Contrasting Depositional Processes of Sub-Clarksville and Woodbine Reservoir Sandstones: ABSTRACT

ABSTRACT The Sub-Clarksville and Woodbine sandstones are newly discovered gas reservoirs in Grimes County. Although the sandstones accumulated in a dominantly shallow marine environment, depositional processes and transport mechanisms were different for each sand body. The Sub-Clarksville represents rapid deposition by storm-generated bottom currents. Sand accumulations are lens-shaped and were restricted to lows by shelf topography. Sedimentary structures grade from laminated to massive sandstone above a sharp basal contact, to churned shaly sandstone at the top overlain by Austin micrite. Mean grain size decreases upward with an average composition of 83 percent quartz, 6 percent rock fragments, 9 percent matrix, and abundant carbonate cement. Quartz content decreases and matrix increases upward. Bedding sequence and textural gradation suggest that sand was eroded from preexisting Woodbine sediments to the east, and transported to the shelf during passage of a major storm. In contrast, Woodbine sandstones represent a prograding deltaic environment. The sandstones are highly quartzose (80-94 percent) and show an increasing grain size upward. The "C" sandstone at Hill field is directly overlain by the Austin and grades from bioturbated mudstone at the base to massive or faintly laminated sandstone near the top. Woodbine sandstones "C" and "D" thicken significantly eastward as an extension of a giant prograding deltaic system to the northeast. Conversely, Woodbine "A" and "B" sandstones were deposited as offshore bars farther west at Kurten (Bell, 1980; Turner and Conger, 1981). Distribution of both the Sub-Clarksville and Woodbine sandstones was affected by deep-seated salt movement at Hill field and the unconformity associated with this local high. Future exploration will depend on a detailed knowledge of the depositional framework in these downdip areas.