Fluid flow through carbonate platforms as evidence for deep-seated reservoirs in Northwest Australia

Fluid flow features in carbonate platforms of the outer Browse Basin, Northwest Australia, are investigated using 3D seismic and borehole data. During the Cenozoic, the study area evolved from a carbonate ramp to a rimmed platform with isolated carbonate build-ups; as such it played a key role in focusing fluid on their buoyant flow to the surface. Statistical analyses of direct hydrocarbon indicators show fluid flow to be focused in elevated areas i.e., the greater is the focusing of fluid, the larger is the carbonate platform or isolated build-up. Locally, karst systems created regions of enhanced permeability through the evolving carbonate stratigraphy. Karstified horizons are located on the top of carbonate build-ups and platform clinoforms as fluid buoyantly concentrates and diffuses up into topographic highs. In turn, the putative migration of gas and fluid generated hypogenic karst systems, enhancing permeability in otherwise lower permeability rock. Based on the interpreted data, we suggest a pivotal relationship between the presence of carbonate build-ups (and karsts) in the Browse Basin and hydrocarbon accumulations at depth. Areas of elevated topography focus sub-surface fluid, enhancing the permeability of carbonate successions. In turn, focused fluid flow can lead to the generation of methanogenic carbonates, promoting the growth of isolated build-ups and leading to further generation of elevated features. This grants the identification of similar features on seismic data from Northwest Australia, and other Equatorial margins in the world, as a valid proxy for the recognition of hydrocarbon accumulations below thick carbonate successions

The role of gravitational collapse in controlling the evolution of crestal fault systems (Espírito Santo Basin, SE Brazil)

A high-quality 3D seismic volume from offshore Espírito Santo Basin (SE Brazil) is used to assess the importance of gravitational collapse to the formation of crestal faults above salt structures. A crestal fault system is imaged in detail using seismic attributes such as curvature and variance, which are later complemented by analyses of throw vs. distance (T-D) and throw vs. depth (T-Z). In the study area, crestal faults comprise closely spaced arrays and are bounded by large listric faults, herein called border faults. Two episodes of growth are identified in two opposite-dipping fault families separated by a transverse accommodation zone. Statistical analyses for eighty-four (84) faults show that fault spacing is < 250 m, with border faults showing the larger throw values. Fault throw varies between 8 ms and 80 ms two-way time for crestal faults, and 60–80 ms two-way time for border faults. Fault length varies between ∼410 m and 1750 m, with border faults ranging from 1250 m to 1750 m. This work shows that border faults accommodated most of the strain associated with salt growth and collapse. The growth history of crestal faults favours an isolated fault propagation model with fault segment linkage being associated with the lateral propagation of discrete fault segments. Importantly, two episodes of fault growth are identified as synchronous to two phases of seafloor erosion, rendering local unconformities as competent markers of fault reactivation at a local scale. This paper has crucial implications for the understanding of fault growth as a means to assess drilling risk and oil and gas migration on continental margins. As a corollary, this work demonstrates that: 1) a certain degree of spatial organisation occurs in crestal fault systems; 2) transverse accommodation zones can form regions in which fault propagation is enhanced and regional dips of faults change in 4D

Pitfalls and limitations in seismic attribute interpretation of tectonic features

Seismic attributes are routinely used to accelerate and quantify the interpretation of tectonic features in 3D seismic data. Coherence (or variance) cubes delineate the edges of megablocks and faulted strata, curvature delineates folds and flexures, while spectral components delineate lateral changes in thickness and lithology. Seismic attributes are at their best in extracting subtle and easy to overlook features on high-quality seismic data. However, seismic attributes can also exacerbate otherwise subtle effects such as acquisition footprint and velocity pull-up/push-down, as well as small processing and velocity errors in seismic imaging. As a result, the chance that an interpreter will suffer a pitfall is inversely proportional to his or her experience. Interpreters with a history of making conventional maps from vertical seismic sections will have previously encountered problems associated with acquisition, processing, and imaging. Because they know that attributes are a direct measure of the seismic amplitude data, they are not surprised that such attributes “accurately” represent these familiar errors. Less experienced interpreters may encounter these errors for the first time. Regardless of their level of experience, all interpreters are faced with increasingly larger seismic data volumes in which seismic attributes become valuable tools that aid in mapping and communicating geologic features of interest to their colleagues. In terms of attributes, structural pitfalls fall into two general categories: false structures due to seismic noise and processing errors including velocity pull-up/push-down due to lateral variations in the overburden and errors made in attribute computation by not accounting for structural dip. We evaluate these errors using 3D data volumes and find areas where present-day attributes do not provide the images we want

