四川盆地涪陵南地区奥陶系五峰组—志留系龙马溪组页岩古构造应力场及裂缝特征

Paleotectonic stress field and fracture characteristics of shales of Ordovician Wufeng Formation to Silurian Longmaxi Formation in southern Fuling area,Sichuan Basin

通过岩石力学实验、声发射实验和地震资料综合解释,利用有限元数值模拟方法,对涪陵南地区奥陶系五峰组—志留系龙马溪组海相页岩古构造应力场进行了数值模拟,并结合岩心实测结果,预测了构造裂缝的发育特征。研究结果表明:①涪陵南地区裂缝主要形成期(燕山晚期)构造应力和裂缝的分布受断层、岩石力学性质和构造应力的影响,断层广泛发育的区域容易出现应力集中,并引起较大的应力梯度。②利用库伦-莫尔破裂准则预测了研究区构造裂缝的发育,同时引入剪切破裂指数R定量表征裂缝发育的强度,其值越大反映裂缝发育程度越高。窄陡断背斜和断层附近裂缝发育,以高角度剪切裂缝为主,而在宽缓向斜部位裂缝发育程度最弱,研究区五峰组—龙马溪组底部硅质页岩应力集中,裂缝最发育。③涪陵南地区海相页岩气保存有利区主要为3个区带:远离大断层的凤来向斜内部,主要构造变形期未遭受大的破坏作用,其位于应力低值区,具有良好的保存条件,为Ⅰ类有利区;白马向斜内部小断层发育,应力值位于中等水平,保存条件中等,为Ⅱ类较有利区;石门—金坪断背斜遭受到强烈改造作用,容易产生大规模剪切裂缝,破坏了保存条件,为Ⅲ类不利区。

Through rock mechanics experiment,acoustic emission experiment,comprehensive interpretation of seismic data,the finite element numerical simulation method was used to numerically simulate the paleotectonic stress field of marine shales of Ordovician Wufeng Formation to Silurian Longmaxi Formation in southern Fuling area.Combined with the measured results of cores,the development characteristics of structural fractures were predicted.The results show that:(1)The distribution of tectonic stress and fractures in southern Fuling area during the main formation period of fractures(the Late Yanshanian)was influenced by faults,rock mechanical properties,and tectonic stress.In areas where faults are widely developed,stress concentration can easily occur and cause large stress gradients.(2)Coulomb-Moore fracture criterion was used to predict the development of structural fractures in the study area,and the shear fracture index R was introduced to quantitatively characterize the strength of fracture development.The larger the value,the higher the degree of fracture development.Narrow and steep fault anticlines and fractures are developed near faults,mainly with high angle shear fractures,while the development of fractures is weakest in the wide and gentle synclines.The stress in the siliceous shales at the bottom of the Wufeng Formation Longmaxi Formation in the study area is concentrated,and the fractures are the most developed.(3)There are three favorable zones for the preservation of marine shale gas in the southern Fuling area:The Fenglai syncline,which is far away from the major faults and has not suffered significant damage during the main structural deformation period,is located in the low stress area with good preservation conditions,and is classified as classⅠfavorable zone;The Baima syncline,with small faults developed,stress values at a moder‐ate level and moderate preservation conditions,is classified as classⅡfavorable zone;The Shimen-Jinping fault anti-cline,which has undergone strong transformation and is prone to large-scale shear fractures,making the preservation conditions damaged,is classified as classⅢunfavorable zone.