Well and seismic data are used to constrain the geometry of traps formed in folds adjacent to salt walls/welds in two deep water subsalt oil fields (Fields A and B) in the northern Gulf of Mexico, USA. The fields are combined structural-stratigraphic traps associated with minibasin-scale folds formed at the basin margin. Field A is adjacent to a megaflap fold with an overturned limb that changes abruptly from a thin to a thick sequence of Paleogene strata along trend. The lateral change in fold geometry occurs at an outward bend in the trend of the salt-sediment interface. Field B is a minibasin-scale fold without an associated megaflap and developed along a more linear salt-sediment interface. Middle to Lower Miocene reservoir sandstones in both fields are moderately dipping-to-overturned and pinch-out stratigraphically prior to intersecting the salt-sediment interface. Trap scale folding and variations in megaflap characteristics are interpreted as primary controls on subseismic deformation, comprising primarily of deformation bands (DBs) observed in core and image logs. Spatial distributions, orientations, and frequencies of DBs are documented from well data in Fields A and B and placed into the trap-scale context to consider their origin. One DB set trends parallel to the strike of the folds in both Field A and Field B and formed in response to bed-parallel shortening. The second set of DBs trends at a high angle to the fold strike and formed to accommodate a change in Field A fold geometry. Flow measurements, along with petrophysical characterization, are used to estimate the impact of DBs on subsurface fluid flow, which is then compared to observed flow measured during well-tests. Despite the high density of DBs throughout the fields, they only impact reservoir flow where both sets are present at a distinct change in Field A megaflap and reservoir geometry.

Summary We use core and image logs to characterize sub-seismic deformation at two adjacent sub-salt hydrocarbon fields, both of which are 3-way structural traps against salt. Microstructural analysis indicates that most of the deformation bands (DBs) are cataclasites, with a lesser amount of protocataclasites. Core calibration with high-resolution oil-based image logs allows interpretation of DBs that were previously unrecognized. Spatial distributions, orientations, and frequencies of DBs are established from structural interpretation of image logs acquired in wells at different structural positions along the salt-sediment interface of each field. The kinematics of the DBs are interpreted from orientation and displacements, and placed in a larger trap scale context to understand their origin and effect on subsurface fluid flow. Bulk effective permeabilities are estimated for each well by incorporating the host sand and DB permeabilities, as well as the reservoir thickness and volumetric fraction of DBs, and then compared to collected pressure and production data.

Summary The oil field described in this study is located subsalt adjacent to a salt wall in the northern Gulf of Mexico. Data from field development wells are used in conjunction with seismic data to constrain a structural interpretation of the field. The field occurs in a dipping halokinetic sequence (dip magnitudes range from ~20 to overturned). The interpretation indicates that the reservoir sand interval pinches out before intersecting the salt-sediment interface (SSI). Two general populations of sub-seismic scale structures (deformation bands and small offset faults) are observed in core and image logs from wells adjacent to the SSI. One population in which deformation bands and faults trend approximately parallel to the strike of the SSI (observed where the SSI is relatively linear), and one in which these features trend at a high angle to the SSI. The population that trends at a high angle to the SSI occurs in the vicinity of an along strike change in trend of the SSI and an associated change in flap geometry.

We present a case study from Gulf of Mexico showing improved sub-salt seismic images achieved from an iterative salt model building workflow including the applications of Reflection Full Waveform Inversion (RFWI) using full-azimuth (FAZ) and wide azimuth (WAZ) seismic data. RFWI is used to guide the salt scenario interpretation, and to directly update the salt velocity. RFWI is first applied to a starting salt model produced from a conventional image-based salt interpretation workflow of seismic tomography, seismic migration, and manually salt picking. A new salt scenario is then proposed based on the suggested directions of the velocity update from RFWI. This iterative process of RFWI velocity update and RFWI-guided salt scenario is completed by either a final round of the former or latter depending on which one shows a more sensible velocity model, flatter gathers, and more focused sub-salt image. The final salt model is different from the starting one in terms of salt geometry, salt velocity including dirty salts and near-salt sediment velocity. These salt model differences result in significant enough improvements of the sub-salt imaging, in terms of better energy focusing and more continuous reflectivity, to the reservoir interpretation. This paper was accepted into the Technical Program but was not presented at the 2018 SEG Annual Meeting in Anaheim, California.

