The Effect Of The Mineral Compositions To Shale Gas Of Longmaxi Formation At Sichuan Basin

In order to study the mineral properties, composition and the relationship with the organic matter characteristics in Lower Silurian Longmaxi Formation shale,30samples from Longmaxi Formation fresh outcrop cross sections located in Shuanghe town, Changning County in Sichuan basin were analyzed by scanning electron microscopy (SEM), electron probe micro analyzer, x-ray diffraction (XRD) and Cathode luminescence analysis(CL), then discuss the reservoir space characteristics of Longmaxi formation. In order to explore the influence of mineral composition for shale gas. The main conclusions are summarized below:1. From the bottom up to top developed black-dark gray graptolite shale, carbonaceous shale, siliceous shale and silty shale pelitic siltstone. The brittle(fragment) minerals include:quartz, feldspars, mica, pyrite; siliciclastic and calcite minerals: Chalcedony, calcite and dolomite; and clay minerals:Illite, chlorite and montmorillonite. The content of quartz decreased upward vertically, is about20%near the top; The content of carbonate mineral increased significantly vertically, is about30～40%near the top; clay minerals has no significant variation in the vertical; The content of chlorite increased upward, mostly about5%; montmorillonite and chlorite have a similar variation in the vertical direction, increasing from the bottom to upwards.2. The two brittleness indexes of the Longmaxi formation at Sichuan Basin are all high, it is believed that the sets of the shales are excellent fracturing formation. It has a great importance for shale gas exploration. Identified5kinds of organic matter occurrence in the rock:①occurs in late filling veinlets and quartz or calcite veins;②qartz and calcite overgrowth or secondary particles inside;③brittle fracture and the surrounding mineral particles;④the cleavage cracks of native calcite and muscovite;⑤clay mineral content is high, it can absorb large organic matter. The organic matter occurrence modes have important relationship with the mineral character.3. Six types of reservoir space, including structural shear fracture, interlayer lamellation fracture, pyrite pore, the inter-crystal micro-pore and micro-crack in clay mineral, the edge around secondary quartz and organic matter pore, were found in the Longmaxi shale. The development of reservoir space is strongly controlled by the lithofacies, mineral compositon and diagenesis.4. The contents of sodium feldspar,carbonate and clays(illite and chlorite) have an effect on the shale gas content. Clay has a strong main gas adsorption, the gap of brittle mineral is beneficial for the storage of gas. When the illite content is in the range of 0%-10%, the shale gas content will be relatively high. When the chlorite content is in the range of0%-5%, the shale gas content will be relatively high. When the carbonate content is over20%, the shale gas content will be generally high. The shale gas content decreases along with the increasing sodium feldspar content. The finer fracture in the fragment minerals have huge influence to the produce and travel of the shale gas. The better shale fractures are developed, the greater the gas accumulation and therefore the higher the gas production.