Mechanisms Controlling Porosity in Red River (Upper Ordovician) Carbonate Reservoir, Cabin Creek Field, Montana: ABSTRACT

Abstract

Cabin Creek field, on the Cedar Creek anticline in southeastern Montana, produces oil from Mississippian, Silurian, and Ordovician carbonate rocks. The major producing reservoir is the Upper Ordovician Red River Formation. The Red River of Cabin Creek field is a 500-ft (152 m) thick limestone-dolomite sequence; the upper Red River consists of several cycles of uniformly thick peritidal shelf carbonate rocks. Favorable porosity is restricted to the dolomites; the limestones are nonporous and serve as cap rocks. Porosity and permeability are controlled both by dolomitization and by silica and anhydrite cements. Lateral and vertical variations of dolomitization are mainly responsible for variations in reservoir properties and effective pay thickness. Average porosity is 13%, with a maximum of 25%. Production is from porosity zones in the upper 150 ft (45 m) of the Red River. Upper Red River limestones consist of biomicrite-wackestones which contain diverse faunal assemblages, and are interpreted as shallow, open-marine shelf deposits. The limestones are pelletal in part, poorly bedded, burrowed, stylolitic, with widespread dolomitic mottling. Producing dolomite zones consist of two distinct facies. The lower facies, interpreted as subtidal, consists of biomicrite-wackestone and skeletal dolomite with moldic porosity. The upper facies, interpreted as intertidal, consists of wavy-laminated, unfossiliferous, microcrystalline dolomite with intercrystalline porosity. The upper facies is abruptly truncated by overlying pelletal limestone and (locally) thin black shale. Conglomerates End_Page 777------------------------------ commonly top the upper facies. In general, producing-zone porosity increases upward from subtidal to intertidal. End_of_Article - Last_Page 778------------