Natural tectonic fractures and their formation stages in tight reservoirs of Permian Lucaogou Formation, Jimsar Sag, southern Junggar Basin, NW China

Abstract

This study investigates natural tectonic fractures in 22 wells that penetrated the tight oil reservoirs in the middle Permian Lucaogou Formation in the southeastern Junggar Basin, China. Analysis was conducted using conventional image logs of wellbores, acoustic emissions (AEs) data collected from core rock mechanics testing, fluid inclusion analysis, and C–O stable isotopes and Ir element analyses of fracture-filling minerals, in order to constrain the relative timing of fracture development. The geochemical properties of fracture-filling minerals and Kaiser effect points, combined with the regional tectonic analysis, revealed a four-stage fracture development in the tested tight reservoirs. During Stage 1, continuous N–S compression occurred at the end of Late Triassic, producing many NW-trending extensional fractures. Calcite fillings were formed within these fractures at temperatures between 31.7 and 45.0 °C based on fluid inclusion analysis. SW–NE compressive stress was produced in the Middle−Late Jurassic during Stage 2, forming ENE-trending shear fractures. The calcite fillings within these fractures formed at temperatures between 50.1 and 64.6 °C. During Stage 3, intense N–S compression formed NNW-trending shear fractures in the Early Cretaceous, and calcite fillings formed at temperatures between 80.0 and 89.9 °C. Finally, Stage 4 corresponds to nearly N–S compression due to the tectonic activities occurring from Tertiary to nowadays, creating NNW-trending shear fractures; no or minimal partial filling of calcite occurred during this Stage. Based on the relation between the oil generation windows and stages of fracture development, it is proposed that the fractures that developed within the Lucaogou Formation during Stages 2–4 have minimal mineral filling, which is most favorable for oil and gas migration and accumulation.