川东地区海陆过渡沉积环境演变过程中泥页岩孔隙系统变化规律

Compared with the single superior marine depositional environment, the marine-continental transitional sedimentary has frequent changes, and the source of organic matter and the evolution of clay minerals in mudrocks are significantly different, which would result in the pore constitution and controlling factors unclear. The previous studies on pore system in mudrocks are mainly focusing on the aspects of organic matter abundance, maturity, composition and content of minerals, lacking analyzing the features of hydrocarbon-forming biological diversity and the transformation of clay minerals in the marine-continental transitional mudrocks, which would restrict the exploration progress of transitional shale gas. This project intends to carry out on the mudrock samples from Longtan formation with different sedimentary environments in Eastern Sichuan, aiming to reveal the differences in mineral composition, organic matter abundance, maturity, kerogen type etc. and their effects on shale physical property. Typical samples will be selected to analyze the crystal morphology and internal pores in different types of clay minerals, illustrating the influence of clay mineral types and their transformation on relevant pores. The maceral composition and hydrocarbon-forming organism in mudrocks will be investigated in detail, and the differences of pore development in different maceral composition and hydrocarbon-forming organism will be analyzed, in order to illustrate the control mechanism for the development of organic pores. Finally, in terms of the relationship between pore constitution and shale composition, a dynamic model of pore system of mudrock will be constructed theoretically to reveal the variation rule of pore system of shale in the process of sedimentary evolution of the Longtan formation in East Sichuan Basin, which could provide theoretical support for favorable zone optimization of the transitional shale gas in research area.

与海相单一优越的沉积环境相比，海陆过渡沉积环境变化频繁、泥页岩中有机质来源和粘土矿物演化差异明显，导致孔隙构成及控制因素不清晰。前人多从有机质丰度、成熟度、矿物组成及含量等角度来研究泥页岩孔隙系统，缺乏对海陆过渡相成烃生物差异和粘土矿物种类转变等特色的刻画，制约着海陆过渡相页岩气的勘探进展。本项目拟对川东龙潭组不同沉积环境钻井样品分析，揭示泥页岩矿物组成、有机质丰度-成熟度-类型等差异及对物性的影响；选取典型样品，解析不同类型粘土矿物的晶体形态及内部孔隙，阐明粘土矿物种类和转化对相关孔隙的影响；详细调查泥页岩显微组成、成烃生物类型，分析不同显微组分和成烃生物内孔隙发育的差异，进而确定其对有机孔发育的控制机理；最后依据孔隙构成与泥页岩组成之间的关系，理论上构建孔隙系统动态变化模型，揭示川东地区龙潭组沉积环境演变过程中泥页岩孔隙系统的变化规律，为研究区海陆过渡相页岩气有利区优选提供理论支撑。

我国海陆过渡相页岩气层分布广泛，海陆过渡相泥页岩具有单层厚度薄、岩性变化快、非均质性强、脆性含量低、干酪根类型差、液态烃产量低等特点，但至今仍未取得较大的勘探突破。通过对龙潭组/吴家坪组泥页岩开展地球化学、储层孔隙类型、孔隙结构参数和甲烷吸附等测试，分析了有机碳含量等参数对吸附能力的影响。自川东南龙潭组至川东北吴家坪组，富有机质泥页岩形成的环境依次为滨岸沼泽、潮坪泻湖、浅水陆棚相和深水陆棚。龙潭组岩性主要为泥岩、粉砂岩和煤层，有机碳含量较高，多在2%以上；矿物组成以黏土矿物为主，平均含量在42%～70%，随沉积环境变化，黏土矿物组合面貌由以伊蒙混层+高岭石+伊利石为主转变为伊蒙混层+伊利石为主；吴家坪组岩性为硅质页岩和灰岩，有机碳含量为6.56%～7.15%；矿物组成主要为石英，其次为黏土矿物和方解石。龙潭组泥页岩干酪根类型为III 型，有机质孔在镜质体和惰质体中发育较差、黏土矿物孔为优势孔隙类型；吴家坪组有机质为Ⅱ1型，有机质孔为主要的孔隙类型，主要发育在腐泥质和固体沥青中。进一步深入研究表明采用龙潭组泥页岩的兰氏体积普遍较高，介于2.19～6.98 cm3/g，吴家坪组泥页岩的兰氏体积为2.08～3.96 cm3/g，龙潭组泥页岩较高的兰氏体积与其较高的比表面积有关。TOC含量和黏土矿物含量是龙潭组/吴家坪组泥页岩兰氏体积的主控因素。由于这两套泥页岩中有机质孔发育存在显著差异，有机质对甲烷的吸附方式可能明显不同。吴家坪组泥页岩中有机质通过提供大量的吸附位点促进甲烷吸附，而龙潭组Ⅲ型干酪根表面吸附及长链结构线性分配提高了甲烷吸附能力。海陆过渡相页岩中富黏土矿物的吸附作用使其吸附能力显著增加，陆源碎屑石英含量的增加会降低泥页岩的吸附能力。