Effects of lacustrine hydrothermal activity on the organic matter input of source rocks during the Yanchang period in the Ordos Basin

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 translucent 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 oxygen of lake bottom and formed a reducing environment which was helpful for the preservation of organic matter and deposition of excellent source rocks.