Although it has been said that "oil is where you find it," yet any regional or worldwide comparative evaluation of producing carbonate lithologies will show that certain lithostratigraphic units have proved to be more prospective than others. Furthermore, it can be demonstrated that certain important time-equivalent reservoir rocks are remarkably similar in lithologic composition on a worldwide basis. This paper focuses on these worldwide time-equivalent developments of lithologically similar or identical reservoir facies.

The following different lithofacies constitute the most important types of carbonate reservoir rocks: reefs and banks (i.e. biogenic structures); oolites and calcarenites; detrital accumulations; conglomerates and breccias; and shallow-water shelf limestones.

These may occur in the form of unaltered limestones, recrystallized or dolomitized limestones, and replacement dolomites. On a worldwide basis, reefs and banks, together with oolite and calcarenite deposits, form by far the most productive carbonate reservoir facies.

At certain periods of geologic time certain specific lithofacies dominate in forming potential reservoir rocks. During Ordovician and Silurian times, replacement dolomites and dolomitized biogenic limestones prevailed. The Devonian System exhibits major reservoir potentials in unaltered or dolomitized reefoid facies. Mississippian, Pennsylvanian, and Lower Permian carbonate reservoirs represent primarily different types of biogenic bank deposits. Middle to Upper Permian producing lithologies encompass reef facies, as well as

End_Page 1822------------------------------

dolomitized oolite and calcarenite deposits and dolomitized shallow-water shelf limestones. In Late Jurassic time, oolite and calcarenite accumulations, together with minor occurrences of reefoid facies, predominated in forming important petroleum reservoirs in the Vera Cruz region, Mexico, in the Smackover trend, and in the Arab D limestone of Saudi Arabia, Kuwait, and in its time equivalent in Israel. In the Cretaceous, reef facies are dominant. In the Tertiary, coral reefs and calcarenite deposits (Asmari Limestone, Iran) form the most important carbonate reservoirs. Such a generalized categorization by lithofacies refers strictly to major producing lithologies and does not exclude simultaneous production from minor carbonate reservoirs on a local basis.

A comparative evaluation of the reservoir potentialities of carbonate lithologies likely to be found in prospective new oil provinces should form a basis for more realistic profitability analyses.

End_of_Article - Last_Page 1823------------