EVAPORITES AS OIL AND GAS SOURCE ROCKS

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Abstract
Shale‐ and micritic limestone‐bound organics have long been considered as hydrocarbon source rocks. However, a conceptual model of hydrocarbon generation in salt basins does not require the entrapment of hydrocarbons in impervious strata, shales or micritic limestones as source rocks.In evaporite basins, organic matter derived from surface waters is preserved beneath the pycnocline of a density‐stratification in anaerobic hypersaline bottom brines. Not being recycled, it is decomposed by bacteria, goes into solution or settles on the bottom. Montmorillonites, calcites, gypsum and potash deposits can alter organic matter catalytically. Impervious clays and micritic limestones trap large hydrocarbon molecules permanently; smaller molecules are flushed through a permeable substrate into aquifers and delivered to reservoirs. After burial, maturation equal to other hydrocarbons is achieved at a lower vitrinite index.