Bruce Trudgill

Controls on fluviolacustrine reservoir distribution and architecture in passive salt-diapir provinces: Insights from outcrop analogs

Fluviolacustrine strata host significant hydrocarbon volumes in basins characterized by syndepositional growth of passive salt diapirs. An understanding of salt-sediment interaction is critical to the prediction of reservoir distribution and architecture in these strata. Large-scale stratal geometries and thickness changes resulting from salt movement are commonly apparent on seismic data, but to date, there are few predictive models for facies architecture at subseismic, reservoir scale. This article uses a high-quality outcrop data set of fluviolacustrine strata in an exhumed salt basin (Upper Triassic Chinle Formation, Paradox Basin, Utah) as an analog for improved understanding of subsurface data sets of similar structural and sedimentological setting. Salt-sediment interaction in the Chinle Formation is expressed by localized lateral variations in stratigraphic thickness, angular stratal relationships, and changes in facies architecture. Based on these criteria, there is evidence for salt-sediment interaction across a series of syndepositional salt structures, including anticlines above buried salt pillows, salt walls exposed at surface, and salt-withdrawal minibasins. Stratigraphy and facies architecture across these structures reflect the following controls: regional subsidence, localized differential accommodation space, and localized paleogeomorphology. Both localized controls were driven by syndepositional salt movement, which exhibited subtle spatial and temporal variations during the deposition of the Chinle Formation. The outcrop data set is used to develop generic predictive models of facies distributions and architectures resulting from different conditions of regional tectonic subsidence and/or fluvial energy. Analysis of stratigraphic expansion across syndepositional passive diapirs suggests that the outcrop-derived models are applicable to many subsurface data sets.

Four-dimensional analysis of the Sembo relay system, offshore Angola: Implications for fault growth in salt-detached settings

Subsurface mapping of several relay ramps from a raft-related fault array in the lower Congo Basin, offshore Angola, reveals a full spectrum of fault linkage styles. A comprehensive three-dimensional geometric and kinematic appraisal of a more complex relay system within the fault array, the Sembo relay system (SRS) highlights differences with current structural models of relay ramp genesis, evolution, and breaching. The SRS is a rearward (upper ramp) breached relay system with a maximum fault overlap and spacing of 9500 and 3600 m (~31,000 and 11,800 ft), respectively. This system is characterized by a structural geometry that becomes increasingly complex with depth as the relay system is gradually assimilated into the structural architecture of a separate and structurally deeper fault system. A detailed kinematic appraisal of the SRS indicates that the throw patterns on the frontal and rearward segments are intrinsically different. Throw backstripping reveals different initiation ages for the rearward and frontal segments of about 5.5 and 2.5 Ma, respectively. Mutual overlap is estimated to first occur about 2.5 Ma at the 5.0 Ma structural level, with fault linkage and ramp breaching occurring subsequently. The SRS therefore represents a complex amalgamation of faults that have each developed independently at different times. The genesis and evolution of the SRS have been governed through time by both salt withdrawal and associated fault detachment histories, in conjunction with increased Congo Fan progradation and sedimentation rates and phases of tectonic tilting of the underlying salt detachment surface.

Relay and accommodation zones in the Dobe and Hanle grabens, central Afar, Ethiopia and Djibouti

The right-stepping Dobe and Hanle grabens display a variety of structures that serve to transfer extensional displacement. These structures range from relay zones between overlapping fault segments to accommodation zones between interacting rift segments. The study reveals the presence of three examples of displacement transfer structures: a breached relay zone, a large-scale accommodation zone that is partially breached, and a composite zone that combines elements of both. All three examples exhibit common structural elements. First, dipping ramps develop between horizontal horst blocks and graben floors. Second, these ramps are cut by numerous faults, most of which are antithetic to the ramps and the graben boundary faults. The antithetic faults bound elongate blocks that are rotated into the grabens. Third, crosscutting faults partially or completely link the en echelon or overlapping graben boundary faults. The identification of precursory structures (mode I fractures) at the leading edge of the Dobe–Hanle accommodation zone breaching faults suggest that the breaching process may be continuing. The spatial alignment from north to south of the crosscutting faults, open fractures and lineaments indicates that the breaching process is progressing from the Dobe graben towards the Hanle graben.

