Print this pageMixed Carbonate-Evaporite Systems - Stratigraphy and Economic Importance in Marine and Lacustrine Settings
World class hydrocarbon source rocks, and oil and gas accumulations occur in many ancient evaporitic marine and lacustrine basins. Seals and traps are, in many cases, controlled by stratigraphic distribution of carbonate-evaporite facies transitions. Marine saline giants represent the lowstand systems of 2nd-order supersequence sets (20-50 my), and provide the regional seal for the hydrocarbons contained within these basins.
Major marine evaporites have been deposited in the Phanerozoic when tectonic-eustatic-climatic conditions provide basin restriction and net evaporative conditions. These saline giants generally occur in low-latitude regions at times in earth history that are characterized by widespread aridity, withdrawal of marine waters from continental shelves, and where basin architecture provided restriction of marine waters. The saline giants have occurred under both greenhouse and icehouse conditions. They are not sudden events, but are preceded by cyclic carbonate-evaporite sequences that reflect progressive climatic deterioration and basin restriction. Carbonate reservoirs are developed during transgressive and highstand systems. Seals are provided by lateral and vertical facies changes to more evaporite-rich facies. Condensed sections deposited during 2nd-order transgressions are oil-rich source rocks.
Restricted lacustrine basins can develop oil-rich source facies during times of aridity, in enclosed lake basins. The best known examples of this occurs in the early Eocene, Green River Formation that occupies three lacustrine basins in the western USA, the Greater Green River basin, Wyoming, the Uinta basin, Utah, and the Piceance basin, Colorado. These basins contain the richest oil shale deposits in the world. The depositional history and facies of these three basins are controlled by evolving regional drainage patterns that reflect the downstream record of weathering patterns controlled by tectonics and climate. Evaporative phases are present in these lake systems, and are associated with rich oil shale units. In the oil-rich Piceance basin, the lake margin sequences are composed of basal siliciclastic sandstone units deposited during periods of falling lake-level. During a number of these low lake levels, evaporates were deposited in the lake center. Rising lake level was characterized by deposits of mixed limestone and oil shale. High lake levels are represented by lean marlstone units that often become organic-rich upward.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009