Subseismic Structures Research Articles

Interaction and linkage of basin-boundary fault segments control deformation bands distribution and damage zone permeability

Analyzing the factors that affect the fault damage zone parameters and the impact of deformation on host rock permeability is fundamental for predicting the fluid flow behavior in subsurface reservoirs. This study combines 3D seismic data with field-based structural and petrophysical analyses to determine the influence of a basin-boundary fault geometry on (1) the damage zone thickness, (2) the spatial distribution of deformation bands, and (3) the damage zone permeability. Seismic interpretation and outcrop investigation on the Portalegre Fault, NE Brazil, revealed complexity in the boundary fault geometry and subsidiary structures. The boundary fault exhibits significant local variation in its strike (from N45°E up to N85°), while minor faults and deformation bands establish cross-cutting relationships with NW–SE-oriented structures, displacing NE–SW- and E–W-oriented structures. By analyzing deformation bands frequency, we identified the presence of three subsequent fault damage zones, totaling approximately 275 m in width. The first damage zone was developed near the basin-bounding Portalegre Fault, and secondary rift faults developed the other two damage zones during breaching and segment overlap. We observed that the permeability distribution recorded successive increases and decreases as we moved away from one fault and approached another. This structural and petrophysical complexity led us to interpret this fault damage zone as a linking damage zone. These observed patterns contrast with the structural and petrophysical behavior of a wall damage zone, wherein the frequency of subseismic structures decreases and permeability increases with distance from the fault. Thus, our study highlights the effect of fault tip interaction and linkage on the distribution of deformation bands and permeability, clarifying aspects related to structural heterogeneities of siliciclastic reservoirs affected by faults.

Structural core observations in a siliciclastic reservoir-scale framework

Abstract Detailed knowledge of subseismic structures and their influence on reservoir production performance is important for optimal reservoir management. Predicting subseismic structures from seismic-scale structural interpretations is inherently difficult without the use of core data. Cores allow direct measurements of porosity and permeability of deformed rocks and enable researchers to make detailed investigation of deformation mechanisms and cementation processes. Frequency, distribution and sometimes orientation of planar structures can be constrained. Such observations are then used together with location relative to seismically mapped fault and fold structures, and with respect to lithology and stratigraphy. However, a significant gap exists between the scale of core observation and the size of structures mappable from seismic data. Bridging this gap requires a sound general understanding of the different structures that occur in reservoirs, which, in addition to faults, includes drag folds, veins, fractures and the many types of deformation bands that can exist in porous rocks. Proper understanding of such subseismic structures is primarily based on outcrop-based observations and analyses, aided by physical experiments and numerical modelling. We stress that integrating such cutting-edge knowledge with core, seismic and other case-specific subsurface data in an appropriate tectonic context is paramount for realistic reservoir characterization and successful reservoir management.

Exploring seismic detection and resolution thresholds of fault zones and gas seeps in the shallow subsurface using seismic modelling

Seismic resolution and illumination issues are sources of challenges in the detailed imaging and detection of subsurface fault architecture and fluid migration. Improved constraints on resolution can provide input into monitoring requirements and detectability of CO2 leakage, and fault-sealing properties during subsurface visualization of migration pathways. This study explores detection and resolution thresholds via synthetic seismic imaging of a detailed shallow normal fault model with realistic fault architecture including sub-seismic structures of damage zones. The base fault model is built from interpretations of high-resolution P-Cable seismic data and is further developed and conditioned by outcrop-based observations and empirical laws for fracture and deformation band distribution. Damage zones of sandstone layers host deformation bands contrary to shale layers with fractures, while mixed lithologies (shaley sandstone and sandy shale) are subjected to a combination of the two deformation mechanisms. We utilize a 2D point-spread function based convolution seismic modelling to produce the synthetic seismic images. Test scenarios include one baseline fault model without a damage zone (M1), and five more advanced/detailed fault models incorporating features known from outcrops (M2-M6; damage zones, an isolated fracture corridor, and gas seeps (CO2) along damage zones of faults and in the fracture corridor). Furthermore, sensitivity analyses on two selected models test the effect of changing the illumination angle and wavelet. The results show that: (1) faults with damage zones have larger disturbances in seismic signals than faults without damage zones, (2) stronger amplitudes are distinguished for models with CO2-filled fractures, (3) a fracture corridor (5 m at it widest) is clearly visible where it crosses horizons bounding horizontal layers, (4) sensitivity tests show good imaging for illumination ≥45°, which is the average dip of the main fault segment, and (5) learnings from fault modelling offer guidance for seismic monitoring.

