塔里木盆地西北缘下寒武统黑色岩系差异性成岩演化及其影响因素

The differential diagenetic evolution and its influencing factors of Lower Cambrian black rock series in the northwestern margin of Tarim Basin

尽管泥页岩在埋藏过程中经历的复杂成岩改造会显著影响页岩油气储层发育潜力,但对其成岩作用,特别是泥页岩中成岩矿物的类型、成岩演化及其影响因素、成岩矿物元素在低孔低渗孔隙系统中的迁移等问题的认识仍很薄弱,因而制约了对泥页岩储层的深入认识。为此,本文以塔里木盆地西北缘下寒武统玉尔吐斯组发育的两套黑色岩系为例,在沉积环境认识的基础上,通过矿物组分、黏土矿物组合、有机碳含量及有机质成熟度等测定分析,着重研究了黑色岩系自生矿物的分布及在成岩演化阶段的变化特征,阐述了成岩变化的时空分布及其影响因素。研究结果表明:(1)在浅水缓坡台地环境下,玉尔吐斯组受两个较为完整的海进-海退沉积旋回控制而形成的两套黑色岩系主要经历了早成岩阶段和中成岩阶段;(2)受沉积环境和热液作用的影响,这两套黑色岩系在石英、重晶石、碳酸盐胶结物和黏土矿物等自生矿物的组合、分布及形成顺序上存在显著的差异,呈现出差异性成岩演化的特点;(3)伊利石、伊蒙混层、绿泥石、白云石、石英胶结等中成岩阶段成岩产物的广泛分布,反映出在成岩过程中特别是中成岩阶段低孔低渗的泥页岩仍存在一定程度的内部和外部来源的矿物元素迁移。本次研究对深入认识塔里木西北缘下寒武统黑色岩系成岩机制,以及分析页岩储层发育特征提供了重要的依据。

Despite shales have undergone complex diagenetic transformation during deep burial and thus can significantly affect the development potential of shale oil and gas reservoirs,the understanding of the diagenetic processes is still relatively poor. The main issues that need to be clarified contain the types of diagenetic minerals in shales,diagenetic evolution together with its impact factors,and the extent of element mobility behavior in such low porosity and permeability pore system,which restricts further understanding of shale reservoirs. Two black rock series of the Lower Cambrian Yurtus Formation in the northwestern Tarim Basin are utilized to examine authigenic mineral distribution and diagenetic evolution characteristics,on the basis of determination of sedimentary environment,mineral composition,clay mineral assemblage,organic carbon content and organic matter maturity. During the diagenetic evolution stage,the temporal and spatial distribution of diagenetic changes and their influencing factors are thus described. This study shows that the two black rock series are developed and controlled by two relatively complete transgressive-regressive sedimentary cycles in the shallow shelf environment,and the dominant diagenetic regime is interpreted to correspond to eodiagenesis and mesodiagenesis.Significant differences in the combination,distribution,and formation sequence of diagenetic minerals such as quartz,barite,carbonates and clay minerals occur in the two sets of black rock series,showing different diagenetic evolution affected by the sedimentary environment and hydrothermal effect. The extensive distribution of diagenetic products in the mesodiagenesis further verify a certain degree of element fluxes can be sourced both internally and externally in low porosity and permeability black shales. This study provides an important basis for further understanding the diagenetic mechanism of the Lower Cambrian black series and analyzing the development characteristics of shale reservoirs in the northwestern margin of Tarim Basin.