain the observed variability in folding style with application to different parts of the basin. Ancient fault systems with dip angles greater than 45° were readily reactivated and produced tight monocline structures with high strain (ε > 0.35) and fault throw (>60 m) (e.g. Moonie Fault System), while faults with small dip (<30°) or large apical angles generated folds with low strain and uplift of the hanging wall. This variability in fault reactivation according to differences in fault throw potentially influences across-fault juxtaposition of strata and sealing potential for CO2 storage. Modelling such as this should be undertaken elsewhere in situations where trishear structural domains are expected, to better understand and predict the deformation of overlying stratal successions.

Influence of host rock composition on permeability reduction in shallow fault zones – implications for fault seal analysis (Vienna Basin, Austria)

In order to calibrate equations for fault seal capacities to a specific basin, faults were analysed using core material from several Neogene hydrocarbon fields in the Vienna Basin, Austria. All studied specimens are siliciclastic rocks that were sampled from a depth interval of &lt;2000 m, and share a similar depth at time of faulting, diagenetic conditions and maximum burial depth. Laboratory results showed a permeability reduction in all fault rocks compared to the host rocks. Both the highest and the lowest fault seal capacities were observed in the same fault rock type with a low phyllosilicate and clay content, and classifying as cataclastic deformation bands. Investigating the strong permeability variations within these fault rocks, microscopic analyses revealed that the fault seal potential is strongly linked to the detrital dolomite content in the host rock. Grain-size reduction processes occur preferably in the dolomite grains, accompanied by cementation. Our study suggests that – in addition to using standard fault seal analysis algorithms – accounting for host rock composition and grain-size reduction therein might help to further constrain fault seal behaviour in shallow depths. Fault seal mechanisms need to be understood on field, formation and micro scales before drawing conclusions for a full basin calibration. Thematic collection: This article is part of the Fault and top seals collection available at: https://www.lyellcollection.org/cc/fault-and-top-seals-2019

Coupled stratigraphic and fault seal modelling used to describe trap integrity in the frontier Bight Basin, Australia

Top seals and faults represent key risks to trap integrity and therefore preservation of hydrocarbons in the frontier Ceduna Sub-basin, offshore Southern Australia. Due to a paucity of well data in the basin, to provide constraint to the stratigraphic distribution of the prospective Cretaceous deltaic and marine sequences, stratigraphic forward modelling was utilised to create facies, grain size and Vshale volumes. These modelled Vshale volumes were subsequently used to investigate the structural control(s) on potential hydrocarbon leakage and migration within key stratigraphic sequences in the sub-basin.A set of coarse (20 km horizontal resolution), large scale (1100 × 600 km) stratigraphic forward models simulated the deposition of Late Jurassic to Tertiary stratigraphic sequences in the sub-basin with an initial 1 Ma interval. Smaller (80 × 60 km), finer scale (0.5 km horizontal resolution, 200 ka interval), models focussing on the Tiger and Hammerhead Supersequences over the Trim 3D seismic survey were used to investigate fault seal and top seal frameworks, using shale gouge ratio and silt and shale thicknesses from Vshale volume. Four stratigraphic forward models were produced to match a range of estimates of Vshale derived from the Gnarlyknots-1A well, the only well penetrating the central Ceduna Sub-basin. These stratigraphies were in turn integrated into a geological model interpreted from the Trim 3D seismic survey creating a geocellular model to test potential migration and trapping scenarios for potential hydrocarbons generated in the sub-basin.Fault and top seal models from the most likely scenario suggest (i) restricted potential for structural trapping near the base of the Tiger Supersequence, (ii) the possible presence of a regional migration pathway associated with sandy shoreface deposits at the transition between the Tiger and Hammerhead Supersequences, and (iii) the association of intraformational top seals and increasing fault seal potential in the deltaic sediments of the Hammerhead Supersequence feasibly resulting in a series of stacked structural traps.

Toward the creation of models to predict static and dynamic fault-seal potential in carbonates

In contrast to faults in clastic reservoirs, rules to predict the exploration and production timescale fault-seal potential in carbonates are lacking. This paper provides a summary of carbonate reservoirs with cross-fault column height differences that represent examples of apparent static fault seal, and a summary of observed examples of dynamic fault seal in carbonate reservoirs and aquifers. These include cross-fault differences in water table depths across carbonate–carbonate juxtapositions, cross-fault pressure differences in carbonate aquifers separated by faults, production-induced cross-fault pressure differences in carbonate hydrocarbon reservoirs, sealing behaviour of faults in carbonate reservoirs inferred from well tests, and examples of low fault transmissibilities from history-matching exercises from carbonate reservoirs. This paper also documents the range of compositions of fault rocks in carbonates and the range of permeabilities that have been reported from low-permeability fault cores in carbonate fault zones, as well as the implications of the observed range of fault permeabilities in carbonates for sealing behaviours. The purpose of this paper is not to argue that every fault in a carbonate reservoir will seal or will even be capable of sealing. There are, however, enough examples of faults in carbonates that are sealing in a dynamic sense, and in a static sense, that the topic of carbonate fault seal should warrant much more study. Creation of predictive models will ultimately require a considerable amount of subsurface data, but these models should be created.

