Nature and distribution of porosity and permeability in jurassic carbonate reservoirs of the Arabian Gulf basin

Summary

The Middle-Upper Jurassic section in the Arabian Gulf basin forms one of the most prolific sequences in the world, in which an excellent combination of source, reservoir and seal rocks was developed within a major sedimentary cycle. The sequence consists of a) relatively quiet deep-water mudstone, wackestone and shale (source facies), b) shallow-water high enery grainstone and packstone (reservoir facies), and c) very shallow supratidal anhydrite (seal facies). The principal factors, which controlled the sedimentation of this sequence, are considered to have been eustatic sea-level change and epeirogenic movement of carbonate shelves.

The Jurassic reservoirs of the major oil fields in this region show exceptionally high porosity up to 30% for their relatively old geologic age (some 150 million years old) and depths of burial in the range between 1,200 and more than 2,700 m. Porosity occurs most commonly as intergranular/remnant primary pore spaces, but its distribution is quite uneven and very complicated. To account for the existence of such high porosity (and permeability) in the Jurassic reservoirs, probable geological, physical and chemical factors for preserving and enhancing porosity (and permeability), such as acidic formation fluids, reduced fluid mobility, tectonic forces, ductility of intercalated beds (e.g. anhydrite), and dolomitization were examined.

It has been observed in various fields in the region that oilsaturated portions of the Jurassic reservoirs tend to retain higher porosity than the surrounding water-saturated zones. Porosity preservation by hydrocarbons is possible primarily because of excess hydrocarbon pressure and of reduced mobility of water in such oil-saturated zones. To continue sediment diagenesis, a steady supply of minerals by formation water and the mobility of the water may have been essential. Because the entrapment of oil in the Jurassic reservoirs in the region is considered to have been as late as early Tertiary, some other (pre-migration) mechanisms which may have worked in the earlier geologic stages for preserving and creating porosity (and permeability) seem to be necessary.