川中古隆起超压分布与形成的地温场因素

The effect of temperature on the overpressure distribution and formation in the Central Paleo-Uplift of the Sichuan Basin

温度和压力是沉积盆地两个重要的物理场,温度影响着超压的形成和分布.本文根据钻孔实测温度和压力数据分析了川中古隆起现今压力与温度的关系;在实验室对封闭流体进行了多组温-压关系实验;利用等效镜质体反射率和包裹体测温数据恢复了川中古隆起不同井区在白垩纪抬升之前的最大古地温,并在此基础上分析了温度降低对研究区超压的影响;最后探讨了生烃增压和欠压实超压形成过程中温度的作用.研究结果表明,川中古隆起现今超压层的压力系数与温度呈正相关关系;在绝对密封的条件下,当压力大于15 MPa时,温度每变化1℃,压力变化1.076 MPa.川中地区不同井区自晚白垩世以来的差异性降温是现今同一超压层系超压强度不同的主要因素,此外超压层还应发生了流体的横向压力传递和泄漏.下古生界原油裂解形成超压的时间是180～110 Ma;气态烃伴生的盐水包裹体均一温度暗示了在90 Ma超压发生调整.盆地模拟结果显示温度对上三叠统须家河组的欠压实增压影响微弱.

Temperature and pore pressure are two important physical fields in sedimentary basins.Some potential mechanisms of overpressure,such as aquathermal expansion,diagenesis and hydrocarbon generation,are related to temperature.Furthermore,temperature may control evolution and distribution characteristics of overpressure. The Sichuan Basin,located in southwest China,is a typical overpressuring basin.Combining physical simulation results with practical geological condition,the impact of temperature on overpressures in the Central PaleoUplift of the Sichuan Basin is analyzed.The measured temperature and pore pressure data,which were obtained from boreholes in different gas fields,were collected to analyze the present relationship between temperature andpressure.Physical simulation experiments were carried out in laboratory to study the temperaturepressure relationship in absolutely sealed condition.The Easy%Ro model for vitrinite reflectance and micro-thermometry of fluid inclusions were applied to reconstruct the maximum paleotemperatures of various formations for different regions in the Central Paleo-Uplift.Based on the thermal history,the impacts of temperature on overpressures generation through oil cracking and disequilibrium compaction were discussed.Multiple overpressure systems exist vertically in the Central Paleo-Uplift in the Sichuan Basin and the main mechanisms for each overpressure system are different.According to petrophysical properties of overpressuring formations,the Upper Triassic overpressure is mainly generated by disequilibrium compaction and the Cambrian overpressure is mainly caused by gas generation,respectively.For the same overpressuring formation,overpressure is positively correlated with temperature in the lateral direction.The pressure-temperature gradient is1.075 MPa/℃for the Cambrian overpressure system and 1.24 MPa/℃ for the Upper Triassic overpressure system.Physical simulation experiment results show that fluid pressure is closely related to temperature in absolutely sealed condition.The pressure-temperature gradient is relatively small in low pressure phase and such relationship is almost linear as pressure is higher than 15 MPa,with gradient value about 1.076 MPa/℃.Formations in the Sichuan Basin have experienced high temperature and the values of Rofor the Cambrian Formation in the Central Paleo-Uplift are higher than 3%.Maximum temperatures reconstructed by Roand fluid inclusions indicate that,before the Later Cretaceous uplift,the Cambrian Formation was 225 ℃in MoxiGaoshiti area and 208 ℃in Weiyuan area and the Upper Triassic Formation was 158 ℃in MoxiGaoshiti area and 148 ℃in Bajiaochang area,respectively.Seals in the Sichuan Basin have very low porosity and permeability because of lithological character and intense compaction.Therefore,the overpressure systems could be deemed absolutely sealed.Combining the physical simulation experiments with temperature decrease since the Late Cretaceous,the pressure of the Cambrian Formation decreased 121.6MPa in Weiyuan area and 91.5MPa in Moxi-Gaoshiti area,and the pressure of the Upper Triassic Formation decreased 48 MPa in Bajiaochang area and 79 MPa in Moxi-Gaoshiti area.Maturity evolution of organic matter and hydrocarbon generation are mainly controlled by temperature.Oil cracking in the Cambrian reservoirs was mainly occurred during 180～110 Ma,and adjusted in 90 Ma.In this period,the Cambrian overpressure formed gradually.Based on basin modeling,the effect of temperature on disequilibrium compaction overpressure can be negligible.However,the Upper Triassic overpressure must also reach the maximum in 90 Ma,because of the deepest burial depth reached in this period.Through this study,we can obtain the following conclusions：（1）Multiple overpressure systems caused by different mechanisms are developed in the Central Paleo-Uplift in the Sichuan Basin and positive correlations between pressure and temperature exist in each pressure systems.（2）When pressure is greater than 15 MPa,it would change 1.076 MPa for a temperature change of 1 ℃in an absolutely sealed condition.The difference in temperature reduction can be regarded as the primary reason for various intensity of pressure within the Central Paleo-Uplift.Besides that,some degree of lateral transfer and leakage of pressure must occur.（3）Controlled by temperature,the Lower Paleozoic overpressure caused by oil cracking formed during 180～110 Ma and redistributed in 90 Ma.However,the effect of temperature on the Upper Triassic disequilibrium compaction overpressure generation is negligible.