四川盆地天宫堂地区下志留统龙马溪组天然裂缝特征及形成期次

Characteristics and formation stages of natural fractures in Lower Silurian Longmaxi Formation in Tiangongtang area of Sichuan Basin

四川盆地西南缘下志留统龙马溪组页岩气储量巨大,裂缝发育特征与形成期次对页岩气富集及产能具有重要影响。以天宫堂地区龙马溪组为研究对象,采用岩心分析、FMI成像测井、岩石声发射实验、裂缝充填物碳氧同位素测试、包裹体均一温度测试和埋藏—热演化史分析等分析技术和综合地质方法,研究页岩中天然裂缝的发育特征和形成时期。结果显示,研究区龙马溪组天然裂缝以构造成因的直立缝与低角度缝共同发育为特征,岩心裂缝具有发育程度高、延伸短、充填程度高等特征。成像测井裂缝倾角比对、岩心裂缝交切关系、裂缝充填物流体包裹体测试及岩石声发射实验等结果表明,天宫堂地区龙马溪组裂缝形成与3期构造运动有关。结合埋藏—热演化史分析可证实:第一期为NW向、NNE向平面剪切缝和NEE向剖面剪切缝,形成于燕山中期—晚期(130~62 Ma),构造应力方位为近SN向(345°±5°),包裹体均一温度为185~206℃;第二期为NE向、NWW向平面剪切缝和NNW向剖面剪切缝,形成于燕山晚期—喜马拉雅中期(62~34 Ma),构造应力方位为EW向(80°±5°),包裹体均一温度为165~184℃;第三期为近SN向、NEE向平面剪切缝和NE向剖面剪切缝,形成于喜马拉雅中期—现今(34 Ma至今),构造应力方位为近NW向(315°±5°),包裹体均一温度为125~162℃。结合地质力学背景,建立了天宫堂地区龙马溪组3期构造裂缝演化模式。

The Lower Silurian Longmaxi Formation on the southwestern margin of the Sichuan Basin holds significant shale gas reserves.The characteristics and formation stages of fractures play a crucial role in shale gas accumula-tion and productivity.Focusing on the Longmaxi Formation in the Tiangongtang area,this study employed core analysis,FMI(Formation Micro-Imager)logging,rock acoustic emission experiments,carbon-oxygen isotope analysis of fracture fillings,fluid inclusion homogenization temperature testing,and burial-thermal evolution history analysis to investigate the development characteristics and formation periods of natural fractures in the shale.The results indicate that the natural fractures in the Longmaxi Formation in the study area are characterized by the coexistence of tectonic vertical and low-angle fractures.The core fractures exhibit high development density,short extension,and high filling degree.Comparison of imaging log fracture dip angles,core fracture cross-cutting relationships,fracture filling fluid inclusion tests,and rock acoustic emission experiments suggested that the frac-tures in the Longmaxi Formation in the Tiangongtang area were associated with three tectonic events.Combined with burial-thermal evolution history analysis,the formation periods were confirmed as follows:the first stage involved NW-oriented and NNE-oriented planar shear fractures,and NEE-oriented cross-sectional shear fractures formed during the mid-late Yanshanian period(130-62 Ma)with tectonic stress orientation near SN(345°±5°)and inclusion homogenization temperatures of 185-206℃;the second stage involved NE-oriented and NW-oriented planar shear fractures,and NNW-trending cross-sectional shear fractures formed during the late Yanshanian to mid-Himalayan period(62-34 Ma)with tectonic stress orientation near EW(80°±5°)and inclusion homogeni-zation temperatures of 165-184℃;the third stage involved near SN-oriented and NEE-oriented planar shear fractures,and NE-oriented cross-sectional shear fractures formed from the mid-Himalayan period to present(34 Ma to present)with tectonic stress orientation near NW(315°±5°)and inclusion homogenization temperatures of 125-162℃.Based on the geomechanical background,a three-stage tectonic fracture evolution model for the Long-maxi Formation in the Tiangongtang area was established.