数值模拟研究二叠纪峨眉山玄武岩浆在四川盆地的热效应

四川盆地是我国重要的含油气盆地，其西南部位于峨眉山大火成岩省的外带，二叠纪峨眉山玄武岩浆对四川盆地热历史及烃源岩热演化的影响一直备受关注.近年来，盆地古温标结果揭示出盆地在二叠纪存在高古热流（75~85 mW·m^-2），甚至部分点位存在超高古热流（97~114 mW·m^-2），被认为和峨眉山玄武岩浆的热效应有关.为了解这些高-超高古热流的成因机制，以及溢流到地表的玄武岩浆对二叠系及以下地层和烃源岩的热影响，本文采用二维有限元方法对二叠纪峨眉山玄武岩浆的热效应进行了模拟，得出如下结论：（1）置于岩石圈底部的地幔柱头高温异常体和挤入地壳底部的高温玄武岩浆在短期内（4 Ma内）对地表热流的扰动分别小于5 mW·m^-2和20 mW·m^-2，均无法解释四川盆地二叠纪的异常古热流.（2）古热流与侵入到地壳内部的岩浆有关，中心在7~17 km深度的不同形态的岩浆都有可能造成高或超高古热流的形成，引起超高古热流的水平状岩浆囊厚度在2~10 km，表层距地表在6~12 km之间.（3）地表岩浆越厚、下伏地层越浅，岩浆对该地层产生的热扰动越大，其中烃源岩所受影响也越大.如，上覆岩浆厚度为300 m时，在深度300 m（二叠系）、800 m（奥陶系）、1250 m（寒武系）、2000 m（震旦系）地层引起的最大升温分别是241℃、77℃、40℃和19℃，所需时间分别为2100年、6100年、1.17万年和2.56万年.（4）相变热的存在对二叠系和奥陶系地层不可忽略，如300 m厚岩浆产生的相变热可以使二叠系地层额外增温达55℃.

Numerical simulation study on the thermal effect of Emeishan basaltic magma in the Sichuan Basin

Sichuan Basin is an important petroliferous basin in China, of which the southwest is located in the outer zone of the Emeishan large igneous province. The influence of Permian Emeishan basaltic magma on the thermal history of Sichuan Basin and the thermal evolution of hydrocarbon source rocks has always been of great concern. In recent years, paleothermometric results from the basin have revealed the presence of high paleo-heat flow (75~85 mW·m-2), and even ultra-high paleo-heat flow (97~114 mW·m-2) in some parts of the basin during the Permian, which are thought to be related to the thermal effect of Emeishan basaltic magma. In order to understand the genetic mechanism of the high- and ultra-high paleo-heat flow and the thermal effect of basaltic magma overflowing to the surface on hydrocarbon source rocks and the strata of Permian and below, the thermal effect of Permian Emeishan basaltic magma has been simulated by two-dimensional finite element method, and the following conclusions have been drawn: (1) The disturbance of high-temperature mantle plume head at the bottom of the lithosphere and the high-temperature basaltic magma extruded into the base of the crust disturbed the surface heat flow less than 5 mW·m-2 and 20 mW·m-2, respectively, in a short time (within 4 Ma), neither of which can explain the Permian abnormal paleo-heat flow in the Sichuan Basin. (2) The paleo-heat flow is related to the magma intruded into the interior of the crust. Magma of different forms at the depth of 7~17 km may cause the formation of high or ultra-high paleo-heat flow. The thickness of the horizontal magma chamber causing the ultra-high paleo-heat flow is 2~10 km, and the surface of the chamber is between 6~12 km from the surface. (3) The thicker the overlying magma and the shallower the underlying stratum, the greater the thermal disturbance of the magma to the stratum, and the greater the impact on the hydrocarbon source rocks. For example, when the overlying magma is 300 m thick, the maximum temperature rise caused in the strata at depths of 300 m (Permian), 800 m (Ordovician), 1250 m (Cambrian) and 2000 m (Sinian) is 241℃, 77℃, 40℃ and 19℃, respectively, and the time required is 2100, 6100, 11700 and 25600 years. (4) The existence of phase change heat cannot be ignored for the Permian and Ordovician strata, for example, the phase change heat generated by 300 m thick magma can increase the temperature of the Permian strata by up to 55℃.