Abstract

The Ismay field produces oil principally from bioclastic carbonate build-ups containing abundant debris of calcareous algae in the Ismay zone of the Paradox Formation (Middle Pennsylvanian)). Individual build-ups, generally elongated northeast-southwest, occur stacked one above another in three intervals in the Ismay zone, collectively forming a productive trend oriented northwest-southeast along a low structural nose. Avalable well control indicates that the build-ups are biostromal, flat-bottomed, mound-like deposits up to several thousand feet across, at least 10,000 feet long, and about 40 feet in maximum thickness. Their geometry and internal characteristics suggest accumulation as local, low, bar-like carbonate banks.

Ten major facies are recognized in the Ismay zone in the vicinity of the field: build-up facies (algal calcarenite and calcirudite, and sedimentary breccia), laminated carbonate bound-stone, dolomite, anhydrite, black shale, dark carbonate mudstone, shelly calcilutite (locally dolomitized), pelletal and foraminiferal limestone, calcareous siltstone and sandstone, and sponge-bearing calcilutite. These facies are believed to be marine in origin. Many of them (e.g., build-up facies) are interpreted as relativly shallow-water deposits, and one facies (boundstone) probably represents intertidal conditions. The major facies are interpreted in terms of variations in salinity, oxidizing or reducing conditions, water turbulence, and depth of water. Thickness relationships suggest that, despite the variety of facies found in the Ismay zone, differences in depth of water during their deposition may not have exceeded a few tens of feet.

The Ismay zone facies show cyclic repetition; three dominant cyclic patterns appear within or adjacent to the productive trend: (1) an evaporitic cycle characterizing basinal facies north and east of the field; (2) a cycle containing productive algal build-ups; and (3) a cycle occurring in areas between build-ups. Facies in cycles of types 2 and 3 are normal marine in origin in the Lower Ismay zone and range from normal marine to penesaline in the Upper Ismay.

Production comes chiefly from porous and permeable zones within the build-ups. Optimum effective porosity occurs in well-sorted, grain-supported, algal-rich carbonates, chiefly as inter-particle pores. Some of these are probably of primary origin, but much of the porosity in the build-ups is secondary, caused by selective or indiscriminate leaching. Variations in porosity and permeability are related mainly to depositional fabrics (abundance and positions of grains and carbonate mud), extent of leaching, and degree of secondary pore filling by calcite and anhydrite. Some of the build-ups and associated rocks are dolomitic: dolomitization and anhydrite mineralization appear to have been spatially and perhaps genetically related. Leaching, dolomitization, anhydrite mineralization, and calcite infill are interpreted as early diagenetic processes.