Data from the United States Oil and Gas File (TOTL) developed by the University of Oklahoma, Norman, Oklahoma, are used to characterize the lithology, location (state and basin), geologic age, year of discovery, depth to top of pay, porosity, permeability, water saturation, volume of crude oil and nonassociated gas originally in place, and net-pay thickness of limestone and dolomite reservoirs in the United States. Distributions of these parameters, representing thousands of reservoirs, establish a framework to which individual carbonate reservoirs can be compared, and provide insights into geologic processes affecting reservoir characteristics.
Limestone reservoirs are more numerous in the United States than dolomite reservoirs (by a ratio of about 3 to 1) because limestones are more abundant than dolomites. However, in the eight states that account for over 90% of United States carbonate reservoirs, there is a statistical tendency for carbonate reservoirs to occur preferentially in dolomites. Dolomite reservoirs, on the average, are larger and deeper than those of limestone, yet they often have lower matrix porosities and permeabilities. This line of investigation offers supplemental evidence that dolomitization tends to improve the reservoir properties of a given formation, and that effective fracture systems at reservoir depths are more likely to occur in dolomites than in limestones.