The Tuscarora fold system (TFS) is located in the Pennsylvania salient in the foreland of the Valley and Ridge province. The TFS is imaged in high quality 3D seismic data and comprises a system of small-scale folds within relatively flat-lying Lower Silurian Tuscarora Formation strata. We characterize the TFS structures and infer layer parallel shortening (LPS) directions and magnitudes associated with deformation during the Alleghany Orogeny. Previously reported LPS data in our study area are from shallow Devonian and Carboniferous strata (based on outcrop and core analyses) above the shallowest of three major detachments recognized in the region. Seismic data allows us to characterize LPS at depth in strata beneath the shallow detachment. Our LPS data (orientations and inferred magnitudes) are consistent with the shallow data leading us to surmise that LPS during Alleghanian deformation fanned around the salient and was distributed throughout the stratigraphic section – and not isolated to strata above the shallow detachment. We propose that a NW-SE oriented Alleghanian maximum principal stress was perturbed by deep structure associated with the non-linear margin of Laurentia resulting in fanning of shortening directions within the salient.

Advances in the understanding of salt tectonics generally result from interpretations of modern seismic data, field studies of exposed salt basins, experimental and numerical modeling, and cross-section restoration. In this special section of Interpretation , a journal emphasizing the mutual

We report on subsurface deformation features measured from recently acquired core and image logs from the Marcellus Formation in north-central Pennsylvania, supplemented with data collected from Valley and Ridge outcrop. The subsurface data are from an area that bridges a gap in existing published data between outcrop in the Valley and Ridge province of Pennsylvania and the relatively undeformed outcrop exposed on the Appalachian Plateau of New York. We find one distinct set of vertical veins that strike orthogonal to Appalachian Plateau fold axes and an associated set of low-angle reverse faults and stylolites that strike parallel to fold axes. As the trend of the fold axes changes around the Pennsylvania salient, the trend of associated mesostructures changes to maintain kinematic compatibility with the shortening direction change along the salient. These structures are interpreted to support a single, although possibly protracted, strain event during Alleghanian deformation, as opposed to multiple events previously interpreted to occur in different regions and stratigraphic levels within the fold belt. The veins are associated with clusters of bedding-plane slip surfaces, which are found at distinct mechanical stratigraphic positions where the competence contrast between shale and stiff limestone units is greatest. We interpret this association to indicate that bed parallel-shear within detachment zones provides the sufficient driving force required to nucleate vertical veins, and the decoupling of beds accommodates extension orthogonal to the shortening direction. Although these veins are oriented orthogonal to the present-day maximum principal horizontal stress, they remain propped open by crystalline cements. Homogenization temperatures of fluid inclusions trapped in the veins, combined with one-dimensional burial and thermal history models, suggest that the pervasive vein set formed during the Late Pennsylvanian Permian during the Alleghanian orogeny.

New seismic and well data from hydrocarbon exploration and development activity associated with the Marcellus Formation shale gas play in north-central Pennsylvania provide insight to the structural style of the Appalachian Plateau fold belt in the region north and northwest of the Allegheny structural front in Potter, Tioga, Bradford, Sullivan, Lycoming, Clinton and Centre counties. The Plateau fold belt in this area developed over a detachment in Upper Silurian Salina Group evaporites during the Permian Alleghanian Orogeny in response to north-northwest directed shortening. At the Allegheny structural front, a deep detachment in Cambrian shales that underlies the Valley and Ridge province to the south-southeast, ramps up-section through Cambro-Ordovician carbonates and Lower-Middle Silurian clastics to a shallow detachment in Upper Silurian evaporites. At the northeastern plunge of the Nittany Anticline (south and east of Williamsport, PA), only a small amount of slip is interpreted to have been transmitted into the foreland on the shallow Upper Silurian detachment. Instead most slip was consumed in fault-propagation folds immediately north of the Allegheny structural front. The Plateau fold belt, developed above the Upper Silurian evaporites, can be divided into structural domains based on fold characteristics. Domain 1 folds have short wavelengths and low amplitudes. Domain 2 salt-cored anticlines have long wavelengths and large amplitudes. Domain 3 comprises large synclines, located between Domain 2 anticlines. Halite originally beneath Domain 3 synclines is interpreted to have been mobilized, or evacuated, into the cores of adjacent Domain 2 anticlines during folding. Seismic data indicate that the base of the salt detachment underlying Plateau folds is a non-planar, stepped surface. Possible scenarios for the development of the non-planar detachment include: 1) mobilization of halite from an evaporite sequence that contained an originally non-uniform distribution of halite, 2) it is an erosion surface that existed prior to deposition of the evaporite sequence, or 3) it developed in response to buckle folding of the stratigraphic layer overlying the evaporite sequence.