Stratigraphic and structural control over permeability distribution in poorly consolidated siliciclastic rocks

Permeability models are very relevant for the characterization of oil systems. However, limitations related to the resolution of seismic data make it difficult to identify subseismic, sedimentary, and tectonic structures, which can significantly impact the flow pattern. This study analyzed the spatial variability of permeability according to stratigraphic and structural geology control to propose a useful model for poorly consolidated, fractured, and faulted siliciclastic reservoirs. In an outcrop analogue to this type of reservoir, air permeability was measured in 3 orthogonal directions at 24 points, spaced 2 m apart.The models were obtained by sequential Gaussian simulation (SGS) after statistical data treatment. The models were validated to ensure the consistency of the generated scenarios. Permeability values showed a positive asymmetric distribution and reduced medians toward tectonic structures. The fitted semivariogram model was exponential, with higher spatial continuity in the horizontal flow direction and lower in the vertical one. The permeability models emphasized the importance of considering subseismic structures in the flow analysis of reservoirs since they have proven to play a significant role in the permeability distribution in the outcrop assessed.

Analysis of deformation bands associated with the Trachyte Mesa intrusion, Henry Mountains, Utah: implications for reservoir connectivity and fluid flow around sill intrusions

Abstract. Shallow-level igneous intrusions are a common feature of many sedimentary basins, and there is increased recognition of the syn-emplacement deformation structures in the host rock that help to accommodate this magma addition. However, the sub-seismic structure and reservoir-scale implications of igneous intrusions remain poorly understood. The Trachyte Mesa intrusion is a small (∼1.5 km2), NE–SW trending satellite intrusion to the Oligocene-age Mount Hillers intrusive complex in the Henry Mountains, Utah. It is emplaced within the highly porous, aeolian Entrada Sandstone Formation (Jurassic), producing a network of conjugate sets of NE–SW striking deformation bands trending parallel to the intrusion margins. The network was characterized by defining a series of nodes and branches, from which the topology, frequency, intensity, spacing, characteristic length, and dimensionless intensity of the deformation band traces and branches were determined. These quantitative geometric and topological measures were supplemented by petrological, porosity and microstructural analyses. Results show a marked increase in deformation band intensity and significant porosity reduction with increasing proximity to the intrusion. The deformation bands are likely to impede fluid flow, forming barriers and baffles within the Entrada reservoir unit. A corresponding increase in Y- and X-nodes highlights the significant increase in deformation band connectivity, which in turn will significantly reduce the permeability of the sandstone. This study indicates that fluid flow in deformed host rocks around igneous bodies may vary significantly from that in the undeformed host rock. A better understanding of the variability of deformation structures, and their association with intrusion geometry, will have important implications for industries where fluid flow within naturally fractured reservoirs adds value (e.g. hydrocarbon reservoir deliverability, hydrology, geothermal energy and carbon sequestration).

Subseismic deformation in the Vaza-Barris Transfer Zone in the Cretaceous Recôncavo-Tucano-Jatobá rift system, NE Brazil

We investigate the subseismic structural expression of the major Vaza-Barris Transfer Zone in the Early Cretaceous Tucano rift basin, NE Brazil based on field observations. Subseismic structures in the Tucano rift fill encompass cataclastic deformation bands, deformation band clusters and deformation band faults. In general, these subseismic structures indicate a ∼120° extension direction and document oblique extension across the N-S Tucano Rift, consistent with the movement direction inferred from plate-scale reconstructions. The transfer zone itself is dominated by a large population of NE-SW trending deformation band structures that developed into deformation band faults making a high angle to the transfer zone. The deformation band faults are quite evenly distributed along the transfer zone, which we attribute to extension related to its arcuate cross-sectional shape with flanks dipping toward the rift margins. Additional subordinate structures, many of which are oriented parallel to the transfer zone, show strike-slip dominated motion, and indicate that the finite strain field in the transfer zone involves a component of NNW-SSE shortening in addition to the main extension along the transfer zone. In terms of subseismic fault prediction, however, the evenly distributed zone-perpendicular structures dominate and could impose restrictions on fluid flow along the zone. This macro-permeability anisotropy should be considered in an injection/production scenario in a transfer zone of the type described here. Although other transfer zones may have different geometries and therefore different subseismic characters, an important observation made here is the close correspondence between first-order geometry (in our case an arch-shaped transfer zone) and subseismic deformation (arch extension). In addition, the observation of deformation band faults in the Banzaê Member of the Marizal Formation shows that rifting lasted through the Aptian and possibly into the Albian.

