Abstract Permian Rotliegend reservoir rocks are generally characterized by high net/gross (N/G) ratios, and faults in such sand-dominated lithologies are typically not considered likely to seal. Nevertheless, many examples of membrane sealing are present in Rotliegend gas fields in the Southern Permian Basin. This manuscript reviews examples of membrane sealing in the Dutch Rotliegend; it presents an extensive dataset of petrophysical properties of Rotliegend fault rocks and analyses two case studies using commonly used workflows. Fault (membrane) seal studies have been carried out on two Rotliegend fields to test the level of confidence and uncertainty of prediction of ‘across fault pressure differences’ (AFPD) based on existing SGR-based algorithms. From the field studies it is concluded that observable small AFPDs are present and that these are likely pre-production AFPDs due to exploration-time scale trapping and retention of hydrocarbons. Two shale gouge ratio (SGR)-based empirical algorithms have been used here to estimate AFPDs in lower N/G reservoir intervals with the aim of predicting membrane seal behaviour, and these results are compared to field data. It is concluded the selected SGR-based tools predict AFPD for Upper Rotliegend lower N/G reservoir rocks with reasonable results. Nonetheless, the core sample datasets show a much wider range of permeability and capillary entry pressure than predicted by the selected SGR transforms. This highlights the potential to modify existing workflows for application to faults in high N/G lithologies. Data sharing and collaboration between industry and academics is encouraged, so that in the long run workflows can be developed specifically for faults in high N/G lithologies.

Abstract To contribute to the understanding of the impacts of fault reactivation induced by reservoir exploitation, we describe the final series of laboratory experiments, numerical simulations and microstructural analysis conducted during the ‘Fault Reactivation in Carbonates’ research project. In the project, the structure and hydromechanical properties of carbonate-hosted fault zones were investigated. For the analyses here reported, faults were artificially generated by direct shearing composite blocks made of layers of reservoir analogue rocks (outcrop travertine or synthetic grainstone) intercalated with one layer of a sealing analogue rock (synthetic carbonate mudstone). Post-direct shearing, cylindrical plugs containing the fault zone and parts of intact rock were cored out from the blocks and tested in a triaxial test rig, simulating fault reactivation. Varied stress paths and pore-pressure conditions representative of fluid depletion and injection were considered. In parallel, two-dimensional mechanical models representative of the direct shear experiments were developed using smoothed particle hydrodynamics (SPH). We observed a continuous reduction in fault transmissibility during direct shearing, followed by a permeability reduction of 50–80% with increasing mean effective stress in the subsequent fault reactivation tests. Experimental fault zone geometries produced during direct shear were broadly reproduced by the two-dimensional modelling approach. We also detected that the inclusion of the carbonate mud sealing rock into the fault zone caused greater compaction of the fault materials when compared to experiments conducted without carbonate mud layers. We conclude that with fault displacement, increasing incorporation of carbonate mud sealing material into the fault zone and the concomitant development of gouge results in the continuous reduction of fault transmissibility/permeability. This occurs in the two very different limestone host-rock types and for all the stress configurations investigated. Discussions on these results and also on the outcomes of the research project as a whole are presented in the paper.

Abstract Fault zones are complex, and show considerable variability in both structure and the distribution of associated fault rocks within the fault core: the zone that localizes most strain and displacement. It is the fault-core gouge zone and associated slip surfaces which provide the cross-fault seal when permeable layers are juxtaposed. Predicting the sealing properties of fault gouge zones is difficult but often required when evaluating faults in exploration prospects. A stochastic modelling approach is described to help better understand the compositional controls on fault gouge seal potential. The model is populated with a random assemblage of four fault rock components: shale smears, shaly gouge, cataclastic gouge and low-strain host-rock lenses. Harmonic averaging of permeability and arithmetic averaging of V shale are then used to upscale the properties, and to propose a simple permeability– V shale model for fault rocks. Practical application of the model is discussed by developing an empirical link between standard well-log data and associated fault rock effective permeability. This new approach has the potential to offer a simple well-log-based fault seal model. The utility of the model is demonstrated with a case study, comparing the results to those generated using other published techniques.

Abstract Hanging-wall traps are successful trapping styles with discoveries made in many sedimentary basins worldwide. Examples of hanging-wall traps are documented in the literature but very few describe the role played by fault-rock seal on trap integrity. This contribution focuses on hanging-wall traps that are dependent on fault-rock seal. Analysis of 18 examples of hanging-wall traps has revealed that the hydrocarbon column height trapped by fault-rock seal is typically less than 190 m. Cross-plots of shale gouge ratio (SGR) and buoyancy pressure from hanging-wall traps have a similar data distribution to published SGR–buoyancy pressure calibration plots. The similarity in data distribution indicates a similarity in the overall fault-sealing mechanism: namely, the capillary fault sealing through the incorporation of clay/shale material into the fault zone. Published ‘global’ calibration plots of SGR v. buoyancy pressure can be used to evaluate the sealing or non-sealing risk of hanging-wall traps in the same manner as for footwall traps.

Abstract An understanding of trap and fault seal quality is critical for assessing hydrocarbon prospectivity. To achieve this, modern analytical techniques leverage well data and conventional industry-standard 3D seismic data to evaluate the trap, and any faults displacing the reservoir and top seal intervals. Above all, geological interpretation provides the framework of trap and fault seal analyses, but can be hindered by the data resolution, quality and acquisition style of the conventional seismic data. Furthermore, limiting the analysis to only the petroleum system at depth may lead to erroneous perceptions because interpreting overburden features, such as shallow faults or gas chimneys, can provide valuable observations with respect to container performance, and can to help validate trap and fault seal predictions. A supplement to conventional 3D data are high-resolution 3D seismic (HR3D) data, which provide detailed images of the overburden geology. This study utilizes an HR3D seismic volume in the San Luis Pass area of the Texas inner shelf, where shallow fault tips and a sizeable gas chimney are interpreted over an unsuccessful hydrocarbon prospect. Static post-drill fault seal and trap analyses suggest that the primary fault displacing the structural closure could have withheld columns of gas c. 100 m high, but disagree with our HR3D seismic interpretations and dry-well analyses. From our results, we hypothesize that tertiary gas migration through fault conduits reduced the hydrocarbon column in the prospective Early Miocene reservoir, and may have resulted from continued movement along the intersecting faults. Overall, this study reinforces the importance of understanding the overburden geology and geohistory of faulted prospects, and demonstrates the utility of pre-drill HR3D acquisition when conducting trap and fault seal analyses.