Dadong Liu , Hui Tian , Chengzao Jia , Qingqing Fan , Xuesong Lu , Mingyang Xu , Yan Song , Chen Zhang
{"title":"富含有机质的页岩中方解石 UPb 定年和断裂开放的地球化学制约因素","authors":"Dadong Liu , Hui Tian , Chengzao Jia , Qingqing Fan , Xuesong Lu , Mingyang Xu , Yan Song , Chen Zhang","doi":"10.1016/j.coal.2024.104621","DOIUrl":null,"url":null,"abstract":"<div><div>Gas-bearing, organic-rich shales commonly host numerous opening-mode fractures; however, their formation mechanism remains controversial, with competing arguments of tectonic-origin and/or hydrocarbon generation pressurization-origin. Here, we studied fracture fillings in shale reservoirs of the lower Silurian Longmaxi Formation in the Luzhou area, southern Sichuan Basin, SW China. Using in-situ U<img>Pb geochronology, rare earth elements (REEs) and C-O-Sr isotope geochemistry, and fluid inclusion analyses, we investigated the timing and geochemical attributions of fracture fills and identify the mechanism of fracture formation. The results show that, the cements that occupy fractures in the Longmaxi Formation shales contain mainly calcite and quartz. The calcite cements show crack-seal and fibrous textures, indicating that they are <em>syn</em>-kinematic mineral deposits. The <sup>87</sup>Sr/<sup>86</sup>Sr values of the calcite cements essentially overlap with those of their proximal host shales. This result, combined with slight depletions in δ<sup>13</sup>C<sub>PDB</sub> and relatively uniform fluid δ<sup>18</sup>O<sub>SMOW</sub> isotopic features, indicate that the fluids from which the calcite precipitated were largely derived from their surrounding host shales. Abundant methane inclusions are present in fracture cements, with trapping pressures of 104.5–157.5 MPa and pressure coefficients of 1.92–2.43, suggesting they were trapped in an overpressurized fluid system. In-situ U<img>Pb dating of calcite cements yielded ages of ca. 160 Ma and ca. 110 Ma, which coincide with the timing of thermal cracking of oil to gas during burial. In combination with the overpressurized, geochemically closed fluid system, the fractures were most likely triggered by gas generation. Our study emphasizes that natural fracturing induced by hydrocarbon generation overpressurization is an essential mode of brittle failure in tectonically quiescent basins worldwide.</div></div>","PeriodicalId":13864,"journal":{"name":"International Journal of Coal Geology","volume":"294 ","pages":"Article 104621"},"PeriodicalIF":5.6000,"publicationDate":"2024-10-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":"{\"title\":\"Calcite UPb dating and geochemical constraints on fracture opening in organic-rich shales\",\"authors\":\"Dadong Liu , Hui Tian , Chengzao Jia , Qingqing Fan , Xuesong Lu , Mingyang Xu , Yan Song , Chen Zhang\",\"doi\":\"10.1016/j.coal.2024.104621\",\"DOIUrl\":null,\"url\":null,\"abstract\":\"<div><div>Gas-bearing, organic-rich shales commonly host numerous opening-mode fractures; however, their formation mechanism remains controversial, with competing arguments of tectonic-origin and/or hydrocarbon generation pressurization-origin. Here, we studied fracture fillings in shale reservoirs of the lower Silurian Longmaxi Formation in the Luzhou area, southern Sichuan Basin, SW China. Using in-situ U<img>Pb geochronology, rare earth elements (REEs) and C-O-Sr isotope geochemistry, and fluid inclusion analyses, we investigated the timing and geochemical attributions of fracture fills and identify the mechanism of fracture formation. The results show that, the cements that occupy fractures in the Longmaxi Formation shales contain mainly calcite and quartz. The calcite cements show crack-seal and fibrous textures, indicating that they are <em>syn</em>-kinematic mineral deposits. The <sup>87</sup>Sr/<sup>86</sup>Sr values of the calcite cements essentially overlap with those of their proximal host shales. This result, combined with slight depletions in δ<sup>13</sup>C<sub>PDB</sub> and relatively uniform fluid δ<sup>18</sup>O<sub>SMOW</sub> isotopic features, indicate that the fluids from which the calcite precipitated were largely derived from their surrounding host shales. Abundant methane