Mesozoic and Cenozoic tectonism and its control on hydrocarbon accumulation in the northern Qaidam Basin of China

The northern area of Qaidam Basin was considered as a compressive basin in the Jurassic and Cretaceous times.In reality,it had involved into a flexural basin as a result of weak compression since the Early-Mid Jurassic.This inference is based upon the restoration study of several main structural cross sections,regional geological data,and analysis of petroliferous circumstances. The Jurassic source rocks still possessed an oil-bearing potential in the Cenozoic time,because they did not reach the threshold of hydrocarbon generation during the Mesozoic burial history.Two periods of strong contractile deformation occurred in the Late Eocene and since the Pliocene respectively were recognized.The deformations had obvious influence on the formation of oil-gas pools.Correspondingly,there existed two types of pools,e.g.primary and secondary pools.The primary pools were developed in traps formed during the first-epoch and previous compression deformation, while the secondary pools only occurred during the second epoch in the shallow-level and fault-bounded fold traps and would be generated only in the regions strongly modified by intensive faults that clearly penetrated into the Pliocene and underlying strata.