--> Toward a Higher Resolution Understanding of Coarse Grain Fluvial Point Bars as Resolved by GPR

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Toward a Higher Resolution Understanding of Coarse Grain Fluvial Point Bars as Resolved by GPR

Abstract

Flood plain deposition within incised alluvial valleys is widely recognized as complex. As a result of the dynamic nature of the processes that drive alluvial valley deposition, floodplain deposits are 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, a well utilized conceptual model in hydrocarbon exploration. As the major reservoir target in alluvial valley hydrocarbon systems, predicting the occurrence and scale of point bar deposits in the subsurface, as well as understanding their internal structure and relationship to other flood plain deposits remains an important topic for research. A series of GPR lines were acquired to image the internal structure of a coarse grained point bar deposit. 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 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 low-stand surface of erosion for the alluvial valley. 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 will assist in locating and predicting the scale and reservoir quality of these deposits in the subsurface, as well as navigating them. Similarly, these images are of sufficient resolution to test and refine the empirical relationship between bed set thickness and channel and channel belt width established by Bridge and Tye (2000).