古气候恢复及其对沉积的控制作用——以四川盆地上三叠统须家河组为例

Restoration of Paleoclimate and Its Geological Significance:As an example from Upper Triassic Xujiahe Formation in Sichuan Basin

四川盆地上三叠统须家河组是盆地的主力产层,该组发育的6段地层表现为砂—泥间互的"三明治"式沉积结构。为了探讨古气候对这种沉积结构的控制作用,文章首次利用孢粉学方法、元素比值法和自然伽马曲线法分别恢复了四川盆地晚三叠世须家河期各段地层沉积时期的古气候。孢粉植被法识别出蕨类和裸子类孢粉化石共计78属,呈现"拟网叶蕨孢属—凹边孢属—苏铁粉属"的须家河组孢粉组合特色,这种孢粉组合反映温暖潮湿的热带、亚热带气候类型。元素比值法中的钙镁比和锶钡比分别反映古温度和古盐度(古湿度),其中,钙镁比值与古温度正相关,锶钡比值与古盐度正相关而与古湿度负相关。自然伽马值是反映气候的又一种方法,GR值高代表古气候潮湿、沉积水体深;反之亦然。在孢粉法恢复的气候背景上,元素比值法和自然伽马曲线法反映了各段地层沉积时期的气候变化细节。综合三种方法的结果,认为:须家河组沉积于温暖潮湿的热带亚热带环境,须1、3、5段沉积时期,气候炎热潮湿;须2、4、6段沉积时期,气候温和干燥。须5段地层最为炎热潮湿,须2段地层最为温和干爽。受控于这种周期性的温、湿度升降,须家河组奇数段地层以泥质沉积为主,偶数段地层以砂质沉积为主,从而形成了砂泥间互的"三明治"式沉积结构,这样"近水楼台先得月"的源储搭配关系为油气成藏提供了有利条件。

Upper Triassic Xujiahe Formation in Sichuan Basin is the principal producing formation.Six members of Xujiahe Formation form sandwich-like sedimentation structure with interbeded sandstone and mudstone.In order to discuss the contribution of paleoclimate on this kind of sedimentation structure,three commonly-used methods of restoring paleoclimate,namely sporopollen,elements and GR-ray logging were used to restore the paleoclimate of the time when each member of Xujiahe Formation was deposited.78 genera of sporopollen were found in the samples of Xujiahe Formation,forming the sporopollen combination of Dictyophyllidites-Concavisporites-Cycadopites.The combination indicates warm and humid climate in tropic and/or subtropic zone.The ratio of Ca/Mg has a positive correlation with paleotemperature while the ratio of Sr/Ba has a negative correlation with salinity（paleomoisture）.The Gamma Ray can also illustrate the change of paleoclimate.The high GR data indicate humid climate,and vice versa.The proofs from sporopollen provided the climate background,and the other two methods described the details of paleoclimate. Combined all the three methods,we can find that the paleoclimate when Xujiahe Formation was sediment belongs to warm-humid tropics-subtropics zone.Paleoclimate when T3 x 1,T 3 x3and T3 x 5were formed showed hot and humid.In the meantime,when T 3x 2,T 3x 4 and T3 x 6 were formed it was warm and relatively dry.The climate of T3 x 5 was outstanding hot and humid,while the temperature and moisture of T 3x 2 was lowest.Controlled by the regular changes of paleoclimate,even numbered members were dominated by mudstone,and odd numbered members were dominated by sandstones.This kind of sandwich-like sedimentation structure formed by alternating layers of mudstone and sandstone provided favorite conditions for the formation of hydrocarbon reservoirs.