安县运动对四川盆地中西部上三叠统地层划分对比与油气勘探的意义

Significance of the Anxian Movement to the stratigraphic division and correlation of the Upper Triassic formations and their hydrocarbon exploration in the western-central Sichuan Basin

四川盆地上三叠统包括原来川西北地区所称的上三叠统须家河煤系(组)和盆地其余地区所称香溪群。一直以来,对这2套地层各段间的对比关系存在着激烈争论,有多种对比意见,但都缺乏有力证据。安县运动的发现并被证实广泛存在于四川盆地中西部地区,为解决上述问题找到了一条路径。分析认为:盆地中西部地区上三叠统原来划分的香一段中海相化石层的广泛分布以及孢粉的组合特征,表明这些地区存在卡尼克—诺利克期的沉积;香一段顶部侵蚀面的被发现、香二段砂岩超覆在香一段的不同层位上,以及区域地震大剖面解释成果,均表明安县运动也强烈影响到这些地区。以安县运动所形成的侵蚀面为界,可将该区上三叠统划分为上、下两部分沉积。由此可知,川西北地区的须二段砂岩应在川中和川西地区南部香二段砂岩之下,改变了原来"砂对砂"、"泥对泥"的格局,出现了须二段大套砂岩往川中方向尖灭,往川西地区南部变为原香一段中上部的粉、细砂岩夹泥岩的现象;须四、须五段横向上的岩性也有明显变化,并往川西北方向尖灭。这一对比结果,为在该区寻找地层尖灭油气藏、岩性油气藏指示了方向。

The Triassic formations in the Sichuan Basin include the previously so-called Xujiahe coal measure in the northeast and the Xiangxi Group in the rest regions of the basin.A violent controversy always exists over the stratigraphic correlation of the above-said formations,but each discussion lacks strong evidences.The discovery of the Anxian Movement and its wide influence on the whole western-central Sichuan Basin provides a solution to the said discussion.The extensive distribution of marine fossil beds in the formerly recognized 'the first member of the Xiangxi Group' of the Upper Triassic in the western-central Sichuan Basin and the characteristics of sporopollen combinations reveal that deposits of the Kanike-Nolike stage exist in these areas.The existence of erosion surface on top of 'the first member of the Xiangxi Group',the overlapping of the second member of the Xiangxi Group on its first member,and regional seismic profiles all show that the Anxian Movement also strongly influenced these areas.With the erosion surface caused by the Anxian Movement regarded as the boundary,the Upper Triassic strata in the study area can be divided into an upper and a lower interval.The sandstone of the second member of the Xujiahe Formation in the northwestern Sichuan Basin should occur under the sandstone of the second member of the Xiangxi Group in the central Sichuan Basin and the southern part of the western Sichuan Basin.This phenomenon changes the previous stratigraphic framework of 'sandstone correlating with sandstones' and 'mudstone correlating with mudstones'.The thick sandstones of the second member of the Xujiahe Formation pinchout toward the central Sichuan Basin and change into siltstones and fine sandstones with interbedded mudstones in the middle-upper of the former first member of the Xiangxi Group toward the western Sichuan Basin.The 4th and 5th members of the Xujiahe Formation also show obvious lateral lithologic variation and pinchout toward the northwestern Sichuan Basin.These understandings are helpful for the exploration of stratigraphic pinch-out traps and lithologic traps in the study area.