China is abundant in shale gas resource, the marine shale gas resource extent is about 885.82 Tcf. The Lower Paleozoic in Sichuan Basin is one of the most important areas of marine shale gas exploration and development in China. Lower Silurian Longmaxi Formation and Lower Cambrian Qiongzhusi Formation are the main strata for researching, and the total reserve of shale gas resources is from 240 to 296 Tcf. CNPC plans to exploit Longmaxi Formation in 2015, with a full production capacity of 2.6×1010m3. However, less number of wells, no further research and the delicate study on sedimentary facies, there are the main three problems for Qiongzhusi strata. Comparing with previous research, this paper focuses on analysis of lithofacies, paleontological fossils and electrical characteristics, classify finely the sedimentary facies and microfacies of Qiongzhusi Formation. Moreover, the depositional model of the research area is established, and we discuss the controlling effects of sedimentary environment to organic-rich shale.
The shelf depositional system of shale was developed in Qiongzhusi Formation of Lower Cambrian in Sichuan Basin, divided into deep-water shelf and shallow-water shelf. The lithology of deep shelf is black shale and gray argillaceous siltstone, within numerous paleontological fossils and pyrite, could be divided into 3 microfacies including mud deep shelf, sand-mud deep shelf and turbidite sand. The lithology of shallow shelf is black shale and the interbed of argillaceous siltstone and calcareous siltstone, which can be divided into sand-mud shallow shelf and beach bar form the shallow shelf.
The Qiongzhusi Formation experienced two times of large-scale transgression and the environment of depositional is deep-water shelf. Because of the storm during transgression, the hydrodynamic is stronger which lead to the development of sand-mud deep shelf. The mud deep shale, which deposited in weak hydrodynamic and stable sedimentary environment, has the average TOC of 1%~4%, the stratigraphic thickness from 20m to 200m, the average porosity of 2.36%, the gas content of 2.03m3/t, the Ro of 2.5%~3.5%, the siliceous content of 35%~56%. Great conditions for shale gas formation indicated the mud deep shelf is a favorable environment for shale deposition. The first transgression occurred in early Qiongzhusi Formation, depositing a high-quality shale in mud deep shelf. The second major transgression occurred in the middle or late Qiongzhusi Formation, and this transgression lasted for a long time and oscillated multi periods, which can be divided into three small-scale transgressions and identified three sets of high-quality shale deposits.
