Abstract: Hydrocarbon-Induced Diagenetic Aureoles (HIDA): Indicators of Deeper Leaky Reservoirs

Zuhair Al-Shaieb, Janet Cairns, James Puckette

The Permian Redbeds 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 and native sulfur are the major diagenetic minerals. The innermost zone of the aureole (zone 1) is characterized by abundant carbonate cementation and generally coincides with a major fault system. Zone 2 is composed of altered (bleached) redbeds with minimal calcite cement. Pyrite ce ent (zone 3) is commonly associated with the carbonate-cemented zones and is disseminated in some bleached sandstones. Zone 4 represents the unaltered red beds.

^dgrC¹³ values of carbonate cements indicate 3 major sources of carbon: (1) an organic source with ^dgrC¹³ values approximately -32 ^pmil vs. PDB, (2) a freshwater source with an average ^dgrC¹³ value of -8.0 ± 3 ^pmil, and (3) a hybrid source (freshwater and organic). A mixing model was developed to calculate the proportion of organic carbon in carbonate cement.

^dgrS³⁴ values of pyrite average 6.1 ^pmil and range from -9 ^pmil to +16 ^pmil. The isotopic composition of sulfides is similar to that of oil in the underlying reservoirs. Formation of diagenetic pyrite is explained by reduction of iron oxides in redbeds by hydrogen sulfide or other organic material associated with hydrocarbons.

Gas chromatography and isotopic analysis suggests oils in the shallow Permian sandstones in Cement field leaked from deeper Pennsylvanian reservoirs. Leakage from deeper hydrocarbon-bearing reservoirs formed shallow petroleum accumulations and contributed to the alteration of the redbeds. Faults were the likely conduits that carried leaked hydrocarbons to the shallow rocks.

The development of the oil fields with alteration halos indicate that the HIDA can be used to identify potential shallow oil and deep gas accumulations. Therefore, the HIDA concept can be used in the exploration for oil and gas, especially in faulted structural settings.

AAPG Search and Discovery Article #90960©1995 AAPG Southwest Section Meeting, Dallas, Texas