Study On The Seismicity And Source Characteristics In The Weiyuan And Rongxian Region Of Sichuan Basin

Hydraulic fracturing technology,as one of stimulation technology,is currently used in shale gas production to increase the porosity and permeability of production block.The final result is that recoverable oil and gas production will increase.The process of injecting fluid into the ground at a high pressure will cause some microseismic activities or even larger magnitude earthquakes.As an important petroliferous basin in China,Sichuan Basin is rich in shale gas resources.Thus,it is one of the important shale gas production areas in China.The research area of this paper is the Weiyuan and Rongxian region(29~30°N,104~105°E).It is located in Weiyuan shale gas production demonstration area in Sichuan Basin,which has a low level of historical seismicity.With the large-scale and commercial exploitation of shale gas in the region since 2018,a large number of earthquakes have occurred,and even two earthquakes above magnitude 5 happened.Therefore,it is very urgent and significant for us to study the seismicity and source parameters in the region.The research period in this paper is from 2018 to 2020,after the beginning of shale gas large-scale exploitation in the study area.In this paper,our research content includes:First of all,this paper adopts the double difference hypocenter locations which introducing relative arrival time difference limitation in order to improve the accuracy of position and analysis of seismic activity in the study area.After relocating of 11245 earthquakes,we can conclude that the earthquakes in the study area distribute at a very low depth,and concentrate in depth between 2 and 6 km.The phenomenon of earthquake clustering is obvious in space and time,and the distribution of earthquakes can be divided into the East and West earthquake clusters.The East cluster mainly orients in NE and NNE direction,and the earthquakes distribution in the West cluster is complex.We calculate the hydraulic diffusion coefficient of several different stages in the East and West earthquake clusters using the linear diffusion formula.It is speculated that the seismicity in this area may be triggered by fluid diffusion.After earthquake relocation,we calculate the b value of the Weiyuan and Rongxian region using the maximum likelihood method.Firstly,the aftershocks in the study area are removed by using the Reasonberg method to ensure the accuracy and stability of b value.The results show that:(1)After spatial scanning,the b value in the study area varies from 0.6 to 1.9 and concentrates in the range of 0.6 to 1.4;(2)On the time scale,relatively large earthquakes in the study area corresponds to low b value or a downward trend of b value;(3)On the spatial scale,the low b value areas mainly distribute in the south wing of the Weiyuan anticline on the Molin fault,and the zone with the conjugate strike of Molin fault orienting in nearly NNE direction.These areas may be in the state of stress concentration,and there is high possibility of massive earthquake could occur in the future.The high b value areas mainly distribute in the east of the study area.In this paper,We use the empirical Green's function method to calculate the source parameters in the Weiyuan and Rongxian region: corner frequency fc and zero frequency limit ?0 and finally calculate the the stress drop ??.We can draw the conclusions as follows:(1)Seismic moment M0 ranges from 1012 to 1015 N·m;(2)The corner frequency fc of different magnitude varies greatly,and the upper limit decreases with the increase of magnitude;(3)The focal radius is between 160 and 980 m;(4)The stress drop ?? ranges from 0.006 to 17 MPa.The average stress drop ?? is 2.5MPa,the median number is 0.66 MPa,which is consistent with the average stress drop ?? of medium and small earthquakes in China(0.1~10 MPa).The stress drop increases with increasing magnitude and seismic moment.On the depth section,the stress drop ?? does not increase with the depth,and the larger stress drop events distribute slightly deeper than the fracturing depth.The distribution of stress drop is uneven in space.In the west of study area,the high stress drop events distribute in the south of Molin fault,the interior of Weiyuan anticline and the zone with the conjugate strike of Molin fault orienting in nearly NE direction.It is speculated that there may be a unknown fault on the southern end of the Molin fault,where the b value is at a low level.In the east of study area,the high stress drop areas corresponds to the high b value.In time scale,the stress drop increase after a large earthquake.Based on researches all above,the following conclusions can be drawn:(1)earthquakes in the study area distribute at a very low depth.It is obvious to see earthquake clustering phenomenon in the study area.(2)The seismicity in the Weiyuan and Rongxian region may be dominated by both fluid and fault reactivation.The low b value and high stress drop region on the Molin fault and areas conjugated with the southern end of the Molin fault are still worthy of further attention in the future exploitation.