Abstract: Petroleum Geology of Carbonate Rocks

Saleh M. Billo

Where oil and gas supervene in reservoirs consisting of both limestone and dolomite, the dolomite and dolomitic rocks are usually the more prolific producers of petroleum. Even the dismissal by some oil explorers of primary or evaporitic dolostones from the category of reservoir rocks have recently been challenged; for example, by the discovery of more than 500 million barrels of oil in a primary dolomite and associated dolomitized portion of the Trenton (Ordovician) limestone of the Lima-Indiana field across the Cincinnati and Findlay arches. Permeability decreased updip where oil in the magnesian phase of the limestone disposed a stratigraphic trap. Oil geologists found that both porosity and permeability developed during dolomitization. Temperature and pressure, time, H, Eh, and salinity are all important controls. Evaporation of sea water past the point of calcium sulphate precipitation suppresses the chemically inhibiting influence of calcium sulphate in solution on dolomite precipitation and increases the Mg/Ca ration from 1:1 at low salinities to over 5:1 or 10:1 in a hypersaline environment.

AAPG Search and Discovery Article #90957©1995 AAPG Mid-Continent Section Meeting, Tulsa, Oklahoma