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The Xihu Depression is the largest hydrocarbon-bearing depression of the East China Sea Shelf Basin (also referred to as the ECSSB). However, the depositional systems and reservoir distribution of the Oligocene Huagang Formation in the Xihu Depression are still controversial. Under the guidance of sedimentology and stratigraphy, this study documented a marine-terrestrial transitional environment in the restricted bay setting of the Oligocene Huagang Formation through core description, well logging, and seismic data analysis. This study also revealed that the Oligocene Huagang Formation is dominated by tidal delta, estuary, and gravity flow deposits in the central anticline zone of the Xihu Depression. The new understanding of the sedimentary systems and the discovery of the transgressive gap in the eastern Diaoyu Islands uplift explain the origin of fine-grained sediments and the EW-trending sand bodies in the central depression and the sand bodies parallel to shoreline in the west slope belt, which cannot be explained by previous study results, such as southern transgression or fluvial deltas and even lacustrine deposition. Moreover, the tidal channels, tidal sand flats, and gravity flow sand bodies formed by the transgressive tides are high-quality reservoirs. The study will provide a basis for well placement and serve as guidance for the selection of favorable hydrocarbon exploration areas in the Xihu Depression. [ABSTRACT FROM AUTHOR]

At present, the architecture modeling method of fluvial reservoirs are still developing. Traditional methods usually use grids to characterize architecture interbeds within the reservoir. Due to the thin thickness of this type of the interlayers, the number of the model grids must be greatly expanded. The number of grids in the tens of millions often makes an expensive computation; however, upscaling the model will generate a misleading model. The above confusion is the major reason that restricts the largescale industrialization of fluvial reservoir architecture models in oilfield development and production. Therefore, this paper explores an intelligent architecture modeling method for multilevel fluvial reservoirs based on architecture interface and element. Based on the superpositional relationship of different architectural elements within the fluvial reservoir, this method uses a combination of multilevel interface constraints and non-uniform grid techniques to build a high-resolution 3D geological model for reservoir architecture. Through the grid upscaling technology of heterogeneous architecture elements, different upscaling densities are given to the lateral-accretion bedding and lateral-accretion bodies to simplify the model gridding. This new method greatly reduces the number of model grids while ensuring the accuracy of lateral-accretion bedding models, laying a foundation for large-scale numerical simulation of the subsequent industrialization of the architecture model. This method has been validated in A layer of X oilfield with meandering fluvial channel sands as reservoirs and B layer of Y oilfield with braided river sands as reservoirs. The simulation results show that it has a higher accuracy of production history matching and remaining oil distribution forecast of the targeted sand body. The numerical simulation results show that in the actual development process of oilfield, the injected water will not displace oil in a uniform diffusive manner as traditionally assumed, but in a more complex pattern with oil in upper part of sand body being left behind as residual oil due to the influences of different levels of architecture interfaces. This investigation is important to guiding reservoir evaluation, remaining oil analysis, profile control and potential tapping and well pattern adjustment. [ABSTRACT FROM AUTHOR]