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

Abstract

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. 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. 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 throught attendant precipitation of gypsum. Calcite associated with this early dolomitization phase occupied intercrystalline positions between dolomite rhombs. 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 associated 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.more » Previous models proposed to explain porosity distribution within Red River reservoirs should be modified to include the overprint of deep diagenetic effects.« less