Organic Geochemical Characteristics and Thermal Evolution Characteristics of Paleogene to Neogene Source Rocks in Mangai Area, Qaidam Basin
Shiling Shi, Changan Shan, Ze Zhao, G. R. Ho, Jiaqi Zhang
Abstract
The Paleogene-Neogene lacustrine source rocks in the Mangai area of the Qaidam Basin are significant exploration targets. A comprehensive evaluation of four major intervals (E$_3^2$, N$_1$, N$_2^1$, N$_2^2$) was conducted through systematic organic geochemistry, maceral identification, thermal maturity analysis, and burial-thermal history modeling. Results show TOC contents of 0.203~1.28% (avg. 0.74%), chloroform bitumen "A" of 0.0164~0.2495% (avg. 0.132%), and total hydrocarbon contents of 69.29~1637.48 $\mu$g/g (avg. 853.39 $\mu$g/g), indicating fair to good quality. Kerogen elemental analysis (H/C=0.97~1.21), maceral composition (sapropelinite+exinite=70~98%), and saturated hydrocarbon chromatography (dominant peaks at C17, C19, C21, C23) collectively indicate predominantly Type II$_1$ organic matter, with locally developed Type I. Thermal evolution exhibits distinct vertical zonation controlled by Neogene tectonic-thermal events: an immature-low maturity zone (R$_0$<0.7%) dominated by soluble organic matter degradation; a mature zone (R$_0$=0.7~1.0%) with active kerogen cracking (HC/TOC up to 224.95%); and a high maturity zone (R$_0$>1.0%) characterized by gas generation via cracking. The saline to semi-saline reducing environment favored organic matter preservation, while mixed algal-terrestrial inputs (Type II$_1$) determined mainly oil-prone characteristics. The thick succession in the central depression, especially the Kaitemilike-Fenghuangtai region, shows the highest hydrocarbon potential. The mature zone (R$_0$=0.7~1.0%) is optimal for conventional oil exploration, while the high maturity zone (R$_0$>1.0%) holds promise for natural gas. This multi-parameter study deepens the understanding of hydrocarbon generation mechanisms and provides an important case for evaluating Paleogene-Neogene lacustrine source rocks in the northern Tibetan Plateau.