Abstract

The Avalon Shale is the uppermost member of the Permian Bone Spring Formation, and is located throughout the interior of the Delaware Basin in west Texas and southeast New Mexico. It is primarily a self-sourced, fine-grained quartz-rich siltstone, and has been subdivided into three zones; upper, middle and lower shales bifurcated by carbonate sediment gravity flows. The three distinct Avalon zones have well defined core development areas that migrate spatially from east to west across the basin and at places overlap, enhancing the resource density. More than 400 horizontal wells targeting the Avalon have been drilled since 2008. The reservoir is largely gas-filled at the western edge of the basin, transitioning to a light oil and condensate reservoir along the eastern edge. A "Goldilocks Zone" for production exists at the toe of depositional slope, where quartz-rich, high TOC (~2–10 wt%) deposits are interbedded with channelized to lobate, laterally discontinuous carbonate gravity flows. At a high level, third-order lowstands have been identified as the primary target intervals, where siliciclastics delivered through deepwater channel deposits and rainout of fine-grained aeolian dust during lowstands deposited sediments with a relatively high storage capacity. Sediment input rates were low enough and waters deep enough that TOC concentrations remained high during these lowstands. Carbonate input, which degrades reservoir quality, remained low while the carbonate platforms were exposed during periods of low sea level. Relationships are observed, spatially and temporally, between depositional facies and productivity. 1000+ BOPD rates have been seen from the Upper Avalon, which is characterized as a 250'–400' thick, mixed carbonate-siliciclastic package. In the most highly productive regions, a >50' thick, laminated, silty high-porosity zone is observable at its base. Similar results have also been observed in the Lower Avalon, where the depositional setting and reservoir character are similar. Each Avalon zone has spatially variable rock properties caused by depositional variations throughout the basin. Three-dimensional reservoir modeling has allowed for the definition of these variations within the overarching "reciprocal sedimentation" stratigraphic architecture. Associated production curves have been applied to these geologic "neighborhoods", allowing for a narrowing of uncertainty ranges during initiation of focused development strategies.

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