We make the case for Early Cretaceous transfer zones that segment the obliquely rifted Atlantic margin of southeastern Brazil. Our interpretation is based on published literature, Bouguer-corrected gravity, regional reflection seismic profiles, and well data. In the Santos and Campos basins, Neocomian rift architecture was strongly influenced by preexisting fabric and structures of the Late Proterozoic (Brasiliano orogeny). The Atlantic margin inherited an east-northeast-west-southwest orientation so that rifting was oblique to the margin.On a regional map of Bouguer-corrected gravity, a nearshore belt of positive anomalies correlates with an interpreted broad Moho uplift in the footwall of Neocomian extensional faults. Farther offshore, a second belt of positive anomalies correlates with a presalt ridge of eroded volcanic or basement anticlines covered by thin Aptian evaporites, interpreted as a failed spreading center. Intervening negative anomalies coincide with the main rift basin. All three belts show apparent offsets along linear zones trending west-northwest-east-southeast, which we interpret as transfer zones. The vergence of half rifts tends to change across transfer zones, compartmentalizing the rifted margin into subbasins.Our results have implications for the risks associated with distribution, maturation, and migration of hydrocarbons within the prolific Early Cretaceous lacustrine petroleum system of the Campos and Santos basins.

ABSTRACT Three quantitative regional transects across the Saih Hatat and Jebel Akhdar anticlines in the Central and Southern Oman Mountains and the Northern Ghaba Basin have been constructed based on surface, well and seismic data. Interpreted large-scale structural geometries suggest that the Saih Hatat and Jebel Akhdar anticlines are basement-involved compressional structures, underlain by north-dipping, high-angle, blind, reverse faults located beneath their southern limbs. A compressional deformation event initiated in the Oligocene (constrained by apatite fission track data) involving the high-angle reverse faults is interpreted in which pre-Permian strata and Permian-through-Lower Cretaceous strata, exposed in the Saih Hatat and Jebel Akhdar anticlines, were parautochthonous - uplifted over the underlying reverse faults, and not displaced a great distance laterally. The allochthonous Hawasina and Sumeini sedimentary rocks and the Semail Ophiolite complex are interpreted to have been emplaced onto the carbonate platform during the Late Cretaceous, and have subsequently been parautochthonous during the Tertiary deformation. The upper portion of the pre-Permian section in the Ghaba Basin consists predominantly of a thick (>4 kilometers) sequence of Cambrian-through-Silurian, predominantly non-marine to shallow-marine, clastics of the Haima Supergroup. In contrast, out of the Ghaba Basin proper in the Central Platform or Musallim High region, the Haima Supergroup is generally less than 2 kilometers thick, and interpreted to thin to the north. The fundamental difference in pre-Permian strata exposed in the Saih Hatat and Jebel Akhdar Anticline windows is the thick (>3.4 kilometers) section of Ordovician age, shallow-marine strata (Amdeh Formation) present in the Saih Hatat Anticline, but absent in the Jebel Akhdar Anticline. In our interpretation, the shallow-marine clastics exposed in the Saih Hatat Anticline represent the northern extension of the Early Paleozoic Ghaba Basin, which have been uplifted over a high-angle reverse fault in the Early Tertiary deformation event. The cross-section through Jebel Akhdar is located to the northwest of the Ghaba Salt Basin, along the Musallim High. In this area the thickness of the Ordovician strata deposited is interpreted to be less than in the Ghaba Basin. The Ordovician section is not present in the Jebel Akhdar structure - the thinned section likely eroded in a Late Paleozoic deformation event.

Outcrop observations and laboratory experiments show that many small chevron folds form by interference of monoclinal kink bands in multilayer buckling. Kink-band interference has also been proposed for some map-scale folds. Furthermore, fault-related folding provides additional mechanisms of monoclinal fold generation, other than buckling, and thus makes kink-band interference all the more conceptually plausible as a significant large-scale process. In this paper we document several relatively simple examples of map-scale monoclinal fold interference, including three interfering monoclines of the Colorado Plateau, a seismically-imaged example in the Perdido foldbelt of the Gulf of Mexico, and a more complex example from seismic mapping in the Santa Barbara Channel, California. Kink-band interference has normally been analyzed in cross section. Here we emphasize the map-view phenomena and present a simple balanced three-dimensional model of the interference geometry, treating the monoclines as two independent kink bands, which does not depend of the kink-band folding mechanism. This model predicts the shape of the jog produced by crossing monoclines and is used to help evaluate the role of interference in the map-view geometry of our examples. The documentation of these simple examples supports the concept that more complex monoclinal fold interference could be a significant phenomenon in the upper crust.

