The Sudair Formation: From Seal to Reservoir, Oman

The carbonate Sudair Formation (Lower Triassic) was originally considered as top seal of the oil and gas-bearing Khuff Formation due to the tight and argillaceous nature of its sediments. Nonetheless, the transition to grainier fabrics towards the north-east of Oman (platform margin) resulted in good reservoir facies, which are now the new focus play for exploration in Oman. This study provides a regional, integrated sedimentological, diagenetic and reservoir quality evaluation of the subsurface Sudair Formation, which has only been described previously in outcrop (Middle Mahil Member). The work has been performed at regional scale, across several fields, and provides an in-depth evaluation of the vertical and lateral facies distribution and related reservoir properties. This bridges an important knowledge gap in the region. Detailed core sedimentological characterisation of four wells capturing the various members of the Sudair Formation (Lower, Middle and Upper) was carried out. Utilising core-to-log calibration, these wells were correlated with nine uncored wells to better constrain the stratigraphic architecture of the reservoir units. In-depth petrographic evaluation was performed on thin-sections and results were integrated with SEM, XRD, QEMSCAN analyses and available porosity/permeability measurements to unravel mineralogical composition and reservoir quality controls. Depositional maps were also used to enhance the characterisation of the reservoir in the inter-well areas by improving the spatial prediction of the rock properties. The sedimentological architecture shows a series of stacked sequence stratigraphic cycles comprising intertidal (IT) and supratidal (NS) domains deepening upwards into high-energy inner ramp environments (IHE) – transgressive hemicycles, which then shallow upwards into intertidal and nearshore towards the cycle tops – regressive hemicycles. The comparison with neighbouring fields in the study area has highlighted that the formation underwent extensive erosion towards the east and south because of the Triassic/Jurassic regional unconformity. The truncation derived from this event caused the loss of the entire Jilh Formation and Upper Sudair Member in many of the eastern wells. Thin-section analysis highlights a complex diagenetic overprint and pervasively dolomitised facies. Dominantly mimetic dolomitisation preserved most of the sedimentary features and allochems of the precursor limestone. Overall, grainier fabrics present higher reservoir quality than mud-supported facies. Early and late dissolution phases created macroporosity and enhanced microporosity, along with fracturing, which locally increased pore connectivity. Cementation is overall weak across the studied wells. This core,log and thin-section-based integrated work provides key insights into the Sudair Formation. These allow the modelling and prediction of facies heterogeneity as well as identification of the best-connected sweet spots, which greatly help exploration and development strategies.