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Ismay-Hovenweep Petroleum System, Blanding Basin, Utah

Edward Coalson1 and Harvey R. DuChene2
1Coyote Oil & Gas Company, LLC, Conifer, CO
2HNK Energy, LLC, Lake City, CO

Oil and gas in the Blanding sub-basin are produced mainly from upper Ismay (Desmoinesian) carbonate mounds. The mounds form linear, subparallel trends; are immediately underlain by thickened sections of Hovenweep Shale; are flanked by thick salina anhydrites; and are immediately overlain by thin upper Ismay sabkha deposits and marine carbonates of the basal Honaker Trail Formation. Correlative, but thinner, marine-shelf upper Ismay carbonates are underlain by thin Hovenweep Shale.

Based mainly on mapping of these thickness trends and on serial stratigraphic cross sections, we propose that the anomalously thick, productive upper Ismay carbonate mounds result from: 1) regional base-level changes, 2) differential subsidence during Hovenweep time due to salt movements, 3) loading of the salt by thickened upper Ismay carbonates and evaporites, and 4) differential compaction of the evaporites surrounding the carbonate-mound masses during latest Ismay and earliest Honaker Trail deposition. We propose an indirect linkage between basement tectonics, salt movements, sediment loading, and deposition of the mounds.

When mapped regionally, the dominant pore fluids in the Ismay and underlying Desert Creek zones follow trends consistent with the more-theoretical results of geochemical measurements and modeling, specifically kerogen typing and thermal maturity indicators. Basinward (northeast) of the Blanding sub-basin, Ismay and Desert Creek hydrocarbons are dominantly natural gas with little mobile water. Shelfward (southwest) of the Blanding sub-basin, Ismay and Desert Creek pore fluids are oil with associated gas and abundant producible water. In the Blanding Basin itself, pore fluids are mainly oil and associated gas, with relatively little mobile formation water. Inferred source-rock maturity trends reflect mainly Laramide burial and inferred regional differences in kerogen types.

AAPG Search and Discovery Article #90092©2009 AAPG Rocky Mountain Section, July 9-11, 2008, Denver, Colorado