Porosity enhancement potential through dolomitization of carbonate reservoirs, a case of study from the Euphrates Graben fields, East Syria

Abstract

The dolostones and dolomitic limestones of the lower Cretaceous Judea formation are a key target of hydrocarbons in most of the Euphrates Graben fields. Core materials investigation, thin sections petrographically examinations, and petrophysical evaluations were obtained to determine enhancement of the porosity through dolomitization. Results showed that the lagoon-shallow marine carbonates of the Judea formation are subdivided into two main zones; the upper “limestone zone” is micritic limestones dominated with no reservoir potential, and the lower “dolomite zone” is dolomitic limestones and dolostones dominated with good to very good reservoir potential. Dolomitization of the mudstones and wackestones of the micritic limestones resulted in formation of microcrystalline dolomicrite and early fabric destructive dolomites. Conversely, dolomitization of the packstones and grainstones resulted in formation of the fabric destructive and saddle dolomites. Based on petrography data, dolomitization of the “limestone zone” is interpreted by the seawater dolomitization mechanism at low-temperatures, while dolomitization of the “dolomite zone” is interpreted by the burial dolomitization mechanism under high temperature and pressure conditions. The “limestone zone” is characterized by the poorest reservoir quality, while the “dolomite zone” is characterized by the best reservoir quality. The seawater dolomitization did not significantly enhance the porosity, while the burial dolomitization contributes to enhancing the preserved secondary porosity. Stylolites microfractures and dissolution seams associated with dolomitization played as major factors in porosity enhancement of the dolostones and dolomitic limestones and serving as pathways for hydrocarbon migration. Dissolution processes increased the porosity and more permeability unless they are filled with the precipitated dolomite and/or calcite. Calcification had significant effects on the porosity by blocking the cavities and channels and decreased the effective pore volume.