塔里木溢流玄武岩火山通道的三维结构及其热成因气体释放

塔里木上奥陶统-志留系沉积地层中广泛发育早二叠世溢流玄武岩的火山通道相岩床-岩墙网络。三维地震数据解释结果显示,这些火山通道以平行围岩地层的岩床和斜切围岩地层的"碟状岩床"为主要特征。在玄武岩喷发过程中,火山通道岩浆的热量可以导致沉积围岩发生热接触变质并将沉积围岩中的有机质转化为"热成因气体"。在塔北英买2井区火山通道烘烤沉积围岩模型基础上,利用有限元热模拟方法确定了该区早二叠世玄武岩喷发时火山通道热烘烤影响范围随时间的变化。基于沉积围岩有机质丰度估算,该区热烘烤成因甲烷释放量可达11.3Gt(即113亿吨)。如果整个塔里木溢流玄武岩省具有与英买2地区相同的释放强度,则塔里木溢流玄武岩省活动期间释放的甲烷总量可达7062.5Gt,必然导致非常显著的环境效应。同时,玄武岩火山通道岩浆引起的热接触变质作用对已存在的油藏具有明显的破坏作用,塔里木盆地古生界总量约8~10Gt的油藏破坏和大量沥青的形成可能与此有关。

Volcanic conduits of the Tarim Flood Basalt Province: 3D structure and thermogenic gas release

The Early Permian sill-dyke networks as volcanic conduit facies of flood basalt were widely developed in the sequences of Upper Ordovician to Silurian systems in the Tarim Basin, NW China. The 3D seismological interpretation revealed their structures of the volcanic conduits mainly included horizontal sills and "saucer-shaped sills" parallel to their host rocks. During the eruption of basalt lava, the heat released from the volcanic conduits was able to transform the organic materials in the sedimentary wall rocks to "thermogenic gas" by contact metamorphism. By using finite element modeling, we modeled the range influenced by contact metamorphism and calculated the release intensity of methane corresponding to the organic abundance of the sedimentary wall rocks, which is 11.3Gt. If all the Tarim flood basalts have methane release capability similar to Yingmai-2 area, the total thermogenic methane released during the eruption of the basalts would be 7062.5Gt. Such a huge methane release was able to trigger remarkable environmental effects. Contact metamorphism induced by basalt volcanic conduits can also destroy the existing petroleum reservoirs. The reported 8~10Gt of oil destruction and widely-spread bitumen in the Paleozoic reservoirs were the results of the volcanic conduit heating.