A change from a humid to an arid climate occurred during the Middle Jurassic. Information about this transitional phase is stored in two oil shales: the lower and upper units of the shale member of the Shimengou Formation (J2s) in the northern Qaidam Basin (NQB), Northwest China. The drastic changes in lithofacies are also reflected in the organic and inorganic inventory of the two oil shales, and suggests the existence of this aridification event. Thus, the two units varied greatly in organic matter source, productivities and depositional environments. The lower unit is interpreted as lacustrine sediment deposited under a humid climate with a high terrigenous organic matter portion besides algal biomass. Furthermore, the high sedimentation rate and large organic matter size prevented organic matter degradation. In contrast, deposition of the upper unit was coincident with progressive aridification, which resulted in minor terrigenous organic matter input, and enhanced algal biomass deposition. Its preservation was favored by an enhanced salinity and stratification of water column with an underlying anoxic bottom water condition. Today, the sediments of the upper unit are characterized by a high total organic carbon (TOC) content and type I kerogen.
In summary, two depositional scenarios of source rock formation under different climatic conditions are introduced to highlight the effect of progressive aridification on the evolution of source rocks. Moderate aridification in the transition period (early stage of aridification) is suggested to have promoted the formation of lacustrine source rocks with the potential to generate waxy oil. The occurrence of similar lacustrine source rocks is also predictable when deposited while similar aridification events, particularly in the Jurassic with its frequent climate changes.