Integrated Workflow for Understanding the Sedimentary Basin and the Rock Quality of the Cretaceous Upper Shuaiba, Bab Member, in the Northwest of Oman Through Thickness Mapping and Rock Typing at Regional Scale

Abstract

Abstract A regional scale study of the Cretaceous Upper Shuaiba, Bab member, in the northwest part of Oman was carried out to understand the sedimentary basin and rock quality for further reservoir potential. This study utilized data from over 1300 wells drilled in the area targeting the same reservoir or deeper targets. One of the key outcomes of the study was an integrated workflow comparing isopach maps and rock typing to understand the sedimentary basin evolution for rock quality prediction. The results were key in understanding the geological concept behind the reservoir distribution within the various upper Shuaiba clinoforms which are separated by clay rich units. After completing a consistence correlation of all the individual upper Shuaiba units, two set of thickness maps were created i.e., cumulative thickness maps in reference to the base of upper Shuaiba and individual unit thickness maps. The former ones were very useful in understanding the sedimentary basin evolution while the other set of maps were useful in understanding the sedimentary processes which controlled the facies distribution and hence reservoir potential identification. In parallel to the correlation and the thickness mapping a rock type scheme was defined from core data (CRT: Core Rock Typing). The analysis was conducted over 44 out of 60 cored wells. The rock type scheme provides an optimum characterisation of porosity, permeability, and saturation which via geological concepts can then be distributed in 3D. Most rock types can be related to depositional facies and their distribution in a depositional model. The overlay of rock type distribution with the thickness maps largely confirms the existing depositional models of prograding clinoforms, mainly from the NW to the SE. An increase in thickness of each unit from deeper basin to shallower water and the sub-crop below the unconformity is matched by the facies/rock type distribution and associated rock properties. Increased data control was required to consolidate the concepts and to map rock type distribution in more detail. A workflow was therefore developed to interpret rock type from log response (LRT: Log Rock Typing) calibrated by the core-based rock type control. A subsequent more rigorous workflow was adopted for LRT interpretation tied to log-based saturation data and a consistent set of Saturation Height Functions (SHF) along with other verification steps such as the comparison of rock type inferred permeability and MDT derived permeability/ mobility. Integration of the correlation, thickness mapping and rock typing has showed its value in understanding rock type distribution and developing geological concepts. This integrated workflow was utilised and applied in the static model for the field development planning (FDP) of several fields which have undergone successful development. The rock type distribution impacted volumetrics, well placement, well performance analysis, forecasting, and well, reservoir, facility management (WRFM) activities.