Carbonate reservoir characteristics and porosity distribution in Souedih Oilfield, northeast Syria

The Souedih (also spelled Suwaidiyah) Oilfield is located in the extreme northeast of Syria in part of the Mesopotamian Basin. The principal reservoir of most fields in this region is the Upper Cretaceous, carbonate-rich Massive Formation. Using data from 25 wells and 36 samples/thin sections, this study focuses on the nature and distribution of porosity in the main Souedih reservoir. The Massive A reservoir is 100-120 m thick and represented by uniform, bioturbated bioclastic packstone and bioclastic packstonegrainstone, deposited in a well-oxygenated, moderate to high-energy, shallow-marine environment. It is generally well cemented by microsparite and micrite, and more rarely by sparite. Porosity is highly variable, ranging from < 1% to 20%. Mouldic porosity is the most common type, with rare channel and fracture porosity. Average porosity values tend to decrease eastward across the reservoir, which can also be divided vertically into five zones. The uppermost of these shows the highest average porosity > 15%. The dominance of mouldic porosity throughout the study area indicates that secondary dissolution was the primary cause and that pre-existing bioclasts were the principal targets for this dissolution. The source of these diagenetic fluids is still unclear, although our data do lend some support to the karstification theory. These characteristics are important for understanding and managing reservoir production, not only for Souedih but for the region in general.