湘西北地区下寒武统牛蹄塘组页岩有机地球化学与矿物组成特征

ORGANIC GEOCHEMISTRY AND MINERAL COMPOSITION CHARACTERISTICS IN SHALES OF NIUTITANG FORMATION, NORTHWESTERN HUNAN

文章以湘西北地区下寒武统牛蹄塘组页岩为研究对象,分析其有机地球化学及无机矿物特征,重点对其烃源岩生物标志化合物特征进行分析。结果表明:该套页岩残余有机碳(TOC)含量高、氯仿沥青"A"含量低、生烃潜量"S1+S2"较小,镜质组反射率Ro在2.3%~3.19%之间,达到了过成熟阶段,以生成干气为主。分子生物标志化合物结果显示正构烷烃呈双峰型,碳数大于C22的中-高碳数烃占优势;Pr/Ph平均值为0.31,植烷优势明显,指示强还原的海相沉积环境;萜烷丰度顺序为五环三萜烷>三环萜烷>四环萜烷,γ-环蜡烷与C30霍烷的比值普遍大于0.3,指示其沉积母质以浮游生物及海洋性自养菌为主,含有部分藻类;Ts/(Ts+Tm)在0.35~0.38之间,与镜质组反射率对成熟度的检测结果相互印证;规则甾烷C27>C29>C28,指示其母质来源以浅海生物来源为主,所有样品中均检测出孕甾烷、升孕甾烷、重排甾烷及4-甲基甾烷,表明其经历过微生物降解作用。全岩矿物组分分析显示该套页岩具备较好的开发物性条件和较强的吸附性能。综合以上分析结果,提出研究区牛蹄塘组页岩具备较好的页岩气成烃成藏的物质条件,并为该区深入开展页岩气勘探目标区优选奠定了很好的工作基础。

The article aims to analyze the organic geochemistry and inorganic mineral characteristics of the Lower Cambrian Niutitang Formation shale in northwestern Hunan Province.The main research methods include carbon-sulfur measurement method, rock pyrolysis method, chloroform asphalt "A" measurement method, clay mineral, and whole rock X-ray diffraction analysis method.The results show that the content of TOC in the Niutitang Formation is high, the content of "A" in chloroform asphalt is low, and the potential of generating hydrocarbon "S1+S2" is low.Most of the vitrinite reflectances Ro range from 2.3% to 3.19%, which have reached the mature stage and mainly generate dry gas.The results of biomarkers show that the n-alkane presents bimodal pattern and the medium-and high-carbon number hydrocarbons with carbon number more than C 22 are dominant.With Pr/Ph average value of 0.31, the advantage of phytate is obvious, indicating the strong reduced marine sedimentary environment.The right angle diagram of Pr/nC 17 and Ph/nC 18 shows that the organic matter types of shale are mainly type I and II1;The terpene abundance order is pentacyclic triterpeneane>tricyclic terpane>tetracyclic terpene.The ratio of γ-alkane to C 30 Hopane is generally greater than 0.3, indicating that its sedimentary parent materials are dominated by plankton and marine autotrophic bacteria, with part of algae;Ts/(Ts+Tm) range from 0.53 to 0.60, which is consistent with the detection results of vitrinite reflectance on maturity.The mass chromatogram of saturated hydrocarbon m/z 217 is C 27 >C 29 >C 28 and the ratio of C 27 /C 29 is more than 1, indicating that its parent materials are dominated by the shallow sea creatures.Pregnane, homo-pregnane, diasterane, and 4-methyl sterane were detected in all samples, indicating that they had undergone microbial degradation.Based on the above analysis results, it is proposed that shales of the Niutitang Formation in the study area have good material conditions for shale gas hydrocarbon accumulation.This research work has laid a good working foundation to carry out the shale gas exploration in the target area.