Sediment Gravity Flow Deposits in the Avalon Shale Play of the Delaware Basin, West Texas and Southeast New Mexico

Stolz, Dustin; Mouton, Kemit; Goldstein, Robert H.; Franseen, Evan; Doveton, John

Mass transport processes can distribute large quantities of shelfal carbonates into deeper basinal settings and have a profound effect on reservoir properties. Influx of carbonate detritus may develop both conventional and unconventional hydrocarbon reservoirs, or even have a negative impact on unconventional reservoir properties. In the Delaware Basin the upper Leonardian (Lower Permian) strata of the Avalon Shale (1st Bone Springs Carbonate) consist of hundreds of meters of dark, organic-rich mudstones interbedded with carbonate-rich sediment-gravity-flow (SGF) deposits. Much remains to be learned about what controls the sweet spots in this largely unconventional oil and gas system.

This study integrates core and well-log data to understand the distribution of SGFs and their affect on reservoir properties of the Avalon Shale around Reeves County, Texas and Eddy County, New Mexico. Well logs were qualitatively calibrated using corresponding core data. The carbonate SGF deposits can generally be recognized by their lower gamma ray and higher resistivity, compared to the surrounding mudstones. Individual SGFs can stack to form packages 10s-100s of centimeters thick. These packages were the focus of correlations and regional mapping in this study.

Core and thin sections show that these SGF-rich units are primarily composed of fine-very fine sand-sized grains of broken and abraded skeletal detritus, sponge spicules, and silt-sized detrital quartz. Texturally, deposits are grainstones to packstones with varying amounts of clay and organic content. Correlation of the thicker SGF packages shows that the Avalon Shale exhibits a complex stratigraphic architecture. Mapping shows preferred transport pathways from sources located in all directions around the basin. Predominance of certain pathways and depositional loci changed through time, reflecting evolution of bathymetry and its control over SGF transport. Given the high volume of SGFs in the system, these units can have a profound effect on this unconventional reservoir. Core petrophysics show carbonate influx from SGF deposition is generally linked to lower porosity, permeability, and TOC than the mudstones. Thus, SGFs typically produce poor-quality reservoir, although others may provide conduits for flow of hydrocarbons or water. Understanding the distribution of SGFs in the Avalon Shale play, and what controls reservoir properties, will improve exploitation of this resource.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013