四川盆地及邻区奥陶纪构造-沉积环境与原型盆地演化

奥陶系是中国重要的海相碳酸盐岩层系。目前,在塔里木、鄂尔多斯盆地奥陶系发现一系列大型油气田,但在四川盆地奥陶系至今仍未取得突破。研究和恢复奥陶纪的构造-沉积环境是研究四川盆地及邻区的构造历史和预测奥陶系有利储集相带的基础。根据野外露头、钻井、测井、地震和地球化学资料,从周缘大地构造背景、沉积相、沉积充填、古地理、古生态、构造变形出发,将盆地充填演化与周缘隆起、海盆演化相结合,重建了四川盆地奥陶纪不同时期的构造-沉积环境,并探讨了隆起性质和原型盆地演化。四川盆地及邻区在奥陶纪表现为克拉通内坳陷和克拉通边缘坳陷相复合的盆地性质,周缘动力学环境经历了拉张→挤压的演变,主要体现在基底沉降以及隆起/隆升两种机制上。盆地周缘为克拉通边缘坳陷盆地,主要为深水盆地相。研究区中部为克拉通内坳陷盆地,经历了两次大的海平面升降旋回,沉积序列由下向上表现为早奥陶世镶边台地→中奥陶世混积陆棚—镶边台地→晚奥陶世缓坡—深水陆棚相沉积。中奥陶世古华南海盆的消亡,华南板块陆内造山,导致川中隆起、黔中隆起等盆地边缘隆起不断隆升和扩展,海平面持续上升,至晚奥陶世五峰组沉积期,基底快速沉降,形成深水陆棚沉积环境,标志着盆地性质的急剧转变,结束了自震旦纪以来的碳酸盐台地沉积历史。

Tectonic-depositional environment and proto-type basins evolution of the Ordovician in Sichuan Basin and adjacent areas

The Ordovician strata are important marine carbonates in China. At present, a series of large oil and gas fields have been discovered in Tarim Basin and Ordos Basin, but no breakthrough has been achieved in the Ordovician strata of Sichuan Basin. The research and restoration of Ordovician tectonic-sedimentary environment provides a basis for studying the tectonic history of Sichuan Basin and adjacent areas and predict the favorable Ordovician reservoir facies belt. According to field outcrop, drilling, logging, seismic and geochemical data, as well as the surrounding tectonic environment, sedimentary facies and filling, paleo-geography, paleo-ecology and tectonic deformation, the tectonic-sedimentary environment of Sichuan Basin in different Ordovician periods was rebuilt by combining basin-filling evolution with the surrounding uplift and trough evolution. Meanwhile, the uplift property and prototype basin evolution were explored. Sichuan Basin and adjacent areas present the composite properties of intra-craton depression and craton-margin depression facies in Ordovician. The surrounding dynamic environment has evolved from tension to extrusion, mainly shown in two kinds of mechanism, i.e., basement subsidence and uplift. Basin surroundings are craton-margin depression basins, dominated by deep-water basin facies. The central part of study area is intra-craton depression basin, experiencing two large cycles of sea level eustasy. The sedimentary sequence is shown as the Early Ordovician rimmed platform→ the Middle Ordovician hybrid continental shelf-rimmed platform→ the Late Ordovician slope-deep water continental shelf facies sediments from bottom to top. The demise of the Middle Ordovician paleo-South China Sea basin and the intra-continent orogeny of South China Plate led to constant lifting and propagation of the central Sichuan uplift, central Guizhou uplift and other basin margin uplifts as well as the continuous rising of sea level. Till the sedimentary period of the Late Ordovician Wufeng Formation, the basement settled rapidly to form the deep-water continental-shelf sedimentary environment, indicating an abrupt change in basin properties and the termination of carbonate platform sedimentary history since Sinian.