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Toward a Higher Resolution Understanding of Fluvial to Shallow Marine Clastic Reservoir Analogues as Resolved by GPR


Clastic deposition in the continental and near shore environment is widely recognized as complex, and is controlled by allogenic and autogenic processes that include eustasy and tectonics, as well as grade, gradient, discharge, and surface morphology. The dynamic nature of the processes that drive fluvial to shallow marine deposition results in deposits that exhibit abrupt lateral and vertical discontinuities in facies that juxtapose porous and permeable units with impermeable ones. The complex geometries and stacking patterns that form the depositional architecture of preserved continental and near shore depositional systems represent significant obstacles to correctly applying many of the conceptual models most utilized by exploration geologists. Close examination of modern fluvial to shallow marine deposits of reservoir quality sand can assist in predicting the occurrence of these deposits in the subsurface, as well as understanding their internal structure and relationship to other passive margin deposits. A series of GPR lines were acquired in the coastal plain of South Carolina to investigate the internal structure and stratigraphy of a variety of fluvial to shallow marine sand deposits, as well as to serve as an experiment in the application of GPR to sedimentological and stratigraphic research. Strike and dip oriented reflection lines, as well as common midpoint surveys were collected in 100 and 200 MHz on fluvial point bars, overbank sand bodies, flood tidal deltas, barrier islands, and estuarine channel bars. The resulting data were processed, depth converted, and interpreted using a radar stratigraphic approach. The processed and interpreted radar sections resolve a variety of bedform types and scales, as well as internal surfaces and dipping interfaces corresponding to unconformity, flooding, and accretionary surfaces. Stratigraphic units are differentiable in section, with observed changes in radar velocity correlative to structural and/or stratigraphic interfaces. Bedding packages are well imaged and can be used to predict the scale of reservoir units using empirical relationships. The results establish the suitability of the imaged deposits as reservoir analogues for ancient continental to near shore equivalents. The high resolution imaging made possible by GPR investigations of modern deposits will assist in locating and predicting the scale and reservoir quality of these deposits in the subsurface, as well as navigating them.