The Reservoir Characteristics Study Of Lower Paleozoic Shale In Northwestern Hunan, China

A breakthrough has been achieved in China's marine shale gas exploration at the Eastern Sichuan area and experience for the development of shale gas in complicated structural area has been accumulated. As a target zone of the new round of the national petroleum resources assessment, the Northwestern Hunan area deposited a set of lower Paleozoic with similar deposit environment and tectonic evolution as the Southeastern Sichuan Longmaxi shale. Moreover, the Northwestern Hunan area has more favorable surface conditions and competitive development costs. Therefore, it is important to undertake an integrated evaluation on lower Paleozoic shale at the Northwestern Hunan area. Futhermore, gas shale is fine grained and layered reservoir with strong anisotropy. The permeability of the fractures and diffusivity of the matrix are important flow parameters for shale gas production. Both parameters maintain close ties with shale pore structure and are affected by its anisotropy. Hence, study of anisotropy of shale pore structure and diffusivity is an important aspect in evaluating the production behavior of shale reservoir. Previous laboratory studies on anisotropy were usually performed on core samples taken from different orientations on the same rock block; however, the sample difference may impact on the results of anisotropy.This work combined field investigation and testing of more than 150 samples to study the petrology, geochemistry, reservoir and adsorption characteristic of the Longmaxi shale at the Northwestern Hunan area. Comparison was also performed on the geological setting and reservoir properties between the two areas using the data publically available for the Southeastern Sichuan lower Paleozoic shale. At the same time, a new experimental method is presented to investigate the anisotropy of shale porosity and diffusivity. Instead of using different core samples, cube samples are prepared and used to study the diffusion and porosity anisotropy. Multiple shale cubes were cut from the shale block.Results show that lower Paleozoic shale at Northwestern Hunan area is comparable to that at Southeastern Sichuan area on hydrocarbon generation ability, organic matter maturity, sealing capacity and reservoirs reconstruction. Thus the lower Paleozoic shale at the Northwestern Hunan area shows early signs of exploration potential and drilling of exploration wells is thus warranted to further evaluate its gas content and other reservoir properties for possible shale gas development in the near future. Meanwhile, the diffusivity is much higher at the parallel to the bedding direction than that at perpendicular direction. Furthermore, porosity measurement also showed different results measured at different directions, suggesting that there are pores only accessible from one direction and the results suggest more such pores in the parallel to bedding direction. Moreover, different cube samples show different anisotropy even they were cut from the same shale block, suggesting that the shale block is heterogeneous and the cube approach is better to study anisotropy on heterogeneous rocks. Finally, scanning electron microscope (SEM) was used to study the pore structure and the relationship between the SEM observation and the diffusivity and porosity results was discussed.