Character of the Avalon Shale (Bone Spring Formation) of the Delaware Basin, West Texas and Southeast New Mexico: Effect of Carbonate-rich Sediment Gravity Flows
Evan K. Franseen, Dustin J. Stolz, and Robert H. Goldstein
Sediment-gravity-flows (SGFs) can distribute large quantities of shelf or slope carbonates to deeper basinal settings, forming heterogeneous deposits, which may develop conventional and unconventional hydrocarbon reservoirs, or even negatively impact reservoir properties. The upper Leonardian (Lower Permian) Avalon Shale (1st Bone Springs Carbonate) in the Delaware Basin consists of hundreds of meters of organic-rich siliciclastic mudstones interbedded with carbonate-rich SGF deposits. Much remains to be learned about what controls the sweet spots in this largely unconventional hydrocarbon system.
This study integrates core and well-log data to understand the distribution of SGFs and their effect on Avalon Shale reservoir properties in Culberson, Reeves and Loving Counties, Texas and Eddy County, New Mexico. Carbonate platforms (ramps) that surrounded this basinal area during the Leonardian gave rise to apron, sheet, submarine fan, and linear SGF geobodies throughout the basin. Individual source areas varied in prominence through time, creating two phases of deposition dominated by submarine fan development in the north separated by a phase dominated by apron development in the northwest and sheet deposition in basin-central areas. Sea-level fluctuations, reflected in sequences identified on the shelf, are a possible control for the regional three-phase fan-apron-fan succession in the basin. Backstepping geometries identified in the apron phase suggest development during transgression/highstand, when production moved proximally on the ramp, whereas fans were deposited during regression/lowstand when production moved distally on the ramp.
Reservoir potential of this system is largely controlled by carbonate content. Increased carbonate content is linked to poorer reservoir quality compared to surrounding mudstones, and shows that carbonate-rich facies do not add a conventional component to this unconventional system. The multiple sources that varied spatially and temporally controlled the location of the better reservoir rock. Submarine fans that formed during regression/lowstand were not deposited during transgression/highstand, and subsequently were covered in thick muddy deposits. Submarine fans created positive relief that limited the extent of other SGFs, resulting in better mudstone reservoir facies being deposited in relatively proximal positions around the margins of the submarine fans.