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Ubiquity of Wave-Dominated Deltas in Growth-Fault Compartments

Abstract

Growth fault compartments hold typically good rock reservoirs and combined with structural traps make good hydrocarbon reservoirs. However, the depositional environments in the growth compartment are rarely determined because the lack of direct observations. In this study is proposed a model to evaluate the process regime of the shoreline deposits trapped in growth compartments. Growth faults and deltaic coastlines are common features along basin margins with a high flux of sediment delivery. Wave-dominated deltas have been recognized to be characteristic and long lived coastline type within such growth faulted, extensional basins. During the progradation of the deltas the structural and depositional systems migrate and adjust as sedimentation rates keep pace with fault displacement rates, usually without there being a surface topographic indication of the fault. A numerical model indicates that increased subsidence along growth faults slows the progradation rate of the delta complex as new accommodation is being created. The decrease in forward delta growth results in a longer period of wave reworking and because subsidence (fault growth) is maintained through time, sediments tend to accumulate sub-vertically, creating thick (expanded thickness) wave-dominated deltaic successions on the downthrown side of the faults. In the subsurface it is difficult to assess the type of the shoreline without cores. It is proposed here to use the expansion index which can be calculated from seismic reflection profiles or well logs as an indirect indicator of the likelihood of “wave dominance” on the delta shoreline. The numerical model finding is also supported by examples. Detailed correlation within the growth-faulted depocenters of the Oligocene Frio Fm. shows growth faults affecting multiple shoreline transits. The sedimentary successions in the downthrown basins are characterized by repeated vertical stacking of shoreface sequences and a pronounced increase in sediment thickness towards the fault. Core samples from different sub-basins and different time intervals show the shoreline deposits to have overwhelming storm-wave signals.