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