Porosities of Carbonate Reservoirs of the Mesopotamian Basin: An Insight into Their Origin

Al-Hashimi, Wissam S., Iraq Ministry of Oil, Baghdad, Iraq

Studies of Mesozoic carbonate reservoirs of the Mesopotamian Basin demonstrate that porosities of these rocks are significantly enhanced or modified by the major diagenetic processes of solution and leaching of evaporates.

Porosities of peritidal and lagoonal dolomites are dominantly of solution origin, indicat­ed by the serrated outlines of individual pores and locally by the presence of solution vugs or evaporate relics. However, in dolomites of late diagenetic and epigenetic origins, dolomi­tization porosity, if not affected by later cementation, appears to be the prevalent type and forms good reservoirs.

These findings bear on the nature of the dolomitization process and on dolomitizing flu­ids. It seems that dolomitization from hypersaline sea water may proceed under surface and near surface conditions on volume-for-volume basis through pseudomorphic or mimetic replacement of host rock, whereas subsurface dolomitization under deep burial conditions may proceed on mole-for-mole basis, thus allowing porosity to develop while obliterating original rock texture. Similarly, limestones in the basin owe part of their moldic porosities to the dissolution of replacive evaporite cement based on petrographic evidence.

Evidence of vanished evaporates are ubiquitous within platform carbonate rocks of the Mesopotamian Basin. They seem to have played a major role in the development of porosi­ties within these rocks. Careful petrographic investigations of porosity evolution within these rocks could be helpful in delineating porosity trends and thus can be used as a tool in pre­dicting possible reservoir location, geometry and continuity.