Formation evaluation in Dezful embayment of Iran using oil-based-mud imaging techniques

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Structural delineation and fracture characterization are two of the main issues for the evaluation of carbonate reservoirs in structurally complex areas. Some of such fields are located in the southwest (Dezful embayment) of Iran. Getting to the Asmari reservoir is not so easy in some cases due to structural complexities where a thick pile of evaporites of Gachsaran Formation overly the reservoir. In such cases structural dip information plays an important role in mapping the structure in the area around the wells. In addition to structural complications, fracture characterization is quite important for Asmari carbonates and in particular for Sarvak carbonates. In most fields of the Dezful embayment, it is not feasible to drill wells with water-based mud, hence getting information on structural dip and fractures is not so easy with conventional borehole imaging tools. A combination of oil-based-mud imaging tools provided a new way of imaging the characteristics of formations drilled with oil-based-mud. Complicated structures were resolved utilizing the dip data gathered with such techniques. Fractures were characterized for their aperture (open or closed), intensity of fracturing, and directional attributes. The information helped to understand reservoir characteristics of tight carbonates of Sarvak formation. In addition to structural details, the OBM imaging provided quantitative Rxo (resistivity of invaded zone) measurement in oil-based-mud environment. It helped to understand invasion profiles, which generally is a function of permeability, in carbonate and sandstone reservoirs of Asmari Formation, and helped reservoir engineers and petrophysicists to understand response of reservoir formation tester (RFT) measurements for formation pressure and reservoir fluid mobility. The paper describes applications of OBM imaging in Asmari and Bangestan reservoirs of Dezful embayment of Iran.