Facies analysis of the Middle Cretaceous Mishrif Formation in southern Iraq borehole image logs and core thin-sections as a tool

Abstract

The Middle Cretaceous Mishrif Formation is one of the most important oil reservoirs in central and southern Iraq. The major goal of this study is to calibrate the FMI imaging log in front of the depth intervals where core data is available (thin sections), and then utilize the calibrated FMI behavior to predict the types of microfacies in the wells where core data is not available. Hence, this study focuses on distinguishing and correlating carbonate facies in four wells from the Mishrif Formation in the West Qurna and North Rumalia oilfields, southern Iraq, using borehole image logs and thin sections. Thin-section data from the cored wells were used to identify six carbonate environments based on their constituent carbonate microfacies and grain types. A succession of carbonate-environment-facies units was determined in the cores relative to already established members (mB2, mB1, CRII, mA, CRI) and used to discern how the borehole image logs respond to respective carbonate facies. These borehole image responses to changing carbonate-facies units were then used to identify similar behaviors in the wells without core and for correlation across the two oilfields. Thin-section analysis of microfacies and grain types led to the interpretation of six carbonate-environment facies in the Mishrif Formation, including mid-ramp, shoal, back-shoal, lagoonal and intertidal facies; a rudist-bioherm subfacies also occurred locally in shoal, back-shoal, and lagoonal facie units. The organization of the carbonate-environmental-facies units and their correlation across the two oilfields confirms that the Mishrif Formation in southern Iraq comprises two progradational depositional cycles, separated by a major transgressive flooding surface. Clearly, the combined analyses of thin sections from cored wells and borehole image responses from the same wells provide a powerful tool in understanding and correlating facies and sequences across nearby oilfields.