FACIES- AND WIRELINE LOG-BASED HIGH-RESOLUTION SUBSURFACE SEQUENCE STRATIGRAPHY OF THE LATE ORDOVICIAN RICHMONDIAN RED RIVER "B" INTERVAL, WILLISTON BASIN, NORTH DAKOTA

The Richmondian (mid-Katian) Red River Formation, North Dakota was deposited within the epicratonic Williston basin, in the interior of the shallow and very broad, tropical, arid carbonate shelf that developed along the western passive margin of North America. The mixed carbonate-evaporite sequence developed during a mid-Katian warm period between the cool early Katian (Guttenberg C-isotope excursion) and major Hirnantian glaciation. This study focuses on the “B” Interval of the Red River Formation, which corresponds to the late Katian Aphelognathus divergens Subzone of the Aphelognathus ordovicicus conodont Zone. Twenty-five cored wells and all available combined neutron porosity-bulk density logs from North Dakota Geological Survey were studied to define the sequence stratigraphic development.

Red River “B” Interval represents an overall shallowing (and “brining”) upward long-term depositional sequence characterized by an upward progressively shallower, more restricted and hypersaline facies. Its maximum thickness in North Dakota is 20 meters, and it progressively thins eastwards/southeastwards. The transgressive systems tract (TST) consists of a single incomplete upward-shallowing cycle with a thin transgressive laminite overlain by a thick skeletal mudstone that grades-up into skeletal wacke-packstone with abundant burrow mottling (subtidal, lagoonal facies). The porosity within the “B” burrowed member ranges from 0.04-5%, and is primarily concentrated in dolomitized burrows as vug and between-crystal type. Permeability generally ranges from 0.01-10 mD. The maximum flooding surface occurs at the base of the regional grainstone-packstone bed (high-energy sand shoal); the grainstone porosity is low (1.1-5.9%), and permeability ranges from 0.01-1.4 mD. The HST is much thicker than TST and contains up to four shallowing upward parasequences. The early highstand systems tract (HST) is composed of porous (1.5-33.1%), peritidal laminated dolomite which is a major reservoir. It is characterized by between-crystal and vug porosity with permeability of up to 44mD. The late HSTs/lowstand systems tract consist of 0.15 cm to 9 meters thick laminated to nodular anhydrite (subaqueous and supratidal setting), locally interbedded with dololaminite.