Evaluation of the Shale Impact on Reservoir Characterization, the Jeribe Carbonate Reservoir in an Oilfield Northern Iraq as a Case Study

The shale content and mode of distribution, along with their impact on the reservoir properties of the Jeribe Formation, were investigated using the available log data in the two selected wells, NET-10 and NET-12, of an oilfield northern Iraq. The dolomite and dolomitic limestone lithology of the formation contains different ratios of shale, with the highest near the middle part of the formation. Horizons of 70 to 99% shale content were identified, but the general ratios are ranging between 10% and 50%. The data from the Spectral Gamma ray log revealed that the shale content of the formation is mostly composed of low Potassium minerals such as Chlorite and Montimorlinite, with appreciable percentages of Illite. The low Th/U ratios along the formation indicated a reduced condition of deposition except for about 2-3 meters of the upper part of the formation in the well NET-10, which looks to be precipitated in a natural depositional environment. Dispersed, Laminar, and Structural modes of shale distribution co-exist within the formation in both studied wells. As the different modes of shale distribution have different impacts on the porosity and permeability of the reservoir rocks, the decrease and increase in the shale content did not perfectly correspond with an opposite fluctuation in the porosity values of the formation. The shale content in the formation has an impact on the porosity calculation by overestimating it by about 4–5% and subsequently underestimating the water saturation by 9% in the well NET-10 and 7% in the well NET-12.