A review of deformation bands in reservoir sandstones: geometries, mechanisms and distribution

Abstract Deformation bands are common subseismic structures in porous sandstones that vary with respect to deformation mechanisms, geometries and distribution. The amount of cataclasis involved largely determines how they impact fluid flow, and cataclasis is generally promoted by coarse grain size, good sorting, high porosity and overburden (usually >500–1000 m). Most bands involve a combination of shear and compaction, and a distinction can be made between those where shear displacement greatly exceeds compaction (compactional shear bands or CSB), where the two are of similar magnitude (shear-enhanced compaction bands or SECB), and pure compaction bands (PCB). The latter two only occur in the contractional regime, are characterized by high (70–100°) dihedral angles (SECB) or perpendicularity (PCB) to σ 1 (the maximum principal stress) and are restricted to layers with very high porosity. Contraction generally tends to produce populations of well-distributed deformation bands, whereas in the extensional regime the majority of bands are clustered around faults. Deformation bands also favour highly porous parts of a reservoir, which may result in a homogenization of the overall reservoir permeability and enhance sweep during hydrocarbon production. A number of intrinsic and external variables must therefore be considered when assessing the influence of deformation bands on reservoir performance.

Simulation of subseismic joint and fault networks using a heuristic mechanical model

Abstract Flow simulations of fractured and faulted reservoirs require representation of subseismic structures about which subsurface data are limited. We describe a method for simulating fracture growth that is mechanically based but heuristic, allowing for realistic modelling of fracture networks with reasonable run times. The method takes a triangulated meshed surface as input, together with an initial stress field. Fractures initiate and grow based on the stress field, and the growing fractures relieve the stress in the mesh. We show that a wide range of bedding-plane joint networks can be modelled simply by varying the distribution and anisotropy of the initial stress field. The results are in good qualitative agreement with natural joint patterns. We then apply the method to a set of parallel veins and demonstrate how the variations in thickness of the veins can be represented. Lastly, we apply the method to the simulation of normal fault patterns on salt domes. We derive the stress field on the bedding surface using the horizon curvature. The modelled fault network shows both radial and concentric faults. The new method provides an effective means of modelling joint and fault networks that can be imported to the flow simulator.

Field-based structural studies as analogues to sub-surface reservoirs

Abstract This paper examines a number of field analogues to structures in the sub-surface, based mainly on examples from SW Britain. Whereas sedimentological analogues are largely interpreted in terms of a clearly defined sequence of depositional events related to stratigraphy, structures can develop throughout the history of the reservoir and their timing is poorly constrained stratigraphically. This leads to fundamental differences in how the two types of analogues are used. The simplest and most direct use of structural analogues comes from comparison of field structures with those sampled in the sub-surface, where they can guide conceptual development of reservoir models. Many examples of this approach exist from the development of stratabound fracture systems to the formation of linked fault systems and fracture networks. Field analogues also play an important role in the study of sub-seismic structures, providing a link to evidence from core. One new area is the application of topology to the study of fault and fracture networks. Not only does this provide insight into fundamental relationships between structures, such as their relative age and connectivity, but it also provides new methods of analysis of great relevance to the spatial variation and scaling of structures.

Characterization of the Devonian Kharyaga carbonate platform (Russia): Integrated and multiscale approach

