Predicting Shallow Marine Reservoir Heterogeneity Using a High Resolution Mapping Approach, Brigadier Formation, NWS, Australia

Abstract

Abstract Reservoir delineation and characterisation of the shallow marine Triassic Brigadier Formation (Fm.), northwest shelf (NWS), Australia is challenged by the thin-bedded nature of the reservoir. These reservoirs are generally below seismic resolution in stark contrast to the underlying, seismically well-imaged Mungaroo Formation fluvial channel belt reservoirs. In the absence of clear seismic definition of reservoir distribution, development of reservoir models for the Brigadier Fm. rely heavily on conceptual depositional models refined by reference to depositional analogues. The Brigadier Fm. intervals currently under development in the Northern Carnarvon Basin are typically 120 m thick with overall 50–80% NTG. Strata in this interval are generally structurally inclined in tilted fault blocks and sub-crop an angular unconformity (IJU) with overlying sealing shales. Although the general geometry of the inclined strata is imaged on seismic, poor impedance contrasts do not allow definition of individual sheet-like, five to 10 m thick depositional elements or internal lithic variations that could be used to confidently target development wells. Thick stacked successions of thin reservoirs separated by shale produce correlation uncertainties between wells and make it challenging to predict spatial trends in reservoir quality. In order to better predict vertical and lateral facies patterns within the Brigadier Fm., a novel approach of reservoir mapping and model construction was developed that deterministically predicts reservoir distribution and connectivity based on conceptual depositional models. A revised Brigadier Fm. stratigraphic framework was created across the sub-regional (88 km x 64 km) study area by integrating the latest sedimentology, biostratigraphy, chemostratigraphy and seismic data. Depositional models of the Brigadier Fm. were reviewed by re-examining core using a depositional-process classification that defined reservoir-element-scale bodies within correlated stratigraphic intervals. Two key depositional scenarios were developed; an asymmetrical, wave-dominated delta model, and an alternative fluvial-dominated delta model. The Mitchell River Delta, north-west Queensland, Australia, was used as a partial analogue to define shapes and scaling of the subsurface depositional elements and their internal lithic patterns. The sub-regional model produced for the study was built in flattened stratigraphic space. Depositional geometries were extracted from this model and morphed into structured space over specific fields. The hierarchical nature of the model enables seamless analysis of exploration to development scale issues. The model is currently being tested by predicting and verifying the vertical facies successions for exploration, appraisal and development wells, and dynamic flow data from well tests.