Geochemical Characteristics Of Hydrocarbon Source Rocks In Nima Basin, Tibet

Nima basin is a Mesozoic-Cenozoic continental basin of east-west orientation, located in the central part of the palteau, Bangong-Nujiang suture zone. Adjacent to Lhasa terrane on the south and Qiangtang on the north, joins Lunpola Basin on the east.The research focus mainly on continental cap rocks and marine basement, collect samples to analyse and discern the geologic features of souce rocks. Paleocene-Eocene Nubao Group, Oligocene Dingjinghu Group and early cretaceous Duoni and Niubao Group are the main source rocks in the area.This paper was divided into 5 chapters, including preface and geological background of the study region, and was totally about 50 thousand words and 46 illustrations. The main conclusions and results are summarized as follows:On the basis of the research predecessor, this paper studies the samples collect from continental basement and marine cap rocks of Nima basin, and analyse sedimentary facies of filed section. In Niubao Group, it composed of 5 sedimentary facies, including fan-delta facies, lacustrine facies, alluvial-fan facies, delta facies and fluvial facies. Besides, the oil shale formed in fan-delta facies and lacustrine facies.Based on souce rock evaluation criterion of Lunpola basin and Qiangtang basin, this paper choose 0.3% to be the mudstone'minimum amount, and 0.1% to be the carbonate's minimum amount. In continental souce rocks, the oil shale of Ejiuxiang Niubao Group have the highest amount of TOC, and the average reaches 15.23%, which is at good level; The oil shale of Xintunacuo rank second, and the average amount of TOC is 0.52%, is considered fairly good; The mudstone samples of Xintunacuo and Cha'angba are considered to be relatively poor; And the shale of Kangmaerle and limestone of Jiwasha can not reach the minimum amount for TOC, so they are not source rock. Langshan Group of marine cap rocks has relatively poor source rock, source rock of Duoni Group barely has no hydrocarbon generating potential.According to carbon isotope geochemistry of kerogen and kerogen maceral characteristics mirroring comprehensive analysis, Cha'angba mudstone have the best organic matter type with the features of type I—II1, and organic matter type of oil shale of Ejiuxiang and Xintunacuo are not good, with the features of type II2 mostly, slight type III. From biomakers aspects, organic matter type of marine source rock is studied, conclusions are the source rock of Langshan Group is formed in salt water, which is deep and lack of oxygen, organic matter is type II1—II2, most of which is from aquatic organism. Source rock of Duoni Group display deep water, lack of oxygen and half salt water environment, the organic matter is type II1—II2, which is effected both by aquatic and land sourced organism.The vitrinite refletance of continental source rock is between 0.6 and 0.86, there are no significant difference in maturity of each samples, they are all in early mature stages. According to isomer rate of biomaker and OEP values, it is considered that samples of Langshan Group are on the stage of high maturity, while samples from Duoni Group not having reach high maturity yet, being early-maturity.From the lithology, source rock organic matter abundance, organic matter type, maturity of organic matter and organic geochemical studies on the preliminary evaluation of source rocks. The oil shale of Niubao Group at Ejiuxiang have high abundance of organic matter, organic matter type is moderate, and just reach early-maturity, which is the region's main source rock strata; While the oil shale of Niubao Group at Xintunaruo is of high organic matter abundance, it is considered to be substantial source rock strata in Nima basin; Although the organic matter abundance of Cha'angba mudstone is not high enough, but the type of organic matter is great, so in some area there may be a favorable section of hydrocarbon source rocks; For the reason that souce rocks of Duoni Group and Langshan Group have low amount of TOC and reach high maturity, they are not source rocks.Comparison analysis is made between oil shale of Nima basin and Lunpola basin, the result is the oil shale of Nima basin has higher TOC and lower hydrocarbon generation substantial, and oil shale of Lunpola basin has better organic matter type, which is formed in salt water reductive environment.The organic matter type of oil shale of Nima basin is not so good, mostly type II2, few type III, which is formed in fresh water oxide environment. The maturity of organic matter in Nima basin is in early-mature stage, while Lunpola basin is in immature-low mature stage.To make assessment comprehensively, the oil shale of Lunpola has better quality than Nima basin.