Evaluation of the effectiveness of tectonic fractures in shale reservoirs of Fengcheng Formation in Mabei area and its geological significance for oil and gas
The effectiveness of natural fractures in shale oil reservoirs not only significantly in-fluences the efficient enrichment of shale oil but also greatly affects the permeability of the res-ervoir and the formation of the fracture network.However,current research on the mechanical effectiveness of natural fractures in shale oil reservoirs is relatively limited.Based on cores and thin-sections observation,and logging,and also combined with three-dimensional laser scanning of fracture surfaces,CT scanning,and stress-sensitive flow experiments of the frac-ture-containing samples,the mechanical genesis types and developmental characteristics of the tectonic fractures in the study area in shale oil reservoirs of the Fengcheng Formation in the Mabei area were firstly presented.And thus,the main controlling factors of the mechanical ef-fectiveness of the tectonic fractures were studied,and finally the influence of different mechan-ical types of the fractures on the enrichment and high production of shale oil were confirmed.The results indicate that the tectonic fractures can be categorized into three groups:oblique shear fractures predominantly in the NNW and NNE directions with a high angle;bedding slip shear fractures without a dominant strike;and vertical tensile fractures mainly with a strike of NW direction.Among these,the oblique shear fractures show the greatest longitudinal exten-sion.The current mechanical effectiveness of tectonic fractures is primarily influenced by four factors:adjustment of in-situ stress orientation,the magnitude of normal stress at the fracture surface,fracture stress sensitivity,and the structure of lithofacies assembles.NNW and NNE oriented high-angle oblique shear fractures are influenced by the orientations of paleo-stress and present in-situ stress fields,respectively.This results in weak filling,low normal stresses on the fracture surface,and a high degree of longitudinal connectivity.Additionally,the oblique shear fractures are characterized by a low roughness of the fracture surface(Root mean square of fracture surface height hRMS=2.379 mm)and a low stress sensitivity(Kψ=0.038 MPa-1).Therefore,it offers the best mechanical effectiveness,improved accumulation and permeability,and easy shear activation after fracturing,and the formation of a complex net-work of fractures with artificial fractures.It also significantly controls high shale oil production in the later stages.In addition,the highest fracture network connectivity(Average number of connections fractures:CL=4.97,Average number of connections fractures branches:CB=1.69)was observed within the lithologic structure of the dolomitic silty shale,and the thicker mechanical layers inhibited the longitudinal connectivity of the fractures.
Mabei areaFengcheng Formationtectonic fracturesmechanical effectivenessshale oil development
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