Discovery of Late Permian pollen and spores in TD-2 borehole in Tuquan Basin of Hinggan League of Inner Mongolia and their oil and gas geological implications

Abstract

Tuquan Basin near the west boundary of the Songliao Basin is a Mesozoic sedimentary basin formed above the Paleozoic strata, which has aroused much attention from geologists due to the discovery of light crude oil from the Jurassic strata in recent years. In the past, systematic research on the oil and gas resources from the Paleozoic was very insufficient. In this paper, the methods such as stratigraphic (lithostratic) and sporopollen analysis and source rock test were conducted so as to analyze synthetically the core samples collected from TD-2 well in the Mangniuhai depression of Tuquan Basin. A stratigraphic unit composed of grayish black mudstone and siltstone beneath the Middle Jurassic conglomerate was discovered in TD-2 well, and some spores and pollen were extracted from the unit, which consist of 26 species and 19 genera, including Leiotriletes adnatus，Punctatisporites debilis，Alisporites auritus，A. communis, A. stenoholcus, Klausipollenites schaubergeri, Pityosporites evolutus, Sulcatisporites dalongkouensis, Hamiapollenites indistinctus, Piceaepollenites opimus etc. indicating Late Permian. Combined with the research results such as the stratigraphy (lithostratic) and source rock test, it is reasonably assigned to Late Permian Linxi Formation, and some high-quality source rocks are discovered in this unit. This discovery is significant for confirming the existence of Permian strata in the Tuquan Basin and also for exploring the late Paleozoic oil and gas resources in the Songliao Basin and adjacent regions.