沁水盆地煤系页岩气的赋存富集机理及地质模型

Organic-rich shale in the Carboniferous-Permian coal-measure strata in Qinshui Basin develops widely and contains a huge amount of shale gas resources, but it is difficult to determine its exploration and development potential because of insufficient research. The project, aiming at the geological and geochemical characteristics of the coal-measure shale, with the combination of current research situation at home and abroad, proposes to carry out the research on the gas occurrence and enrichment mechanism, gas content and its main controlling factors. The key scientific issues addressed include: the mechanism of oil retention in the shale, and its influence on gas-generating potentials and the formation and evolution of organic nano-pores; occurrence mechanism and controlling factors of the shale connated bound water, and its influence on adsorption capacity; characterization of total pore size and adsorption phase volume of the shale. The purpose of the project is to reveal the main controlling factors of shale gas in the coal-measure strata in Qinshui Basin and construct a geological model of gas in place approximating to geological conditions for the selecting blocks. The marine-continental transitional facies coal-measure shale contains mainly Type III kerogen, with the typical Chinese characteristics. The innovation of the project is embodied in the shale adsorption gas model which is taken account of the influence of the connated bound water, and the shale free gas model which deducts the adsorption phase, making it more consistent with the geological conditions. The research results would develop and perfect the coal-measure shale gas accumulation theory, and can be directly applied to the evaluation and exploration of the coal-measure shale gas of Qinshui Basin as well as other areas in the Shanxi province.

沁水盆地石炭—二叠系海陆过渡相煤系富有机质页岩发育，储集了丰富的页岩气资源,但研究程度低，尚难确定其勘探开发潜力。本项目针对这套煤系页岩的地质地球化学特征，结合国内外现状，拟开展页岩气的赋存富集机理、页岩含气性及主控因素的研究，所涉及的关键科学问题包括：页岩滞留油机理及其对生气潜力、有机质纳米孔隙形成与演化的影响；页岩原生束缚水的赋存机理、控制因素及其对吸附能力的影响；页岩全孔径与吸附相体积的表征。本研究的目的是揭示沁水盆地煤系页岩气的主控因素，针对典型区块构建逼近地质条件下页岩的含气性模型。本项目选择的海陆过渡相煤系页岩以III型干酪根为主体，具有典型的中国特色，其创新性体现在页岩吸附气模型的构建考虑了原生束缚水的影响，页岩游离气模型考虑了吸附相体积的影响，使其更加吻合地质条件。研究成果可发展我国煤系页岩气富集成藏理论，并可直接应用于沁水盆地及山西省其它煤系页岩气的评价与勘探。

我国煤系页岩气资源丰富,但研究程度低，勘探开发潜力不落实。本项目以沁水盆地高-过成熟煤系页岩为重点，并通过与其它盆地煤系页岩与南方海相页岩的对比，研究了煤系页岩的储集物性及控制因素、生排油演化及生气潜力、孔隙水的赋存机制及影响因素、甲烷吸附机制与原地气量的地质模型，取得了如下主要进展：.（1）煤系页岩有机质以III型为主，含少量IIb型；富含粘土矿物，石英等脆性含量低，脆性指数主要介于0.2-0.4；孔隙以无机孔为主，微裂缝发育，而有机质粒内孔欠发育；纳米孔隙结构以微孔与较小的介孔(<10nm)占优势，并主要受控于TOC与黏土矿物。随成熟度增加，有机质总孔与微孔的孔容和比表面积下降的拐点Ro可高达3.9-4.5%，明显高于海相页岩（拐点的Ro介于3.0-3.5%）。 .（2）煤系页岩具有一定的生排油能力，其生成的液态石油族组成富含沥青质，其生油过程主要为干酪根的解聚，先形成沥青质，再裂解为烃类；煤系页岩的生气潜力低，介于100-200ml/gTOC，为海相页岩的50%左右。 .（3）煤系页岩孔隙水含量变化大，但均处于超低含水饱和度；孔隙水可赋存于有机与无有孔中，但在无有孔中更为富集，其平衡水饱和度明显高于有机孔；孔隙水在不同孔径的纳米孔隙均有分布，但以无机孔的非微孔为主，并大幅度降低其比表面积，尤其是介孔的比表面积。孔隙水的含量与分布主要受控于黏土矿物，其次是TOC含量。.（4）煤系页岩孔隙水明显降低了其甲烷吸附性能，与干燥样品相比，甲烷吸附容量降低幅度为10-30%；煤系页岩的原地气量远低于四川盆地海相优质龙马溪页岩，一般不到2m3/t,以吸附气为主，并主要受TOC与黏土矿物含量的共同控制。.上述研究成果揭示了煤系页岩的储集与含气性特征及其制约因素与机制，丰富与发展了页岩气的地质学理论，不仅具有重要的科学价值，而且对于沁水盆地及其它盆地煤系页岩气的评价与勘探具有的指导作用。