Diagenesis in Thick Ice-House Carbonate Cycles: Implications to Reservoir Development

Saller, Arthur H.¹ (1) Chevron, Houston, TX

Cores from the Pleistocene of Indonesia and Pennsylvanian of west Texas show similar diagenetic features in thick carbonate cycles. The upper Pleistocene from offshore East Kalimantan (Borneo), Indonesia has 60-80 m thick carbonate cycles deposited during each of the last three 110,000-year sea-level cycles. Diagenesis of those cycles includes abundant early calcite cement and pervasive early dissolution of depositional aragonite in the upper 1-4 meters of cycles. The middle parts of Pleistocene cycles contain unaltered aragonite and little petrographic alteration of calcite. Micrite near the maximum flooding surfaces in the middle of cycles has been partially dolomitized.

Pennsylvanian cycles at Reinecke field (Horseshoe Atoll, west Texas) are 25-30 m thick. Porosity of 8-12% occurs throughout cycles. Intense meteoric diagenesis of grainstones in the upper parts of cycles resulted in intergranular, moldic and vuggy porosity. The middle parts of cycles show little early diagenetic alteration other than dolomitization. As a result, microporosity of micrite-rich fabrics dominates in limestones in the middle of cycles. Early dolomitization of micrite is common in the middle of cycles. Some early dolomite was recrystallized by deep burial fluids. Those deep burial fluids dolomitized additional limestone. Deep burial dolomitization greatly increased permeability creating “raceways” through the reservoir. Late dolomitizing fluids were derived from highly saline fluids generated when salts accumulated in the Permian Basin at the end of the Permian. Those dense saline brines percolated down into lower Paleozoic strata the Midland Basin, and then convected up into Pennsylvanian carbonates due to geothermal heating.