Depositional settings and history of the Lower Miocene Fleming Group, Refugio County, Texas, as defined using seismic geomorphology

Abstract

Abstract A study of the Lower Miocene Fleming Group in Refugio County, Texas from 3-D seismic and wireline-log data documents continuous strandplain and associated lower-coastal-plain fluvial systems along a wave-dominated shoreline. This study delineates and characterizes, through the interpretation of seismic attributes and seismic geomorphology maps from 15 horizons, three principal styles of stratal patterns: (1) parallel and subparallel, strike-continuous sets of beach ridge and swale deposits within a strandplain system that prograded at least 12 km, (2) sinuous channel belts in a coeval mixed-load meanderbelt system, and (3) acoustically dim, nearly featureless seismic attribute patterns that record homogenous and topographically subdued, shallow-marine mudstones associated with transgressive marine deposits and flooding surfaces. Holocene depositional analogs for these Lower Miocene sandy strandplain deposits include the Nayarit coastline in Mexico, whereas the lower coastal plain between Galveston and Corpus Christi, Texas serves as an appropriate analog for mixed-load meanderbelt systems. The Lower Miocene coastline in Refugio County consists of a regressive phase that was punctuated by two transgressive phases. These episodes of marine transgression correspond to the Anahuac Formation and the Marginulina A shale, respectively. Succeeding events record complex, amalgamated beach-ridge deposits and coeval fluvial-channel systems that exhibit patterns of channel migration, erosion, incision, and downdip fusion of channel belts. This study concludes that potential reservoir geometries and architectures in the Lower Miocene succession are much more complex than previously inferred, the result of rapidly fluctuating and evolving strike-fed shoreline systems and contemporaneous fluvial dip-dispersal systems, some manifested by shore-parallel trends confined in swales. Improved reservoir-development strategies in this prolific hydrocarbon-bearing play should be guided by distribution of sandy depositional elements and the paleogeographic setting.