Fault-seal potential of the Palaeozoic in the northwest Canning Basin

As part of a Geological Survey of Western Australia organised review of the Canning Basin involving UWA and CSIRO, the fault-seal potential for the northwest Canning Basin has been analysed. This study has two foci: firstly identifying potential for fault-bound hydrocarbon reservoirs in the Early Permian (Poole Sandstone and Upper Grant Group). Secondly, James Price Point, 55 km north of Broome, is the chosen location for an LNG facility to service the northern North West Shelf gas fields. As such, the study aims to highlight potential CO2 sequestration reservoir sequences occurring inside 200 km of James Price Point, the economically feasible distance for CO2 delivery to an injection site. Historically, hydrocarbon exploration drilling in the Fitzroy Trough targeted anticlinal structures, which proved unsuccessful due to localised, but significant, erosion of the Permian sequence including the Noonkanbah Formation top-seal on anticlinal crests. Given there is potential for untested, fault-bound traps to exist, which might provide an alternative to the anticlinal traps, it will be useful to identify the distribution of shale-rich, top-seal and fault-seal prone sequences, and where these occur at suitable reservoir depths. The study shows the Early Permian sequences on the flanking terraces of the Fitzroy Trough commonly have suitable top-seal and fault-seal prone sediments. In wells analysed in the Fitzroy Trough itself, the Early Permian sequence is poorly represented, but Permo-Carboniferous sediments observed indicate some sealing potential might exist there. Moving south onto the Broome Platform and into the Wiluna Sub-basin, the Early Permian sequences still display some sealing potential, but Ordovician units might provide more suitable targets for sequestration in these areas.

Trap integrity framework for the offshore northern Perth Basin

The presence of hydrocarbon accumulations and several palaeo-oil columns in Permian reservoirs across the offshore northern Perth Basin attests to a widespread charge system and suggests that trap integrity is a critical exploration risk. The local structural evolution between the Beagle Ridge and the Wittecarra Terrace and the impact of renewed mid-Jurassic extension on a series of prospects (Cliff Head, Dunsborough, Lilac and Morangie) is assessed to define a regional trap integrity framework. The focus is on the potential development of pathways through the Early Triassic Kockatea Shale regional top seal caused by reactivation of trap bounding faults and/or interaction with newly formed overlying structures. A series of 3D geo-mechanical models are constructed to simulate the Mid-Jurassic deformation and to evaluate the first-order factors controlling the partitioning of reactivation stresses/strains and the development of seal bypass. In parallel, the local fault seal potential is assessed by focusing on the likelihood of the fault surfaces to reactivate and conduct fluid within the Jurassic stress regimes. The stress data are also used to compute the risk of reactivation in terms of the increase in pore fluid pressure required to bring a fault segment to a state of instability. The overall risk of top seal bypass is primarily driven by the reservoir-bounding faults orientation and dip. The distribution of jogs, the interactions between fault tips, the fault density, the location of regional major fault, the fault propagation mechanisms and top seal brittleness also control locally the integrity of the top seal.

Spectral decomposition of 3D megasurvey for definition of hydrocarbon remigration systems and prediction of traps integrity

To advance our understanding of fluid remigration and assess fault seal potential, an investigation of the near seabed energy content (from 3D seismic data) has been carried out on the Australian NW Shelf. The ability to determine fluid remigration pathways and to predict trap integrity are amongst the most critical components of hydrocarbon exploration. Faults are widely cited as conduits for fluid flow and seal breach by fault reactivation represents a well known critical exploration risk in many petroleum provinces including the Gulf of Mexico, the North Sea and the Australian NW Shelf. The occurrence of fault-hosted mineralisation, fault-related migration of fluids and direct measurements of fluid flow and deformation all attest to the role of active or critically-stressed faults in promoting fluid flow by enhancement of structural permeability.

The petrophysical properties of deformation bands in relation to their microstructure

Deformation bands are significant discontinuities in reservoir sandstones providing baffles to the migration of fluids and acting as seals for hydrocarbon accumulations. Their contribution to these processes is dependent upon a range of inter-related factors including lateral continuity, relative displacement, reservoir geohistories and rock microstructure. Deformation bands have millimetre-scale displacements, which are smaller than the seismic resolution, and hence the study of their microstructure from reservoir cores is a necessary tool in the prediction of their fault seal potential. The latter is only achieved if detailed petrophysical measurements of fault microstructure are carried out using techniques, which are suitable for measurement of very low permeability fault rock at the relevant scale. Conventional techniques of fault rock porosity and permeability determination invite host rock bias in measurement and inevitably underestimate the potential of these structures as fluid barriers. Pressure decay probe permeametry used in this study has the advantage of measuring small volumes of rock at finer spatial resolutions and detailed permeability distributions around common types of deformation bands are obtained. Together with supporting data provided by conventional core analysis techniques, these measurements show that the greatest differences in petrophysical properties relative to host rock occur in those bands, which have experienced cataclasis. These results provide valuable input into reservoir simulation models and help reduce much of the uncertainty regarding the role of different types of deformation bands in fault seal.

