--> --> Regional Correlation of Carboniferous Heath and Tyler Strata from Central Montana to the Williston Basin, North Dakota, USA

AAPG Rocky Mountain Section Meeting

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Regional Correlation of Carboniferous Heath and Tyler Strata from Central Montana to the Williston Basin, North Dakota, USA

Abstract

Correlation of the mid-Carboniferous Heath and Tyler formations from the type area in central Montana east to the Williston Basin in North Dakota and Montana has been historically challenging. New cores and well penetrations along with detailed analyses of new and existing cores facilitate a more robust regional evaluation of this interval. A newly completed 671 BOPD Heath horizontal well in central Montana and several oil-saturated Tyler sandstones identified in vertical sections of deeper horizontal wells in North Dakota are reviving interest in these source-rock rich intervals. The Heath-Tyler interval thins from greater than 700 feet thick in central Montana to 200 feet thick in North Dakota. The Mississippian-Pennsylvanian boundary is probably in the upper part of the Stonehouse Canyon, above the top of the Bear Gulch, or at the base of the Cameron Creek, in central Montana and near the contact between the lower and upper Tyler in southwestern North Dakota. Recent analyses lead to the following conclusions: a) the HGR shales in the Central Basin area of North Dakota correlate to high-gamma ray shales in the lower and middle part of the Heath in central Montana, specifically HGR shale A correlates with the lowest units in the Van Dusen zone, HGR shale B correlates with the lower Cox Ranch shale, and HGR bed C correlates with the upper Cox Ranch shale; b) the abundance of wood and plant debris, lateral discontinuity of markers due to numerous erosion surfaces (incised valleys), and late Mississippian spores and conodonts found in the lower Tyler in the southwestern North Dakota area indicate that it is generally correlative with the lower part of the Stonehouse Canyon of central Montana, c) the presence of dark gray to black shales and marine limestones, and Pennsylvanian fossils and spores found in the upper Tyler in the southwestern North Dakota area suggests that it is correlative to the uppermost part of the Stonehouse Canyon of central Montana (above the Bear Gulch Limestone), and d) the lithological similarity of the uppermost upper Tyler in North Dakota, dominated by red shales, sandstones, and breccias, to the Cameron Creek of central Montana suggests that these units are correlative. SEDIMENTOLOGY-STRATIGRAPHY OF THE LOWER TYLER FORMATION (UPPER MISSISSIPPIAN?) – WILLISTON BASIN: IMPLICATIONS FOR HYDROCARBON EXPLORATION AND DEVELOPMENT Timothy O. Nesheim 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.