In situ stress characteristics of the NE Sichuan basin based on acoustic emission test and imaging logging

This study presents the distribution rule of in situ stress in the northeast Sichuan basin and its relationship with fracture. Sixty-seven sets of core samples of 21 Wells from the terrigenous clastic rock formation (Shaximiao, Qianfoya, Xujiahe) and marine carbonate formation (Jialingjiang, Leikoupo, Feixianguan) in the northeast Sichuan basin were tested by acoustic emission experiment. The in situ stress variation with the depth was established and the corresponding regression analysis was done. The horizontal principal stress direction of terrigenous clastic rock formation and marine carbonate rock formation was obtained by combining the dual diameter data of 6 wells and the imaging logging data of 3 wells. The results show that the vertical stress in the northeast of Sichuan basin has a linear relationship with the depth, and there is little difference between the vertical stress and the overburden weight of rocks. The maximum and minimum horizontal principal stress and horizontal shear stress increase with the burial depth. The divergence degree of horizontal shear stress with depth greater than 3000 m is greater than that of the stratum smaller than 3000 m. The horizontal stress plays a dominant role in the northeast Sichuan basin. With the increase in depth, the influence of tectonic stress field decreases and the vertical stress increases. Impacted by Dabashan and Qinling plate tectonic movement, the direction of in situ stress in marine carbonate strata is nearly east–west. The direction of maximum horizontal principal stress in terrigenous clastic rock formation is basically northwest–southeast. The imaging logging data show that the fracture direction is consistent with the horizontal principal stress direction, and the present in situ stress direction is favorable to the secondary reconstruction of natural fractures, and the fractures keep good opening. The distribution law of in situ stress in northeast Sichuan basin shows σH > σV > σh, indicating that the fault activity in this area is dominated by strike-slip type, the tectonic stress field is dominated by horizontal tectonic stress, in addition that the stress state is conducive to reverse fault activity.

Impact of coexisted clay mineral and organic matter on pore growth in Lower Jurassic Da'anzhai lacustrine shale reservoir in the Northeast Sichuan Basin, West China

Understanding pore growth is of great significance to investigating reservoir performance in shale-gas systems. However, different from the marine shale reservoir, the lacustrine shale reservoir is commonly rich in clay minerals, resulting in a complicated and poorly understood pore system. We have investigated the impact of coexisting clay mineral and organic matter on pore growth in the Lower Jurassic Da’anzhai Shale in the Northeast Sichuan Basin, West China, through performing total organic carbon (TOC) analysis, XRD, field-emission scanning electron microscopy, focused ion beam-scanning electron microscopy (FIB-SEM), N2 and CO2 adsorption experiment, and high-pressure mercury intrusion porosimetry. Our results indicate that the Da’anzhai Shale is dominated by clay-mineral-hosted pores, which are commonly filled or partly filled by pyrobitumen. Controlled by organic maceral, organic pores are poorly and heterogeneously developed in pyrobitumen, and minor or even no organic pores grow in vitrinite. Mesopore and macropore are popular in the Da’anzhai Shale reservoir with complex shapes, e.g., slit- or plate-like shapes combined with “ink-bottle” shapes, confirming a pore system dominated by clay-mineral-hosted pores. The weak positive correlation between the clay mineral content and the meso/macropore volume confirms that the clay mineral is a positive contributor to the storage space, and the weak negative correlation between the TOC and the mesopore volume suggests that infilling of pyrobitumen decreases the pore volume significantly. Similar correlations occur between specific surface area and clay mineral/TOC. FIB-SEM observation confirms that the pore system, e.g., the pore size, pore shape, and pore volume, is controlled by the coexisting clay mineral and pyrobitumen filling in a later stage. The calculated plane porosity of the initial inorganic pore and the unfilled inorganic pore in the Da’anzhai Shale is in the range of 3.66%–10.95% and 0.79%–1.46%, respectively, suggesting that 76.66% of inorganic pores is inactive due to pyrobitumen filling. All of this evidence suggests that pore growth in the Da’anzhai Shale is positively contributed by clay minerals, but it is negatively contributed by pyrobitumen filling. Further discussion suggests that pyrobitumen infilling between clay minerals in the Da’anzhai lacustrine shale can decrease the original pore volume significantly, which work together to govern the pore system in shale reservoirs.