Organic Geochemical Characteristics Of Upper Triassic And Lower Jurassic Strata In Qiangtang Basin And Source Rock Evaluation

The Upper Triassic and Lower Jurassic strata in Qiangtang Basin are regarded as the most potential area for hydrocarbon exploration in the Qinghai Tibet Plateau region.Previous studies on organic geochemistry were mainly based on surface outcrop samples that were weathered in various degree.This study used records of organic geochemistry,sedimentary environment and lithofacies palaeogeography to evaluate the source rocks,with the purpose to provide the drilling well data for the oil and gas exploration in Qiangtang BasinThe Upper Triassic Xiaochaka Formation is mainly composed by a sequences of sand and silt clastic rocks,lack of thick and dark mudstone layers.There are 5 single silty mudstone layers with 1m thickness.On the other hand,4 dark mudstone layers were seen in the Lower Jurassic Quse Formation.Single dark mudstone layer with maximum thickness of 11.36 m also occurred.It obtained the organic geochemical records of these two formations.For the Upper Triassic Xiaochaka Formation,the average content TOC value is 0.75%.,and 8sections were belong to the intermediate hydrocarbon source rocks;The values of abundance coefficient of organic matter(24?10)indicated the Humic sludge type(II2),Mean vitrinite reflectance values(Ro = 0.98 %)indicate that organic matter from the sampled sections is in a mature stage.For the Lower Jurassic Quse Formation,the average high content TOC(4.20 %)were obtained.The high values of abundance coefficient of organic matter(24?10)indicated the Humic sludge type(II1)?Mean low Ro value(Ro = 0.51 %)indicate that organic matter from the sampled sections is in an immature or low immature stage.A preliminary analysis of sedimentary environment suggested a short transport distances of sediments during the both deposition periods of the Late Triassic and Early Jurassic periods.However,sediment sources were very abundant with fast deposition rates in the Late Triassic.Preliminary evaluation suggested that the Upper Triassic Xiaochaka Formation might belongs to poor to middle grading source rocks.The Lower Jurassic Quse Formation is a favorable source rock in the Qiangtang Basin...