四川盆地早三叠世多尺度海平面变化旋回及对储层发育的影响

Multiple scale fluctuations of the Early Triassic sea level and its influence on reservoirs in the Sichuan Basin

四川盆地东北部鸡唱峭壁剖面发育了下三叠统碳酸盐鲕粒浅滩层序。为揭示该层序形成过程中的海平面变化,进行了一系列同位素和元素地球化学分析。C和O同位素变化反映了高频的海平面变化,可用于地层层序的划分。δ^(13)C增加和δ^(18)O的减少,表明海平面的上升,发育退积准层序组;δ^(13)C的减少和δ^(18)O的增多表明海平面下降,发育进积准层序组。样品采自飞一段至飞二段,这两段共识别了5个准层序组,即pss1-pss5。整个研究层段共划分了5个五级海平面变化旋回及2个四级海平面变化旋回。Sr和Mn含量变化反映了低频的海平面变化,也可用于背景分析以及验证样品是否保持原始沉积成岩环境。Sr同位素的上升趋势,与全球海平面的上升同步;Mn的含量与古水深有关,常富集于深水区,表现为Mn的含量在离岸深水区较岸边浅水区高。Sr、Mn含量的变化揭示了长期的海平面变化趋势。

Multiple scale sequences of carbonate oolitic bank developed in Jichang profile of the northeastern Sichuan Basin in the Early Triassic. Several isotopic and elemental proxies were used to reveal the sea-level fluctuation when those sequences formed. The changes of carbon and oxygen isotopes, which are coincident with the high frequency sea level fluctuation, could be used for stratigraphic sequence division. δ13C increasing and δ18O decreasing are used as the proxies of sea level rising, showed in regressive parasequence sets. δ13C decreasing and δ18O increasing are used as the proxies of sea level falling, detected from trangressive parasequence sets. Five parasequence sets, named as pss1-pss5, were distinguished from the first and second members of the Feixianguan Formation from where we sampled. 5 fifth order cycles of sea level fluctuation were recognized and grouped into 2 forth order cycles of sea level fluctuation. The Sr and Mn contents indicate the low frequency fluctuation of sea level, indicating the background of sedimentary and diagenetic environment. The increasing contents of Sr isotope are coincide with the long-term fluctuation of global sea level. Mn contents are related to water depth, enriching in deep water and poor in shallow littoral water. The Sr, Mn content changes indicate the long-term trend of the sea level fluctuation.