Facies and Burial Diagenesis of a Carbonate Reservoir: Chapman Deep (Atoka) Field, Delaware Basin, Texas

Abstract

Hydrocarbon production at Chapman Deep Atoka field is from a complex microfacies mosaic of shallow-water bank limestones deposited along the northern hingeline of the Delaware basin. Reservoir localization is essentially stratigraphic in terms of depositional and diagenetic facies, although regional draping and a system of vertical fractures are significant structural aspects of the field. The bank facies consist of cyclic alternations of Donezella (algal) bioherms, oolite-biograinstone shoals, and low-energy interbank deposits. Laterally equivalent slope and basinal facies include spiculitic and crinoidal argillaceous limestones and shales, with interbedded lenses of fine-grained carbonate and siliciclastic turbidites. Early diagenetic effects include incipient marine ce entation and the formation of secondary porosity, most of which was occluded by calcite cements, internal sediments, and dolomitization. In contrast, reservoir evolution is principally related to diagenesis in the deep subsurface (mesogenetic) environment. Bulk-volume reduction by chemical and physical compaction was counterbalanced by porosity rejuvenation through the selective dissolution of allochems, cements, and stylolite surfaces, and the formation of open-gash fractures and adjoining stylolites. Although this limited pore system is of inherently low permeability, effective communication within and between individual reservoir lenses was enhanced by later fracturing. Although potential reservoir facies can be mapped regionally and burial diagenetic effects can be recognized petrogr phically, exploration for similar reservoirs in the Delaware basin is hindered by our limited knowledge of mesodiagenesis.