陆相湖盆页岩岩相分类方案及其意义——以沧东凹陷孔二段为例

Shale lithofacies classification scheme of continental lake basin and its significance:a case of E_(2) Member of Kongdian Formation in Cangdong Sag

与常规碎屑岩相比,页岩具有矿物组成复杂、颗粒细小、富含有机质和强非均质性的特点,岩心描述和普通薄片鉴定的准确性受到很大限制,传统的岩石分类和命名方法需进行改进以满足页岩岩相划分的理论研究与实际工作的需要。首先,分析页岩岩相划分的难点,确定特殊组分和粒度对页岩岩性的影响;其次,通过对沧东凹陷孔二段页岩矿物组分的X射线衍射、有机碳质量分数w(TOC)测试,提出考虑有机质体积分数、基于岩石无机矿物的“五组分三端元”岩相划分方案;最后,在岩相划分的基础上,剖析该页岩岩相划分方案的地质与工程意义。研究结果表明:沧东凹陷孔二段页岩主要表现出多种矿物混积的特点,可划分为4个大类岩相和21个小类岩相。该页岩岩相划分方案对岩石组分真实体积进行归一化处理,以长英质矿物、灰云质矿物和黏土矿物作为三角图分类三端元,并结合方沸石和有机质体积分数来划分页岩岩相,能够体现页岩组构的分布规律、指示页岩的沉积环境和表征页岩的脆性特征。

Compared with conventional clastic rocks,shale is characterized by complex mineral composition,fine particles,rich organic matter and strong heterogeneity.Therefore,the accuracy of core description and ordinary thin section identification are greatly limited.The traditional rock classification and naming methods need to be improved to meet the needs of theoretical research and practical work of shale lithofacies classification.Firstly,the difficulty of shale lithofacies classification was analyzed,and the influence of special components and particle size on shale lithology was determined.Secondly,based on the analysis of X−ray diffraction and mass fraction of organic matter w(TOC)determination test data in the E_(2) Member of Kongdian Formation in Cangdong Sag,a lithofacies classification scheme of"five components and three end-members"was proposed,which includes volume fraction of organic matter and inorganic minerals.Finally,on the basis of lithofacies classification,the geological and engineering significance of the shale lithofacies classification scheme was analyzed.The results show that the shale of the E_(2) Member of Kongdian Formation in Cangdong Sag mainly shows the characteristics of mixed sedimentation of various minerals,which can be classified into four major lithofacies and 21 minor lithofacies.In this classification scheme,the real volume of rock components is normalized.Felsic minerals,calcareous-dolomitic minerals and clay minerals are used as triangulars to classify the three end-members,and the volume fraction of analcime and organic matter were combined to classify shale lithofacies.Shale lithofacies classification scheme can reflect the distribution law of shale fabric,indicate the sedimentary environment of shale and characterize the brittleness of shale.