The Sedimentary Evolution During Late Triassic-Jurassic Period and the Hydrocarbon Exploration Direction in the Eastern Part of the Qiangtang Basin, Tibet

摘要

Based on the field outcrops surveyed,combined with recent published the regional tectonic evolution and geochronology data,we analyzed the lithologies and rock associations of strata,identified the sedimentary facies types,and discussed the distribution sedimentary facies and the hydrocarbon accumulation in the eastern Qiangtang basin during the Late Triassic–Jurassic.Marked by regional unconformities,there are two tectono-stratigraphic units(from the Carnian to the Norian and from the Rhaetian to the Kimmeridgian,respectively)in the eastern part of Qiangtang basin.We systematically described the distribution range,thickness variation and lithological characteristics of different formations in the tectonostratigraphic units.The Late Triassic-Jurassic is dominated by marine facies and marine-continental transitional facies.The marine-continental transitional facies include deltaic and tidallagoon facies.Marine facies including gentle carbonate slope,evaporative platform,restricted platform,littoral,neritic,bathyal and abysmal facies.The Carnian stage is dominated by littoral–neritic–bathyal–abysmal facies in the north Qiangtang depression otherwise the littoral–neritic facies in the south Qiangtang depression.The early Norian stage is dominated by carbonate gentle slope-mixed continental shelf facies.The late Norian,Bajocian,Callovian and Kimmeridgian stage are dominated by tidal flat-delta facies in the north Qiangtang depression and littoral-neritic facies in the south Qiangtang depression.The Bathonian and Oxfordian stage are dominated by evaporative platform-restricted platform-mixed continental shelf facies.The sedimentary facies formed zones from north to south and extended in an E–W direction.The Eastern Lower Uplift(ELU)played an important role in the division zones of sedimentary facies from north to south.During the Bathonian and Oxfordian,the ELU developed below the sea level and controlled the distribution of restricted platform,evaporative platform and platform margin.We analyzed 20 source rock samples from the upper Triassic-Jurassic.The total organic carbon(TOC)value from Qoimaco,Buqu and Adula Formations.of late TriassicJurassic in the eastern Qiangtang basin are ranges from 0.17~0.33%(average 0.28%),0.05~0.25%(average 0.15%)and 10.32~28.78%(average 19.33%),respectively.Obviously,the Adula Fm.developed good source rocks.The values of Tmax and S1+S2 in the Adula formation are 459-461℃(average 460℃)and 6.75-28.55 mg/g(average 18.18 mg/g),indicating that the Adula source rock has reached high-over-maturity stage.The bathyal,gentle slop and platform facie belts of the Upper Triassic can configurate the good hydrocarbon prospects in the northeastern area of the Qiangtang basin.