Reservoir 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

Abstract

Sediment gravity flows (SGFs) can distribute large quantities of shelfal or slope carbonates into deeper basinal settings, forming complex heterogeneous deposits. Such deposits may have a negative or positive impact on hydrocarbon reservoir properties. In the Delaware Basin the upper Leonardian (Lower Permian) strata of the Avalon shale play (first Bone Spring carbonate) consist of hundreds of meters of dark, organic-rich siliciclastic mudstones interbedded with carbonate-rich SGF deposits. This project investigates carbonate-rich SGFs in the Avalon shale, integrating core and well log data, to determine the local-to-regional depositional controls on deep-water carbonates, as well as to understand the depositional architecture of the Avalon shale and how carbonate influx affected its reservoir potential. Over 500 well logs along with two cores were utilized for this study. The carbonates are interpreted to have been deposited by carbonate-rich SGFs with the mudstones primarily deposited in the distal, waning portions of the SGFs. These deposits stack to form thick SGF packages. Correlation of these packages shows that SGF source areas were located in all cardinal directions around the basin. The amount and timing of sediment input from the source areas varied throughout Avalon deposition, which resulted in a complex stratigraphic architecture. Two phases of carbonate fan development occurred and are separated by a phase of apron development. Backstepping geometries within the apron deposits suggest deposition during transgression and highstand. Fans were deposited in absence of major apron deposits and are distributed farther into the basin relative to the apron deposits, which suggests deposition during regressions and lowstands. Core petrophysics show the carbonate facies generally have lower porosity, permeability, and TOC than the mudstones. Thus, the carbonate facies typically produce poor-quality reservoir and mudstones produce high-quality reservoir. The thickest mudstones, forming the best-quality reservoir, were deposited during transgression and highstand in areas of reduced carbonate SGF influx and along the margins of older fan lobes. Understanding the distribution of carbonate SGFs throughout the Avalon shale will improve exploitation of this resource and enhance understanding of the controlling factors on the deposition of deep-water carbonates and shales.