SHALLOW-WATER VS. DEEP-WATER STRATIGRAPHIC GEOMETRIES IN THE ORGANIC-RICH SHALE/MUDSTONE DEBATE

Abstract

In the central Williston basin, USA, the Bakken Formation and overlying lower Lodgepole Formation both have fine-grained, organic-rich stratigraphic units that have been interpreted sedimentologically to represent deep-water deposition in a low-energy, distal-marine environment; how-ever, these formations display vastly different stratigraphic geometries that challenge the conventional sedimentology interpretations. The Bakken Formation spans the Devonian-Carboniferous boundary and includes black, organic-rich (2%–26% total organic carbon [TOC]) shale units. Stratigraphic characteristics strongly support deposition of all Bakken sediments in shallow water, as indicated by (1) the Bakken stratigraphic position overlying a major subaerial unconformity; (2) the restriction of Bakken strata to basinal areas; (3) the absence of shale-equivalent landward deposits; (4) a layer-cake, onlap, landward-thinning stratigraphic geometry for all Bakken units; (5) gradual landward shale pinchouts that occur by intra-shale onlap and stratal thinning, not erosional truncation; (6) unequivocal evidence for very shallow-water middle Bakken deposition; and (7) the absence of evidence for large intra-Bakken sea-level changes. Lower Lodgepole strata in the Williston basin are characterized by prominent sigmoidal clinoforms. In the lower Virden clinoform, argillaceous mudstone, laminated microcrystalline dolostone, microbial-peloidal-intraclastic packstone, and skeletal-oolitic limestone form a shelf facies that transitions seaward into a thick (maximum 80 m), skeletal-peloidal mudstone to packstone slope facies, which transitions seaward into seaward-thinning (10 m to 1 m), black, organic-rich (1%–8% TOC) carbonate mudstone in a basin-floor facies,