Evaluation of the oil and gas preservation conditions, source rocks, and hydrocarbongenerating potential of the Qiangtang Basin: New evidence from the scientific drilling project

The Qiangtang Basin of the Tibetan Plateau,located in the eastern Tethys tectonic domain,is the largest new marine petroliferous region for exploration in China.The scientific drilling project consisting primarily of well QK-1 and its supporting shallow boreholes for geological surveys(also referred to as the Project)completed in recent years contributes to a series of new discoveries and insights into the oil and gas preservation conditions and source rock evaluation of the Qiangtang Basin.These findings differ from previous views that the Qiangtang Basin has poor oil and gas preservation conditions and lacks high-quality source rocks.As revealed by well QK-1 and its supporting shallow boreholes in the Project,the Qiangtang Basin hosts two sets of high-quality regional seals,namely an anhydrite layer in the Quemo Co Formation and the gypsum-bearing mudstones in the Xiali Formation.Moreover,the Qiangtang Basin has favorable oil and gas preservation conditions,as verified by the comprehensive study of the sealing capacity of seals,basin structure,tectonic uplift,magmatic activity,and groundwater motion.Furthermore,the shallow boreholes have also revealed that the Qiangtang Basin has high-quality hydrocarbon source rocks in the Upper Triassic Bagong Formation,which are thick and widely distributed according to the geological and geophysical data.In addition,the petroleum geological conditions,such as the type,abundance,and thermal evolution of organic matter,indicate that the Qiangtang Basin has great hydrocarbon-generating potential.

Geochemistry of upper Permian siliceous rocks from the Lower Yangtze region, southeastern China:implications for the origin of chert and Permian ocean chemistry

The Permian Chert Event is of great significance to understanding the geological evolution of the entire Permian; however,the origin of widespread chert formation is debated. We report new geochemical data from deep-marine siliceous rocks of the upper Permian Da-long Formation, Lower Yangtze region, southeastern China. Their geochemical results show that these thin-bedded siliceous rocks have a clear biologic origin, with rare to no evidence of hydrothermal influence. The values of Al/(Al + Fe + Mn) and Eu/Eu~* are 0.60-0.84(mean = 0.72) and 0.45-1.08(mean = 0.77), respectively, and Mn/Ti ratios are relatively low(mean = 0.72). The correlations of LaN/CeN, LaN/YbN, and Fe203/Ti02 with Al_2 O_3/(Al_2 O_3 + Fe_2 O_3), along with the Ce anomaly, indicate that the Da-long siliceous rocks were deposited at a transitional zone between a continental margin and the open ocean; i.e., relatively close to terrestrial sediment input and far from hydrothermal activity. The accumulation of chert is related to its unique paleogeographic location in an equatorial setting with many submarine paleo-highlands.Intense upwelling and frequent local volcanism are the main factors that promoted the development of siliceous rocks in the studied area. Ocean acidification triggered by large-scale volcanism(Large Igneous Province) during the late Permian led to extensive silica precipitation and preservation.

Study of an Early Silurian–Early Permian paleo-weathering profile in Zunyi, north Guizhou Province, China

To understand and help settle the controversy around the living time of Pinnatiramosus qianensis Geng, a paleo-weathering profile situated in the town of Yongle near the city of Zunyi, north Guizhou Province, China, was analyzed. The profile formed during a break in sedimentation between the Early Silurian and the Early Permian.Paleosol developed with a depth of several meters. The fossil plant P. qianensis Geng is present, but only in the lower portions(Layer 2) of the paleosol. Another plant with an irregularly branching system is found in Layers 2and 3. The distinct geochemical characteristics of the lower and upper portions of the Gaojiayan paleosol indicate a compound genesis. Its lower portions(Layers 1 and 2)resulted from in situ weathering of silty mudstone of the lower Silurian Hanjiadian Formation. The upper portions(Layer 3) are allochthonous. Transgression brought substantial concentrations K and Na, and led to K- and Naenrichment in the profile. Pumping of vascular plants and downward leaching enhanced the K enrichment in the middle portions. A superior preservation of P. qianensis Geng was observed in an exposure of Layer 2. Mass balance calculation indicates a great K enrichment related to bioaccumulation in the top of Layer 2 and a K loss in Layer3. Fossil plants(e.g., P. qianensis Geng) preserved in the paleosol are Permian rooting systems growing down into the lower Silurian rocks.