AAPG Middle East Region Geoscience Technology Workshop

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Multi-Disciplinary Approach for Shammar Clastic Reservoir Characterization, North Oman


Following the discovery of a clastic reservoir at the base of the Tertiary Shammar Member in 2014, more than 30 exploration and production wells were drilled and extensively sampled, via coring, sidewall plugs, and cuttings. The aim of sampling was to evaluate the poorly understood reservoir in terms of facies types, geometries, distribution, connectivity and reservoir properties. Core-based sedimentological studies were essential to understand the reservoir stratigraphy and to predict its distribution, since it is below seismic resolution and it varies in thickness from 0.5m to 15 m. A multi-disciplinary, field-scale reservoir evaluation was carried out through integration of sedimentological description and petrophysical well logs evaluation, complemented by petrological, diagenetic, mineralogical, sediment provenance, and biostratigraphical studies. Detailed biostratigraphic analyses indicate an Early-Middle Paleocene age for the base Shammar clastic reservoir. Cores and wireline logs show that the oil-bearing reservoir unconformably overlies Early Cretaceous Nahr Umr Formation shales. The reservoir is sealed by overlying lower Shammar shales. Sedimentology and petrology of the base Shammar clastics reservoir suggest highly variable lithofacies including pebbly conglomerates and very fine-grained sandstones, which fine upwards into carbonaceous shales, heterolithics, calcareous bioturbated sandstones and sandy limestones. Sedimentological architecture and biostratigraphic analyses suggest that basal clastics were probably deposited in restricted coastal to open marine environments during a sea level lowstand (forced regression). They show an upwards increase in marine influence and water depth, resulting from rapid marine transgression. Clast and sand composition, Qemscan mineralogical analysis, heavy mineral ratios, and detrital zircon geochronology all support clastic sediment provenance from the Semail Ophiolite and the Hawasina Nappes to the north. Integrated sedimentological-wireline log correlations, supported by seismic data, suggest that the presence and distribution of the reservoir lithofacies was controlled by pre-existing topography. Incised valleys preferentially formed on Nahr Umr Formation shales, which frame comparatively resistant Cretaceous carbonates of the Shuaiba and Natih Formations at the core of structural highs formed by Late Cretaceous inversion. Further Tertiary inversion resulted in a combined stratigraphic-structural trap. The presence of accommodation space does not necessarily associate with reservoir lithofacies such as conglomerates and sandstones. Occasionally the accommodation space was filled with non-reservoir sandy claystones. Petrology and mineralogy analyses, including Qemscan, XRD, and Scanning Electron Microscopy (SEM), integrated with gamma ray, neutron and density logs indicate that the reservoir quality is strongly controlled by primary facies (clay content) and rarely by diagenesis.