About This Item
Share This Item
During the Wolfcampian, sediment gravity flows were common events at some shelf margins in the Permian basin. These mass flows transported large volumes of shoal-water bank and reef carbonates downslope into the Midland and Delaware basins, forming a wide variety of redeposited lithofacies. For example, along a segment of the Eastern shelf margin at least 40 km (25 mi) long, redeposited carbonates extend into the Midland basin 25 km (16 mi) or more. Within this basin margin setting, several petroleum pay zones occur in mass-transported debris.
In designing exploration strategies for these types of frontier deep-water reservoirs, whether within the Permian basin or elsewhere, one must develop appropriate depositional models. Some questions come to mind. Do these deposits represent episodic, widespread, single-pulse debris sheets, debris aprons dominated by numerous but rather random pulses of areally extensive sheet-flow calcarenites, or more systematically developed submarine fan facies having both channelized deposits in inner and mid-fan settings as well as sheet-flow calcarenites deposited as outer-fan lobes?
Redeposited Wolfcampian carbonates are subdivided into three major lithofacies. (1) Limestone and dolomite conglomerate debris flows and turbidites with dark interstitial micrite. Individual beds are as much as 8 m (26 ft) thick, normal to massively graded, and some beds are arranged by thinning-upward sequences. These carbonates from on of the reservoir facies with intercrystalline, solution interparticle, fracture, and vuggy porosity. (2) Wackestone to packstone calcarenite turbidites consisting largely of biotic grains. This lithofacies forms the most abundant type of redeposited sediment. The calcarenites occur in beds a few cm to 2.5 m (8 ft) thick that exhibit a variety of Bouma turbidite divisions and in some localities are arranged in thickening-upward units. Calcarenite turbi ite locally form petroleum reservoirs with solution interparticle, intrabiotic, biomoldic, and fracture porosity. (3) Wackestone to packstone calcisiltite and calcarenite turbidites that occur in less than 2 cm (1 in.) thick beds. This facies does not exhibit vertical cycles of bed thickness nor good reservoir qualities.
Analyses of cores from 12 wells both within and outside the petroleum fields suggest that these redeposited carbonates may represent a combination of debris sheet and submarine fan depositional processes. The conglomerates could be genetically unrelated to the calcarenites and represent episodic debris sheet pulses; or alternatively, these conglomerates may be channelized deposits in inner fan to mid-fan positions near the basin margin. Some of the thick-bedded calcarenites possibly represent mid-fan channelized deposits whereas the more basinward thickening-upward calcarenites resemble unchannelized outer-fan calcarenite lobes. Thin-bedded calcisiltite turbidites appear to occupy basin plain, outer-fan fringe, and interchannel settings. If these reservoirs are developed within one or more fan facies, the size and spatial arrangement of the individual fans still remain to be determined. A better knowledge of appropriate depositional models should enhance future exploration efficiency.
End_of_Article - Last_Page 442------------