--> ABSTRACT: Lakota Paleogeography and Search for Paleogeomorphically Entrapped Hydrocarbons--A Second Look, by Davis S. Muller and John C. Dolson; #91030 (2010)

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Lakota Paleogeography and Search for Paleogeomorphically Entrapped Hydrocarbons--A Second Look

Davis S. Muller, John C. Dolson

Fair to excellent-quality Lower Cretaceous Lakota sands and chert pebble conglomerates unconformably overlie the Jurassic Morrison Formation over most of northern, central, and eastern Wyoming. This apparent uniform blanket of reservoir facies has discouraged stratigraphic exploration. Most Lakota production has been established within structural closure.

A regional look at the landscape of the Lower Cretaceous of Wyoming provides a model whereby this apparent blanket of sand and gravel masks attractive regional exploration objectives. The initial Lakota sedimentary event in central Wyoming was the deposition of Lakota sands and gravels on a northward-sloping pediment. Paleocurrent and rock provenance evidence support the theory that many of the isopach thicks observed in the Lakota interval represent second-stage dissection of this older pediment surface. Regional drainages cut across the state, some deflected eastward by the forebulge developed in response to thrust-loading the Sevier orogenic belt. Tributaries to this through-going regional drainage network also incised into the pediment surface.

Valley-cutting was followed by aggradational valley-filling. Coarse-grained valley fill was deposited in the major drainages and their tributaries.

The coarse tributary fill represents second-generation deposition of sands and gravels pirated from the dissected pediment. Finer-grained fluvial and fluvio-lacustrine fill completed the continental phase of the cycle. A tongue of the Skull Creek Shale unconformably overlies the Lakota over much of central Wyoming, providing a potential petroleum source for Lakota objectives. Paleogeomorphically entrapped Lakota hydrocarbons may await the explorationist within incised valley fill, in pediment gravels adjacent to fine-grained valley fill, and adjacent to Morrison paleohills.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.