Subsurface Architecture, Sedimentological Properties, and Geomodeling of Meandering Channels Incised Late Cretaceous Intertidal Carbonates of Mesopotamian Basin

Summary Channels exist in both terrigenous and marine environments. Understanding of fluvial clastic channels is well-established among geologists due to their abundance. In contrast, carbonate channels are rarely encountered, or their sizes are not considerable enough to be important for hydrocarbon exploration and development activities. Therefore, marine-origin channel characteristics are yet to be studied and understood. One of such unique appearances is observed within shallow water Mishrif carbonates deposited during the Late Cretaceous in the southern part of the Mesopotamian basin. This basin holds significant hydrocarbon accumulations trapped in structures formed during the Late Cretaceous orogeny and Miocene Zagros folding within the Arabian plate. The first impression was surprising to detect these massive meandering geobodies that vastly propagated laterally throughout the study area. In this paper, we present a methodological framework for characterization, interpretation, and modeling of geometrical and sedimentological attributes of carbonate channels. Similar to terrachannels, marine channels cannot be clearly concluded based solely on well log or core information. Seismic and well data integration is a must to make a final conclusion that a well encountered a channel geobody. The existence of a single well that penetrated the channel was enough to confirm their existence. Description of channels’ subsurface architecture shows that their width achieves ~1–1.9 km in traverse. Geographically, the incision is at least 15 km as they go beyond the study area. Two stacked channels were discovered throughout the research area with varying height between ~9 m and 43 m. Both thin-section and core descriptions confirm that the incision fill-up sediments are carbonates. Reservoir facies filled up the channel incision with porosity ranging between 14% and 27%, while background facies are intertidal carbonates with ~10% porosity on average. Once it became confident that channels existed, all drilled wells went through rigorous rechecks if they encountered similar geobodies to identify lateral variations of height, width, and heterogeneity of infill sediments. Nine additional wells showed potential of penetrating channel fill-ups based on log data. Seismic data provided confirmation that meandering channels indeed exist at those drilled locations. Sequence stratigraphic knowledge helped to interpret the process of how these channels formed. Background facies were deposited at an intertidal shallow water environment that resulted in nonreservoir micrite-dominant fabric. Subsequent sea-level fall carved the background and formed channel incisions. Subsequent highstand systems tract related sea-level rise filled up the incisions with reservoir facies. At the modeling stage, the main challenge is the difference in resolution of well and seismic inversion data. Seismic data were used to guide lateral heterogeneity, while well data were used to capture vertical property changes. This study contributes uniquely to the understanding of carbonate reservoirs by (1) explaining the origin, morphology, and evolutionary history of rarely documented meandering carbonate channels and (2) introducing a deterministic seismic-driven methodology for 3D geobody modeling that reduces uncertainties in contrast to conventional stochastic approaches. The identification of these channels provides new opportunities to target potential hydrocarbon accumulations beyond crestal areas, where appraisal drilling along meander belts may add significant reserves.