ABSTRACT The Devonian carbonate platform (Obj-2) of the Kharyaga field (Russia) has been operated by Total since 1999 and produces 42° API oil, rich in H2S. A multidisciplinary (sedimentological, structural, stratigraphic, and geomechanical modeling) and multiscale study was made of the field, integrating seismic, micro-resistivity log, and core data to characterize the reservoir. The Kharyaga carbonate platform is characterized by complex relationships among sea-level variations, subsidence, and sedimentation rates. The field is divided into a platform edge (barrier zone) in the southern area, and back barrier and lagoon environments to the north, each characterized by different facies distribution and dynamic behavior. The platform shelf and the reef flat show dome-shaped thrombolites; the back barrier is characterized by grainstone shoals associated with columnar stromatolites, whereas the lagoon comprises peloidal wackestones and packstones associated with laminated fenestral stromatolites. Significant and prolonged subsidence, in both the lagoon and the platform edge, is associated with partial emergence of the platform reef, as shown by karst development. Seismic interpretation highlighted both underlying north-south structures and syndepositional east-west collapse faults along the southern barrier edge. Two-dimensional and three-dimensional sedimentological modeling of the Obj-2, calibrated by well data and core analyses, allowed the simulation of alternative scenarios to explain the evolution of the carbonate platform during the Devonian and suggested the observed facies distribution is compatible with the presence of a fixed outer platform edge represented by east-west faults. The carbonate platform of the Kharyaga field exhibits complex behavior; hydrocarbon (HC) accumulation and flow is controlled by the interplay of primary porosity, karst distribution, and structural discontinuities (faults and fractures). Besides the role of the karstification, this paper mainly focuses on the role played by tectonic structures on the evolution of the carbonate platform and on the HC flow. From drill stem test data, east-west faults and karsts are considered as major permeability heterogeneities, but interference tests showed more complex fluid pathways at the smaller scale. Microresistivity images show dominant northeast-southwest and northwest-southeast fracture sets in the central barrier, forming an interconnected flow network with the seismic faults. Geomechanical modeling suggests that these fractures are subseismic structures linked to inherited, deep-seated faults active at the time of fracturing, in addition to the listric reservoir faults.

Factors controlling permeability of cataclastic deformation bands and faults in porous sandstone reservoirs

Improving the prediction of sub-seismic structures and their petrophysical properties is essential for realistic characterization of deformed sandstone reservoirs. In the present paper, we describe permeability contrasts induced by cataclastic deformation bands and faults in porous sandstones (766 data synthesized from field examples and the literature). We also discuss the influence of several factors, including tectonic regime, presence of a fault, burial depth, host sandstone porosity, and grain size and sorting for their initiation and permeability. This analysis confirms that permeability decrease is as a function of grain-crushing intensity in bands. Permeability reduction ranges from very limited in crush-microbreccia of compaction bands to high permeability reduction in cataclasites and ultracataclasites of shear-dominated bands, band clusters and faults. Tectonic regime, and especially normal-fault regime, with its tendency to localize strain and generate faults, is identified as the most important factor, leading to the formation of cataclastic bands with high permeability contrasts. Moreover, moderate burial depth (1–3 km) favors cataclastic bands with high permeability contrasts with respect to the host sandstone. High porosity, coarse-grain size and good grain sorting can slightly amplify the permeability reductions recorded in bands.

Developing a model discrete fracture network, drilling, and enhanced oil recovery strategy in an unconventional naturally fractured reservoir using integrated field, image log, and three-dimensional seismic data

In this study, we develop a model discrete fracture network (DFN) for the unconventional, naturally fractured Tensleep Sandstone oil reservoir at Teapot Dome, Wyoming. Reservoir characterization is based on three-dimensional (3D) seismic data, fracture image logs from Teapot Dome, and field observations of the Tensleep exposure in the Alcova anticline and Fremont Canyon areas. Image logs reveal that the dominant reservoir fracture set trends parallel to the present-day maximum horizontal compressive stress () inferred from drilling induced fractures. Analog field studies of the Alcova anticline and Fremont Canyon suggest fracture heights and lengths are power-law distributed, while the fracture spacing distribution is best described as log-normal. Image-log–derived fracture apertures are also log-normally distributed. These properties are incorporated into a model DFN. We assume subseismic folds, faults, and fracture zones control fracture intensity distribution and use composite 3D seismic attributes to locate subtle changes in seismic response interpreted to result from subseismic structure. Directional curvature defines aperture-opening strain normal to the dominant reservoir fracture set. Seismic attributes are scaled and combined to control fracture intensity variations in the model. Grid-cell porosity and permeability distributions derived from the DFN suggest the presence of northeast–southwest-trending reservoir compartments. We suggest that enhanced oil recovery operations may be optimized using lateral injection and production wells oriented along interpreted compartment boundaries at high angles to . This combination of injection and production laterals could help maximize storage and hydrocarbon recovery in depleted reservoirs and in down-dip residual oil zones.

Progressive evolution of deformation band populations during Laramide fault-propagation folding: Navajo Sandstone, San Rafael monocline, Utah, U.S.A.

