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The AAPG/Datapages Combined Publications Database

Williston Basin Symposium

Abstract

NDGS/SKGS-AAPG

Fourth International Williston Basin Symposium, October 5, 1982 (SP6)

Pages 93 - 100

POROSITY TYPES, GEOMETRY AND INTERPORE MINERALS OF THE LOWER DUPEROW FORMATION, BILLINGS NOSE AREA, WILLISTON BASIN, NORTH DAKOTA

RANDOLPH B. BURKE, North Dakota Geological Survey, University Station, Grand Forks, ND 58202-8156
GARY L. STEFANOVSKY, Department of Geology, University of North Dakota, Grand Forks, ND 58202

ABSTRACT

Three types of porosity, 1) intercrystalline, 2) moldic and 3) vuggy, are recognized in cores from wells drilled in the lower Duperow Formation in the Billings Nose area of North Dakota. The principal type of porosity is intercrystalline, formed during dolomitization. Dolomitization appears to be fabric selective, preferential to mudstones, mudstones and wackestones associated with intraclasts, oncolites(?) and stromatoporoids. Dolomite rhombs range in size from an average of 20 to 30 microns to 50 microns. Scanning electron photomicrographs of epoxy casts of intercrystal porosity show the pores to haye regular, mostly polyhedral shapes ranging in size from 70 microns down to a few microns. The pores are joined by irregular-tabular-polyhedral pore throats a few microns in size. Total porosity is estimated to be 20 to 30%. Local, scattered limpid rhombs of dolomite in some mudstones suggest that dolomitization was eogenetic having formed shortly after deposition of the sediments. A replacive origin for other dolomites is indicated by the clouded dark centers in some rhombs and the penetration of skeletal allochems by rhombs.

Most moldic porosity results from the dissolution of mollusc shells and anhydrite pseudomorphs. Permeability is limited by clay laminae and the micritic matrix. Pore size varies from tens of microns up to a few millimeters except where enlarged by dissolution to form vugs.

Vugular porosity is generally less than 7 millimeters in size and irregular in shape. Most vugs appear to have formed in the mesogenetic environment by the enlargement of moldic pores, and from the dissolution of anhydrite and unreplaced limestone patches that survived earlier pervasive dolomitization.

A variety of minerals is found within the pores on the faces of dolomite rhombs and in pore throats, including quartz, (both authigenic and detrital), anhydrite, calcite, ilmenite, possibly muscovite, and clays of the chlorite, illite and smectite (montmorillonite) families. The presence of these clays within pores can be misleading to both the exploration geologist trying to interpret the high water saturations calculated from electric logs, and also the production engineer trying to optimize hydrocarbon recovery.

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