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Finding producing reservoirs within the Pennsylvanian Leo Member of the Minnelusa Formation is a challenging problem for exploration geologists. This study of the "Leo" sandstones provides depositional models for use in predicting occurrences of stratigraphic hydrocarbon traps.

The study area lies in the southern portion of the Powder River Basin, between townships 31N and 43N, ranges 61W and 79W in Wyoming and includes the westernmost parts of South Dakota and Nebraska. The study area encompasses 10,908 square miles within which approximately 980 wells have penetrated the "Leo."

Data from Leo outcrops, cores, and wells are compared with data from

analogous modern settings to develop a thorough understanding of the depositional processes active during "Leo" time. Isopach and net sandstone maps that accompany the report display the dominant trends of reservoir potential sandstones.

Using predictive models of regional sandstone sedimentation patterns, six "Leo" oil and gas fields were examined stratigraphically and sedimentologically in detail. These fields include Alum Creek, Bright, Indian Creek, Buck Creek/Leimser, Redbird, and Barker Dome. The study of these fields was undertaken to develop smaller scale rock models that can aid in understanding the distribution of reservoir potential strata within the regional sandstone trends.

Database

980	Study Wells	29	Cores Described
9	Regional Stratographic Markers Correlated	179	Thinsections
24	Measured Outcrop Section Described

Maps

Regional Maps (1:500,000) (1:96000)		Field Maps (1:24000)
4 Interval Isopach Maps 1 Structure Maps 6 Sandstone Maps 1 Penetration Map 1 Outcrop Location Map		6 Interval Isopach Maps 5 Sandstone Maps 3 Structure Maps

Cross Sections		Field Studies
7 Regional Stratigraphic Sections 13 Field Cross Sections		Alum Creek Barker Dome Bright Buck Creek/ Leimser Indian Creek Redbird

Report Contents

TEXT AND FIGURES-This extensively illustrated report includes discussion, figures, and color plates on such topics as: Executive Summary, History of Study, Previous Work, Procedures, Stratigraphy/Facies, Depositional Facies, Petrography, Depositional Setting, Modern Analogs, Studies of Selected Oil and Gas Fields, Synthesis, and Bibliography.

STRATIGRAPHIC DATA-This file contains the stratigraphic data (9 stratigraphic markers) net sandstone values, and isopach values, as well as information on well name, operator, location, API number, completion date, datum, and production status for the 980 study wells.

CORES AND OUTCROP DESCRIPTIONS - This appendix contains the descriptions of 29 cores keyed to geophysical logs. These are cores from wells penetrating the Middle Member of the Minnelusa Formation. In addition, this section contains the descriptions of 24 measured outcrop locations from around the outer perimeter of the study area. These were examined to discern the geometries, relative size, and sedimentological character of sandstone bodies for comparison with those identified in the subsurface.

MAPS AND CROSS SECTIONS - These appendices contain 13 regional maps at scales of 1 : 96,000 and 1 : 500,000 and 14 field maps at a scale of 1 : 24,000. This includes structure, net sandstone, isopach, and facies maps. In addition, these appendices include seven regional stratigraphic cross sections and 13 field cross sections.