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The Sedimentary Structure of a Course Grained Point Bar as Resolved by GPR, Congaree River Valley, SC

Patrick Duff
University of South Carolina

Flood plain deposition within incised alluvial valleys is widely recognized as complex, and is controlled by grade, gradient, discharge, channel sinuosity, bed load/suspended load ratio, and channel morphology. As a result of the dynamic nature of the processes that drive alluvial valley deposition, floodplain deposits are often characterized by abrupt lateral and vertical discontinuities in facies. The complex geometries and stacking patterns of flood plain deposits represent a significant obstacle to correctly applying the Incised Valley Fill Model, one of the most utilized conceptual models in hydrocarbon exploration. As the major reservoir target in hydrocarbon systems involving incised alluvial valleys, predicting the occurrence of point bar deposits in the subsurface, as well as understanding their internal structure and relationship to other flood plain deposits remains a topic of active research.

A series of GPR lines were acquired to investigate the internal structure of a coarse grained point bar deposit, as well as to serve as an experiment in the application of GPR to sedimentological and stratigraphic research. Strike and dip oriented GPR reflection lines, as well as a common midpoint survey were collected in 50, 100, and 200 MHz on point bar number two, Conagree River Valley, South Carolina. Point bar two is located approximately 11 km south of Columbia, SC; it is a coarse grained arkosic sand body containing semi-saturated sediments and the water table. 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 dipping interfaces interpreted to be lateral accretion surfaces. Stratigraphic units are differentiable in section, with an abrupt change in radar velocity observed. The results shed light on the depositional character of the Congaree River Valley, establish the suitability of point bar two as a reservoir analogue, and suggest a broad role for GPR technology in sedimentological and stratigraphic investigations.

AAPG Search and Discovery Article #90182©2013 AAPG/SEG Student Expo, Houston, Texas, September 16-17, 2013