Monoclinal fault propagation folds are a common type of structure in orogenic foreland settings, particularly on the Colorado Plateau. We have studied a portion of the San Rafael monocline, Utah, assumed to have formed through pure thrust- or reverse-slip (blind) fault movement, and mapped a particular sequence of subseismic cataclastic deformation structures (deformation bands) that can be related in terms of geometry, density and orientation to the dip of the forelimb or fold interlimb angle. In simple terms, deformation bands parallel to bedding are the first structures to form, increasing exponentially in number as the forelimb gets steeper. At about 30° rotation of the forelimb, bands forming ladder structures start to cross-cut bedding, consolidating themselves into a well-defined and regularly spaced network of deformation band zones that rotate with the layering during further deformation. In summary, we demonstrate a close relationship between limb dip and deformation band density that can be used to predict the distribution and orientation of such subseismic structures in subsurface reservoirs of similar type. Furthermore, given the fact that these cataclastic deformation bands compartmentalize fluid flow, this relationship can be used to predict or model fluid flow across and along comparable fault-propagation folds.

Fault interaction in porous sandstone and implications for reservoir management; examples from southern Utah

Different types of fault interaction are examined and compared to a single fault situation with respect to density, distribution, and orientation of subseismic structures. Fault branch points are found to be considerably more complex than single faults. The damage zone in these areas shows a wider range in orientation of deformation bands and fractures, and the damaged volume extends far into the fault blocks. Overlapping structures develop wide damage zones at early stages, typically with structures that are oblique to the faults and, thus, represent potential flow barriers. The damage associated with relay structures is inherited by later stages, when the fault segments are coalesced and behave as a single fault. At advanced stages, the damage zones are uncommonly wide in breached relay locations. Such locations can be recognized as places where faults make abrupt steps or bends. The extent to which complications associated with both single-tip and double-tip interactions affect reservoir performance depends on the nature of the minor structures in the damage zone. It is thus crucial that the physical nature of minor structures is investigated so that their influence on reservoir performance can be evaluated.

The use of dipmeter data to constrain the structural geology of the Gullfaks Field, northern North Sea

Analyses of 23 km of dipmeter data from 48 wells on the Gullfaks Field, northern North Sea, have helped to constrain and understand the structural geology of the area. The analyses have verified the general structure obtained by seismic interpretation; i.e. a main, western domino system of rotated fault blocks and non-planar bedding and an eastern horst complex, separated by a collapsed anticline structure. In addition, subseismic structures have been revealed by the integrated use of dipmeter data, well log correlation, seismic data and core data. Within the domino system, dip of bedding decreases towards the main faults to the west (large-scale drag). The effect of such large-scale drag decreases with depth, probably due to a vertical increase in consolidation and mechanical strength at the time of deformation. Within the accommodation zone, a modified fold structure is seen, whereas strata within the horst complex are generally subhorizontal. More than half of all faults on the Gullfaks Field have developed a zone of local drag. Generally, only the northerly-trending faults are associated with drag. The interval affected by drag is typically some tens of meters wide, and is consistently wider in the hanging wall than the footwall. Due to the effect of drag, total offset may be up to one order of magnitude larger than the amount of missing section identified from well log correlation. There is no apparent relationship between interval affected by drag and lithology. However, drag is less abundant in more consolidated rocks at deeper stratigraphic levels. Minor faults are less abundant than expected from a power-law down-scaling of seismically detectable faults, and are not particularly common within the large-scale drag zones. It is suggested that a significant amount of subseismic deformation occurs on a scale below resolution of dipmeter data, probably by ductile flow rather than discrete faulting. North-trending minor faults are most common and will restrict fluid flow in an E–W direction due to cataclasis and phyllosilicate smear associated with abundant deformation bands (micro-faults).

Detection of Zones of Abnormal Strains in Structures Using Gaussian Curvature Analysis

Whereas some folds, such as those produced by flexural slip, do not theoretically entail strain within the folded surfaces, any surface involving double curvature (such as domes and saddles) cannot form without some stretching or contraction of the bedding. Whether straining of the surfaces is required during folding depends on the three-dimensional fold shape and, in particular, on the Gaussian curvature at points on the folded surface. Using this as a basis, I present a method for detecting zones of anomalously high strain in oil-field structures from Gaussian curvature analysis (GCA) of natural structures. The new method of GCA is suitable for analyzing surfaces that have been mapped seismically. A Gaussian curvature map of the structure is a principal outcome of the a alysis and can be used to predict the density of strain-related subseismic structures, such as small-scale fracturing. The Goose Egg dome, near Casper, Wyoming, is analyzed and provides an example of GCA. In this structure, a relationship is observed between fracture densities and Gaussian curvature.