Cache Field--A Pennsylvanian Algal Reservoir in Southwestern Colorado

Abstract

Oil and gas accumulation in the Cache field in southwestern Colorado is chiefly in reservoirs developed in a northwest-trending group of stacked algal carbonate mounds and related carbonate facies in the Ismay zone (Des Moines age) of the Paradox Formation. Cache field is the most productive Pennsylvanian field in the Colorado part of the Paradox basin. Characteristic cyclic deposition which is developed in the zones of the Paradox Formation is shown on a smaller scale within the Ismay of the Cache field. The Ismay has an average thickness of 180 ft and consists of nine carbonate lithofacies, anhydrite, siltstone, and black shale, which are recognized in one or more of the three cycles in the Ismay. The carbonate lithofacies are: (1) algal calcirudite breccia; (2) calcirudite breccia; (3) foraminiferal-pelletal limestone; (4) laminated boundstone; (5) crinoidal limestone; (6) earthy dolomite; (7) carbonate mudstone; (8) shelly mudstone; and (9) dolomite. The depositional environment of the normal-marine buildup cycle was dominated by the formation of algal mounds in relatively shallow water where phylloid Ivanovia algae grew in profusion on broad banks. In slightly deeper water flanking the mounds, and in channelways, fossiliferous carbonate mudstone was deposited. One facies (boundstone) probably formed in intertidal conditions. The evaporitic cycle which followed the normal-marine buildup cycle probably was begun by the lowering of sea level. During the evaporitic cycle, dolomite and massive anhydrite (gypsum) were deposited in an intertidal-supratidal environment. Favorable zones of porosity and permeability are present within the algal calcirudite breccia and the dolomitized shelly mudstone. Porosity is predominantly secondary (early diagenetic) and is the result of leaching of the algae and solution brecciation (vugs) in the algal limestone, and leaching and recrystallization to dolomite in the dolomitized limestone.