Study on in-Situ Stress Distribution in the East and West of Sichuan Basin

This paper statistically analyzed 112 acoustic emission experimental data of cores from 38 wells in the Upper Permian Changxing Formation, Triassic Xujiahe Formation, Leikoupo Formation, Jialingjiang Formation, and Feixianguan Formation in the northeastern Sichuan Basin, and 106 acoustic emission test data of cores from 32 wells of the Jurassic Shaximiao Formation and Penglai Formation in western Sichuan, eliminated data with large errors, established scatter plots of in-situ stress changing with depth in northeastern and western Sichuan, and carried out corresponding regression analysis. Studies show that the in-situ stress distribution rules in northeastern Sichuan and western Sichuan are quite different, which are mainly related to the evolution and tectonic movement of basin. The vertical stress of two blocks is generally linear with the depth, and the vertical stress value is related to the rock. The difference between overburden weight and vertical stress is small. The maximum and minimum horizontal principal stresses gradually increase with buried depth, and the degree that horizontal principal stress increase with buried depth in northeastern Sichuan is generally greater than that in western Sichuan. The horizontal shear stress is also linear with buried depth. The corresponding linear relationship is stronger in the western part of Sichuan, while the dispersion in the eastern part of Sichuan has an increasing trend as the buried depth increases. The elastic modulus of rock directly affects stress value of rock. Generally, the higher the elastic modulus of rock, the better it is to maintain higher stress. The elastic modulus of rocks of different genesis is obviously different, and the relationship between the stress value of rock and the elastic modulus is quite different. Because strength of carbonate rock is greater than that of sandstone, the change of elastic modulus is the maximum level of carbonate rock. The influence of principal stress is greater than that of sandstone, and the influence of minimum principal stress of sandstone is greater than that of carbonate rock. In carbonate rock, the effect of elastic modulus on the minimum horizontal principal stress is greater than the maximum horizontal principal stress, while in sandstone it is the opposite.

A contribution to the knowledge of cavernicolous ground beetles from Sichuan Province, southwestern China (Coleoptera, Carabidae, Trechini, Platynini)

Two new genera and four new species of cave-adapted ground beetles are described from Sichuan Province, southwestern China. Uenoaphaenopsgen. nov. is established to place the trechine species Qianotrechus fani Uéno, 2003 occurring in the limestone cave Hua’er Dong, southeastern Sichuan (Luzhou: Gulin). Chu pheggomisetoidesgen. nov. & sp. nov., from the limestone cave Hanwang Dong, northeastern Sichuan (Guangyuan: Chaotian), is somewhat like the European cavernicolous trechine genus Pheggomisetes Knirsch, 1923, from Bulgaria and Serbia, in particular in the configurations of head and pronotum. Boreaphaenops liyuanisp. nov., also from Hanwang Dong, is the second representative of the genus and the first record in Sichuan. Agonotrechus sinotroglophilus Deuve, 1999, a troglophile, is reported from Sichuan for the first time. The other two new species belong to the platynine genus Jujiroa Uéno, 1952: J. uenoisp. nov. from the cave Banche Dong on the northern side of the Dadu River (Leshan: Shawan) and J. wangzhenisp. nov. from the cave Hua’er Dong, which is sympatric with Uenoaphaenops fani (Uéno, 2003) comb. nov. A distribution map for the localities of all abovementioned caves and a key to Jujiroa species known in Sichuan are provided.

Origin and paleoenvironment of organic matter in the Wufeng–Longmaxi shales in the northeastern Sichuan basin

The formation environment and preservation conditions of sedimentary organic matter (OM) play an important role in the accumulation of shale gas. In the present study, inorganic and organic geochemical data were analyzed to determine the origin and preservation environment of sedimentary OM in the Wc-1 well of the Wufeng–Longmaxi (WF–LMX) Formation in northeastern Chongqing, China. In a biomarkers analysis, the numerical characteristics of n-alkanes ( n-C17/ n-C31>4.0), tricyclic terpenes (C23TT/C30H>1.0), and steranes (C27/C29St>1.0) suggested that the main origin of OM in the black shale was planktonic algae. High values of P/Ti and BaXS in the paleoproductivity indices suggested that primary productivity in the WF–LMX Formation was relatively high, peaking in the lower LMX Formation. Relative enrichment in U, V, and Mo, and the changing trends in V/(V+Ni) and Ni/Co suggested that the redox conditions for the bottom water, which changed from the WF Formation to the lower and upper LMX Formation, were oxic/dysoxic to anoxic and dysoxic, respectively. The relationship between total organic carbon and the above indexes indicates that different key factors controlled OM enrichment in the WF–LMX Formation. In the WF Formation, oxic bottom water was not conducive to the preservation of sedimentary OM. In the lower LMX Formation, the highest paleoproductivity and anoxic bottom water conditions promoted the enrichment and preservation of sedimentary OM. In the upper LMX Formation, excessive terrigenous inputs and relatively low paleoproductivity limited the enrichment of sedimentary OM.

Performance of free gases during the recovery enhancement of shale gas by CO2 injection: a case study on the depleted Wufeng–Longmaxi shale in northeastern Sichuan Basin, China

In this work, a novel thermal–hydraulic–mechanical (THM) coupling model is developed, where the real geological parameters of the reservoir properties are embedded. Accordingly, nine schemes of CO2 injection well (IW) and CH4 production well (PW) are established, aiming to explore the behavior of free gases after CO2 is injected into the depleted Wufeng–Longmaxi shale. The results indicate the free CH4 or CO2 content in the shale fractures/matrix is invariably heterogeneous. The CO2 involvement facilitates the ratio of free CH4/CO2 in the matrix to that in the fractures declines and tends to be stable with time. Different combinations of IW–PWs induce a difference in the ratio of the free CH4 to the free CO2, in the ratio of the free CH4/CO2 in the matrix to that in the fractures, in the content of the recovered free CH4, and in the content of the trapped free CO2. Basically, when the IW locates at the bottom Wufeng–Longmaxi shale, a farther IW–PWs distance allows more CO2 in the free phase to be trapped; furthermore, no matter where the IW is, a shorter IW–PWs distance benefits by getting more CH4 in the free phase recovered from the depleted Wufeng–Longmaxi shale. Hopefully, this work is helpful in gaining knowledge about the shale-based CO2 injection technique.