At present, the understanding of the sedimentary paleoenvironment and organic matter enrichment law of black shale in the Wufeng-Longmaxi Formation is still insufficient. This time, we investigated the total organic carbon (TOC) content, X-ray diffraction mineralogical composition, and the major and trace element abundances of the newly recovered cores of three shale gas exploration wells in Changning, Qianjiang, and Xiushan areas which were, respectively, selected to discuss and compare the varying paleoenvironmental conditions and the factors that control organic matter accumulation, such as terrigenous input, redox conditions, primary productivity, and the degree of water retention in the basin. The results show that black siliceous shale lithofacies, gray-black shale lithofacies, gray-black silty shale lithofacies, gray argillaceous siltstone lithofacies, and gray shell marl lithofacies are mainly deposited in the Wufeng-Longmaxi Formation in the southeastern Sichuan Basin. Organic-rich black shale is mainly concentrated in the lower part of the Wufeng Formation and Longmaxi Formation. The continuous thickness of organic carbon content greater than 2% in the Changning area is 32 m, the continuous thickness in the Qianjiang area is 22 m, and the continuous thickness in the Xiushan area is 11 m. The hydrocarbon generation potential gradually weakens from west to east. The identification of the geochemical behavior characteristics of major and trace elements shows that enrichment of the organic matter black shale section in the Changning-Qianjiang-Xiushan areas is mainly formation in the anoxic-moderate water mass restriction environment with high primary productivity and less terrigenous detritus input under the low hydrodynamic conditions. However, the correlations of terrigenous detrital proxies Ti/Al, redox proxies V/Cr, Ni/Co, and paleoproductivity proxies Babio with the TOC contents show that the Changning area is a favorable area for the formation of organic-rich black shale and the Qianjiang area is affected by the Wuling underwater uplift; the thickness of the black shale deposits becomes thinner; the Xiushan area is adjacent to the Xuefeng uplift, and the deposition conditions of the accumulation of organic matter black shale deteriorate, which is unfavorable for shale gas exploration.
The shale gas exploration in Changning, Weiyuan, Fuling Jiaoshiba, and other areas has achieved great success in the Sichuan Basin, and the basin margin of the Sichuan Basin will be an important target in the future. In view of the lack of understanding of shale gas reservoir characteristics of the basin margin area of the Sichuan Basin, this paper conducts a study on reservoir characteristics and gas-bearing properties of this area using qualitative and quantitative techniques. The results show that the thickness of the black shale reservoir in the Qianjiang area is up to 33 m, the kerogen sapropelic type (I) is dominant, and Ro ranges from 2.2 wt% to 3.39 wt%. The mineral composition is relatively complex, mainly composed of quartz and clay minerals. Quartz content is in the range between 9.4 wt% and 79 wt% ( average = 47.48 wt % ), while clay content is in the range between 10.02 wt% and 43.6 wt% ( average = 29.41 wt % ). There are various types of reservoir spaces, mainly including organic pores dominated by matrix pores, intergranular and intragranular pores, intergranular and intragranular dissolution pores, secondary dissolution pores, and microfractures, among which organic pores are the main pore types. The pore volume of nanopores is 0.009897-0.017177 cm3/g ( average = 0.01304 c m 3 / g ), and the specific surface area ranges from 12.1 m2/g to 35.1 m2/g ( average = 20.47 m 2 / g ). Micropores and mesopores are the main pore sizes for the development of organic pores. Quartz particles promote the development of reservoir space, while clay minerals are not conducive to the development of pores. The total gas content ranges from 0.5 m3/t to 4.43 m3/t ( average = 1.69 m 3 / t ). Its enrichment is controlled by many factors. The increase of organic matter abundance is the material basis for shale gas enrichment. The development of organic pores provides high-quality storage space for shale gas preservation. The development of different mineral components restricts the enrichment of shale gas.
Considering that karst caves, underground rivers and dissolution fractures in shallow carbonate formation in Sichuan Basin are extremely developed, leakage, failure and plugging difficulties are easy to occur in the drilling process. The TDEM was used to carry out the exploration of hidden karst geological bodies in well QM2, and the quasi-three-dimensional inversion based on lateral constrain was used to invert the TDEM data. Three NW trending anomalous bands were identified in the lower Triassic Jialingjiang formation within the range of drilling, consisting of seven relatively low resistivity anomalous zones. Under the guidance of TDEM quasi-three-dimensional inversion resistivity data, the low-resistivity karst development area is avoided and the specific drilling location of well QM2 is determined. No karst cave and underground river were drilled in the later drilling process of well QM2, as well as no instability phenomenon occurred. It indicates that the TDEM detection results are consistent with the actual drilling, and the quasi-three-dimensional TDEM inversion interpretation data based on lateral constraints is reliable, and can accurately detect the buried karst in the well site.
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