ABSTRACT: A New Method For Calculating Fault Seal Potential by Shale Smearing

ABSTRACT: A New Method For Calculating Fault Seal Potential by Shale Smearing

Characterization of Fault Zones for Reservoir Modeling: An Example from the Gullfaks Field, Northern North Sea

A fault-seal study was performed for part of the Gullfaks field (North Sea) as an aid to improve reservoir management of this complexly faulted structure. The operator's (Statoil) map data were used to build a three-dimensional model of the fault network. Together with petrophysical logs, this model was used to compute the variation of fault-seal potential (shale gouge ratio or fault-zone percent shale) on each fault surface. Pressure data from exploration and production wells have been projected onto the modeled fault surfaces. The preproduction pressure differences at sealing faults (separating different hydrocarbon columns) provide information about the capillary seal at the faults. Across-fault pressure drops at particular times during production also have been displayed. These dynamic pressure drops provide a guide to the permeability of the fault zones after flow has started. The calculated fault parameters (displacement and shale gouge ratio) can be converted to fault-zone thickness and permeability and can be used to derive fault transmissibility modifiers for reservoir simulations. Maps have been produced showing reservoir juxtaposition areas and calculated fault permeabilities along faults throughout the study area. These parameters are compatible with the recorded pressure history and tracer movement between wells, and will enable key reservoir management decisions to be tested and optimized.

The relationship between faults and pressure solution seams in carbonate rocks and the implications for fluid flow

Abstract Pressure solution plays a major role in fault initiation and development in carbonate rocks. Strike-slip faults in the Triassic and Jurassic limestones of Somerset, UK, initiated as en echelon extension fractures, which became linked by pressure solution seams. Shear occurred along the pressure solution seams as the bridges between the veins rotated. The linked vein-pressure solution seam systems developed into pull-aparts and eventually into through-going faults. Normal fault planes in the Cretaceous Chalk at Flamborough Head, Yorkshire, UK, commonly have the pitted appearance of slickolites. Phyllosilicates are often concentrated along faults in Chalk, the thickness of the phyllosilicate gouge being proportional to fault displacement. The enrichment of phyllosilicates along the faults is due to pressure solution rather than simply to phyllosilicate smear. Pressure solution can be concentrated at the contractional quadrants of faults, particularly where there is a contractional overstep onto an adjacent fault. Metre-scale oversteps between strike-slip faults in Somerset often have pressure solution seams, while contractional oversteps and bends at Flamborough Head are accommodated by compaction of beds, with pressure solution apparently being important. The concentration of phyllosilicates by pressure solution can hinder fluid flow along and across faults in carbonate rocks. A high density of pressure solution seams between overstepping faults can effectively link these faults, increasing both the effective length of the barrier to fluid flow and the fault seal potential.

Analysis of fault geometry and displacement patterns

Abstract Interpretation of the subsurface is still often performed on suites of vertical seismic sections with structures being correlated from line to line to build up a map view at a particular horizon. The advent of 3D seismic surveys and routine use of interpretation software has made it common to produce ‘horizon-based’ interpretations, using auto-tracking techniques. Neither of these approaches guarantees a three-dimensionally consistent interpretation because of ambiguities in lateral and vertical correlation of structures. More reliable descriptions of a faulted surface can be obtained if the three-dimensional properties of a fault network are observed directly. Fault traces picked on vertical sections can be used to model the fault surfaces, which are then viewed in perspective to give an immediate assessment of the plausibility of the interpreted fault geometry. By associating horizon terminations with their respective fault segments this cut-off information can be shown on the perspective view of the fault surface, allowing the interpreter to examine in detail how reservoir layers are disrupted and juxtaposed by the fault network. The horizon cut-off information can also be used to calculate displacement-related values (throw, heave, separation) which may be gridded over the entire picked surface in order to produce displacement maps, also in the perspective view. Isolated single faults have simple displacement patterns with a maximum near the fault centre. Fault arrays show partitioning of displacement between the various splays, with relatively abrupt changes in the displacement at branchlines. Long ‘single faults’ are frequently shown to be segmented into en-echelon arrays rather than being simple continuous structures. Antithetic faults exhibit a downward decrease in displacement towards a tip line near their master fault. In areas of cross-cutting fault trends, displacement patterns can discriminate between different possible kinematic histories. Fault displacement can also be used to calculate other attributes such as fault seal potential, which can also be posted onto the perspective view of the fault surface. These analyses of fault geometry and displacement allow a more objective assessment of subsurface interpretation. They do not rely on particular assumptions about deformation mechanisms, only on the observation that displacement variation on individual fault surfaces is systematic.