Study on development mechanism and variability of strike-slip fault-controlled reservoirs regulated by multi-stage structural stress: a case study of the Shunbei area, Tarim Basin

Abstract

The Shunbei oil-and-gas field in the Tarim Basin features a typical strike-slip fault-controlled fractured-vuggy reservoir. The formation of these reservoirs is primarily influenced by fracturing related to structural stress during periods of fault activity. This contrasts with reservoir types such as matrix vugs controlled by original sedimentary facies and caves modified by karst. To study the development mechanism and distribution patterns of strike-slip fault-controlled reservoirs under the influence of multi-stage structural stress, comprehensive analysis of field observations, core samples, well logging data, seismic surveys, and drilling dynamic data was conducted to characterize the development characteristics of fault-controlled reservoirs. This included different strike-slip faults, different parts along a single strike-slip fault, and different stratigraphic levels vertically. Combined with stress field numerical modeling, multi-stage structural stress recovery was carried out to predict the main development periods and distribution patterns of fault-controlled reservoirs. Significant variations in internal stress states were observed across different segments during periods of strike-slip fault activity. Tensile stress predominated in pull-apart segments, resulting in predominantly tensile fractures, whereas compressional stress state in push-up segments led to a variety of fracture types. The fault-controlled fractures in the top of the Yijianfang Formation in the Shunbei area were mainly developed during episode Ⅲ of the Middle Caledonian period and the Late Caledonian-Early Hercynian period. Few fractures were developed during the Middle to Late Hercynian period and thereafter. Compared to the Shunbei No.1 fault, the Shunbei No.18 fault exhibited higher fracture opening degree and density with a large displacement. Strike-slip fault-controlled reservoirs exhibit a cluster-like structure. Structural differences of the reservoirs are influenced by internal stress states in different segments during fault activity. Pull-apart segments typically feature a large fault core-damage zone with more cavities and 'double-cluster' structures, whereas push-up segments display more diverse fault core-damage zones with greater separability and 'multi-cluster' structures. The stress intensity during the strike-slip fault activity controls the types and scale of reservoir spaces, with smaller fault zones dominated by fractures and larger fault zones developing extensive fault core-damage zone architectures. The scale of fault-controlled reservoirs is positively correlated with fault activity intensity. Early fault activity promotes significant fracture development, while later stages, characterized by increased burial depths, result in few newly derived fractures due to reduced rock susceptibility to fracturing.

Keywords

• stress state
• multi-stage tectonic stress
• segmentation
• fault-controlled fractured-vuggy reservoir
• shunbei area
• tarim basin