Study On Shale Gas Accumulation Conditions Of Longmaxi Formation In Northern Hebei Province

Sichuan basin, located at the western part of China, is an important oil&gas accumulation area. As one of the best source rocks in sichuan basin, the Lower Silurian Longmaxi Formation black shale located into the northern of Guizhou Province is generally regarded as a potential reservoir for shale gas. However, there are only few studies on the accumulation mechanism. Based on the organic geochemistry, mineralogy and adsorbed gas capacity etc. we have conducted a systematic research on accumulational geological of the black shale in the the northern of Guizhou Province.The sedimentary environment of Longmaxi formation is the shallow-water shelf and deepwater shelf in turn. The black shale of the bottom Longmaxi Formation is the deep-water shelf, The mudstone-limestone of the top Longmaxi Formation is the shallow-water shelf, In the guizhou central uplift of the Longmaxi Formation is the tidal flat. In the study area, the black shale of the Longmaxi Formation dominated by I types of kerogen, rich aboundent of organic contents(TOC on average 3.5%). Vitrinite reflectance(Ro) was greater than 1.7%, has been in over mature evolutionary stages. The minerals in the black shale of the Longmaxi Formation are mainly composed of detrital minerals, the minerals in the clay minerals are mainly composed of illite and montmorillonite. Four pore types are recognized in the black shale of the Longmaxi Formation:in tergranular pore,intragranular pore,fracture and organic pore.Intergranular pore and intragranular pore were the main pore types. The shale pore(usually 1 ~ 10 microns) and fracture(slit width was 0.5 ~ 10 microns) commonly more development in brittle minerals and clay minerals content.In the study area, the gas content of the black shale was more than 1m3/t(Maximum of 4m3/t). The gas content gradually was reduced from base to top. Siliceous minerals(mainly quartz) and the organic matter in the black shale was the main internal control cause of the microporous. At the same time, there is a positive correlation between the pore volume and the methane adsorption, which further confirmed that the Organic matter microporous was the main factor influencing the gas content of the black shale. For the unconventional oil and gas exploration, Lower Silurian black shale, which is characterized high-quality, moderate maturity, micro-porosity welldevelopment in muddy siltstone and very fine sandstone, is with good geological conditions for the formation of shale accumulation.