中扬子区下古生界页岩气关键评价指标限定值研究

The evaluation and regional selection of shale gas under the high-over matured condition of complicated structural area in South China is a challenging task. The current shale gas evaluation criterion which based on North American shale gas exploration experience is not applicable to shale with high-over thermal maturity. After the high-over thermal evolution process, it is worth reminding that the total organic carbon (TOC) of type I-II shale would be decreased, and the thermal stage of shale might reach or over the dead line of gas generation, and the organic porosity and adsorption capacity would be reduced. The organic-rich shales from Lower Cambrian Niutitang Formation and Lower Silurian Longmaxi Formation in the middle Yangtze region were selected as the research objects in this study. From hydrocarbon- generating potential and residual hydrocarbon content standpoint, by analyzing the restitution of original TOC, and the relationship of residual gas content and TOC, the lower limit value of residual TOC of shale gas resource evaluation at high-over thermal stage is proposed. Based on the natural evolution profile and physical diagenesis simulation respectively, the change rules of organic porosity character and adsorption capacity of shale at high-over thermal stage with the increase of maturity will be studied. Combined with the dead line of gas generation obtained by hydrocarbon generation kinetics method, the upper limit value of organic matter maturity of shale gas resource evaluation under the high-over matured condition is proposed. Research results will provide important theoretical basis for exploration and regional selection of shale gas resource under the high-over matured condition.

我国南方复杂构造区高-过热演化条件下的页岩气选区评价是当今难题，以北美页岩气勘探经验为依据的现行页岩气选区评价标准并不适用于高-过热演化页岩。经过高-过成熟演化历程，海相I-II型页岩有机碳含量会有所损耗，可能会达到或超过生气上限，页岩有机质孔隙度和吸附能力也会有所降低，这些是评价高-过热演化含气页岩需要注意的问题。本课题以中扬子区牛蹄塘组和龙马溪组富有机质页岩为研究对象，从生烃能力和残留烃量的角度出发，通过原始有机碳恢复、含气量与有机碳含量相关关系分析等方法，指出高-过热演化页岩气评价的残余有机碳下限值。同时，分别以自然演化剖面和成岩物理模拟为依据，研究高-过热演化阶段页岩有机质孔隙特征和吸附能力随成熟度增加的变化规律，结合生烃动力学方法确定的页岩生气上限，最终指明高-过热演化条件下页岩气评价的有机质成熟度上限值。该研究成果将为高-过热演化区页岩气的勘探选区提供重要的理论依据。

页岩气资源潜力评价关键指标限定值的取值具有重要意义，它不仅直接决定了评价区资源潜力预测的可靠性和目标区的圈定，更会影响页岩气勘探的成败。中扬子下古生界海相页岩地质背景十分复杂、热演化程度高、构造改造强烈，这些地质条件给页岩气选区评价造成困难，并影响评价指标限定值的取值。针对热演化程度过高这一问题，本项目以中扬子牛蹄塘组和龙马溪组富有机质页岩为研究对象，通过原始有机碳含量恢复、含气量相关性分析、生烃动力学模型建立、热演化程度与有机质孔隙孔径分布、孔容、吸附能力的耦合关系分析等相关研究，阐明了该区有机碳含量、生烃转化率和有机质孔隙结构特征的演化规律，指出适用于中扬子下古生界高-过热演化条件下页岩气评价的残余有机碳下限值为1.80%，成熟度上限值为Ro等于3.0%。.本项目主要成果认识包括以下三个方面。一是建立了研究区富有机质页岩数据库，总结了中扬子地区牛蹄塘组和龙马溪组页岩有机质丰度、类型和成熟度在平面上和垂向上的分布特征。二是用Rock-Eval开放体系模拟进行页岩的生烃动力学研究，获取了平均生烃活化能（57.10Kcal/mol）和指前因子（1.0×1016 s-1）等动力学参数，结合有机质损失和岩石压实数学模型，计算出研究区原始有机碳含量恢复系数为1.125，建立的生烃动力学模型表明海相页岩“生气死亡线”为Ro等于3.0%。三是通过对自然演化剖面样品的氩离子抛光-高分辨率扫描电镜分析，总结了页岩有机质孔隙的大小、形状、结构特征随热演化程度增加的变化规律，发现当Ro大于3.0%时有机质孔隙发生明显的收缩变形，利用低温氮气吸附实验进一步证实该阶段页岩介孔由管状孔和墨水瓶形孔收缩变形为锥形孔和狭缝孔。利用BJH模型计算了页岩介孔孔容，发现页岩BJH孔容与Ro呈负相关关系，且在Ro达到3.0%时出现急剧降低的趋势。通过等温吸附实验发现海相页岩Ro一旦超过3.0%，其吸附能力会大幅度下降。项目研究成果为高-过热演化区页岩气的勘探选区提供了重要的理论依据。