Lithofacies paleogeography mapping and reservoir prediction in tight sandstone strata: A case study from central Sichuan Basin, China

Sand-rich tight sandstone reservoirs are potential areas for oil and gas exploration. However, the high ratio of sandstone thickness to that of the strata in the formation poses many challenges and uncertainties to traditional lithofacies paleogeography mapping. Therefore, the prediction of reservoir sweet spots has remained problematic in the field of petroleum exploration. This study provides new insight into resolving this problem, based on the analyses of depositional characteristics of a typical modern sand-rich formation in a shallow braided river delta of the central Sichuan Basin, China. The varieties of sand-rich strata in the braided river delta environment include primary braided channels,secondary distributary channels and the distribution of sediments is controlled by the successive superposed strata deposited in paleogeomorphic valleys. The primary distributary channels have stronger hydrodynamic forces with higher proportions of coarse sand deposits than the secondary distributary channels. Therefore, lithofacies paleogeography mapping is controlled by the geomorphology, valley locations, and the migration of channels. We reconstructed the paleogeomorphology and valley systems that existed prior to the deposition of the Xujiahe Formation. Following this, rock-electro identification model for coarse skeletal sand bodies was constructed based on coring data. The results suggest that skeletal sand bodies in primary distributary channels occur mainly in the valleys and low-lying areas,whereas secondary distributary channels and fine deposits generally occur in the highland areas. The thickness distribution of skeletal sand bodies and lithofacies paleogeography map indicate a positive correlation in primary distributary channels and reservoir thickness. A significant correlation exists between different sedimentary facies and petrophysical properties. In addition, the degree of reservoir development in different sedimentary facies indicates that the mapping method reliably predicts the distribution of sweet spots. The application and understanding of the mapping method provide a reference for exploring tight sandstone reservoirs on a regional basis.

Review and prospect on the botryoidal structures from the Sinian Dengying Formation,Sichuan Basin,China

Fabric of carbonate rock is the important foundation and one of main research contents for study on carbonate sedimentology,and has always been the attention of the academic circles.Botryoidal structures from the Sinian Dengying Formation in the Sichuan Basin is a kind of special carbonate fabric,the fabric is named after the shape of a grape.In this paper,from four aspects of the research status,the definition of the botryoidal structures and the related terms,the construction characteristics of the botryoidal structures,the component of the botryoidal structures,geochemical characteristics and the genesis of the botryoidal structures are reviewed.It points out the current research issues of botryoidal structures from the Sinian Dengying Formation in the Sichuan Basin,and put forward that future research should focus on the accurate analysis of its internal construction,precipitation mechanism of the major components,and the construction mechanism of botryoidal structures.