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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 68 (1984)

Issue: 4. (April)

First Page: 494

Last Page: 494

Title: Comparison of Carbonate and Shale Source Rocks: ABSTRACT

Author(s): R. W. Jones

Abstract:

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) is low, with a negligible generative capacity for hydrocarbons, despite a relatively high hydrocarbon/TOC ratio in the immature state. An anoxic depositional and early diagenetic environment can result in the deposition of organic-rich, fine-grained carbonate rocks 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).

Gas-prone organic facies are rare in carbonate rocks because such facies are usually dominated by terrestrial organic matter deposited in a dominantly clay matrix. 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 commonality 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 due 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.

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