Date of Award

1993

Document Type

Thesis

Degree Name

Master of Arts (MA)

Department

Geology

First Advisor

P.E. Videtich

Abstract

Carbonates and evaporites in the "C" zone c: the Red River Formation (Upper Ordovician) in the subsurface of Divide County, North Dakota, were studied in order to determine the geologic framework of porosity evolution in this important hydrocarbon-producing formation. Examination of 28 cores, petrographic analysis, geochemical analysis, and mapping based on 120 geophysical log suites allowed the characterization of "C" zone lithologies and diagenetic features and suggests that the entire "C" zone was deposited subtidally in an increasingly restricted basin, rather than in a subtidal to supratidal regressive sequence as interpreted by others in different parts of the basin.

The "C" zone consists of three informal members: a lower burrowed member, a middle laminated member, and an upper anhydrite member. Economic porosity is limited to the laminated and burrowed members. Previous investigations of "C" zone reservoir geometries demonstrate that prediction of both lateral and vertical porosity in the "C" zone is difficult. Many previous investigators concluded that dolomitization is the dominant control on porosity distribution in the "C" zone, and subsequently tried to interpret porosity distribution on the basis of a single dolomitization model. Evidence uncovered in this study indicates that porosity distribution is controlled by variations in the degree of dolomitization, but also by mesogenetic dissolution of calcite and dolomite associated with the maturation of organic ma_cer and concurrent production of CO2. Highly variable dolomite textures indicate multiple episodes of dolomitization, which might explain the difficulty of predicting reservoir geometry (porosity distribution) on the basis of one dolomitization model.

Carbon and oxygen stable isotope geochemistry suggests that the initial dolomitizing fluid was seawater or seawater-derived brines. This penecontemporaneous phase of dolomitization produced planar dolomites that were subsequently neomorphosed or recrystallized into nonplanar and coarser-planar textures, as indicated by stoichiometric and well-ordered dolomite compositions, nonplanar and coarse-planar textures, depleted Sr and possibly depleted 0180, and uniform luminescence.

Both structural and stratigraphic traps are important in the study area. Consideration of porosity distributions, dolomitization patterns, and production trends indicate that the northwestern portion of Divide County is prospective and the southeastern portion is not prospective for hydrocarbon exploration.

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