四川盆地埃迪卡拉晚期层序地层特征和沉积演化

Sequence Stratigraphic Characteristics and Sedimentary Evolution Model of the Late Ediacaran in the Sichuan Basin

近年来,在四川盆地川中古隆起和外围斜坡区埃迪卡拉晚期地层(震旦系灯影组)天然气勘探相继取得了重大发现和突破。深化埃迪卡拉晚期地层和沉积等方面研究,对四川盆地非继承性构造斜坡区灯影组勘探具有重要指导意义。综合利用地质、地球物理测井和地球化学资料,并结合前人研究成果,对川中古隆起和川北斜坡区灯影组沉积晚期地层SQ4进行IV级层序划分和对比,探讨了体系域的沉积演化。结果表明:1)SQ4可以划分为5个IV级层序。受沉积古地貌影响,川北斜坡区发育SQ4-1、SQ4-2和SQ4-3,古隆起高部位区发育SQ4-3、SQ4-4和SQ4-5;2)川北斜坡区灯影组SQ4由下向上为低位体系域和海侵体系域沉积,低位体系域为多期丘核—凝块石滩沉积演化,海侵体系域为丘间—丘核—砂屑滩—丘盖沉积演化。川中古隆起高石梯—磨溪地区SQ4由下向上为海侵体系域和高位体系域沉积,其中海侵体系域与川北斜坡区为等时沉积,高位体系域为丘间—砂屑滩—凝块石滩—丘坪沉积演化;3)川北斜坡区灯四段低位域具备形成岩性圈闭条件,其中SQ4-1、SQ4-2两套低位域面积分布大,是岩性圈闭勘探的有利区。

Substantial discoveries and breakthroughs have been made in the search for natural gas in the middle Sichuan paleo-uplift and upper Ediacaran strata from the end of the Neoproterozoic(Sinian Dengying Formation)in the peripheral slope area of the Sichuan Basin.This detailed study of late Ediacaran strata and sedimentation in the non-inherited structural slope area of the Sichuan Basin is of significant benefit to Dengying Formation exploration.Comprehensive interpretation of geological and geochemical data and geophysical logging for the region,taking previous research reports into account,enabled fourth-order sequence division and correlation of SQ4.The sedimentary evolution of the system tract is also discussed.The findings show:(1)SQ4 is divided into five fourth-order sequences.Influenced by their sedimentary paleogeomorphology,SQ4-1,SQ4-2 and SQ4-3 developed in the slope area of north Sichuan;SQ4-3,SQ4-4 and SQ4-5 developed in the high region of the paleo-uplift.(2)In the Dengying Formation,SQ4 comprises a low system tract and a transgressive system tract from bottom upwards.The low system tract evolved from multi-stage core agglomerate beach sedimentation.The transgressive system tract indicates evolution of inter-hill core sand debris beach dome cap sedimentation.In the Gaoshiti Moxi area of the middle Sichuan uplift,SQ4 is the result of the evolution of a sedimentary transgressive system tract and a highstand system tract from bottom upwards.The transgressive system tract and the north Sichuan slope area are isochronous deposits,and the highstand system tract represents the sedimentary evolution of inter-hill sand debris beach and conglomerate stone beach near Qiuping.(3)Conditions of the low system tract of Dengying Formation SQ4 may have formed lithological hydrocarbon traps.The SQ4-1 and SQ4-2 low system tracts in the north Sichuan slope region cover large areas,and favor lithological trap exploration.