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.

中文翻译：

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方解石 U[sbnd]Pb 测年和地球化学对富有机页岩裂缝张开的影响


含气、富含有机物的页岩通常有许多张开模式裂缝;然而，它们的形成机制仍然存在争议，关于构造起源和/或碳氢化合物生成加压起源的争论不休。本研究以川南泸州地区下志留统隆马溪组页岩储层为研究对象。使用原位 UPb 地质年代学、稀土元素 （REE） 和 C-O-Sr 同位素地球化学以及流体包裹体分析，我们研究了裂缝填充的时间和地球化学属性，并确定了裂缝形成的机制。结果表明：Longmaxi组页岩中占据裂缝的胶结物主要含有方解石和石英。方解石胶结物显示出裂缝密封和纤维质地，表明它们是同步运动矿物矿床。方解石胶结物的 87Sr/86Sr 值与其近端主页岩的 87Sr/86Sr 值基本重叠。这一结果，再加上 δ13CPDB 的轻微消耗和相对均匀的流体 δ18OSMOW 同位素特征，表明方解石沉淀的流体主要来自周围的主页岩。裂缝胶结中存在丰富的甲烷包裹体，捕集压力为 104.5-157.5 MPa，压力系数为 1.92-2.43，表明它们被困在超压流体系统中。方解石胶结物的原位 UPb 测年得出的年龄约为 160 马 和约 110 马，这与埋藏过程中油气热裂解的时间相吻合。结合超压、地球化学封闭的流体系统，裂缝很可能是由气体产生引发的。 我们的研究强调，由碳氢化合物生成超压引起的自然压裂是全球构造静止盆地脆性破坏的重要模式。