AAPG Geoscience Technology Workshop

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The effectiveness of clastic reservoirs for hydrocarbon migration and accumulation in deep basins

Abstract

It has long been a problem for petroleum geologists to understand how oil and gas can charge in the clastic reservoirs that tends to become tight in deep basins. Taking the reservoirs of fluvial sandstones in the middle Jurassic Sangonghe Formation in the central Junggar basin as the dissection objects, the structure of sedimentary strata and the diagenetic features and processes of the sandstones are studied. The samples obtained from outcrops and borehole cores are observed microscopically and microscopically. Oil charge experiments with mixed wetting pore media are performed to understand the role of capillary force plays in oil and gas charge under deep basin conditions. Along with the oilfield data, the diagenetic processes are analyzed to reconstruct reservoir burial process models, and the charging behaviors and processes of oil in the reservoirs are simulated. The results suggest that the physical properties and oil-gas bearing state of deeply buried clastic reservoirs are very heterogeneous, and the reservoir heterogeneity is fundamentally controlled by sedimentory structures that make spatial hierarchy of sediments. In the reservoirs, oil charged repeatedly in the oil-bearing rocks and never in the adjacent ones. Generally, these oil-bear lithofecies and no-oil lithofecies are separated by two types of barrier strata: depositional barriers and diagenetic ones. The depositional barriers are composed of the argillaceous strata inside the sandstone bodies and/or between them. The diagenetic barriers include two kinds of lithofacies. One kind of barriers contain 10% or more flexible and plastic particles so that the rocks were completely compacted in the early diagenesis stage. Another kind of barriers were the calcareous concretions that were completely cemented in the early diagenesis stage. The distribution of these barriers is also controlled by the sedimentary structures. The microscopic observation results show that the existence of early charged bitumen in the oil-bearing lithofecies affects the digenetic processes in later stages, that makes the process of densification of the oil-bear lithofecies relatively slow. The aforementioned studies make us understand further the effectiveness of tight sandstone reservoirs for hydrocarbon migration and accumulation in deep basin conditions. The different diagenesis behaviors in each lithofecies in a reservoir, controlled by sedimentary structures, result in an architectural heterogeneity in the reservoir. Therefore, the oil-bear rocks are commonly the effective lithofecies in the reservoir that are favorable for oil and gas migration and accumulation under deep basin conditions. The effectiveness of the reservoir depends on the proportion of the effective lithofecies in the reservoir. The higher the proportion, the larger the effective storage space of the deep reservoir, the better the continuity and connectivity among the effective lithofecies, and the more favorable for oil and gas migration and accumulation, as well as for oil and gas production.