Organic geochemistry and organic matter enrichment mechanisms of the lacustrine source rocks from a small-faulted Basin: A case study of the Bayindulan, A'er and Wuliyasitai sags in the Erlian Basin, Northern China

Abstract

Lacustrine organic-rich shales in the lower Tengger member (K1bt1) are the most important source rocks in the Erlian Basin. The Erlian Basin contains many sags with independent subsidence-sedimentation histories, structure styles and hydrocarbon exploration potential. A comprehensive study was carried out to investigate the organic geochemical characteristics of the shales from three typical sags (the Bayindulan, A'er and Wuliyasitai sags) in the Erlian Basin to define the differences to their depositional environment, dominant source material, and hydrocarbon generation potential in small-faulted basins. Kerogen types in these shales in the study area can be classified into type Ⅱ1-Ⅰ kerogen in the Bayindulan Sag, type Ⅱ2 to type Ⅱ1 in the A'er Sag and type Ⅱ2 in the Wuli Sag. Biomarker characteristics indicate that the dominant algae-sourcing organic matter (OM) in the shales in the Bayindulan Sag was formed under the salty and reducing conditions. Source rocks from the A'er Sag were formed under the fresh-brackish and weak reducing-weak oxidizing conditions with a mixing input of algae and higher plants. Organic-rich shales from the Wuli Sag were deposited under the fresh water and oxidizing environment with a terrestrially dominated organic matter input. The correlation among parameters including average chain length (ACL) of C25–C33n-alkanes, the aquatic/macrophytes proxy (Paq), TAR, the proportion of saropelinite, the wax indexes, Pr/Ph, and TOC were used to investigate the main controlling factors of the enrichment of the organic matter in these three sags. The organic matter accumulation models on the formation of high-quality shales in the Erlian Basin are anisotropic due to various depositional environments and biological sources of source rocks in different regions. The characteristics of organic matter accumulation in the Erlian Basin can be classified into three patterns. Firstly, the good preservation condition that favored the accumulation of organic matter is the main controlling factor resulting in the organic matter enrichment in the Bayindulan Sag. Secondly, the bloom of algae in nutrient-rich surface water favored the accumulation of OM in the A'er Sag. Finally, large influx of vascular plants in the oxygen-rich condition is the main factor leading to the formation of the high-quality shales in the Wuli Sag. Further, these three organic matter accumulation models could be used for reference in the assessment of the petroleum potential of the faulted basins that are similar with the Erlian Basin.