Source Rock Characteristics and Petroleum-Geological Significance of Paleogene Niubao Formation in the Nima Basin, Central Tibet, China

Abstract

Nima Basin, as a typical Tethys area, is a Cenozoic oil-bearing basin in the middle of Qinghai-Tibet Plateau, which is expected to become an important target for petroleum exploration. The main controlling factors and models of organic matter enrichment are not yet clear. To identify the organic geochemical characteristics of high-quality source rocks and the main controlling factors of their formation, this study analyzes the paleoclimate and paleo-salinity indexes based on the analysis of the major and trace elements through systematic sampling of the muddy sediments of Niubao Formation in Well Shuangdi 1, Cebuco Depression. The results show that the organic matter of the E_1-2n source rocks in Nima Basin is differentially enriched, which shows that the organic matter of the middle E_1-2n source rocks is more enriched than that of the upper E_1-2n source rocks. Hydrocarbon source rocks of the middle E_1-2n are mainly of type I organic matter, while those of the upper E_1-2n are mainly of type Ⅱ₁ organic matter, both of which are in the mature stage. Organic matter enrichment is primarily governed by a combination of climate and redox conditions, with humid climate and reducing environments emerging as the dominant factors, whereas salinity exerts a lesser influence. On the one hand, this study can supplement the accumulation mechanism of organic matter in the Tethys domain in Asia; on the other hand, it will fill the gap in the geological research of Nima Basin, contributing to the prediction and exploration of oil and gas resources in the Nima Basin.