Simulation of paleotectonic stress field of Silurian Longmaxi shale reservoirs in Wulong area of southeastern Chongqing

Pang Yizhen; Chen Kongquan; Zhang Peixian; He Guisong; Tang Jiguang; Zhang Douzhong; Gao Lingyu; Yan Chunming; Wang Yanfeng

doi:10.11939/jass.20230137

Abstract：The development of the shale reservoir within the Silurian Longmaxi Formation in the Wulong area of southeastern Chongqing is significantly affected by the superposition of mul-tiple tectonic processes.This has led to intricate structural deformations in the region,making it imperative to analyze the principal controlling elements for shale gas preservation.Given the shale reservoir's characteristic low porosity and low permeability,tectonic fractures play a pivotal role as the main controlling factor in the migration and accumulation of shale gas.The paleotectonic stress field determines the development of these tectonic fractures,augmenting the reservoir's storage capacity and enhancing the connectivity among fractures.Consequently,this has a profound impact on the exploration and development of shale gas in the Wulong area.Thus,analyzing the sequence of tectonic evolution,deciphering the direction of the paleo-tectonic stress field,and quantitatively characterizing the features of the tectonic stress field hold substantial theoretical and practical significance for shale gas exploration and exploitation in this area. In this research paper,comprehensive field measurements were carried out on the data of conjugate shear joints in the Wulong area and its adjacent periphery.These data were then sys-tematically classified into distinct stages and carefully matched.By leveraging the trend and superposition relationships of fold axes both within the study area and its surrounding regions,the direction of the paleo tectonic stress was determined.Through the integration of geochronological data and the outcomes of rock acoustic emission experiments,the tectonic evolution process of the Wulong area was accurately reconstructed.This enabled the determina-tion of the tectonic evolution periods and the corresponding stress magnitudes for each period,while also clarifying the key tectonic deformation period and the fracture distribution traits of the study area.Employing finite element software ANSYS,a three-dimensional geological model was established,specifically focusing on the key tectonic deformation period of the Silurian Longmaxi Formation in the Wulong area.In light of the distribution patterns of rock mechanical parameters in the study area,the mechanical units were appropriately divided,and a corresponding mechanical model was constructed.After multiple rounds of simulation correc-tions,the optimal boundary stress application conditions for the model were precisely ascer-tained by analyzing the distinct characteristics of the paleotectonic stress field.Subsequently,numerical simulations of the paleotectonic stress field were executed to elucidate the structural stress characteristics during the key tectonic deformation periods. The research findings reveal that since the Middle Yanshanian,the Wulong area has mainly experienced two phases of structural stress.These phases are characterized by stress dir-ections in the SE-NW orientation and close to the EW direction,with the tectonic stress mani-festing as compressive.During the Middle Yanshanian,under the influence of the progressive expansion of the Xuefeng intracontinental orogenic movement from the SE-NW direction,a series of NE-trending faults and associated folds emerged in the Wulong area and its neighbor-ing areas.These formations coalesced to form the primary structural pattern of the Wulong area,marking the principal structural deformation period.This pattern has since become the dominant tectonic configuration in the region.However,during the early stage of Middle Yan-shanian,the stress environment generated by the Xuefeng intracontinental orogeny underwent a transformation in the study area.The stress changed from the SE-NW compressive state to a nearly EW compressive state.This led to the formation of near SN-trending faults and related folds in the area,which superimposed upon the pre-existing structural morphology.In the pro-cess,L-shaped and T-shaped fold structures materialized in the study area and its environs.Although the tectonic activity during this late stage of Middle Yanshanian did not succeed in changing the main NE-trending tectonic pattern in the Wulong area. Regarding the stress magnitudes,in the middle stage of Middle Yanshanian,the maxim-um horizontal principal stress ranged from 112 MPa to 194 MPa,while the minimum horizont-al principal stress ranged from 60.93 MPa to 147.99 MPa.Notably,the high values of the maximum principal stress were concentrated in the core of the western Wulong syncline.In the late stage of Middle Yanshanian,the maximum horizontal principal stress fluctuated from 75.67 MPa to 168.32 MPa,and the minimum horizontal principal stress varied from 31.19 MPa to 95.56 MPa.Significantly,the stress value of the western Wulong syncline surpassed that of the eastern syncline.It is also worth noting that there exists a discernible correlation between the stress distribution characteristics and the burial depth.The stress contours were distributed in a ring-like fashion,with the maximum stress value at the central region progressively diminish-ing outward.Additionally,the stress gradient of the low-value zone exhibited a close relation-ship with fault intensity.Specifically,the greater the fault intensity,the steeper the stress gradi-ent of the maximum principal stress.At the endpoints and inflection points of the faults,the high stress regions emerged,where stress concentration occurred.Moreover the stress on the hanging wall of the fault was higher than that on the footwall.As a result,the degree of frac-ture development on the hanging wall of the fault was more pronounced than that on the foot-wall.Consequently,the favorable preservation area for shale gas was located farther from the hanging wall of the fault compared to the footwall.Moreover,stress concentration at the end-points and inflection points of the faults led to the relatively developed structural fractures that were easily interconnected,facilitating the escape of shale gas.Hence,these areas were unfa-vorable for shale gas preservation.Furthermore,the deformation degree of the western Wulong syncline was more pronounced than that of the eastern Wulong syncline.The greater the de-formation degree of the syncline,the larger the curvature of its core,making it more suscept-ible to the formation of'Λ'-type fractures.These fractures were conducive to the storage and preservation of shale gas.Therefore,the favorable preservation area for shale gas in the west-ern Wulong syncline was superior to that in the eastern Wulong syncline.Studying the charac-teristics of the paleotectonic stress field lays the foundation for exploring the development de-gree of tectonic fractures and evaluating the effects of deep fracturing,and it is indeed a critical factor influencing high yields.