Abstract: Depositional Environments of Oil Shale

Ronald C. Surdam, Leslie L. Lundell

Oil shale in the Green River Formation of Wyoming and Colorado is associated with domal stromatolites, cross-bedded ooliths and pisoliths, ostracodal lag deposits, flat pebble conglomerates, bedded saline minerals, and barren marlstones with flute casts. In addition, some oil shale units contain mudcracks, breccias, and saline mineral nodules. Obviously neither the depth of water, nor the presence of bottom currents in the depositional environment, is a limiting factor relative to oil shale deposition.

On the other hand, the influx of detrital sediments is a serious constraint on oil shale deposition. Kerogen content of oil shale drops drastically near clastic deltas and prograding clastic shorelines. Much of the oil shale in the Green River Formation was deposited in an environment characterized by shallow water, periodic desiccation, high organic productivity, and a very low sediment influx. These conditions are well satisfied by a playa-lake complex, or in other words, a shallow-water lake surrounded and protected by a broad playa fringe.

Kerogen-rich laminae of the oil shale are the result of a relatively continuous deposition of algal mats and oozes, whereas the carbonate-rich laminae are derived from at least two sources: (1) clastic transport and (2) chemical precipitation. Seasonal flooding of the playa lake with fresher water contributes not only detrital carbonates washed into the lacustrine environment from the playa fringer, but also contributes carbonate as a chemical precipitate.

AAPG Search and Discovery Article #90971©1976 AAPG-SEPM Rocky Mountain Sections 25th Annual Meeting, Billings, Montana