--> --> Sedimentology-Stratigraphy of the Lower Tyler Formation (Upper Mississippian?) – Williston Basin: Implications for Hydrocarbon Exploration and Development for Hydrocarbon Exploration and Development

AAPG Rocky Mountain Section Meeting

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Sedimentology-Stratigraphy of the Lower Tyler Formation (Upper Mississippian?) – Williston Basin: Implications for Hydrocarbon Exploration and Development for Hydrocarbon Exploration and Development

Abstract

The lower Tyler Formation (upper Mississippian?) has recently been documented to contain a series of three highly organic-rich (5-30% TOC), relatively thin (2-8 ft. thick) black shale beds that extend across the central portions of the Williston Basin. Thermal modeling and Tmax data (up to 450°) indicate the lower Tyler extends into the peak oil generation window where fluid overpressure (≥0.55 psi/ft) suggests that generated hydrocarbons are largely in place. Five vertical wells have commercially produced oil from the lower Tyler to date in western North Dakota: 4 from fractured (source bed) reservoir and 1 from conventional, porous and permeable sandstone. Additional free oil recoveries have come from drill stem tests and short-lived production tests. Core-based investigation reveals the lower Tyler contains three general facies associations: 1) distal shale beds (including the black shales), 2) proximal silty mudstone to sandstones, and 3) silty, mottled mudstone beds (paleosol horizons). Stacking patterns and stratigraphic correlations reveal several paleosols can be correlated on a basin scale, which when used as sequence boundaries subdivide the lower Tyler into five distinct sequences. The upper three sequences contain black shale beds, which appear to mark maximum flooding. Macroscopic burrows and diverse marine fauna occur in section below the black shales beds during the transgressive systems tracts, when water conditions were relatively stable with normal marine salinity and well oxygenated conditions. Within the black shales, macroscopic burrows disappear while marine fauna diversity decreases to only including marine brachiopods, sometimes of monospecific assemblages, which is interpreted to indicate normal marine salinity but with substantially decreased free oxygen. Above the black shales, during the highstand systems tract (HST), macroscopic burrows are minimal and marine fauna are negligible suggestive of perhaps variable, brackish water with reduced oxygen. Porous, hydrocarbon-charged, 5-10 ft. thick sandstone beds occur discontinuously in the HST’s. Each cycle is capped by a regionally correlative paleosol horizon, the falling stage systems tract where marine sediments were exposed to subaerial conditions for a prolonged period of time to develop a soil horizon. Towards the margins of the basin, the various distal shale facies (including the black shales) begins to be replaced by various siltstone to sandstone facies, which overlap in part with the black shales and also constitute hydrocarbon reservoir.