鄂尔多斯盆地长7油层组油页岩特征及成因分析

Study on the Characteristics and Origin of the Oil Shale in the Chang 7 Member, Yanchang Formation in Ordos Basin

摘要: 鄂尔多斯盆地油页岩资源丰富，主要发育于三叠系延长组三段长7油层组，分布在盆地南部，矿层厚度一般为5~30 m，最厚可达40 m。在野外剖面实测基础上，对大量地表和井下油页岩样品测试数据进行了地质学统计。结果表明：油页岩含油率主要分布于4%~9%，平均为7%；发热量主要为7~13 MJ/kg，平均为9.10 MJ/kg；灰分分布于45.6%~90.1%，平均为69.5%；全硫含量为0.01%~14.01%，平均为2.82%，总体为高灰分、中–富硫、中品级油页岩。油页岩有机质主要由腐泥型(Ⅰ)和腐殖腐泥型(Ⅱ1)组成，大部分进入成熟度较高的生油阶段，有机质丰度高，具有较高的产烃率。地质地球化学特征表明，长7油页岩形成于气候温暖潮湿，湖泊水深大于15 m的半深湖–深湖，淡水–半咸水，封闭、滞流，缺氧的强还原沉积环境。晚三叠世长7沉积期间，生物繁盛，湖泊自身的生产力为盆内长7油页岩的形成提供了最主要的有机物质来源，稳定的湖水分层形成的湖盆底部缺氧的强还原环境，有利于有机质的堆积和保存，从而形成了有机质类型较好、有机碳含量较高、含油率较高的油页岩。
> In Ordos basin, which is rich in oil shale resource, oil shale is mainly found in the southern part of Chang 7 member of the Triassic Yanchang-3 Formation, with common thickness of 5 to 30 m and 40 m of maximum. In this pa- per, a statistic synthetic study on test data of both surface and borehole oil shale sample was conducted based on the field measures of oil shale sections. Research shows that oil content ranges from 4% to 9% with average of 7%, calo- rific value varies between 7 MJ/kg and 13 MJ/kg with average of 9.1 MJ/kg, ash content is between 45.6% and 90.1% with average of 69.5%, and total sulfur values are between 0.01% and 14.01% with the average of 2.82%. The oil shale in the mass is characterized as high ash, medium to high rich sulfur, and medium grade oil shale. The organic matters of oil shale, with high content and mainly of sapropelic and humic-sapropelic type, are dominantly in the mature stage and have good hydrocarbon generation potential. It is made clear by geological and geochemical study that the Chang 7 oil shale was formed in semi-deep and fresh-half salt lake with water depth more than 15 m under warm-humid climate, where it was a closed, stagnant, anoxic and strong reducing sedimentary environment. During Chang 7 deposition stage of Late Triassic, biological prosperity provided a source of organic matter for the formation of Chang 7 oil shale, the stable water stratification resulted in the strong reducing environment of the lake bottom, which was favorable for the accumulation of organic matter and its preservation and thereafter forming good type oil shale of high organic carbon content and high oil content.