Imperfect rock seals above petroleum deposits may allow large volumes of low-molecular-weight hydrocarbons to slowly leak and diffuse to the surface. The seeping hydrocarbons chemically alter and incorporate into near-surface and surface rocks as pore-filling cements that are isotopically and chemically distinctive and geographically identifiable because their compositions and densities markedly contrast with surrounding rocks. Strong empirical evidence indicates that gases diffuse directly through the overburden, and leakage of liquid hydrocarbons is controlled by salinity variations in formation waters which affects their solubilities and promotes chemical reactivity. Highly reducing hydrocarbons and associated compounds cause discoloration of surface strata by reductio and dissolution of iron. Near the surface, hydrocarbons are oxidized; expansion of depressurized gas evaporates ground waters concentrating and precipitating dissolved solutes with unique isotopic signatures.

Such alteration and mineralization phenomena have been documented in outcrops of a Permian redbed sequence overlying several prolific oil accumulations in southern and central Oklahoma, but especially at the Cement anticline. Recognition of similar phenomena elsewhere could lead to new discoveries.

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