Insights into the Formation Mechanism of Low Water Saturation in Longmaxi Shale in the Jiaoshiba Area, Eastern Sichuan Basin

Investigating the variation of water content in shale reservoir is important to understand shale gas enrichment and evaluate shale gas resource potential.Low water saturation is widely spread in Longmaxi marine organic-rich shale.To illustrate the formation mechanism of low water saturation,this paper analyzed water saturation of Longmaxi shale reservoir,restored the history of natural gas carrying water capacity combining homogenization temperature and trapping pressure of fluid inclusion with simulated thermal history,and established a model to explain pore water displaced by natural gas during the thermal evolution.Results show that the gas-rich Longmaxi shale reservoir is characterized by low water saturation with measured values ranging from 9.81%to 48.21%and an average value of 28.22%.TOC in high-mature to over-high-mature Longmaxi organic-rich shale is negatively correlated with water saturation,indicating that well-connected organic pores are not available for water.However,quartz and clay mineral content are positively correlated with water saturation,which suggests that inorganic-matter-hosted pores are the main storage space for water formation.The water carrying capacity of natural gas varies as a function of gas generation and expulsion history,which displaces bound and movable water in organic pores that are part of bound and movable water from inorganic pores.The process can be divided into two phases.The first phase occurred due to the kerogen degradation into gas at Ro of 1.2%–1.6%with a water carrying capacity of natural gas ranging from 5632.57–7838.73 g/km3.The second phase occurred during the crude oil cracking into gas at Ro>1.6%with a water carrying capacity of natural gas ranging from 10620.04 and 19480.18 g/km3.The water displacement associated with natural gas generation and migration resulted in gas filling organic pores and gas-water coexisting in the brittle-mineral-hosted pores and clay-mineral-hosted pores.