Characteristics and Distribution Patterns of Pore-laminar Fractures in Organic-rich Laminated Shales of the Jiyang Depression

Key words: 
• pore-laminar fractures /
• lithofacies /
• continental shale /
• formation mechanism /
• shale oil

Abstract: [Objective] Pore-laminar fracture system is an important reservoir space and seepage channel of shale oil, which controls the distribution of shale oil. Due to the internal relationship between pores and lamellar fractures in genesis, this paper will take the pore-laminar fracture system as a whole to be researched. [Methods] The shale of the Es3x-Es4s of the Paleogene in Jiyang Depression is selected as an example. Taking the lamina as a unit, a series of experiments were carried out on the two typical lithofacies of organic-rich laminated limestone and organic-rich laminated mudstone by means of microscope observation, scanning electron microscope and SEM-EDS-AMICS analysis technology, and the development distribution mode and control factors of pore-lamellar fracture system were discussed. [Results and Discussions] The results show that the organic-rich laminated limestone mainly developed calcite-associated pores (intercrystalline pores, intergranular pores and dissolved pores), accounting for 32.48% of the total surface pore rate. The boundary between the two laminae is clear and the laminar fractures are well developed. The organic-rich laminated mudstone is most developed with clay mineral-associated pores (intercrystalline pores and intergranular pores), accounting for 37.51 % of the total surface pore rate, and there is also a good development of laminar fractures. [Conclusions] The pore-laminate fracture system of organic-rich laminated limestone is mainly composed of calcite intercrystalline pores, clay mineral intercrystalline pores and laminar fractures. The organic-rich laminated mudstone is mainly composed of clay mineral intercrystalline pores, organic pores and laminar fractures. The development of different pore-laminar fracture system in the two lithofacies is controlled by sedimentation and diagenesis. The depositional environments control the types of laminae and their combinatorial methods. In the process of diagenesis, the organic-rich laminated limestone is mainly affected by calcite recrystallization and dissolution, and the organic-rich laminated mudstone is dominated by clay mineral transformation and thermal evolution of organic matter. It is of great significance to clarify the distribution pattern and development control factors of pore-laminar fracture system for guiding the exploration and development of shale oil in Jiyang Depression.