Natural Gas Genesis, Source and Accumulation Processes in Northwestern Qaidam Basin, China, Revealed by Integrated 3D Basin Modeling and Geochemical Research

The northwestern Qaidam Basin has great potential for natural gas exploration. Research and exploration activities of natural gas conducted over the last two decades have provided abundant information about natural gas geochemistry. However, its genesis, source, and accumulation process are still controversial. This research utilized a published comprehensive natural gas dataset and a 3D basin modeling approach to ascertain the genesis of natural gas in the northwestern Qaidam Basin and to discuss its accumulation process. Moreover, diagrams for classifying the source material type were revised to better understand the origin of natural gas. In the northwestern Qaidam Basin, the source material type of natural gas is chiefly coal-type gas with a few oil-type gases. Natural gases from the Jianshan anticline originated predominantly from the Jurassic source rock, while those in most other regions were derived mainly from the Upper Xiaganchaigou source rocks. However, some oil-associated gas related to oil accumulation came chiefly from the Shangganchaigou source rock. In addition, a small amount of early mature thermogenic gas in the Xiaoliangshan, Yondunzi, and Xianshuiquan regions may have come from the Lower Youshashan source rock. Moreover, this research discovered the secondary cracking gas of oil in the northwestern Qaidam Basin, which has a great significance in broadening the natural gas exploration in an area. Similarly, the gas accumulation models were established for three types of gases, i.e., oil-associated gas, late primary cracking gas, and oil-cracking gas. This study emphasizes geological research based on a comprehensive field dataset analysis and comparing adjacent mature blocks. This methodology can be used in other emerging natural gas exploration areas.