Interpretation On The Nature Of Induced Fractures

Underground fractures has long been recognized to be closely related to regional tectonics,earthquake mechanism and geological resources such as oil,gas and geothermal resources.However,many fractures are undiscernible on seismic reflection profiles and logging data,and their mechanical and kinematic properties remain unstudied.One of the most important ways to study underground fractures is to carry out scientific drilling activities,obtained cores provide a way for in-situ observation and measurement of characteristics of deep fractures.It can accurately obtain the geometric,kinematic and geochemical characteristics of the fracture,so as to avoid multi-solution caused by logging data and supplement seismic data.It can also eliminate fractures caused by later drilling process and correct the simulation results of underground fractures.SK-2 is located in Xujiaweizi fault depression zone in the southeast faulted zone of Songliao Basin.Because of continuous coring,it has high adoption rate,large aperture and deep drilling design depth,the drilling core data of SK-2 provides a better knowledge of fracture mechanism and tectonic stress field in the studied area.In this paper,we take the core of SK-2 as the research object.In combination with polarizing microscope,scanning electron microscope,transmission electron microscope and other instruments,the samples collected by the system are analyzed with the principle of local to whole,dynamic to static,macro to microcosmic and then to nanoscale.Based on the study of regional tectonic setting and previous studies on hydrocarbon accumulation patterns and distribution patterns,the characteristics of major faults in the Songliao Basin,the possible mechanism of fracture and the problem of tectonic evolution and debate in the whole region are supplemented and restricted.The main conclusions are as follows:(1)The induced fractures(including:core disking fracture,petal fracture,etc.)in core are mostly developed in mudstone;(2)Subseismic faults are mostly normal faults indicating multistage movements,which is coincide with regional tectonic events.Combined with gas logging results,we suggest subseismic faults play a significant role in controlling gas migration in the region;(3)Hydrothermal fractures are filled with veins and hydrothermal breccia.