Hydrocarbon generation characteristics and exploration prospects of Proterozoic source rocks in China

A large number of primary oil and gas reservoirs have been discovered in Proterozoic strata all over the globe.Proterozoic sequences are widely distributed in China, and the discovery of large Sinian-aged gas reservoirs in the Sichuan Basin and Mesoproterozoic liquid oil seepages in North China shows that attention should be paid to the exploration potential of Proterozoic strata. In this paper, the main controlling factors of Proterozoic source rocks are discussed. Principally, active atmospheric circulation and astronomical cycles may have driven intense upwelling and runoff to provide nutrients; oxygenated oceanic surface waters could have provided suitable environments for the organisms to thrive; volcanic activity and terrestrial weathering caused by continental break-up would have injected large amounts of nutrients into the ocean, leading to persistent blooms of marine organisms; and extensive anoxic deep waters may have created ideal conditions for the preservation of organic matter. Additionally, the appearance of eukaryotes resulted in diversified hydrocarbon parent material, which effectively improved the generation potential for oil and gas. Through the comparison of Formations across different cratons, seven sets of Proterozoic organic-rich source rocks have been recognized in China, which mainly developed during interglacial periods and are also comparable worldwide. The Hongshuizhuang and Xiamaling Formations in North China have already been identified previously as Mesoproterozoic source rocks. The early Proterozoic Changchengian System is highly promising as a potential source rock in the Ordos Basin. In the Upper Yangtze area, the Neoproterozoic Datangpo and Doushantuo Formations are extensively distributed, and represent the major source rocks for Sinian gas reservoirs in the Sichuan Basin. Moreover, the Nanhuan System may contain abundant shales with high organic matter contents in the Tarim Basin, although this possibility still needs to be verified. Indeed, all three cratons may contain source rocks of Proterozoic strata; thus, these strata represent major exploration targets worthy of great attention.

Sedimentary evolution of Neoproterozoic rift basin in northern Tarim

Based on the outcrop,drilling and seismic data,the sedimentary successions,evolution and hydrocarbon exploration potential of the Neoproterozoic rift basin in the northern Tarim were firstly analyzed.Due to assembly and breakup of the Rodinia supercontinent,the Tabei paleocontinent and the Tarim paleocontinent were developed in the Tarim craton with an EW-trending back-arc rift basin between them during Neoproterozoic time;very thick marine clastic rocks,carbonate rocks and volcanic rocks(including tillite)were deposited in the Kuruktag and Aksu area of the northern Tarim,which experienced deep sea,bathyal sea and littoral sea environments with transitional delta and ice-sea.During the Early Cryogenian and the Late Ediacaran,the northern rift basin evolved from the deep sea to the littoralneritic sea,while the lithology changed from clastic rocks to carbonate rocks.According to the field and production data,the formation and preservation of the source rocks and reservoirs indicate a good exploration potential of the Neoproterozoic rift basin.

Evolution of the Neoproterozoic rift basins and its implication for oil and gas exploration in the Tarim Basin

According to the aeromagnetic,seismic,and geological data,the surface structure,sedimentation,and distribution characteristics as well as the deep dynamic mechanism of the north-south differentiated Neoproterozoic rift basins were well investigated to reveal the tectonic evolution and its control on the distribution of the Early Cambrian sedimentary basin and source rocks in the Tarim Basin.The rift basin in the southern Tarim was a product of superplume activities during the early breakup period of the Rodinia supercontinent.It initiated in the Early Cryogenian(about 780 Ma)and appeared as NE-direction aulacogens extending into the Tarim Basin.The rift basin in the northern Tarim was a back-arc rift basin derived from the subduction of Pan-Rodinian oceanic plate,which initiated in the Late Cryogenian(about 740 Ma)and occurred as a nearly EW-direction narrow band across the Tarim Basin.The northern Tarim back-arc rift basin had similar formation and evolution process to the Late Mesozoic-Cenozoic back-arc rift basins in East Asia,both showing an oceanward migration;however,the Tarim rift basin finally evolved from the fault-depression basins into passive continental margin.The Neoproterozoic rift basins controlled not only the distribution of source rocks in the syn-rift period but also the development of Early Cambrian sedimentary basin.Nearly EW-distributed syn-rift(Cryogenian to Ediacaran)and postrift(Lower Cambrian Yurtusi period)source rocks were likely to develop between the present Tabei uplift and central uplift belt.