Basement-involved structures commonly occur as long, irregular chains of uplifts in foreland basins. These structures commonly contain significant hydrocarbon accumulations, with most major fields located on the broad crests of these structures. The search for complex traps in deeper targets and in subthrust structures requires an improved understanding of the geometry and evolution of these structures. Characteristic features of basement structures include deformation zones within the sedimentary cover that dissipate significant fault slip, and gently dipping frontlimbs and backlimbs. Fault slip in the basement is usually accommodated in the cover by a triangular, widening-upward deformation zone on the forelimb, with the nature of deformation controlled primarily by the mechanical stratigraphy. If the cover contains interlayered competent and incompetent units, the incompetent units are characterized by significant penetrative deformation, whereas the competent units are faulted after a relatively small amount of penetrative deformation. Depending on the competency contrast between the basement and cover, the nature of basement, and the physical conditions of deformation, the deformation zone may also propagate downward into the basement. Gently dipping backlimb and frontlimb panels are related to movement of the hanging wall over synclinal and anticlinal bends in the major fault, respectively. Many basement faults are characterized by a number of synclinal fault bends within the basement, which result in long and gently dipping backlimbs. Forelimb panels are related to anticlinal bends that typically occur at the basement-cover interface, as well as at one or more locations in the sedimentary cover. Case studies of well-constrained examples of structures from the Bighorn and Uinta basins and the Central Basin platform, demonstrate the development of these characteristic features and their strong dependence on the mechanical stratigraphy. These models and case studies will be useful in interpreting foreland basement structures in areas with poor or limited data.

Present‐day stress directions interpreted from well bore breakouts adjacent to two crastal‐scale, active, strike‐slip faults (the San Andreas fault in California and the Great Sumatran fault in Sumatra) indicate that the maximum horizontal stress direction (SH) is oriented at a high angle (70°–90°) to both faults. The regionally defined stress fields spanning these faults show that adjacent young or actively growing folds have formed orthogonal to SH and are therefore in the thrust‐fault orientation. These observations indicate a decoupling of the strike‐slip and compressional components of the deformation within these broadly transpressive zones. Borehole breakouts in 118 wells in western California indicate a regionally consistent stress pattern with SH generally oriented NE‐SW, nearly perpendicular (80°–90°) to the strike of the San Andreas fault. The orientation of SH nearly perpendicular to the San Andreas fault implies low shear stress on the fault and is consistent with geological interpretations of the Coast and Transverse Ranges indicating active compressional deformation, fault plane solutions for recent dip‐slip‐style earthquakes, principal stress directions determined from inversion of earthquake focal mechanisms, and induced hydraulic fracture orientations. A stress trajectory map for western California is computed using an iterative statistical algorithm in which observed directional data, such as breakout directions, are used to obtain a model regional stress field. Analysis of well bore breakouts in 25 wells within the central and southern oil districts of Sumatra indicates that the regionally defined SH adjacent to the active Great Sumatran strike‐slip fault is oriented at a high angle (70°–80°) to the fault This orientation of SH is consistent regionally with geologic stress indicators and focal mechanisms of dip‐slip earthquakes. Preliminary analysis of the stress field in the vicinity of the Philippine and Alpine faults suggests SH is also oriented at a high angle to these active strike‐slip faults. Similarly, the minimum horizontal stress Sh is oriented at a high angle to the the Kane and Dead Sea transforms. The observation of SH and Sh in the vicinity of active strike‐slip faults being oriented nearly perpendicular and parallel to the faults suggests that large, crustal‐scale strike‐slip faults may, in general, be inherently weak surfaces.

A strategy to balance cross sections of complex structures is documented and illustrated by the interpretation of a compressional structure in the deep-water Gulf of Mexico. The strategy is applicable to structures formed in sedimentary rocks under low temperatures in both compressional and extensional environments, and involves the comparison of the observed structure with simple, balanced, forward models. Forward models generated using fault-related fold theory help in understanding the processes and kinematics involved in the deformation. Further, forward models are completely constrained and easy to balance, whereas it is difficult to balance data. Therefore, forward models are useful in evaluating ideas without completely solving the structure. Models are constructed assuming parallel behavior (preservation of layer thickness, no net distortion where layers are horizontal, and conservation of bed length). Unmetamorphosed, sedimentary rocks are generally observed to deform obeying the assumptions of parallel behavior in field, map, well, and seismic data.

The theory and mechanics of a statistical smoothing algorithm for estimating stress fields based on observed data are described. Incorporated into the algorithm are tunable parameters which allow the user to adjust the smoothness and fidelity of the fitted stress field. In addition, the user has the option of using robustness weights, or to weight the predicted stress field by assigning quality ranks to each data point. Fitted stress fields can be displayed as either a gridded map (axes of maximum, or minimum, horizontal stress are shown as short bars located at each point in an evenly spaced grid), or as a stress trajectory map. Examples of predicted stress fields for southern California and western Canada, based on borehole elongation data, are displayed as stress trajectory maps. A predicted paleostress field for the Spanish Peaks intrusion complex in Colorado, based on vertical dike orientations, is displayed as a gridded map. The smoothing algorithm is not limited to the analysis of stress orientation data. Any data consisting of a set of undirected lines measured at discrete locations are appropriate for input; possible candidates include strain data, mineral or intersection lineations, and lineaments.