西藏羌塘盆地下侏罗统曲色组页岩沉积环境及有机质差异富集机理

Depositional environments and mechanism of differential organic matter enrichment for shale of the Lower Jurassic Quse Formation in Qiangtang Basin,Tibet

羌塘盆地下侏罗统曲色组是重要的烃源岩层系,但目前对该套烃源岩中有机质富集机理仍缺乏系统研究,制约了对该区油气成藏物质基础的认识。以羌塘盆地南羌塘坳陷毕洛错地区下侏罗统曲色组厚度约100 m的富有机质页岩为研究对象,综合岩心、TOC、主量元素和微量元素等数据,系统研究了页岩沉积时的古环境和古气候特征,分析了页岩中有机质富集的主控因素,建立了该区曲色组页岩有机质的差异富集模式。结果表明,页岩有机质含量具有2段性,2段有机质含量均呈自下而上递增趋势,其中下段(59~110 m)有机质含量总体较低(TOC处于0.3%~8.1%之间,均值为2.7%),包含3层薄层富有机质段;而上段(0~59 m)有机质含量相对较高(TOC处于0.8%~16%之间,均值为4.9%)。曲色组沉积时期总体为潮湿气候环境,并受到热液作用的影响,下段的有机质富集主要与适量的陆源输入、氧化还原条件和古生产力有关;上段的有机质富集主要受控于氧化还原条件和古生产力。曲色组上段与下段有机质富集机理的差异可能与古气候和海平面变化的驱动密切相关。

The Lower Jurassic Quse Formation in the Qiangtang Basin is an important source rock series in the basin.However,a systematic study on the mechanism of organic matter enrichment in this source rock is still lacking,which limits the understanding of the material basis of hydrocarbon accumulation in this area.In this paper,the 100-meter thick organic matter-rich shale of the Lower Jurassic Quse Formation in the Biluoqiao area of the South Qiangtang Depression is used as a study subject.we systematically investigated the the palaeoenvironmental and palaeoclimatic characteristics of shale at the time of deposition,analyzed the main controlling factors of the organic matter enrichment in the shale,and established models of differential organic matter enrichment in the shale of Quse Formation based on the integrated core,TOC,major and trace elements data.The results show that the shale can be divided into two intervals by the organic matter content which increased upward at both intervals.The lower interval(59~110 m)had a generally low organic matter content(the TOC contents range from 0.3%to 8.1%,with an average of 2.7%),and contained three thin organic matter-rich intervals.However,the organic matter content in the upper interval(0~59 m)was relatively high(the TOC contents range from 0.8%to 16%,with an average of 4.9%).The deposition of Quse Formation generally occurred in a humid environment,and was influenced by hydrothermal processes.The organic matter enrichment in the lower interval was mainly related to moderate terrestrial inputs,redox conditions and palaeoproductivity,and the organic matter enrichment in the upper interval was mainly controlled by redox conditions and palaeoproductivity.The difference in the organic matter enrichment mechanism between the upper and lower intervals of the Quse Formation may be closely related to the driving forces of palaeoclimate and sea level changes.