In-situ stress characteristics from upper Es4 to lower Es3 in Boxing Subsag

Shahejie Formation of Boxing Subsag in Dongying Sag is rich in hydrocarbon resources and holds significant exploration potential. However， the complex geological structure， rapid lateral lithological changes in lacustrine shales， strong reservoir heterogeneity， and significant burial depth pose substantial challenges. Multiple fault zones significantly affect the spatial distribution of the in-situ stress field， leading to frequent drilling issues such as collapse and fluid invasion. Additionally， during reservoir fracturing operations， the interference between wellbore fracture networks and difficulties in fracturing key intervals directly hinder hydrocarbon exploration and development of Boxing Subsag. The Upper Submember of the 4th Member of the Eocene Shahejie Formation （Es4U） to the Lower Submember of the 3rd Member of the Eocene Shahejie Formation（Es3L） （Es4U-Es3L） were taken as research objects， and mechanical experiments， logging interpretation， and seismic attribute analysis were integrated to construct a heterogeneous rock mechanics parameter model and determine the in-situ stress state of individual wells. Based on the finite element geomechanical model， the study employed elastoplastic finite element numerical simulations to characterize the in-situ stress field spatially and analyze the controlling factors and mechanisms of in-situ stress field distribution differences. Results indicate that the central-western part of Boxing Subsag has experienced prolonged structural activity， with major faults exhibiting extension and strike-slip characteristics. The overall trend of structural activity is from the south to the north and from the subsag center towards major faults. The overall in-situ stress state of the Es4U-Es3L falls into Class I but varies with depth in some areas. The maximum horizontal principal stress shows an NE-WN extension of high， low， and high-value variation. High in-situ stress zones are mainly concentrated near the Gaoqing-Pingnan fault corner， dominated by nearly EW-trending horizontal compression. The main controlling factors for in-situ stress field distribution include reservoir lithology， structural morphology， and faults. Reservoir lithology results in uneven distribution of in-situ stress due to different mechanical properties of rock， and structural morphology influences in-situ stress properties. Faults lead to chaotic in-situ stress directions， with stress concentrations at fault tips and bends.