四川盆地深层海相页岩气赋存机理与勘探潜力

深层—超深层页岩气是四川盆地天然气增储上产的战略接替领域。基于志留系页岩气的大量勘探开发实践、实验测试资料和前瞻性研究成果，探讨了深层海相页岩气的赋存状态与聚集机制，指出了四川盆地深层页岩气的有利勘探区。页岩的纳米孔喉系统决定了其内部聚集的天然气呈现吸附态，主要以单分子层在微孔—介孔中吸附聚集。页岩吸附气量除受控于孔、缝的比表面积，还受TOC含量、温度、压力和含水性的影响。深层高温条件下，页岩储层的最大吸附气量较低，随着温度降低页岩的吸附能力增强，最大吸附气量也逐渐增加。页岩气在深层超压条件下主要以游离气赋存，呈超临界状态高密度聚集，游离气含量受储集空间及孔喉结构、埋藏深度、地层温度和压力、超临界流体性质以及页岩含水性等诸多因素综合影响。在抬升过程中，因构造改造的强度、时间和方式不同，页岩气的赋存相态转化及散失机制不同。其中，抬升幅度小且改造强度弱时，页岩气层仍保持深层"游离气为主，超压富气"的特性；抬升至中—浅层时，受断裂开启、剥蚀露头和页岩自封闭性降低等游离气散失机制影响，页岩气层的含气量和游离气量降低、吸附气占比增加，因此，远离剥蚀区、大断裂带的深埋藏区是深层页岩气的最有利富集区。四川盆地内部构造相对稳定，深层海相页岩气普遍保持着"早期滞留，超压富气"的成藏特征，优选宜宾—泸州地区、綦江—涪陵地区、永川—大足地区和垫江—梁平地区为有利勘探区。

Occurrence mechanism and exploration potential of deep marine shale gas in Sichuan Basin

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.