| Shale gas, as an important type of unconventional oil gas, has been attached importance to countries because of its huge amount of potential resources, clean and environmental protection. As the amount of potential resources is huge, China ranks first in the world but because of the immature development technology, digging shale gas is in infancy. Right now, it is particularly important to do some in-depth study of shale gas. According to a large number of studies, the sedimentary environment plays an important role in the mechanism of reservoir forming and preservation conditions.It not only controls the accumulation and preservation of organic matter, but also controls fracturing of the shale gas mining, etc. The primary purpose of the studying shale gas is to find out its characteristics such as sedimentary environment. The sedimentary environment has been controversial on Wufeng Formation of Ordovician.There are basically two views. One is "shallow water theory" and the another one is "deep water theory". Combining sedimentology, petrology, geochemistry and oil-gas geology, etc.,which article starts from the analysis of the sedimentary environment of Wufeng formation analyzing the sedimentary environment,lithofacies paleogeography,mineral composition, organic geochemistry characteristics of northeast of Sichuan province and southeast of Chongqing. On the basis of examining the sedimentary environment of Wufeng Formation, the article will analyze the basic geological conditions which is good for the gathering of shale gas and then mark the useful area taking advantage of multiple factors superposition method.The study areas of Wufeng Formation mainly produce tidal flat facies and nertic shelf facies.And the mainly rock micro facies are carbonaceous siliceous shale and carbonaceous shale,etc.According to the analysis of characteristics of paleontology,the main trace element and rare earth element of Wufeng Formation,the results show that Wufeng Formation come into being in limited shallow-marine environment where the waters is not deep, the surface seawater is desalted, the bottom seawater is serious lack of oxygen, the delimitation is obvious,and the symptoms of viscous flow,revivification and desalt are very strong,not the far abyssal environment.WufengFormation mainly developed siliceous shale that mineral component is given priority to quartz and clay mineral.Quartz content is more than 50%,and clay mineral content is between 30-50%. The organic abundance is high and the distribution of it is uneven but the content is often more than 1%. The organic matter is mainly divided into type I and type II.Its thermal evolution degree is much high that Ro is mainly from 1.5 to3%.In order to find out the control function of sedimentary environment on shale gas,the organic abundance will be divided into four levels:>4%,2-4%,1-2%,<1%.After analyzing the four different black shale,the results show that the strong deoxygenation environment of deepwater shelf facies is most conducive to shale gas accumulation,the amount of organic carbon of carbonaceous siliceous shale and carbonaceous shale is the highest, and the amount of the organic carbon in mineral constituent is increasing with the increase of quartz and the decrease of clay minerals and carbonate mineral. The organic carbon of siliceous shale is the highest, in which the quartz content is more than 50% and the clay mint content is less than 30%, followed by clay shale and then carbonate shale is the lowest. Finally, based on the old map of lithofacies, the article will put the thickness of the organic abundance, organic matter rich shale,Ro and brittleness mineral pitch on the distribution of sedimentary facies to mark Wangcang-Nanjing and Wulong-Daozhen advantageous areas using multiple factors superposition method. |
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