Thermal Alteration Of Diabase Sill Intrusion On Hydrocarbon Generation Of Its Neighboring Shale

Igneous intrusions can not only increase the thermal maturity of the surrounding shale,but also destroy the hydrocarbon generation potential for shale near the contact zone.It is significant for hydrocarbon generation evaluation and favorable area selection of shale gas in the igneous rock-developed areas to reveal the thermal effect of diabase intrusion on the neighbouring shale.In this paper,the diabase sills intruded into the Xiahuayuan Formation and the Xiamaling Formation,as well as their surrounding shales are taken as the research objects.Based on the experiments of Zircon U-Pb dating,element geochemistry and organic geochemistry,the diabase intrusion mechanism is discussed,and the estimating model of the influence of diabase bed on shale hydrocarbon generation ability was established.By using two-dimensional finite element numerical simulation,the thermal effect of the diabase sill on the gound temperature,thermal maturity and shale gas accumulation of the host rock are analyzed.The study of the intrusive mechanism shows that the two sets of diabase were formed in different time and tectonic environments.The diabase in the Jurassic Xiahuayuan formation was formed in an intra-continental environment,its U-Pb zircon age is 159±0.5 1Ma,marking the beginning of the main episode of Yanshan movement in the western part of Yanshan region.The diabase in the Mesoproterozoic Xiamaling Formation was formed in an island arc-related environment.Its intrusion is closely related to the back arc extension.The diabase intrusion will affect the thermal maturity and hydrocarbon generation of the neighbouring shales in different level.The Xiahuayuan diabase sill in ZY1 well is about 221m and can increase the thermal maturity(Ro)of shale within 68m.The Xiamaling diabase sill is about 7m and can increase the thermal maturity of shale within 8.5m.According to the remaining hydrocarbon generation capacity of shale,the surrounding rock is divided into four zones:strong,medium,weak and non thermal metamorphism zones.It is found that the range of the strong,medium,and weak thermal metamorphism zones accounts for 65%,23%and 12%of the influence range of thermal maturity(Ro).The influence range of Ro is directly related to the ground temperature field of the host rock,and is also affected by the parameters of intrusive rock and host rock.The numerical simulation results show that the surrounding rock experienced three stages after diabase intrusion:rapid temperature rise,rapid temperature drop and slow cooling.Through simulating the influence of different parameters of diabase and host rock on thermal maturity,it is shown that the thickness of diabase sill and the specific heat capacity of the host rock are the main factors affecting the maturity of organic matter.The shale gas content of the Xiahuayuan Formation in well ZY1 drilled in the Xuanhua basin indicates that the huge diabase sill significantly increases the maturity of the Xiahuayuan shales(gas-prone),which is now in the mian gas generation stage,with shale gas content of 1.3m3/t on average.However,due to the total loss of hydrocarbon generating capacity within 44m below the diabase sill,the shale gas content is extremely low in this section,only 0.07m3/t.Although it has been proved that the physical properties of shale will become better near the diabase,the hydrocarbon generation capacity is the key factor controlling the formation of shale gas reservoir near the diabase sill.