| Abstract | The Middle Cenomanian Woodbine sandstones act as a major reservoir system for many large oil fields throughout East Texas. Although numerous studies have been completed on Woodbine outcrops within DFW Metroplex, none have used modern techniques or tools, or utilized facies model concepts to study their reservoir characteristics or environment of deposition. Prior studies interpreted these outcrops as a shelf-strandplain coastal setting or a fluvial-dominated delta plain. However, this study of Woodbine outcrops along Lake Grapevine identified evidence of significant tidal influence.
The focus of this project was to determine the depositional environment and obtain a better understanding of the reservoir characteristics of the upper Woodbine (Lewisville) sandstones found in outcrop along the southeastern shores of Lake Grapevine in Tarrant County, TX. A detailed study of the lithofacies, ichnofacies, and biofacies, along with handheld spectral gamma ray and permeameter analyses, from 8 measured sections were conducted to identify representative lithofacies. Oversized thin-sections were made to estimate porosity ranges for each lithofacies, and to identify the types and extent of cement in the sandstones. Photomosaics were utilized to delineate sand body geometries by tracing out the lateral extent of the units and identifying significant surfaces and potential fluid barriers or baffles.
Seven lithofacies were distinguished in the outcrops of the study area: Bioclastic, massive bioturbated sandstone, mudstone, heterolithic sandstone and mudstone, crossbedded sandstone, flaser-bedded sandstone, and cemented sandstone. Two of the most common and laterally continuous lithofacies, the massive bioturbated and crossbedded sandstones, also had the best reservoir characteristics, with average porosities of 26% and 27%, and average measured permeabilities of 6,300 mD and 10,700 mD, respectively. The lower permeabilities in the massive bioturbated sandstone are related to clay-rimmed burrows. The bioclastic, mudstone, and cemented sandstone lithofacies are potential barriers to fluid flow, as they all have low porosities (less than 2%) and permeabilities (less than 200 mD). The data acquired during this study were all consistent with an interpretation of a tidally-influenced estuarine to shallow marine depositional environment for the upper Woodbine in the study area, which differs from previous studies. The high abundance of trace fossils that are commonly found in tidally-influenced depositional systems, including Conichnus/Bergaueria, Cylindrichnus, Planolites, Palaeophycus, Rosselia, Rusophycus, Skolithos, and Thalassinoides, coupled with the presence of heterolithic deposits and commo oyster shells led to this interpretation. |
