--> Stable Carbon and Oxygen Isotope Record and Facies Stacking Pattern of the Subsurface Red River Formation in Eastern North Dakota

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Stable Carbon and Oxygen Isotope Record and Facies Stacking Pattern of the Subsurface Red River Formation in Eastern North Dakota

Abstract

The Ordovician Red River Formation is a major conventional reservoir within the intracratonic Williston Basin, with most of its production coming from the western North Dakota and eastern Montana. Accordingly, its facies are well known from the subsurface of the basin center; both the core material and wireline logs are drastically reduced away from this producing area, and become very rare to virtually non-existent towards the basin margins in the subsurface USA. This study focuses on a recently drilled and fully cored borehole through the Red River Formation in eastern North Dakota. The 126 m (414 ft) core was logged noting depth, porosity, mineral composition, sedimentary structures, texture, grain size and type, and dolomitization. The Red River Formation in the study area is characterized by burrow-mottled texture, with Thalassinoides-like burrow fills within a finely crystalline matrix. Major carbonate lithofacies include: (1) skeletal dolomudstone-wackestone (deeper subtidal); (2) skeletal dolowackepackstone (shallow subtidal); (3) crinoid dolopackstone (high-energy shoal); and (4) thrombolite (upper subtidal-lower intertidal). The cored Red River succession appears to contain 6 complete depositional sequences bounded by grainy intevals (skelelal (dolo)wackepackstone to packstone). The basal three sequences are 50-140 ft thick and dominated by parasequences consisting of skeletal wackemudstone capped by skeletal wackepackstone. The uppermost part of sequence 3, and the remaining sequences 4, 5 and 6 are predominated by grainy lithofacies, with parasequences consisting of skeletal wackepackstone grading up into packstone that is locally capped by thrombolite. Lack of tidal flat facies caps to parasequences indicates incomplete shallowing to intertidal depths, and suggests that some of the parasequences are amalgamated. Samples for stable C-O analysis were collected every three feet (∼0.9 m) from bulk carbonate matrix. The carbon-isotope values vary from -1.6 to +1.1‰ VPDB (average -0.14‰ VPDB), and show two significant positive isotope excursions. The first one occurs at the boundary zone between sequences 2 and 3, and the second one is found in the middle part of sequence 6. The carbon-isotope signature shows overall trend towards more positive values up section. The oxygen-isotope values vary from -6.1 to +3‰ VPDB (average -3.3‰ VPDB). The carbon-oxygen cross-plot pattern suggests that the stable isotope record has been diagenetically altered.