## 1 _Acticle_
###### Abstract  
A total of 70 shale samples from seven core holes, namely, N203, N208, N209, N210, N211, N215 and N217, in Changing area, south Sichuan, were analyzed for major and trace elements. Based on an analysis of longitudinal variation characteristics of major and trace elements, the RECOX conditions, paleoproductivity, relative sea level height and terrigenous detrital injection amount of gas-bearing shale in Wufeng Formation-Submember Long-1\({}_{1}\) in the Changing area were studied. And, the main controlling factors of organic matter enrichment in Wufeng Formation-Submember Long-1\({}_{1}\) were discussed. The results show the following: (1) Trace elements Mo, U, Ba and Ni are relatively enriched in the shale of Wufeng Formation-Submember Long-1\({}_{1}\) in the Changning area, while the Co element is deficient. The flat distribution pattern of rare earth in Wufeng Formation-Submember Long-1\({}_{1}\) shows that the shale deposits are weakly influenced by terrigenous materials. The amount of terrigenous injection in Wufeng Formation is higher than that in Submember Long-1\({}_{1}\), and the amount of terrigenous injection increased gradually from the lower part of Bed Long-1\({}_{1}\)1 to the lower part of Bed Long-1\({}_{1}\)1. (2) The RECOX environment discrimination indexes such as U/Th, Ni/Co, V/Sc and SU show that the water RECOX conditions of Wufeng Formation-Submember Long-1\({}_{1}\) in the Changning area have changed many times during the shale sedimentary period, and the overall RECOX conditions are dominated by an anaerobic environment. The RECOX indexes at the boundary between Long-1\({}_{1}\)1 and Wufeng Formation are relatively small and in the oxidizing environment range. The RECOX index from Bed Long-1\({}_{1}\)1 to Bed Long-1\({}_{1}\)1 decreases gradually, and the anaerobic environment of Bed Long-1\({}_{1}\)1 and Bed Long-1\({}_{1}\)2 gradually changes to the oxygen-poor environment of Bed Long-1\({}_{1}\)1; and, the RECOX index of Bed Long-1\({}_{1}\)1 is the lowest, which is generally oxygen rich. (3) Organic matter enrichment in Wufeng Formation Submember Long1\({}_{1}\) in the Changning area is mainly controlled by RECOX conditions. Low terrigenous injection and a relatively high sea level have positive effects on organic matter enrichment, but there is no obvious relationship between the paleoproductivity and organic matter enrichment._  
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controls the reservoir space and adsorption capacity of shale, and it ultimately determines the output of shale gas [3]. Therefore, it is very important to study the controlling factors of organic matter enrichment for shale gas exploration and development. The main controlling factors of organic matter enrichment in sediments have been discussed a lot in the past 20 years [4; 5; 6]. There are two main factors affecting organic matter enrichment: marine surface primary productivity and favorable organic matter preservation conditions. The geochemical characteristics of shale can realize the reconstruction of a paleo-marine REDOX environment [7; 8]. When the sources of U, V, Mo, Cr, Ni and other elements are few, the enrichment is significantly affected by the REDOX environment during deposition, while the diagenesis has little influence, so they are often used to identify the REDOX conditions of water bodies [9; 10]. The Ba element derives from terrigenous detrital and biogenic origin, and the paleoproductivity index biogenic barium (BaxS) of Ba content can be used to evaluate the paleoproductivity of marine basins [4; 11; 12]. Rare earth elements have strong stability, and the solubility of seawater to rare earth elements is very low, so the content of rare earth elements (\(\sum\)REE) can indicate the supply of terrigenous materials in sediments. By studying the geochemical characteristics of shale elements, the informations relating to the sedimentary environment during deposition can be obtained, providing clues for the controlling factors of organic matter enrichment in shale [13; 14].