Reservoir Evaluation of the Tertiary Succession in Selected Wells at Ajeel Oilfield, Northern Mesopotamian Basin, NE Iraq

Abstract

In the present study, a total of forty-fve rock samples, well-logs data, and 3 crude-oil samples from reservoirs rocks, in addition to 70 rock samples from Jurassic–Cretaceous source rocks, are used to assess the Tertiary succession in the Ajeel oilfeld. Examined thin sections prepared from core rock samples collected from several wells within the Ajeel oilfeld indicated the presence of several microfacies in which diferent types of porosity such as intraparticle, interparticle, moldic, and vugs have been detected. Moreover, diferent diagenetic features, including cementation, dolomitization, recrystallization, dissolution, and microfractures, indicate that the examined formations were deposited in a marine depositional environment. Cross-plots of several well-logging data showed that Tertiary reservoir rocks can be divided into eight reservoir units composed predominantly of limestone, dolomitic limestone, and thin beds of anhydrite beds. In the majority of wells, these units had a total and efective porosity of up to 32.0% and 30%, respectively. A wide range of variation is observed in water saturation with the lowest being 5% and higher hydrocarbon contents, indicating that these formations are the main reservoirs in the Mesopotamian Basin. The geochemical investigation of crude-oils recognized parafnic (medium-light), and sour crude-oils, which are originated mainly from marine-origin organic matters. Palynofacies examination showed that source rocks in the Mesopotamian Basin deposited mainly in distal suboxic-anoxic and distal dysoxic-oxic conditions with kerogen Type II (oil-prone), indicating that Jurassic–Cretaceous succession represents main sources rocks in northern Iraq.