Heterogeneity of shale oil reservoirs in the E2l2(1) in Weixinan Sag, Beibu Gulf Basin
), Abstract
Weixinan Sag, an important area for the exploration and development of both conventional and unconventional hydrocarbon in the Beibu Gulf Basin, South China Sea, has witnessed the success of well WY-1 which tested commercial oil and gas flow after fracturing in July, 2022, marking a major breakthrough in offshore shale oil development in China. An integration of the analysis of core, thin section, mineral composition, organic geochemistry, scanning electron microscope (SEM), matrix porosity and permeability and fine logging, is applied to study the heterogeneous signatures of shale oil reservoirs in well WY-1, Weixinan Sag. The results are shown as follows. Four types of lithofacies can be identified in the lower submember of the second member of Liushagang Formation (E2l2(1)) in well WY-1, including highly siliceous clayey mudstone, mixed mudstone, clay-rich siliceous siltstone and (mixed) siliceous sandstone. Lithofacies is key to the reservoir heterogeneity. An analysis of the physical properties, pore throat characteristics, mobility and brittleness of the four types of lithofacies, indicates that the reservoir space is dominated by intergranular pores, and the (mixed) siliceous sandstone facies is of the highest porosity and permeability, the best compressibility and the highest mobility, a favorable lithofacies, followed by clay-rich siliceous siltstone, and the mixed mudstone and highly siliceous clayey mudstone coming at last though the compressibility and mobility of mixed mudstone are relatively better than the highly siliceous clayey mudstone. It is found that the favorable lithofacies for reservoir development can be sorted as (mixed) siliceous sandstone the first, followed by clay-rich siliceous siltstone, mixed mudstone and highly siliceous clayey mudstone in order. In terms of the vertical distribution of reservoir properties, the bottom interval of the E2l2(1) dominated by (mixed) siliceous sandstone interbedded with clay-rich siliceous siltstone, is the best in reservoir property, mobility and compressibility; the brittleness and compressibility of the interbedded mixed mudstone and highly siliceous clayey mudstone in the middle interval of the E2l2(1) are better than the upper interval. In all, it can be concluded that the sweet-spot section in well WY-1 is mainly composed of (mixed) siliceous sandstone interbedded with clay-rich siliceous siltstone in the lower interval of the E2l2(1), followed by the middle interval where mixed mudstone is well developed.
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