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Abstract: Abstract: Interdisciplinary Analysis of Some Potential Petroleum Source Rocks in East-Central Utah--Implications for Hydrocarbon Exploration in Nonmarine Rocks of Western United States

Thomas D. Fouch, John H. Hanley

Attrital coal, composed largely of unidentifiable organic matter and of mollusk and ostracod skeletal-mud-supported carbonate and calcareous claystone, all of which are rich in organic matter, constitutes a significant potential petroleum source-rock assemblage in a continental setting. Because the beds weather easily and are slope formers, they are difficult to observe in outcrop exposures; many are too thin to be recorded on drill-hole or rock-type logs. As a result, these organic-rich beds commonly are grouped within alluvial facies generally considered unattractive for petroleum exploration. Although internal structure of the coals as viewed in thin section seems to indicate that the organic material is of algal origin, we have not unequivocally identified algal matte . These attrital coals melt when heated and laboratory analyses indicate that they are petroleum-source rocks, whereas coals composed largely of woody plant material are considered to be primarily gas-source units. These petroleum source-rock coals also differ from typical coals in that they may contain untransported species of freshwater mollusks.

Our comprehensive interdisciplinary study involved analysis of rock type, structure, organic geochemistry, palynomorphs, and mollusks for accurate interpretation of environments of deposition. The areas examined in Utah are (1) Price River Canyon, (2) Soldier Summit, and (3) Willow Creek Canyon.

In Price River Canyon, late Paleocene coal of subbituminous rank is intercalated with carbonate and siliciclastic units rich in organic matter. Though the beds are assigned commonly to the North Horn Formation (Cretaceous-Paleocene), lithostratigraphic relations indicate that the units may be part of a tongue of the Flagstaff Member of the Green River Formation (Paleocene-Eocene). Our analysis indicates formation in a fresh, clear, oxygenated, quiet-water, marginal-lacustrine environment that included aquatic vegetation. The organic carbon values of the coals range from 50 to 70%, and coals yield up to 83 gal of oil per ton (357 l/t). Cumulative thickness of the coal beds is greater than 3 ft (1 m) within a 150-ft (45 m) sequence. This sequence can be traced southeast on outcrop for m re than 4 mi (6.5 km) to an erosional edge, and northeast in the subsurface possibly for more than 45 mi (73 km) to the lower part of the productive section at the Altamont oil field.

Outcrops of the lower part of the main body of the Green River Formation near Soldier Summit contain coal interbedded with mud-supported carbonate rock and calcareous claystone, all of which are rich in organic matter. Polygonal cracks in laminated carbonate rocks are infilled with carbonate mud and a shallow lacustrine mollusk association. Our analyses indicate that the beds formed in the early Eocene in a shallow, fresh, oxygenated, periodically exposed, nearshore, open-lacustrine environment containing aquatic plants and away from a terrigenous clastic influx. The coal yields 23 gal of oil per ton (99 l/t), and the organic carbon value is 54%. Coal and associated beds rich in organic matter are also present in the lower part of the main body of the Green River Formation at Willow C eek. The coal yields 79 gal of oil per ton (340 l/t), and the organic carbon value is 70%. The Willow Creek beds formed in a fresh, oxygenated, nearshore, open-lacustrine environment away from terrigenous clastic influx.

Internal drainage systems, such as those in which the organic-rich rocks discussed here formed, existed during the Cretaceous and Tertiary in a large part of the Rocky Mountains and Great Basin. Preliminary analyses indicate that similar potential hydrocarbon source rocks are also present in some nonmarine rocks of these areas. Interdisciplinary analysis of rock types and structures, organic geochemistry, palynomorphs, and mollusk associations is a key to reconstructing the depositional environments and postulating the distribution of additional hydrocarbon source-rock units in the western United States.

AAPG Search and Discovery Article #90969©1977 AAPG-SEPM Rocky Mountain Sections Meeting, Denver, Colorado