南黄海盆地二叠系高-过成熟烃源岩的生物标志化合物特征及其地质意义

Characteristics of biomarkers and the geological significance in highly to over-mature Permian source rocks in the South Yellow Sea Basin

南黄海盆地二叠系烃源岩的生烃层系多、热演化程度高、沉积环境变化大,前人极少从生物标志化合物的角度探讨烃源岩的差异。本文通过对CSDP-2井二叠系16个成熟-过成熟烃源岩样品进行有机地球化学分析,剖析了四套烃源岩的饱和烃、芳香烃馏分中生物标志化合物的组成、演化规律及地质意义。结果表明,二叠系不同层系烃源岩的甾烷系列、三芳甾烷系列、烷基二苯并噻吩系列化合物和β-胡萝卜烷的相对丰度具有显著差异,据此可将其分为栖霞组下段和龙潭组-大隆组泥岩、栖霞组上段和孤峰组硅质岩、孤峰组硅质泥岩3类烃源岩。研究显示,栖霞组下段和龙潭组-大隆组泥岩烃源岩沉积于淡水氧化或微咸水贫氧环境,有机质来源于浮游生物和陆生高等植物;栖霞组上段-孤峰组烃源岩沉积于还原咸水或静水硫化环境,其中硅质岩烃源岩的有机质来源于浮游生物和硅藻,硅质泥岩烃源岩的有机质来源于浮游生物、硅藻和陆生高等植物。此外,甲基菲指数、烷基二苯并噻吩参数(4-MDBT/DBT、MDBI、4,6-/1,4-DMDBT)可作为上二叠统烃源岩的成熟度指标,但不能作为中—下二叠统烃源岩的成熟度指标。

The Permian source rocks in the South Yellow Sea Basin(SYSB)are characterized by multi-sets of hydrocarbon-generating strata,high thermal evolution degrees,and dramatic changes in sedimentary environment.However,at present,little is known about the biomarker differences of the source rocks.Through organic geochemical analysis with 16 mature to over-mature source rock samples in the four sets of Permian source rocks of the CSDP-2 well in the SYSB,the compositions,evolution law,and geological significance of biomarkers in the alkane and aromatic hydrocarbon fractions are clarified.Biomarker parameters show that the relative abundance of the compounds in sterane series,triarylsterane series,alkyl dibenzothiophene series,and theβ-carotene varied greatly in different sets of source rocks in the Permian of SYBS.Three types of source rocks could be classified,namely,mudstones in the Lower Qixia Formation and the Longtan to Dalong Formation,chert in the Upper Qixia to Gufeng Formation,and siliceous mudstones in the Gufeng Formation.The research proved that mudstones in the Lower Qixia Formation,the Longtan Formation,and the Dalong Formation are deposited in oxic fresh or dysoxic brackish water conditions,in which the organic matter mainly derived from plankton and terrestrial higher planters.Source rocks in the Upper Qixia Formation and the Gufeng Formation are deposited in anoxic saline or euxinic sulfidic environment,in which the organic matter in chert is derived from plankton and diatom,whereas that in siliceous mudstones is derived from plankton,diatom,and terrestrial higher planters.Besides that,we proposed that the methylphenanthrene indexes and the alkyl dibenzothiophene parameters(4-MDBT/DBT,MDBI,4,6-/1,4-DMDBT)could be used as maturity scale for the Upper Permian source rock,but cannot be used for the Middle and Lower Permian source rocks.