The Permian red beds at Cement and Chickasha oil fields in southwestern Oklahoma have undergone extensive and intensive alteration. This diagenetic mineralization is a direct expression of hydrocarbon migration along unconformity surfaces and fault zones. The oxidation of seeping hydrocarbons to carbon dioxide is the major source of carbon in diagenetic carbonates, which occur as cement and as replacement of gypsum and detrital grains. Calcite cement with ^dgrC¹³ values up to -39 ppt PDB reflect this hydrocarbon source. However, a few calcite samples analyzed show ^dgrC¹³ values of approximately -6 ppt PDB which indicates a freshwater origin. In addition, isotopically hybrid carbonate cement with a bimodal carbon source also is found throughout the s ratigraphic section.

Bleaching of red beds and formation of pyrite are explained as reduction of iron oxides by hydrogen sulfide associated with hydrocarbons. Sulfur isotope ratios of pyrite are similar to those of crude oil. The ^dgrS³⁴ values of pyrite samples collected from the surface and shallow subsurface tend to be slightly enriched in S³². The enrichment of pyrite with the light isotope S³² may be due either to increases in biological activity or to increases of oxygen fugacity. Authigenic kaolinite and mixed-layered illite-smectite are formed as by-products of

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hydrocarbon migration, which has significant effect on formation-water chemistry. The relationship between hydrocarbon migration and diagenetic minerals may be used as a pathfinder for hydrocarbon accumulation at depth.

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