Subsurface Architecture, Sedimentological Properties and Geomodelling Of Meandering Channels Incised Late Cretaceous Intertidal Carbonates Of Mesopotamian Basin.

Abstract Channels exist in both terrigenous and marine environments. Understanding of fluvial clastic channels are well-established among geologists due to their abundance. In contrary, 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 appearance is observed within shallow water Mishrif carbonates deposited during 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 Arabian plate. First impression was surprising to detect these massive meandering geobodies that vastly propagated laterally throughout the study area. This paper presents a methodological framework for characterization, interpretation and modelling of geometrical and sedimentological attributes of carbonate channels. Similar to terra-channels, the marine-channels cannot be clearly concluded based on solely from well log or core information. Seismic and well data integration is must to make final conclusion that a well encountered a channel geobody. Existence of a single well 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 has 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 to 43 m. Both thin-section and core descriptions confirmed that the incision fill-up sediments are carbonates. Reservoir facies filled-up the channel incision with porosity ranging in between 14 to 27%, while background facies are intertidal carbonates with ~10% porosity in average. Once it became confident that channels exist, all drilled wells went through rigorous re-check if they encountered similar geobodies to identify lateral variations of height, width and heterogeneity of in-fill sediments. Nine additional wells showed potential of penetrating channel fill-ups based on log data. Seismic data provided confirmation that meandering channels in-deed exist at those drilled locations. Sequence stratigraphic knowledge helped to interpret the process how these channels formed. Background facies were deposited at an intertidal shallow water environment that resulted in non-reservoir micrite-dominant fabric. Subsequent sea-level fall carved the background and formed channel incisions. Subsequent HST-related sea-level rise filled-up the incisions with reservoir facies. At the modelling stage, the main challenge is difference of resolution of well and seismic inversion data. Seismic data was used to guide lateral heterogeneity while well data was used to capture vertical property changes. Identified new channels opened new insights to seek possible extended hydrocarbon accumulations away from crestal part of development area. Planning and drilling appraisal wells along meanders could add additional reserves.