A Novel Integrated Approach to Characterize Transition Zone in a Low Relief Marginal Field

Abstract

Abstract Marginal Field developments require robust understanding of reservoir lateral and vertical extension, facies distribution, saturation profiles and uncertainty. This paper highlights a reservoir study of a Lower Cretaceous unit of the Thamama Group, located within the Eastern UAE, close to existing major oil producing fields. The results of an integrated petrophysical approach in modelling a low structural relief, complex carbonate reservoir, complicated by the presence of transition zone and post-sedimentary imbibition effects will be showcased. Static (core, logs) and dynamic data (pressure, sampling, well tests, PLT) from wells in the study area and nearby fields helped build a robust rock classification and saturation model. Saturations below present day FWL were evaluated as residual due to post-sedimentary regional reservoir tilting which is supported by the presence of both tar and bitumen in the core. NMR was used to derive an independent dataset to define petrophysical groups, reservoir saturation modelling, irreducible water saturation and identification of transition zone intervals for well perforation planning. These predictions were complemented by MICP saturation models using paleo-FWL and tested with fluid sampling in new wells. NMR capillary pressure-based saturation profiles showed excellent agreement with log derived (Archie) saturation and drainage SHF model results, supporting the rock typing and reservoir properties evaluation. MICP saturation models, representing drainage cycles, were not used directly but as complementary support to identify transition zones using paleo-FWL depth. Predictions of irreducible Sw and transition zone heights were directly confirmed with fluid sampling stations, demonstrating the additional value of NMR derived capillary pressure data for saturation modelling and alternative rock typing. Saturations below present day FWL were evaluated as trapped residual hydrocarbon due to post-sedimentary reservoir tilting, which occurred regionally. The presence of tar and bitumen on core supported the concept of the reservoir undergoing imbibition. Sampling showed water.