As with shales, the source potential of carbonate rocks depends primarily upon the organic facies rather than the mineral matrix. Where the depositional and early diagenetic environment is highly oxygenated, the total-organic-carbon (TOC) content is low. The remaining kerogen is highly oxygenated, with a negligible generative capacity for hydrocarbons, despite a relatively high hydrocarbon/TOC ratio in the immature state. An anoxic depositional early diagenetic environment can result in the deposition of organic-rich, fine-grained carbonate sediments that are excellent potential source rocks.
Excellent oil-prone source rocks, whether with carbonate- or clay-mineral matrices, have many characteristics in common. Both form in anoxic environments, are generally laminated and heterogeneous, have moderate to high TOC, and contain high-quality organic matter (OM). The latter is exemplified by atomic H/C ratios ≥ 1.2 near a vitrinite reflectance of 0.50% Ro. Although they constitute a small percentage of all carbonate rocks, organic-rich, fine-grained carbonate rocks are widespread in both time and space and are the probable source of 30–40% or more of the petroleum reserves of the world.
Gas-prone organic facies are rare in carbonate rocks because they are usually dominated by terrestrial organic matter deposited in a dominantiy clay matrix. However, gas-prone organic facies may occur in carbonate rocks as a result of turbidite deposition or by a mixture of kerogen types II and IV. Most carbonate rocks contain nongenerative organic facies, as do most siliceous rocks. Oxygen-rich depositional environments for carbonates are found from sea level (reefs) to the ocean depths (Globigerina ooze).
Despite the basic relationship between organic-rich oil-prone carbonate and shale source rocks, some significant differences exist. Oils derived from carbonate rocks are often richer in cyclic hydrocarbons and sulfur compounds than oils derived from shales, owing to the dearth of terrestrial-plant waxes in the OM and less iron in the pore water. In addition, the generally earlier decrease of porosity and permeability and the greater contrast between the physical properties of the OM and the rock matrix in carbonate source rocks often result in different primary migration characteristics.