A synthesis of geologic attributes and reservoir properties is provided for 41 carbonate petroleum fields. From throughout the world, 55 scientists developed detailed case studies of carbonate reservoirs ranging in age from Ordovician to Miocene and in size from 1.7 × 10³ to 32.0 × 10⁹ bbl initial oil in place (IOIP).

The objective was to focus on the detailed depositional and diagenetic history of a significant number of carbonate reservoirs, and secondarily to characterize their petrophysical evolution and reservoir volumetrics. An attempt was made to develop a complete "anatomy" of each field, which includes among many other attributes: regional paleosetting, tectonics, entrapping facies, source rocks, reservoir dimensions, and fluid data.

Using a chronological arrangement, broad trends through time become evident. A progression of reservoir types occurs through the Phanerozoic, from peritidal and subaerially exposed facies in Paleozoic examples, through a long-term sequence of shallow-shelf sands and reefs, to relatively deep-marine facies in the Cretaceous and Tertiary. These last range from periplatform debris flows to deep-shelf pelagic chalks and deep-basinal mixed lithofacies that have been diagenetically altered, mainly to dolomite. No trends are seen, however, in the occurrence of pinnacle or other reefs through the Phanerozoic.

Many of the largest fields occur in Tertiary mobile belts and other regions affected by Tertiary tectonics, principally in carbonates of Cretaceous to Miocene age. Furthermore, at least nine of the largest fields owe their reservoir productivity largely to fracturing, and at least ten smaller fields involve some fracturing.

In contrast, the great number and diversity of reservoirs of shallow-epeiric, platform, peritidal, and sabkha origins are also evident, with reserves between 70 and 700 × 10⁶ bbl IOIP. Shallow reef-mound, atoll, or pinnacle reef fields of large magnitude, all in carbonates of phototropic, warm-water, marine origin, are common over structures, particularly those related to subjacent salt diapirs. Non-salt-supported shallow shelves and reefs have smaller fields of less than 200 × 10⁶ bbl IOIP.

Future requirements include much more knowledge about (1) the extent, diversity, and recognition of deep-marine carbonates; (2) the frequency, size, orientation, and contribution to reservoir productivity of fractures; and (3) a large information base, generated by postmortem studies of the vast number of reservoirs which are destined to become candidates for supplemental recovery methods.

End_of_Article - Last_Page 148------------