A Study On Petroleum Geochemical Characteristics Of The Main Mesozoic Source Rocks In Qiangtang Basin, Tibet, China

Qiangtang Basin is considered to be of good prospect for oil and gas exploration. Due to that it is located in the Qinghai-Tibet Plateau under special geographical and geological conditions, it remains low level explored so that many geoscientifical and exploration methodological problems need to be solved. Based on a series of organic geochemical analyses of 164 outcrop and core source rocks collected from Qiangtang Basin, this thesis reports the results of a study on the petroleum geochemical characteristics of the main Mesozoic source rocks of Qiangtang Basin in order to provide some fundamental references for the studies on the oil generation, migration and accumulation history and patterns in the basin.The organic geochemical data shows that the organic matter in the Jurassic and Triassic source rocks in Qiangtang Basin has a very high maturity at high mature-postmature stage. The vitrinite reflectance of kerogen (Ro) is above 2.8%. The T_max values of Rock-Eval analyses are above 455℃for most of the source rocks. The inertinites and liptinites (formerly called exinite) disappear from kerogen due to that they are derived into secondary maceral groups. Furthermore, the organic matter in the source rocks is basically proved to be of only residues. The Rock-Eval pyrolysis of source rocks shows that the content of S₁ component takes 0.04 mg/g as the maximal, S₂ component ranges 0.01-0.10 mg/g, while S₄ component dorminates with its content up to 11.11 mg/g. The content of chloroform bitumen A is only 0.0005% to 0.0283% as well.The abundance of organic matter in the source rocks is very low, and the TOC value varies within a narrow range. The TOC values of the mudstones and shales range from 0.03% to 0.52%, while those of the carbonate source rocks range from 0.05% to 1.22%. According to the relevant source rock classification criterion, source rocks in Qiangtang Basin belong to medium to good source rock classes.The sedimentary environment of Jurassic source rocks in Qiangtang Basin was marine and reductive. The Pr/Ph ratio ranges from 0.18 to 0.8 while most source rocks have the value smaller than 1. The nC₂₁/nC₂₁+ value lies between 0.84 and 2.18 while most of the values greater than 1. The organic matter is mainly derived from aquatic organisms together with small amount of terrestrial organism input. Most of the source rocks demonstrate a single-peak type distribution pattern of n-alkanes with the highest peak of C₁₇ component. The regular steranes show a "V" type distribution pattern, in which the abundance of C₂₇- and C₂₉-steranes is roughly equal.The high level thermal evolution has seriously affected the molecular geochemical characteristics of the Mesozoic source rocks in Qiangtang Basin. Both steranes and terpanes suffered serious damage, and plenty of polyaromatics and alkenes were derived as the result. Even the carbon isotope composition has been obviously altered as well.The geochemical information of the outcrop source rocks in Qiangtang basin is obviously altered by weathering effection. The outcrop source rocks have clearly lower TOC content and lower abundance of soluble organic matter than those of core source rocks, but the T_max value of Rock-Eval analysis obviously increases. The oxygen and nitrogen in Kerogen of the outcrop source rocks loss so obviously that the composition of the organic elements shows no regularity varying with the formations what is held by the core source rocks.