inclusions are present in fracture cements, with trapping pressures of 104.5–157.5 MPa and pressure coefficients of 1.92–2.43, suggesting they were trapped in an overpressurized fluid system. In-situ U<img>Pb dating of calcite cements yielded ages of ca. 160 Ma and ca. 110 Ma, which coincide with the timing of thermal cracking of oil to gas during burial. In combination with the overpressurized, geochemically closed fluid system, the fractures were most likely triggered by gas generation. Our study emphasizes that natural fracturing induced by hydrocarbon generation overpressurization is an essential mode of brittle failure in tectonically quiescent basins worldwide.</div></div>\",\"PeriodicalId\":13864,\"journal\":{\"name\":\"International Journal of Coal Geology\",\"volume\":\"294 \",\"pages\":\"Article 104621\"},\"PeriodicalIF\":5.6000,\"publicationDate\":\"2024-10-17\",\"publicationTypes\":\"Journal Article\",\"fieldsOfStudy\":null,\"isOpenAccess\":false,\"openAccessPdf\":\"\",\"citationCount\":\"0\",\"resultStr\":null,\"platform\":\"Semanticscholar\",\"paperid\":null,\"PeriodicalName\":\"International Journal of Coal Geology\",\"FirstCategoryId\":\"5\",\"ListUrlMain\":\"https://www.sciencedirect.com/science/article/pii/S0166516224001782\",\"RegionNum\":2,\"RegionCategory\":\"工程技术\",\"ArticlePicture\":[],\"TitleCN\":null,\"AbstractTextCN\":null,\"PMCID\":null,\"EPubDate\":\"\",\"PubModel\":\"\",\"JCR\":\"Q2\",\"JCRName\":\"ENERGY & FUELS\",\"Score\":null,\"Total\":0}","platform":"Semanticscholar","paperid":null,"PeriodicalName":"International Journal of Coal Geology","FirstCategoryId":"5","ListUrlMain":"https://www.sciencedirect.com/science/article/pii/S0166516224001782","RegionNum":2,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q2","JCRName":"ENERGY & FUELS","Score":null,"Total":0}
Calcite UPb dating and geochemical constraints on fracture opening in organic-rich shales
Gas-bearing, organic-rich shales commonly host numerous opening-mode fractures; however, their formation mechanism remains controversial, with competing arguments of tectonic-origin and/or hydrocarbon generation pressurization-origin. Here, we studied fracture fillings in shale reservoirs of the lower Silurian Longmaxi Formation in the Luzhou area, southern Sichuan Basin, SW China. Using in-situ UPb geochronology, rare earth elements (REEs) and C-O-Sr isotope geochemistry, and fluid inclusion analyses, we investigated the timing and geochemical attributions of fracture fills and identify the mechanism of fracture formation. The results show that, the cements that occupy fractures in the Longmaxi Formation shales contain mainly calcite and quartz. The calcite cements show crack-seal and fibrous textures, indicating that they are syn-kinematic mineral deposits. The 87Sr/86Sr values of the calcite cements essentially overlap with those of their proximal host shales. This result, combined with slight depletions in δ13CPDB and relatively uniform fluid δ18OSMOW isotopic features, indicate that the fluids from which the calcite precipitated were largely derived from their surrounding host shales. Abundant methane inclusions are present in fracture cements, with trapping pressures of 104.5–157.5 MPa and pressure coefficients of 1.92–2.43, suggesting they were trapped in an overpressurized fluid system. In-situ UPb dating of calcite cements yielded ages of ca. 160 Ma and ca. 110 Ma, which coincide with the timing of thermal cracking of oil to gas during burial. In combination with the overpressurized, geochemically closed fluid system, the fractures were most likely triggered by gas generation. Our study emphasizes that natural fracturing induced by hydrocarbon generation overpressurization is an essential mode of brittle failure in tectonically quiescent basins worldwide.
期刊介绍:
The International Journal of Coal Geology deals with fundamental and applied aspects of the geology and petrology of coal, oil/gas source rocks and shale gas resources. The journal aims to advance the exploration, exploitation and utilization of these resources, and to stimulate environmental awareness as well as advancement of engineering for effective resource management.