Submarine slide blocks and associated soft-sediment deformation in deep-water basins: A review

Three-dimensional (3D) seismic and outcrop data are used to review the significance of submarine slide blocks and associated soft-sediment deformation structures in deep-water basins. Submarine slide blocks are generated during major instability events in a variety of geological settings and their size exceeds that of boulders, which are 4.5 km long on a number of continental margins, presenting internal folding, thrusting and rolling over basal breccia-conglomerate carpets. In addition, soft-sediment deformation structures such as foliated strata, intrafolial folds, tiling, bookshelf sliding and dilational jogs reflect important shearing within blocks and their basal glide planes. This work proposes that buried blocks and associated coarse-grained debrites are capable of forming prolific reservoir intervals for hydrocarbons and mineralization. Three-dimensional leakage factor models show the bulk of fluid flow to be focused in vertical and horizontal surfaces within, and immediately below displaced blocks. The generation of large slide blocks can also mark the sudden release of overburden pressure, and result in the loss of seal competence above existing hydrocarbon fields. Ultimately, this review clarifies the present-day understanding on the modes of formation of submarine slide blocks, confirming their economic importance in deep-water basins throughout the world

A 3-D morphometric analysis of erosional features in a contourite drift from offshore SE Brazil

A contourite drift from offshore Brazil is mapped in detail and investigated using state-of-the-art 3-D seismic data. The aim was to review the relevance of erosional features in contourite drifts accumulated on continental slopes. Topographically confined by growing salt diapirs, the mapped contourite ridge is limited by two erosional features, a contourite moat and a turbidite channel, showing multiple slide scars on it flanks. Associated with the latter features are thick accumulations of high-amplitude strata, probably comprising sandy/silty sediment of Miocene to Holocene age. The erosional unconformities are mostly observed in a region averaging 3.75 km away from the axes of a channel and a moat, whose deposits interfinger with continuous strata in central parts of the contourite drift. The multiple unconformities observed are mostly related to slide scars and local erosion on the flanks of the drift. This work demonstrates that the existence of widespread unconformities within contourite drifts on continental slopes: (1) may not be as prominent as often documented, (2) are often diachronic and interfinger with correlative hiatuses or aggraded strata in axial regions of contourite drifts. Although less widespread than regional, or ocean-scale unconformities, these diachronous features result in significant hiatuses within contourite drifts and are, therefore, potentially mappable as relevant (regional-scale) unconformities on 2-D/3-D seismic data. Thus, without a full 3-D morphometric analysis of contourite drifts, significant errors may occur when estimating major changes in the dynamics of principal geostrophic currents based on single-site core data, or on direct correlations between stratigraphic surfaces of distinct contourite bodies

Reservoir leakage along concentric faults in the Southern North Sea: implications for the deployment of CCS and EOR techniques

High-quality 3D seismic and borehole data in the Broad Fourteens Basin, Southern North Sea, is used to investigate newly recognised concentric faults formed in salt-withdrawal basins flanking reactivated salt structures. Throw-depth and throw-distance plots were used to understand the growth histories of individual faults. As a result, three families of concentric faults are identified: a) intra-seal faults within a salt-withdrawal basin, b) faults connecting the seal and the reservoir on the crest of an inverted anticline, c) raft-bounding faults propagating into reservoir units. They have moved obliquely and show normal throws, even though they formed during a period of regional compression. Faults in the salt-withdrawal basin and on the inverted anticline are highly segmented, increasing the chances of compartmentalisation or localised fluid flow through fault linkages. Slip tendency analysis was carried out on the distinct fault families to compare the likelihood of slip along a fault at different pore fluid pressures and within different lithologies. Our results show that sections of the faults are optimally oriented with regards to maximum horizontal stresses (σHmax), increasing the slip tendency. The identified faults cut through a variety of lithologies, allowing different values of pore fluid pressures to build up before faults reactivate. Within the Vlieland Sandstones, pore fluid pressures of 30 MPa are not sufficient to reactivate pre-existing faults, whereas in the deeper Posidonia Shales faults might reactivate at pore fluid pressures of 25 MPa. Fluid flow features preferentially occur near fault segments close to failure. Heterogeneity in slip tendency along concentric faults, and high degrees of fault segmentation, present serious hazards when injecting CO2 into the subsurface. This study stresses the importance of high-quality 3D seismic data and the need to evaluate individual fault systems when investigating potential reservoirs for carbon capture and storage and enhanced oil recovery

