中国南方不同埋藏条件下海相页岩气差异保存机理

Differential preservation mechanisms of marine shale gas under varying burial conditions in southern China

保存条件是制约复杂构造区海相页岩气勘探突破的关键因素,其在不同埋藏条件下存在差异,相关机理尚不清楚。本文基于志留系页岩埋藏—生烃—抬升动态演化格架及其温压场、页岩气赋存形式的定量恢复研究,结合关键地质参数与含气量的实验大数据统计分析,将南方海相页岩气埋藏类型分为埋藏—生烃、抬升—改造2个阶段的最大埋藏期、深层和中浅层的超压与常压六种情形,分别进行了保存条件研究,指出各自的保存机理及差异,并建立了页岩气保存条件的综合定量评价指标体系。结果表明:(1)埋藏—生烃阶段,处于页岩气生成和积聚阶段,以高温热裂解气为主,至最大埋藏处(期)形成的页岩气主要以超临界态、游离态原地聚集与保存,呈超压富气,这归因于受顶底板封堵性和烃类吸附作用控制的低排烃效率和高滞留烃量。而开启性断层、不整合面和紧邻高孔渗地层等先天性条件下易产生高排烃效率,页岩气原始资源潜力不足。(2)抬升—改造阶段的深层页岩气,在良好的顶底板致密性和边部分隔性的条件下,页岩气仍保持了最大埋藏期的超压富气特征,即使发生一定量的页岩气散失,主要是在浓度差驱动下沿层理缝侧向扩散;目标层被开启性断层切割则页岩气逸散严重并形成异常低压。(3)抬升—改造阶段的中浅层页岩气,在超压状态下,以游离气为主,吸附气次之,仍呈超压富气,此类保存主要受构造变形强度控制,其中岩石力学行为是关键,页岩气散失属于浓度差驱动下沿层理缝向断层、露头区侧向分子扩散;在常压状态下,构造改造强烈,以吸附气为主,含气量变化大,自封闭和水动力是此类埋藏条件下页岩气保存的主控因素,其中纳米限域空间内吸附和毛管压力封闭机制是关键,页岩气散失处于浓度差下沿微裂缝系统分子扩散或(和)压差和水动力下沿开启性断层、露头区渗流逸散。

The breakthrough of marine shale gas exploration in complex structural areas is hindered by the variability of preservation conditions under different burial scenarios,and the underlying mechanism driving these differences is still unclear.This study investigates the dynamic evolution of Silurian shale burial,hydrocarbon generation,and uplift,considering temperature and pressure fields.We quantitatively restore shale gas occurrence form and content,combined this with statistical analysis of key geological parameters and experimental big data of gas content.Based on these analyses,we divide marine shale gas burial types in southern China into 6 types:The maximum burial period of the burial-hydrocarbon generation stage and uplift-transformation stage,overpressure and normal pressure in deep and middleshallow layers.We individually examine the preservation conditions of each type,elucidating the preservation mechanisms and distinctions,ultimately establishing a comprehensive quantitative evaluation index system for shale gas preservation conditions.The results show that:1.In the burial-hydrocarbon generation stage,shale gas is in the stage of generation and accumulation,primarily dominated by hightemperature pyrolysis gas.Shale gas formed in the maximum buried area(stage)is mainly accumulated and preserved in situ in supercritical and free states,showing overpressure-rich gas.This is attributed to low hydrocarbon expulsion efficiency and high hydrocarbon retention controlled by top and bottom sealing and hydrocarbon adsorption.In addition,the occurrence of high hydrocarbon expulsion efficiency is more likely in congenital conditions such as open faults,unconformities,and adjacent formations with high porosity and permeability,resulting in insufficient original resource potential of shale gas.2.For deep shale gas in the uplift-renovation stage,despite some shale gas loss through lateral diffusion along bedding fractures driven by concentration differences,overpressure-rich gas characteristics persist,similar to those observed during maximum burial periods.However,when the target layer is intersected by an open fault,shale gas escapes seriously and forms abnormally low pressure.3.The medium and shallow shale gas in the uplift-transformation stage is primarily composed of free gas,followed by adsorbed gas under overpressure,and still exhibits the characteristics of overpressure-rich gas.The preservation conditions are mainly governed by the strength of tectonic deformation,with rock mechanical behavior playing a crucial role.Shale gas dissipation occurs through lateral molecular diffusion along bedding fractures in fault and outcrop areas driven by concentration differences.Under normal pressure,the structure transformation isstrong,the shale gas is mainly adsorbed gas,and the gas content changes greatly.The primary factors contributing to the preservation of shale gas are self-sealing and hydrodynamic processes,with the adsorption and capillary pressure sealing mechanisms playing a crucial role.Shale gas loss occurs due to molecular diffusion along microfracture systems driven by concentration gradients,as well as seepage and escape through open faults and outcrop areas under pressure differentials and hydrodynamic forces.