陆相页岩含气性主控地质因素——以辽河西部凹陷沙河街组三段为例

以辽河西部凹陷沙河街组三段页岩为研究对象,在有机地化、岩石矿物组成、扫描电镜、等温吸附等实验分析以及岩心观察的基础上,分析讨论了陆相页岩气形成条件,并着重探讨了陆相页岩含气性主控地质因素以及海、陆相页岩气发育条件差异。研究表明,辽河西部凹陷沙河街组三段泥页岩具有沉积厚度大、有机质丰度高、有机质类型多样、热演化程度较低、储集物性好以及含气量高等特点,具备陆相页岩气形成的基础地质条件;此外,页岩的含气性还受到泥页岩有机组成、无机组成和孔隙等内在因素以及温度、压力和埋藏深度等外在因素的共同影响;吸附态天然气受有机碳含量、有机质成熟度、岩石矿物组成以及温度、压力和深度的影响,表现为吸附气量与有机碳含量、黏土矿物、压力呈正相关关系,其中黏土矿物组成中又以伊/蒙混层的影响为主;与有机质成熟度、碳酸盐矿物含量、温度呈负相关关系,随着埋藏深度增加表现出先增加后降低的趋势;游离态天然气主要受到泥页岩孔隙度大小的影响,表现为孔隙、裂缝越发育,页岩孔隙度越高,游离气量也就越高,其中孔隙发育程度与有机质成熟度以及黏土矿物含量呈正相关关系,与石英、长石及碳酸盐矿物含量呈负相关关系,与有机碳含量关系不明显;裂缝的发育则更多地受到构造应力、岩石矿物组成、岩石力学性质、黏土矿物脱水收缩、成岩作用等内、外在因素的共同影响。通过对比海相和陆相页岩气形成条件后发现,烃源岩成熟度的差异从根本上决定了陆相和海相页岩的含气潜力,并可能进一步影响低成熟度条件下陆相页岩气的赋存方式,即溶解气对于陆相页岩气而言可能具有不可忽视的贡献。

Main-controlling geological factors of gas-bearing property of continental shale gas: a case study of Member 3rd of Shahejie Formation in western Liaohe sag

The shale in Member 3 of Shahejie Formation, western Liaohe sag was taken as the research object. Based on core observations and experimental analyses including organic geochemistry, mineralogy, SEM, isothermal adsorption etc., the forming conditions of continental shale gas were analyzed and discussed, focusing on main-controlling geological factors for the gas-bearing property of continental shale and the differences in development conditions of marine-continental shale gas. Research has shown that the mudstone/shale in Member 3 of Shahejie Formation, western Liaohe sag is characterized by large sedimentary thickness, high abundance of organic matter, diversity of organic matters, low degree of thermal evolution, good reservoir physical property and high gas content, thus providing a basic geological condition for the formation of continental shale gas. In addition, gas-bearing property of shale is also influenced by internal factors (organic-inorganic compositions and pores) and external factors (temperature, pressure and buried depth). Adsorbed natural gas is mainly influenced by organic matter content and maturity, rock mineral compositions, temperature, pressure and depth. This is demonstrated by the fact that adsorbed gas is positively correlated to organic matter content, clay minerals and pressure, of which clay mineral compositions are mainly influenced by illite-montmorillonite mixed-layer, but negatively correlated to organic matter maturity, carbonate mineral content and temperature; with the increase in buried depth, a trend of first increasing and then decreasing is presented. Free natural gas is mainly influenced by shale porosity, showing that the more developed the pores and fissures are, the higher the shale porosity will be, and the higher the free gas content will be. The development degree of pores is positively correlated to organic matter maturity and clay mineral content, whereas negatively correlated to quartz, feldspar and carbonate mineral content, and there was no obvious relationship with organic carbon content. However, the development of fissures was more affected by internal and external factors, such as tectonic stress, rock mineral composition, mechanical properties of rocks, clay mineral dehydration and shrinkage, and diagenesis. Through comparing the forming conditions of marine and continental shale gas, it is found that the gas-bearing potential of continental and marine shale is fundamentally determined by the difference in source rock maturity, which might further affect the occurrence mode of continental shale gas under the condition of low maturity, i.e., the dissolved gas might make a noticeable contribution to continental shale gas.