Geological condition and exploration prospect of shale oil and gas in the Bohai Sea

The Bohai Sea is the Cenozoic subsidence center of the Bohai Bay Basin. Three sets of hydrocarbon source rocks (i.e., Ed3, Es1+2, Es3) developed in the Paleogene of the Basin indicate significant exploration potentials of conventional and unconventional oil and gas resources, especially the shale oil and gas.To further determine the shale oil and gas potential of the area, the exploration and development activities in the Bohai Sea were investigated and the comparison of the petrogeological conditions in the Basin with those in typical shale basins both in China and other countries, was combined with analyses of geochemical parameters and logging data, microscopic observation of thin sections and X-ray diffraction results of whole rock mineral of samples from the Basin, as well as basin simulation, to carry out an in-depth study.It comes out with four conclusions. (1) There are four high-quality hydrocarbon-rich sags in the Bohai Sea. Of which the Bozhong sag has the highest potential of harboring shale gas with gas-prone and highly matured source rocks. The Huanghekou and Liaozhong sags are more oil-prone with relatively low maturity. And the Qikou sag is both oil and gas prone with its low matured Ed3 and Es1+2 source rocks more likely generating oil and highly matured Es3 producing gas. (2) The three sets of source rocks have good reformability with an average brittle mineral content of more than 63.5% and an average clay mineral content of less than 34.7%.The mud shale below 3 200 m was once at the stage B of middle diagenesis, during which the montmorillonite in the montmorillonite-illite mixture was largely converted to illite, thus increasing the plasticity of the shale reservoir. Considering exploration activities in other areas with similar conditions, layers buried between 3 200 m and 4 200 m are suggested to be the targets for shale oil and gas exploration and development in the Bohai Sea. (3) The Bozhong sag has the greatest potential of shale oil and gas resources with an estimated resource of up to 95.8 billion tons, accounting for 45.2% of shale oil and gas resources in the Bohai Sea. The major source rocks in the sag are under overpressure as a whole. Given the economic cost of shale gas and oil exploration and development in the Bohai Sea, the Ed3 and Es1+2 source rocks of the Bozhong sag that are characterized by moderate burial depth and high maturity, are the most promising shale gas exploration targets in the Bohai Sea. (4) The west slope (at the edge) of the Bozhong sag is considered to be the most promising part in the area for current exploration stage as the three sets of source rocks with an average TOC value of more than 3.04% in the slope are under abnormally high pressure and have relatively shallow burial depth. Furthermore, gas logging shows a total gas content as high as 99% and X-ray diffraction results indicate that the clay mineral content in the shale reservoirs is the lowest compared with that of other areas in the Bohai Sea, making them ideal candidates for future fracturing. © 2020, OIL & GAS GEOLOGY Editorial Board. All right reserved.