Alpine-scale 3D geospatial modeling: Applying new techniques to old problems

The investigation of geologically complex settings in Alpine or mountainous terrains is still dominated by traditional data collection and analytical techniques. The application of computer-aided geometric design and three-dimensional (3D) visualization and interpretation is rarely applied to such settings, despite its significant benefits. This contribution uses the Gosau Muttekopf Basin (Eastern Alps, Austria) to demonstrate that the application of 3D geospatial models can both provide new insights into our understanding of such settings and result in a more robust and reproducible synthesis of a complex region. The objective of studying the Muttekopf Basin is to investigate the 3D structural control on the deposition of the deepwater sedimentary basin fill. Data for the investigation only consist of that which would be collected in a traditional field study (e.g., structural mapping, stratigraphic logging, and data localities derived from hand-held GPS [global positioning system]). The 3D basin configuration is initially derived using traditional analysis techniques (e.g., cross-section construction, photo-panel mapping, block diagrams, etc.). Using these analysis techniques, significant thickness variations are observed the basin fill and are related to temporal and spatial variations in displacement of the controlling structure on the southern basin margin. However, there are significant limitations to this approach. In particular, because of the uncertainty in projection and spatial positioning, these techniques can only be used in an illustrative or qualitative fashion. To overcome these limitations, a 3D geospatial model is constructed from the same input data and illustrates that 3D geospatial modeling is a powerful technique for understanding complex geological settings. Integration of map data, stratigraphic section data, photographic images, structural data, and rock property data (gamma ray) into a single geospatial model maximizes the constraints of the limited data set. It also facilitates a deeper data analysis by significantly decreasing the time involved in generating multiple surfaces required for isopach generation. The use of the isopach maps in the Muttekopf Basin provides significant insights into the basin9s evolution. In the Schlenkerkar section, the isopach maps reveal: (1) there was very little sediment thickness variation across the basin during the early basin fill; (2) the intermediate episode was characterized by a very thick accumulation in the basin9s axis with significant thinning onto the southern uplifted margin; and (3) a northward migration of accumulation occurred during the late stage of the basin fill. Overall, the isopach maps suggest that the structure on the southern margin was the primary control on accommodation space creation and that it was most active during the intermediate basin-fill episode. Using similar observations from isopach maps for the entire basin reveals that the change in structural style of the southern margin from a fold- to a fault-dominated system plays a significant role both on internal deformation of the basin as well as the sedimentology of the syngrowth basin fill. Geospatial models, therefore, provide a more robust technique for analyzing and interpreting data within a 3D environment. In addition, they enable analysis that would be impossible with traditional techniques, such as probabilistic geocellular model construction and input models for 3D structural restorations.

Salt diapir reactivation and normal faulting in an oblique extensional system, Vulcan Sub-basin, NW Australia

Oblique extension and salt diapirism have their own distinct mechanisms that control the geometry and kinematics of structures. In this study, we document a geological phenomenon from the Vulcan Sub-basin in NW Australia that combines these processes: a salt diapir reactivated in an oblique extensional system. Detailed structural analysis of this natural example, with a focus on normal fault systems, allows characterization of the oblique extensional system and investigation of how the pre-existing structural fabrics and salt diapir control deformation, and interact with each other under oblique extension. After comparison with forward modelling results and using constraints from geological evidence, our fault strike analysis indicates that the Neogene flexural extension orientation in the oblique extensional system is around 347°, revealing a perpendicular relationship between extension direction and fault strikes from the deformation zone. The salt diapir, reactivated during Neogene extension, strongly influences local structure in the oblique extensional system by altering fault strikes, stepping patterns, deformation zone width and fault density, indicative of a ‘stress–strain concentration’ effect of the salt diapir owing to its extremely low strength. These results provide valuable insights into the understanding of oblique extensional systems, the geomechanical role of salt during extension, and the Neogene tectonic evolution of NW Australia. Supplementary material: Dominant seismic frequency analysis, uninterpreted seismic profiles, additional structural and isochron maps, a 3D view of fault systems, and original measurements of the fault system are available at http://doi.org/10.6084/m9.figshare.c.3148321.