ABSTRACT To reveal the effects of lacustrine hydrothermal activities on the organic matter input of source rocks and the formation of excellent source rocks in the Ordos Basin during the Middle and Late Triassic Yanchang period, samples of the Yanchang Formation in the YK1 well in the southern Ordos Basin were selected; source rock evaluations, element geochemical analysis, and palynofacies studies were systematically conducted. The results show that excellent source rocks are developed in the Chang 7-3 interval. The intensity indicators of hydrothermal activity (IIHAs), namely Al/(Al + Fe + Mn) (IIHA1) and (Fe + Mn)/Ti (IIHA2) show that the lake underwent three episodes of obvious thermal fluid activity during the depositional period of the Chang 7-Chang 6 members of which the Chang 7-3 period was the peak of hydrothermal activity. The main hydrocarbon-generating components in the source rocks of the Yanchang Formation were granular amorphous organic matter (AOM) and algae and gelified particles were the secondary hydrocarbon-generating components. Three palynofacies assemblages have been identified in the source rocks. Assemblage A is distributed in the Chang 8-Chang 9 members and Chang 6-2 + 3 interval. The organic matter present in this assemblage mainly consists of type III and transparent lignocellulosic fragments and gelified particles comprise the dominant organic debris. Assemblage B is distributed in the Chang 7-3 interval which consists mainly of Type I and Type II organic matter and is characterized by high abundances of algae and AOM. Assemblage C is distributed in the Chang 7-1 + 2 interval and consists mainly of Type II organic matter which is characterized by the simultaneous development of AOM and phytoclasts. The correlations between the contents of AOM and algae in the source rocks and the IIHAs indicate that hydrothermal activity may have led to the development of algae and source rock formation. The distribution of AOM and algae show consistently high contents over the entire Chang 7-3 interval which are not only coupled with the peak of total organic carbon content, liquid hydrocarbon potential, hydrogen index, and peak of hydrothermal activity but also shows different degrees of correlation with the IIHAs. Their relationships indicate that the peak of the lake’s hydrothermal activity may have induced algae and plankton blooms, which may have been one of the important controlling factors for the formation of excellent source rocks. The close relationship between the IIHA and sulfur content indicates that hydrothermal fluids may have further deprived the lake bottom of oxygen and formed a reducing environment which was helpful for the preservation of organic matter and deposition of excellent source rocks.

中文翻译：

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鄂尔多斯盆地延长期湖相热液活动对烃源岩有机质输入的影响

摘要 为揭示鄂尔多斯盆地中、晚三叠世延长期湖相热液活动对烃源岩有机质输入和优质烃源岩形成的影响，以鄂尔多斯南部YK1井延长组样品为研究对象。盆地被选中；系统地进行了烃源岩评价、元素地球化学分析和孢粉相研究。结果表明，长7-3层段发育优良烃源岩。热液活动(IIHAs)强度指标Al/(Al+Fe+Mn)(IIHA1)和(Fe+Mn)/Ti(IIHA2)表明，该湖在沉积期经历了3次明显的热流体活动长7-长6段，其中长7-3期为热液活动高峰期。延长组烃源岩主要生烃组分为粒状无定形有机质（AOM），藻类和凝胶化颗粒为次生生烃组分。在烃源岩中已识别出三个孢粉相组合。组合A分布在长8-长9成员和长6-2+3区间。该组合中存在的有机物质主要由 III 型和透明的木质纤维素碎片组成，凝胶化颗粒构成主要的有机碎片。组合B分布于长7-3层段，以Ⅰ类和Ⅱ类有机质为主，藻类和AOM丰度高。组合C分布于长7-1+2区间，主要为Ⅱ类有机质，其特征是AOM与植株同时发育。烃源岩中AOM和藻类含量与IIHAs的相关性表明，热液活动可能导致藻类和烃源岩的发育。AOM和藻类分布在整个长7-3层段均呈现出持续的高含量分布，不仅与总有机碳含量峰值、液态烃潜力、氢指数和热液活动峰值相耦合，而且还表现出不同程度的与 IIHA 的相关性。他们的关系表明，该湖热液活动的高峰期可能导致藻类和浮游生物大量繁殖，这可能是形成优良烃源岩的重要控制因素之一。IIHA与硫含量的密切关系表明，热液可能进一步剥夺了湖底的氧气，形成了有利于有机质保存和优质烃源岩沉积的还原环境。