Survey of Energy Resources 2007
Origin of Oil Shale
Oil shales were deposited in a wide variety of environments, including freshwater to saline ponds and lakes, epicontinental marine basins and related subtidal shelves. They were also deposited in shallow ponds or lakes associated with coal-forming peat in limnic and coastal swamp depositional environments. It is not surprising, therefore, that oil shales exhibit a wide range in organic and mineral composition. Most oil shales were formed under dysaerobic or anaerobic conditions that precluded the presence of burrowing organisms that could have fed on the organic matter. Many oil shales show well-laminated bedding attesting to a low-energy environment free of strong currents and wave action. In the oil shale deposits of the Green River Formation in Colorado and Utah, numerous beds, and even individual laminae, can be traced laterally for many kilometres. Turbiditic sedimentation is evidenced in some deposits as well as contorted bedding, microfractures, and faults.
Most oil shales contain organic matter derived from varied types of marine and lacustrine algae, with some debris of land plants, depending upon the depositional environment and sediment sources. Bacterial processes were probably important during the deposition and early diagenesis of most oil shales. Such processes could produce significant quantities of biogenic methane, carbon dioxide, hydrogen sulphide, and ammonia. These gases in turn could react with dissolved ions in the sediment waters to form authigenic carbonate and sulphide minerals such as calcite, dolomite, pyrite, and even such rare authigenic minerals as buddingtonite, an ammonium feldspar.