滇东北地区下石炭统万寿山组烃源岩生物标志物特征及沉积环境分析

STUDY ON CHARACTERISTICS OF BIOMARKERS AND SEDIMENTARY ENVIRONMENT OF LOWER CARBONIFEROUS WANSHOUSHAN FORMATION SOURCE ROCKS, NORTHEAST YUNNAN

为研究滇东北及邻区下石炭统万寿山组泥质烃源岩的有机质来源及沉积环境特征,共采集了33件样品,开展了烃源岩基本特征及生物标志化合物特征以及微量元素特征的研究。测试结果显示,烃源岩的有机质来源比较复杂。烃源岩正构烷烃图谱表现出机质具有水生生物和陆生植物双重来源;CPI值与OEP值无明显奇偶优势。正构烷烃的ΣC21-/ΣC22+值范围在0.21~7.68,(nC21+nC22)/(nC28+nC29)值范围在0.76~2.72,烃源岩在区域上可能具有陆源植物与海相生物混合来源的特征,甾族C27,C28和C29-ααα-20R关系三角图也证明这一点。同时,干酪根δ13Corg检测结果反映出研究区北部以Ⅲ型腐殖型干酪根为主,南部属于Ⅱ、Ⅲ混合型干酪根。样品的Ts/(Tm+Ts)比值在0.45~0.53,微量元素中的V/(V+Ni)比值在0.63~0.96,结合五环三萜烷的分布特征认为烃源岩形成于还原环境背景。伽玛蜡烷/αβ-C30藿烷比值在0.12~0.18,表明烃源岩沉积时水体盐度为正常盐度。根据样品中Sr/Ba值在0.23~7.9,B/Ga值在0.88~12.68,变化较大,说明万寿山组在研究区的南部属于半咸水的浅海环境,在研究区的北部属于滨浅海环境。

33 samples from the Lower Carboniferous Wanshoushan Formation source rocks in northeast Yunnan Province are collected for study of basic characteristics of hydrocarbon rock,biological marker compound and rare earth elements so as to understand their origin of organic materials and sedimentary characteristics.It shows that the sources of organic matters in Wanshoushan Formation source rocks are complex.The n-alkanes diagram for hydrocarbon source rocks shows that the organic matters come from a mixed origin, both form aquatic organisms and from terrestrial plants. There is no obvious parity advantage between CPI value and OEP value. The n-alkanes ΣC21-/ΣC22+ value ranges from 0.21 to 7.68, and(nC21+nC22)/(nC28+nC29) value from 0.76 to 2.72.The source rocks may have mixed sources of terrestrial plants and marine organisms, which are evidenced by relationship between C27,C28 and C29-ααα-20 R of sterane.Testing results of δ13Corg show that type Ⅲ kerogen is dominant in the northern area,while type Ⅱ and Ⅲ kerogen is dominant in the south area.The Ts/(Tm+Ts) ratio of the samples ranges from 0.45 to 0.53 and the V/(V+Ni) ratio ranges from 0.64 to 0.96.It is considered that the hydrocarbon source rocks were formed in a reducing environment according to the distribution characteristics of pentacyclic triterpenes.The ratios of gammaceranes/αβ-C30 hopanes is between 0.12 and 0.18,reflecting that the hydrocarbon source rocks were formed in the seawater with a normal salinity.Sr/Ba value in the sample is in the range of 0.23~7.9 and B/Ga value is in the range of 0.88~12.68 with a great variation,suggesting that the Wanshoushan Formation was in a shallow sea environment of brackish water in the southern area and in a coastal shallow sea environment in the northern area.