Organic matter abundance and significance of oil geology of the Cretaceous mudstones and shales from Rikaze area, southern Tibet, China

In this paper the research of organic matter abundance of dark mudstones and black shales in the Cretaceous strata which are developed completely and large-thickly in the marine environment from southern Tibet is carried out. The results of composition analysis reveal that the contents of organic carbon vary between 0.05% to 1.1％,and its average value during the Cretaceous period is about 0.42%, approximately 0.531％and 0.265％during the early and late Cretaceous respectively. The variety of average content of organic carbon is related to the paleogeography situation of the study area and sea-level oscillation. The contents of chloroform bitumen ‘A’ range from 0.0009% to 0.0072％, and its average value is about 0.0040％. The ratio between maximum and minimum content of both organic carbon and chloroform bitumen ‘A’ is 22 and 8. The vague negative correlation of organic carbon and chloroform bitumen ‘A’ is concerned with their lost quantities suffered from heavy surface weathering, drastic ultraviolet irradiation and high degree of thermal evolution of organic matters, and the later content is drastically higher than the former. Ten pieces of moderate and good samples of oil resource have organic carbon contents over 0.6% during the Cretaceous period, which accounts for 27% of the total samples. Nine pieces of them are from late Cretaceous, which accounts for 24.3% of the total. This fact demonstrates the Cretaceous mudstones and shales from Dongshan Formation in the north Himalayas area and Gyabula Formation in the Kangma-Longzi area are the main effective oil-bearing resource in the study area.