渝东南盆缘转换带龙马溪组页岩气散失过程、能力及其主控因素

Diffusion process and capacity of Longmaxi shale gas in the basin-margin transition zone of SE Chongqing and their controlling factors

研究页岩气的散失过程、散失能力及其控制因素对于揭示页岩气的成藏机理、指导页岩气的勘探选区等都具有重要的现实意义。为此,以四川盆地东南部及其盆缘转换带(以下简称渝东南盆缘转换带)下志留统龙马溪组页岩为例,通过现场解吸实验模拟页岩气散失过程,并借助X射线衍射分析、有机碳含量测试、低温氮气吸附实验、等温吸附实验、扫描电镜观察等室内研究手段,定性分析页岩气的散失过程,定量评价页岩气的散失能力,并在此基础上探讨影响页岩气散失能力的主控因素。研究结果表明:(1)该转换带龙马溪组上、下段页岩的页岩气散失过程存在着明显的差异,后者的散失能力明显低于前者;(2)页岩气散失能力主要由温度、压力及岩石属性等决定,其中温度、压力是最主要的外在因素;(3)有机质含量是决定页岩气散失能力的最主要内在控制因素,随有机质含量的增加,页岩比表面积增大、吸附能力增强,页岩气散失能力降低;(4)页岩气散失能力在一定程度上受到岩石矿物成分和孔隙结构的影响,石英含量、黄铁矿含量与散失能力呈负相关关系,长石含量与散失能力呈正相关关系,碳酸盐矿物含量和黏土矿物含量与散失能力之间的相关性不明显。

The studies on the diffusion process and capacity of shale gas and their controlling factors are of great significance to reveal the shale gas accumulation mechanism and guide the selection of shale gas exploration areas.In this paper,we took the shale of Lower Silurian Longmaxi Fm in the southeastern(SE)Sichuan Basin and its basin-margin transition zone(hereinafter referred to as the basin-margin transition zone of SE Chongqing)as an example.The diffusion process of shale gas was simulated by using the field desorption experiment.Then,the diffusion process of shale gas was qualitatively analyzed and the diffusion capacity of shale gas was evaluated quantitatively by means of laboratory experiments,e.g.X-ray diffraction(XRD)analysis,total organic carbon(TOC)test,Low-temperature nitrogen adsorption experiment,isothermal adsorption experiment and scanning electron microscope(SEM)observation.Finally,the main factors controlling the diffusion capacity of shale gas was discussed.And the following research results were obtained.First,the diffusion process of shale gas in the upper member of Longmaxi Fm in this transition zone is obviously different from that in the lower member,and the diffusion capacity of shale gas in the lower member of Longmaxi Fm is much lower than that in the upper member.The diffusion capacity of the Lower Longmaxi Fm is obviously weaker than that of the Upper.Second,the diffusion capacity of shale gas is mainly dominated by temperature,pressure and shale properties,among which temperature and pressure are the most important external factors.Third,organic content is the most important internal factor controlling the diffusion capacity of shale gas.As the organic content increases,the specific surface area of shale increases,the absorption capacity of shale enhances and the diffusion capacity of shale gas decreases.Fourth,to some extent,the diffusion capacity of shale gas is affected by rock mineral compositions and pore structures.The diffusion capacity of shale gas is in the negative correlation with quartz contents and pyrite contents,in the positive correlation with feldspar contents,and in no significant correlation with carbonate mineral contents and clay mineral contents.