关键词: 四川盆地, 深层页岩气, 赋存形式, 构造稳定, 勘探潜力

Abstract: Deep to ultra-deep shale gas is a strategic replacement area for increasing the reserve and production of natural gas in Sichuan Basin. Based on a lot of practices, experimental and test data and prospective research results for the exploration and development of Silurian shale gas, this paper explores the occurrence mode and accumulation mechanism of deep marine shale gas, and points out favorable exploration areas of deep shale gas in Sichuan Basin. The nanopore throat system of shale determines that the natural gas gathered inside is mainly in the absorbed state, presented as monolayers absorbed and accumulated in micro-meso pores, and the amount of absorbed shale gas is controlled not only by the specific surface area of pores and joints, but also by TOC content, temperature, pressure and water content. Under the condition of high temperature in deep reservoirs, the maximum amount of absorbed gas in shale reservoir is low, which gradually increases as the temperature is decreased drops and the shale absorption capacity is enhanced. Under the condition of deep overpressure, shale gas is mainly occurred as free gas and densely accumulated under supercritical state, and the free gas content is affected by multiple factors, such as reservoir space and pore-throat structure, burial depth and formation temperature and pressure, supercritical fluid properties and shale water content. In the process of uplifting, the occurrence phase transformation and loss mechanism of shale gas are different due to differences in the intensity, time and mode of tectonic transformation. Specifically, in the case of small uplift amplitude and week transformation, the shale gas is still dominated by free gas and rich in ultra-pressure gas in deep reservoirs; at the shallow-medium uplifting depths, the gas content and free gas content in shale gas reservoir reduce while the proportion of absorbed gas increases due to the loss mechanism of free gas, including fault activity, outcrop denudation, and decrease of shale self-sealing ability. Therefore, the deep buried zones away from denudation area and large fault zone are deemed as the most favorable enrichment zones for deep shale gas. In fact, Sichuan Basin has a relatively stable internal structure, where the deep marine shale gas generally maintains the reservoiring characteristics of "retention in early stage and enrichment in ultra-pressure gas", and Yibin-Luzhou, Qijiang-Fuling, Yongchuan-Dazu and Dianjiang-Liangping areas are deemed as favorable exploration areas.

Key words: Sichuan Basin, deep shale gas, occurrence mode, tectonic stability, exploration potential