Early and Late Diagenetic Alteration of Ordovician Red River Carbonates, Tioga Deep Field, Williston Basin, North Dakota

PERKINS, RONALD D., Duke University, Durham, NC

The Ordovician Red River Formation in the Williston basin is generally subdivided into three restrictive-upward cycles referred to as "A," "B," and "C" zones in descending order. The C-zone has been further subdivided into a C-anhydrite member, a C-laminated member, and a C-burrowed member. Most Red River production in western North Dakota and eastern Montana comes from dolomitized burrowed and laminated members of the C-cycle. Four cores taken by Amerada Hess Corporation within the Tioga Deep field, Williams and Montrail Counties, North Dakota, were studied in detail to establish depositional and diagenetic controls on porosity distribution within the Red River.

The "C" zone sequence in the Tioga Deep field consists of five distinct units including (in ascending order): (1) a basal burrowed subtidal member capped by a distinctive bed exhibiting boudin-like structures, (2) an overlying marine facies consisting of bryozoan (?) and coralline algal boundstone beds, (3) a restricted marine (lagoonal) facies with low organic diversity, (4) laminated mudstone and stromatolitic algal units of schizohaline origin, and (5) organic-rich anhydrite (originally gypsum) precipitated in a hypersaline environment.

Dolomitized portions of the Red River C member are genetically related to downward-descending Mg-rich brines derived from hypersaline basin waters depleted of their sulfate content through attendant precipitation of gypsum. Calcite associated with this early dolomitization phase occupied intercrystalline positions between dolomite rhombs. This calcite may have been precipitated as a coeval mineral phase or left as unreplaced vestiges of original calcite (or aragonite subsequently altered to calcite) during dolomitization. Following burial, limestones tended to chemically compact through pressure solution, whereas late stage fractures were localized in the more brittle dolomitic portions of the Red River. These fractures served as conduits for late stage leaching fluids, possibly assoc ated with hydrocarbon generation, that enhanced porosity in the dolomites through the removal of associated calcite. Late-stage leaching is reflected in "overly porous" dolomite haloes around cemented burrow centers and highly porous dolomite seams along stylolitic contacts. Previous models proposed to explain porosity distribution within Red River reservoirs should be modified to include the overprint of deep diagenetic effects.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)