Effects of sand-shale anisotropy on amplitude variation with angle (AVA) modelling: The Sawan Gas Field (Pakistan) as a key case-study for South Asia's sedimentary basins

Amplitude variation with angle (AVA) is a technique widely used in the characterisation of hydrocarbon reservoirs and assumes the Earth’s crust to be an isotropic medium. Yet, anisotropy is ubiquitous in stratigraphic sequences and has first-order effects on seismic AVA responses when investigating subsurface prospects. This work analyses the effects of anisotropic strata on AVA responses using the Lower Goru Formation, middle Indus basin (Pakistan) as a case study. In the study area, shale intervals are interbedded with reservoir sands of the Sawan gas field. Shales in this field form laminae or are dispersed within reservoir sands, making the Lower Goru Formation an example of a vertically transversely isotropic (VTI) medium. In this work, we calculate the effective (saturated) mechanical properties of the Lower Goru Formation based on rock physics templates; the Backus (1962) average typically designed for layered media, combined with the empirical relations of Brown and Korringa (1975) and Wood (1955). The input data used in our rock physics modelling is based on detailed petrophysical analyses of well data. Using the saturated effective mechanical properties of the Lower Goru Formation, we generate angle-dependent reflection coefficient curves (and seismic AVA responses) based on exact and approximate solutions, for both isotropic and anisotropic reservoir scenarios. Our results suggest that the effects of lithological anisotropy are more pronounced in places with thick shale beds within reservoir sands. Conversely, angle-dependent reflection curves, and seismic AVA responses based on isotropic or anisotropic cases, give similar solutions in the presence of thin shale beds. As a corollary of this work, we present a Bayesian inversion method for the estimation of porosity in VTI media

Crestal fault geometries reveal late halokinesis and collapse of the Samson Dome, Northern Norway: Implications for petroleum systems in the Barents Sea

This paper uses 2D and high-quality 3D seismic reflection data to assess the geometry and kinematics of the Samson Dome, offshore Norway, revising the implications of the new data to hydrocarbon exploration in the Barents Sea. The study area was divided into three (3) zones in terms of fault geometries and predominant strikes. Displacement-length (D-x) and Throw-depth (T-z) plots showed faults to consist of several segments that were later dip-linked. Interpreted faults were categorised into three families, with Type A comprising crestal faults, Type B representing large E-W faults, and Type C consisting of polygonal faults. The Samson Dome was formed in three major stages: a) a first stage recording buckling of the post-salt overburden and generation of radial faults; b) a second stage involving dissolution and collapse of the dome, causing subsidence of the overburden and linkage of initially isolated fault segments; and c) a final stage in which large fault segments were developed. Late Cretaceous faults strike predominantly to the NW, whereas NE-trending faults comprise Triassic structures that were reactivated in a later stage. Our work provides scarce evidence for the escape of hydrocarbons in the Samson Dome. In addition, fault analyses based on present-day stress distributions indicate a tendency for ‘locking’ of faults at depth, with the largest leakage factors occurring close to the surface. The Samson Dome is an analogue to salt structures in the Barents Sea where oil and gas exploration has occurred with varied degrees of success

Spatial and dimensional relationships of submarine slope architectural elements: A seismicscale analysis from the Espírito Santo Basin (SE Brazil)

High-quality 3D seismic data are used to interpret the styles and scale-relationships of architectural elements on the continental slope of Espírito Santo (SE Brazil). Sand-prone architectural elements identified in this work include: a) axial canyons incising a salt-withdrawal basin (Unit 1), b) turbidite lobes intercalated with heterogeneous mass-transport deposits (Unit 2), and c) channel complexes confined by salt-controlled topography (Unit 3). Analyses of width/height (W/H) ratios reveal two distinct dimensional groups: Mass-transport deposits and turbidite lobes with W/H ratios ≥ 100, and channels and blocks with W/H ratios between 1 and 30. Importantly, all buried submarine canyons and channels systems show average W/H ratios of 12-13 for different stratigraphic units. Length-width (L/W) ratios of structural and stratigraphic compartments vary between 1 and 10. A significant result of this work is the confirmation that distributions and dimensions of architectural elements can be controlled by salt-related faults and topography, with higher dimensional variability and lower continuity of sand-prone elements occurring in the vicinity of salt ridges. Our data also shows a marked tendency for clustering, and scale overlaps, between distinct architectural elements. The approach in this paper is relevant for hydrocarbon exploration as it uses quantitative data to predict slope compartmentalisation as a function of basin geometry