ABSTRACT
The Permian red beds that overlie some giant oil fields in southwestern and south-central Oklahoma have undergone extensive mineralogical and chemical diagenesis. The diagenetic minerals occur within a distinctly zoned aureole that delineates the position of the oil field. The geometries of the aureoles strongly reflect the major structural elements that controlled emplacement of hydrocarbons in the underlying rocks. Calcite, ferroan calcite, manganese-rich calcite, dolomite, ankerite, pyrite, marcasite, and native sulfur are the major diagenetic minerals. The innermost zone of each aureole is characterized by abundant carbonate cementation and generally coincides with a major fault system. Pyrite and marcasite cements are commonly associated with carbonate-cemented zones; these minerals occur also in the bleached sandstones.
δC13 values of carbonate cements indicate 3 major sources of carbon: (1) an organic source with δC13 values of approximately –32‰ vs. PDB, (2) a freshwater source with an average δC13 value of –8.0 ± 3‰, and (3) a hybrid source (freshwater and organic). A mixing model was developed to calculate the proportion of organic carbon in carbonate cement.
δS34 values of pyrite and marcasite average 6.1‰ and range from –9 to + 16‰. The isotopic composition of sulfides is similar to that of oil in the underlying reservoirs. Formation of diagenetic pyrite and marcasite is explained by reduction of iron oxides in red beds by hydrogen sulfide, and by other organic material associated with hydrocarbons.
The HIDA concept can be used in exploration for oil and gas, specifically in structurally controlled reservoirs.
