Sedimentary model and its implications for oil and gas exploration of Upper Triassic in Northern Qiangtang Depression

Based on the measured data of the Upper Triassic section in the Northern Qiangtang Depression, We have studied the stratigraphic sequence and sedimentary facies, established the sedimentary evolution model, and revealed the indicative significance for oil and gas exploration in the Qiangtang Basin.The results show that:(1) the Late Triassic Carnian-Norian in the North Qiangtang Depression can be divided into two third-order sequences, with four sets of strata deposited in the area. The first third-order sequence corresponds to the Jiapila Formation(LST) and the Bolila Formation(TST and HST), the second third-order sequence corresponds to The Adula Formation(SMT,TST and HST) and the Duogaila Formation(RST). (2) The main sedimentary facies of the Jiapila Stage in the North Qiangtang Depression are fan delta and delta; the depression during Bolila period shows the coastal reef model, the distal steepening and gentle slope model and carbonate platform model, respectively; the Adula Formation and Duogaila Formation are dominated by shallow sea shallow shelf facies and delta facies, respectively; (3) the outer gentle slope facies of the Bolila Formation and the shelf facies of the Adula Formation are conducive to the formation of good source rocks, among which the widely developed shelf muddy source rocks of the Adula Formation are the most important. The oil in the Bolila Formation is mainly of marine origin and medium to high maturity, and mainly comes from the outer gentle slope facies marl of the Bolila Formation. The late low-maturity oil comes from the shelf-transitional facies Mixed organic matter of the Adula Formation.The North Qiangtang Depression has developed several large hydrocarbon-rich sags, and the Upper Triassic horizon has favorable conditions for hydrocarbon generation and accumulation. The study is of great geological significance to deepen the geological understanding of oil and gas in the Qiangtang and focus on the target formations for oil and gas exploration.