ABSTRACT: Porosity Development in Evaporite Dissolution Breccia

Randolph B. Burke

Angular intraclasts ranging in size from less than 1 cm to greater than 7 cm occur in cores in the bottom 1 m of the lower Duperow Formation (Frasnian) depositional cycle 3, the major Devonian oil-producing zone in southwestern North Dakota. Oil production is primarily from intercrystalline dolomite porosity enhanced by microvugs and fractures.

The breccia is a brown, very finely crystalline replacement dolomite. Dolomite rhombs are larger (less than 250 microns) within intraclasts than those in the matrix (60 to 125 microns). The smaller rhombs are interpreted to have a burial replacement dolomitization history, and the larger rhombs a syndepositional origin, later enlarged during burial dolomitization. The upper contact of the breccia is gradational with the overlying wackestones showing burrowing and reworking. The underlying contact is an abrupt reactivation surface defined by a change in lithology to a gray-green patterned, siliciclastic-rich dolomudstone. The dolomudstone consists of anhedral, microcrystalline (10 micron) dolomite, with very coarse sand and rounded intraclasts. The anhedral dolomite is interpreted to b syndepositional. Locally the dolomudstone shows incipient caliche development and has fluvial sedimentary structures. Rounded clasts of the dolomudstone are locally reworked into the overlying breccia.

The sequence of textures is interpreted to indicate dissolution of the evaporite interbeds from a thin transgressive package of carbonates and evaporites deposited during the initial stages of a slow sea-level rise. The high primary porosity of the breccia served at different times

both as a sink and as a conduit for magnesium-rich brines for dolomitization.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990