Reservoir connectivity of offshore oilfields with a sparse well pattern: a case study of the third member of Weizhou Formation of oilfield A in Weixinan Sag, Beibuwan Basin

To address the prominent injection and production contradiction, complex remaining oil distribution, and utilization issues in the third member of the Weizhou Formation (W₃) of oilfield A in the Weixinan Sag, Beibuwan Basin, the study developed a comprehensive reservoir configuration characterization system for offshore oilfields with a sparse well pattern, involving reservoir configuration interface classification, anatomy of composite sand body, and quantitative characterization of reservoir configuration unit. The reservoir connectivity was studied, aiming to optimize production strategies in the area. Vertically, the different oil groups in the third member of the Weizhou Formation in oilfield A could be divided into 3 to 4 monogenetic sand bodies. For lateral characterization, the study integrated multiple approaches, including reservoir contact pattern analysis, sand body scale constraints, well and seismic combination, and dynamic verification of production data to obtain the reservoir configuration profiles of different injection and production wells in the same period. Combined with variations in sedimentary microfacies of different oil groups and the positive correlations between the thickness and width of subaqueous distributary channels, the reservoir configuration unit of the third member of the Weizhou Formation in oilfield A was further quantitatively characterized. Based on that, the lateral and vertical connectivity of W₃Ⅳ_D and W₃Ⅳ_E oil groups in the main development layers of oilfield A was analyzed. Profile characterization of the W₃Ⅳ_D oil group showed that the subaqueous distributary channel sand bodies were vertically superimposed with moderate inter-channel sand body continuity and overall good lateral connectivity. Planar analysis showed that the river channels in the southwest provenance direction swung slightly with significant scale variations. Small-scale mouth bars were locally developed. The river channels in the northeast provenance direction were relatively straight with few scale variations. For W₃Ⅳ_E oil groups, the northern wells showed rapid changes in vertical sedimentary microfacies of single well and sand body thickness between wells, with moderate sand body continuity. The southern wells generally exhibited weaker connectivity. Planar studies revealed evident swings of river channels in the southwest provenance direction and well-developed mouth bars. The river channels in the northeast provenance direction were relatively straight, and the interdistributary bays were more developed.