Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Forming of Rich and Lean Oil Shale Deposits in the Eocene Green River Lake Basin - Link to Climate Changes: Piceance Creek Basin, Colorado, USA
(1) Department of Geology & Geological Engineering, Colorado School of Mines, Golden, CO.
Lake deposits, formed in the Green River Formation in the Piceance Creek Basin (PCB) during the early to middle Eocene are largely composed of kerogen-rich and kerogen-poor carbonates (rich and lean oil shale). Deltas, shore sandstones, coquinas, and stromatolites formed in the littoral and sublittoral areas, whereas fine-laminated, soft sediment disturbed, and brecciated oil shale deposits were deposited in the deeper parts of the lake. Formation of rich and lean oil shale zones is correlative with PCB stratigraphy. Six evolutionary lake stages and at least two levels of cyclicity occur in the PCB, and are bounded by sequence boundaries or correlative conformities. Lake stages, cyclicity, and occurrence of rich and lean oil shales are controlled by the climatically controlled variations in runoff and vegetation. Six lake stages are defined as: S1-Freshwater Lake, S2-High Siliciclastic Input, S3-Rapidly Fluctuating Lake, S4-Rising Lake, S5-High Lake, and S6-Closing Lake. Lake stages indicate an overall lake evolution and correlate well with climate changes during the early to middle Eocene. This is illustrated by the S3-Rapidly Fluctuating Lake, formed during the peak of the Eocene climate optimum, when conditions were semi-arid, and that contain thin, rich oil shale beds interbedded with evaporites, in the deep lake. The S5-High Lake was cooler with increased runoff resulting in thick, rich oil shale beds that occur across an increased lake area.
Two levels of cyclicity are marked, in many places, by abrupt changes in lithofacies and oil shale richness. Sequence boundaries and their correlative conformities separate rich oil shale zones from lean oil shale zones, and are marked with rapid changes in runoff and vegetation. Sequence boundaries are followed by leaner oil shale zones that suggest low vegetation and low runoff. Fine-laminated oil shale and evaporite deposits dominate in deeper lake areas. Rising lake level is marked by high occurrence of soft sediment deformation and oil shale breccias in deeper lake, suggesting higher energy during lake level rises. In the marginal areas progradation of shore sandstones and deltaic deposits indicate increased runoff. In many places, sandstones are overlain by deepening upward littoral to sublittoral carbonates i.e. oolitic and stromatolitic deposits, that pass upwards into oil shales. These rich oil shale zones, deposited during high lake levels, are interpreted to represent high runoff and high vegetation.