TOWARD A HIGHER RESOLUTION UNDERSTANDING OF COARSE GRAINED FLUVIAL POINT BARS AS RESOLVED BY GPR

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. The processes that drive alluvial valley deposition, produce deposits characterized by abrupt lateral and vertical discontinuities in facies. The complex geometries and stacking patterns of flood plain architecture represent a significant source of geologic uncertainty in hydrocarbon systems involving incised alluvial valleys, and accordingly predicting the occurrence and scale of point bar deposits, the major reservoir target in these systems, 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. The resulting data were processed, depth converted, and interpreted using a radar stratigraphic approach. The resulting radar sections resolve a variety of bedform types and scales, as well as flooding surfaces and dipping interfaces interpreted to be lateral accretion surfaces. Stratigraphic units are differentiable in section, with an abrupt change in radar velocity observed corresponding to the contact between the deposit and the underlying contact with crystalline basement. Flood packages of foreset laminae are well imaged and can be used to predict the scale of point bar reservoir units within the alluvial valley using empirical relationships. The results establish the suitability of point bar two as a reservoir analogue for a coarse grained fluvial point bar deposit. The high resolution imaging made possible by GPR investigations of modern deposits can assist in locating and predicting the scale and reservoir quality of these deposits in the subsurface, as well as navigating them. Also, since data were collected before and after October 2015 flooding of the Congaree, flood sedimentation is estimated for the study area.