Contrasting styles of reservoir development in proximal and distal chert facies: Devonian Thirtyone Formation, Texas

The lower Devonian Thirtyone Formation of west Texas and New Mexico is one of the largest chert reservoir successions in the world, having accounted f or more than 750 million bbl of oil production. As much as 650 million bbl of additional mobile oil remains in these reservoirs, making this play an i mportant target for further exploitation. A major limitation on the recover y of this remaining oil resource is an appreciation of the controls on rese rvoir development and heterogeneity. Although all Thirtyone chert reservoirs have much in common, they can be di vided into proximal and distal settings, each of which is characterized by distinct depositional geometries and styles of reservoir heterogeneity. Pro ximal reservoirs, represented by Three Bar field, are composed of a single, thick, sheetlike chert unit, which extends for hundreds of square miles. H eterogeneity in these reservoirs, which were formed by strike-parallel depo sition on a gently sloping outer platform during regional transgression, is primarily a function of faulting, fracturing, and dissolution of associate d carbonate along unconformities. Small-scale (bed-scale) heterogeneity als o exists within the tabular chert body, resulting from variations in silica deposition and diagenesis between and among beds. By contrast, distal reservoir successions, typified by University Waddell f ield, comprise thin, vertically stacked and laterally discontinuous chert i ntervals whose origin is a function of transport and deposition of siliceou s sediments as debris flows and turbidites. Flow units in these reservoirs are thin (10-20 ft [3-6 m]) and separated vertically and laterally from one another by low-permeability mud-rich, siliceous sediments and hemipelagic deposits. The distribution of flow units is the result of both paleotopogra phy and sea level cyclicity. Chert units are most abundant in transgressive and early highstand legs of sea level rise-fall cycles and display offset stacking suggestive of topographically controlled reciprocal sedimentation. Faults and fractures appear be relatively minor contributors to reservoir heterogeneity in distal reservoirs.