Abstract

Porosity and permeability in carbonate strata result from interactions between certain favored types of original sediment and a variety of diagenetic processes. Alteration may be induced by several generations of hydrographic conditions controlled in turn by the total geologic history of the reservoir area. Reservoir development is therefore highly complex. Carbonate sediment is produced and deposited with high porosity. This is normally reduced from 50% to less than 10% during diagenesis but permeability may be increased by some processes.

In an analysis of hydrocarbon reservoirs, the following combinations of sediment type, stratigraphic relations and favorable diagenetic overprint persistently recur in the geologic record:

1. Lime sand bars and sheets on shelves with histories of incomplete cementation (Chesteran, Oklahoma; Murban on Trucial Coast; Arab D, Saudi Arabia; Smackover of Louisiana).

2. Carbonate organic buildups whose original high porosity and permeability along margins of banks and platforms have not been completely occluded, or whose fabric has been dolomitized and fractured (Devonian buildups in Alberta and the Golden Lane atoll of eastern Mexico).

3. Downslope talus accumulation below major carbonate bank or platform margins (Poza Rica and Reforma pools of Mexico).

4. A stratigraphic trap formed by subtidal grainstone and intertidal dolomitized mudstone reservoir which change facies updip to a sabkha anhydrite seal (Permian San Andres of West Texas and the Mississippian strata of Williston Basin).

5. Leaching and dolomitization at regional unconformities, processes aided by an original grainy texture and/or fracturing (Natih and Fahud Cretaceous fields of Oman; Late Paleozoic phylloid algal limestone of southwestern U.S.A.; basin margin pinnacle reefs of Devonian Zama-Rainbow; and Silurian of Michigan basin).

6. Chalky textured pure carbonate muds which have not been “solution welded” by burial below 1 000 m and have been protected from Mg poor solutions (North Sea Danian; Austin Chalk).