茂名盆地羊角含矿区始新统油柑窝组油页岩有机地球化学特征及沉积环境

Organic Geochemical Characteristics and Depositional Environment of Oil Shale of Eocene of Paleocene Youganwo Formation in Yangjiao Mining Area of Maoming Basin

针对茂名盆地油页岩成矿机理问题,利用茂名盆地羊角含矿区始新统油柑窝组油页岩莱科(Leco)、热解、含油率以及生物标志化合物等分析,研究了其有机地球化学特征,讨论了油页岩有机质来源与沉积环境。结果表明:油柑窝组油页岩有机质丰度高,有机质类型为Ⅰ-Ⅱ1型,含油率平均为5.98%,属品质中等偏好型;碳优势指数(ICP)值(3.73~4.37)、奇偶优势(OEP)值(7.59~9.57)、较小甾烷C29ααα20S/(20S+20R)、较低Ts/(Ts+Tm)以及最高热解峰温(Tmax)值(428.00~434.00℃)均指示油柑窝组油页岩热演化处于未成熟阶段。正构烷烃气相色谱(GC)图显示为单峰式,主碳峰为C27;∑C21-/∑C22+与∑C21+22/∑C28+29均呈现低值,ICP与OEP呈现高值,高碳数藿烷系列含量丰富,三环萜烷含量较低,未检测出奥利烷和羽扇烷等非藿类化合物,综合C27-C28-C29规则甾烷相对含量特征,确定油柑窝组油页岩有机质来源主要为湖相藻类体以及细菌、浮游生物与陆生植物混合来源。总有机碳质量分数/全硫质量分数(TOC/S)值(41.14~170.08)、姥鲛烷/植烷(Pr/Ph)值(0.82~1.30)以及重排甾烷/规则甾烷值(0.06~1.10)指示油页岩沉积时水体为淡水、还原环境。在亚热带-温带温暖-湿润气候背景下,藻类等水生生物勃发引起的高湖泊生产力和淡水、还原环境是茂名盆地油柑窝组油页岩成矿的有利条件。

In view of the forming mechanism of oil shale in Maoming basin, the authors studied the organic geochemical characteristics, and discussed the origin of organic matters and depositional environment of Eocene Youganwo Formation oil shale in Yangjiao mining area based on Leco, rock pyrolysis, oil yield, and biomarker analysis. The results show that the abundance of organic matter is high, the kerogen is type Ⅰ-Ⅱ1, the average oil yield is 5.98%, and the quality of oil shale is fairly high. ICP(3.73-4.37), OEP(7.59-9.57), low C29ααα20 S/(20 S+20 R), low Ts/(Ts+Tm),and Tmax(428-434℃) indicate that the oil shale thermal evolution of Youganwo Formation is in immature stage. The gas chromatography(GC) of n-alkanes has the features of unimodal distributions with nC27 as the mean peak. ∑C21-/∑C22+ and ∑C21+22/∑C28+29 are low, ICP and OEP are high. The content of high-carbon hopanes is high,while the content of tricyclic terpenes is low. No non-hoprii compounds such as oleane and lupane are detected. Combined with the relative content of C27-C28-C29,all the above indicate that lacustrine algae, mixed sources of plankton and terrestrial plants are the main source of organic matter of the oil shale of Youganwo Formation. TOC/S(41.14-170.08), Pr/Ph(0.82-1.30), and rearranged sterane/sterane(0.06-1.10) indicate that the paleo-lake condition was fresh and reductive when oil shale deposited. In a subtropical-temperate warm-humid climate, high lake productivity is caused by aquatic organisms such as algae bloom, and reductive fresh water environment is favorable for oil shale mineralization in Youganwo Formation of Maoming basin.