Abstract: The occurrence phases of marine-terrestrial transitional facies shale gas in China, particularly the proportions of adsorbed and free gas, remain highly debated. This controversy restricts accurate evaluation of in-situ gas content and optimization of development strategies for transitional facies shale gas. Taking the marine-terrestrial transitional shale of Member 2 of Shanxi Formation in the Daning-Jixian block of Ordos Basin as a case study, the pore structure characteristics of the shale were analyzed using scanning electron microscopy (SEM) in combination with nitrogen and carbon dioxide adsorption experiments. Methane isothermal adsorption experiments were conducted to characterize the gas occurrence features. Finally, the proportions of adsorbed and free gas in the transitional shale were validated using two methods: pressure-maintained coring combined with isotope determination, and pore structure analysis coupled with molecular simulation. The study shows that the organic matter in the marine-terrestrial transitional shale is predominantly composed of vitrinite and inertinite. SEM reveals that the shale is dense with limited development of nanoscale organic pores, but micropores with diameters less than 2 nm are well developed. These micropores exhibit strong methane adsorption capacity, where adsorbed gas is primarily stored in organic matter micropores. In contrast, mesopores and macropores are mainly developed in inorganic matter and serve as the primary occurrence space for free gas. During the pressure-maintained gas extraction process, methane carbon isotopes in shale gas exhibit distinct two-stage variations: during the pressure-maintained stage, the carbon isotope values of methane remain stable, primarily reflecting the production of free gas stored in mesopores and macropores; during the desorption stage, the carbon isotope compositions progressively become heavier, indicating the production of adsorbed gas stored in micropores. Based on pressure-maintained coring combined with isotope determination, the adsorbed gas proportion in the shale samples was calculated to be 84.5 %, with free gas accounting for 15.5 % . Furthermore, based on the pore structure data of the marine-terrestrial transitional shale, molecular simulation was employed to calculate the theoretical proportions of adsorbed and free gas in the shale samples. The results are generally consistent with those obtained by pressure-maintained coring combined with isotope determination; adsorbed gas and free gas account for 85.2 % and 14.8 %, respectively. The analyses based on pressure-maintained coring combined with isotope determination and pore structure coupled with molecular simulation both reveal a high proportion of adsorbed gas in marine-terrestrial transitional facies shale gas. As a result of this feature, the production curves of related shale gas wells exhibit low initial output, slow decline rates, and extended stable production periods. The research findings aim to guide the exploration and development of marine-terrestrial transitional facies shale gas. Moreover, the analytical methods employed can be applied to calculate the proportions of adsorbed and free gas in marine shale gas and deep coalbed methane reservoirs.

Key words: Ordos Basin, marine-terrestrial transitional facies, shale gas, occurrence phases, isotope fractionation

摘要： 中国海陆过渡相页岩气的赋存相态(吸附气、游离气的占比)存在较大争议,制约了海陆过渡相页岩气原位含气量评估与开发方案优化。以鄂尔多斯盆地大宁—吉县区块山西组二段海陆过渡相页岩为例,通过扫描电镜观察以及氮气、二氧化碳吸附实验分析了页岩的孔隙结构特征,并通过甲烷等温吸附实验表征了气体的赋存特征,最后采用保压取心+同位素测定法以及孔隙结构+分子模拟法两种方法对海陆过渡相页岩中所含的吸附气、游离气占比进行了验证。研究发现,海陆过渡相页岩中的有机质组分以镜质组和惰质组为主,页岩在扫描电镜下致密且少有纳米级有机质孔发育,但发育孔径小于2 nm的微孔;微孔对甲烷的吸附能力强,储层中的吸附气赋存在有机质微孔中,而介孔与宏孔主要为无机质所贡献,是游离气的主要赋存场所。在保压取气过程中,页岩气中的甲烷碳同位素呈现出明显的两阶段变化:保压阶段,甲烷的碳同位素值保持稳定,主要反映赋存于介孔、宏孔中的游离气产出;解吸阶段,甲烷的碳同位素值逐渐变重,反映赋存于微孔中的吸附气的产出。根据保压取心+同位素测定法,计算获得页岩样品的吸附气占比为84.5%,游离气占比为15.5%。此外,根据海陆过渡相页岩的孔隙结构数据,采用分子模拟法计算了页岩样品中的吸附气、游离气的理论占比,其结果与保压取心+同位素测定法的计算结果基本一致,分别为85.2%和14.8%。保压取心+同位素测定法以及孔隙结构+分子模拟法的分析结果均揭示了海陆过渡相页岩气中吸附气占比高的特性,这种特性使得相关页岩气井的产能曲线呈现出生产初期产量低、衰减较慢、稳产时间较长的特点。研究结果以期指导海陆过渡相页岩气的勘探开发,分析方法也可用于海相页岩气、深部煤层气中吸附气、游离气占比的计算。

关键词: 鄂尔多斯盆地, 海陆过渡相, 页岩气, 赋存相态, 同位素分馏