Regional sedimentary responses to syndepositional faulting and reservoir potential of Upper Cretaceous limestones in the B Oilfield, Mesopotamian Basin

Abstract

Upper Cretaceous limestones represent the primary reservoirs in the B Oilfield, central Mesopotamian Basin. While syndepositional faults have clear influences on the thickness of these strata, their controls on sedimentation across different reservoir intervals remain poorly understood. This study examines the sedimentary responses to syndepositional faulting during the Late Cretaceous through integrating analysis of fault growth rates and facies distribution across fault blocks. Estimated growth rates for two major faults (F1 and F2) range from 0 to 30 m/Ma, with thickness differences of up to 60 m observed between hanging wall and footwall sites. Fault activity varied over time and was more intense during the deposition of the Lower Hartha and Tanuma formations. This enhanced activity resulted in significant thickness variations and promoted the accumulation of foraminifera and bioclastic packstone facies (MF3/MF2) inside tilted fault-blocks, which are associated with favorable reservoir properties. By influencing facies distribution, syndepositional faulting might also contribute to the development of thicker reservoirs within fault zones. In contrast, fault activity diminished during the deposition of the Upper Hartha and Khasib formations, where more uniform stratigraphic thickness and facies distributions are observed. These findings highlight the critical role of syndepositional faulting in creating accommodation space and enhancing reservoir development conditions in the study area, offering valuable implications for petroleum exploration and reservoir modeling in structurally analogous settings.