Fault seal and containment failure analysis of a Lower Miocene structure in the San Luis Pass area, offshore Galveston Island, Texas inner shelf
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Faults that displace siliciclastic reservoirs have been observed to either seal hydrocarbon accumulations in structural traps or serve as conduits for buoyant fluid migration. While many faults located along the Texas Inner Shelf in the Gulf of Mexico do provide adequate lateral seals for the Lower Miocene petroleum system, oil and gas operators targeting the large antiformal structure roughly 7 mi offshore from San Luis Pass have been highly unsuccessful in discovering commercial amounts of methane gas. Images interpreted from 12 mi2 of high-resolution 3-D seismic reflection data (HR3D) has revealed an apparent gas chimney feature directly above the target structure that previously acquired lower-resolution conventional 3-D data failed to identify. Furthermore, the available seismic data show that the 55,000 foot-long normal growth fault displacing the San Luis Pass structure (Fault A) has propagated into the shallow Late Pleistocene (~140 ka) and younger sediment, suggesting recent movement of the hanging wall block has occurred. These three observations call into questions the ability for Fault A to properly seal and contain hydrocarbon accumulations, assuming the structure was sufficiently charged with methane, similarly to the surrounding Lower Miocene structures that have produced. An analysis of fault seal and potential containment failure mechanisms affecting the San Luis Pass structure is conducted here in order to address how hydrocarbons may have escaped into the shallow overburden sediments. 3-D geologic modeling of the Lower Miocene 2 (LM2) reservoir interval and Amph. B Shale top seal against Fault A yields fill-to-spill closure capacities of approximately 686 ft and 992 ft for the footwall and hanging wall closures, respectively. Fault seal membrane limited methane column height estimations are 300 ft and 325 ft from footwall to hanging wall, and were obtained by way of empirically calibrated equations that attempt to account for capillary entry properties of a fault through the estimation of its clay mineral content using the Shale Gouge Ratio (clay volume/fault throw). Although capacity estimations appear to be geologically reasonable in this region, they fail to explain the lack of hydrocarbons in the system, so four potential across-fault migration and leakage scenarios are considered for the purpose of determining pathways from the reservoir interval to the shallow subsurface. Areas where sandstone on sandstone juxtapositions generally pose the greatest risk of across-fault leakage, and 23 individual Lower Miocene 2 and Middle Miocene (MM) sandstone units juxtaposed against Fault A are evaluated. While the ability of Fault A to seal hydrocarbons may be feasible in static conditions, additional mechanisms evaluated using the available data include: top seal membrane leakage, top seal mechanical failure and fault reactivation mechanisms. Top seal thickness ranges between 500 ft and 1,000 ft in the study area, and analogous Lower Miocene mudstones are shown to retain methane columns of about 936 ft. Data limitations significantly reduce the ability to thoroughly investigate top seal mechanical failure and fault reactivation at this time, however, apparent vertical displacement measurements from overlapping seismic datasets suggest that movement along Fault A continued since it originally formed, and that two pulses of increased throw rate may have occurred in Early Miocene, and the Pleistocene. The apparent Pleistocene throw rates range from 0.010 mm/year to 0.125 mm/year, and are significant because the Early Miocene pulse occurred before the formation of the Amph. B top seal. Thus, it is interpreted that fault reactivation may represent the primary containment failure mechanism for the San Luis Pass structure, and that the increased apparent throw rate in the Pleistocene may symbolize a period of hydrocarbon leakage from the LM2 reservoir interval.