Research On Sedimentary Tectonic Evolution And Source Rocks Of Wuyu Basin

Wuyu basin, located in Namling of Shigatse region, is a representative remnant basinrelated to India-Eurasia collision and the intro-continental orogeny of Tibet. However, thepetroleum research has not been carried out yet. This article identifies the type of the basin, themodel of sedimentary tectonic evolution and the Organic geochemistry characteristics ofsource rocks. On this basis, the resource potential is confirmed. These research results wouldprovide great significance to the resources prospectiveness of Wuyu basin and the research onCenozoic tectonic evolution of Tibet. This article obtains ideas as follows:According to the analysis of sedimentary and tectonic feature, we set up the stratumsystem of the basin. Wuyu basin is a Cenozoic intermountain basin, formed from subductionnorthward of the Indian plate after the continent-to-continent collision between the Indian Plateand the Eurasian plate. The basin had successively undergone the initial lacustrine advance ofRigongla Formation, the maximum lacustrine advance of Mangxiang and Gazhacun Formation,the shrinking period of Zongdangcun Formation, which manifests a coarse-fine-coarse grainedcycle on the stratigraphic column. The tectonic evolution is divided into3periods, the initialperiod of the thrust, the strong period of the thrust and the shrinking period of the thrust.Found by geochemical analysis, Mangxiang Formation’s source rocks, which are at thepeak period of hydrocarbon generation with the mean of Ro reaching0.89%, have thecharacteristics of meliorating organic type with the type of II1, high abundance with theaverage of TOC reaching1.09%, superior potential hydrocarbon generation amount with themean of S1+S2reaching3.24mg/g and become the major source rocks in Wuyu Basin. Inaddition, the coal samples of Mangxiang Formation are good gas source rocks which havereached to a mature stage and are abundant in organic material, type III primarily. The averageof nC21-/nC22+is1.2in Mangxiang Formation, which stands for the organic material comesfrom the lower aquatic organisms; Pr/Ph with the average of1.2represents that thesedimentary environment of organic matter is a weak reduction deep or a semi-deep lacustrineone; the index of γ-acerance reaches3.32in average illustrates that water has a stratificationpattern and remains in high salinity situation in the sedimentary period, which is beneficial tothe organic preservation. The source rocks in Zongdangcun Formation are effective but still atlow-mature stage. According to the analysis of burial history, the source rocks collected fromRigongla, Mangxiang and Gazhacun Formation are in the active oil generation window. Partsof the strata in Gazhacun Formation have entered the oil-generating window. The remnant hydrocarbon amount in Wuyu basin has reached150million tons. All thatdiscussed in the article indicate that prospecting in Wuyu basin is valuable.