中上扬子地区早寒武世(纽芬兰世-第二世)岩相古地理
Sedimentary facies and palaeogeography of the middle-upper Yangtze area during the Early Cambrian (Terreneuvian-Series 2)
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摘要: 由于受区域构造作用、海平面变化、上升洋流以及古气候等主控因素的制约,中上扬子地区纽芬兰世-第二世主要表现为浅海环境。早寒武世梅树村期,主要发育潮坪相碎屑岩、磷块岩、白云岩和硅质岩,陆棚相含磷结核硅质岩、泥岩以及陆棚深水盆地相碳质页岩、硅质岩等。筇竹寺期,海侵范围扩大,陆棚深水盆地向西北扩展,其余广大区域为碎屑岩浅海所覆盖。沧浪铺期,主要表现为碎屑岩和碳酸盐交互沉积的混积陆棚环境,盆地相则具有继承性,主要为碳质页岩沉积。早寒武世龙王庙期开始向碳酸盐台地演化,广大区域为碳酸盐沉积,盆地区范围有所缩小。早寒武世的岩相古地理演化表明,从早到晚沿着东南边缘始终存在一较深水沉积区,其沉积物质主要为碳质页岩和硅质岩,为烃源岩有利沉积区,奠定了油气生烃成藏的物质基础。同时在演化过程中还形成了相应的储层以及分布较广的膏盐层。根据这一油气地质条件组合及其空间分布,针对中上扬子地区早古生代海相地层的油气勘探应该首先围绕这一富烃沉积区来开展,集中对膏盐层下部目的层开展油气勘探工作。Abstract: Constrained by regional tectonics, sea-level changes, upwelling oceanic currents and palaeoclimates, the sedimentary environments in the middle-upper Yangtze area appeared as the neritic shelf during the Early Cambrian (Terreneuvian-Series). During the Meishucunian (Early Cambrian), the study area was covered by tidal-flat clastic rocks, phosphorites, dolostones and siliceous rocks; shelf siliceous rocks with phosphatic nodules and mudstones, and shelf deep-water basin carbonaceous shales and siliceous rocks. During the Qiongzhusian, the shelf deep-water basin expanded toward the northwestern part due to constant transgressions. The adjacent areas were still occupied by the shallow siliciclastic sea. During the Canglangpuan, the study area was developed into the mixed siliciclastic-carbonate shelf environments where the carbonaceous shales were accentuated. Finally till the Longwangmiaoan, the study area was graded into the carbonate platforms. The above-mentioned sedimentary facies and palaeogeographic evolution during the Early Cambrian shows the persistence of the deep-water depositional area along the southeastern margin of the Yangtze block, where the dominant carbonaceous shales and siliceous rocks are favourable for the deposition of the marine source rocks and the accumulation of hydrocarbons. Meanwhile, the reservoir rocks and wide-spread gypsum horizons were also formed during these periods. It follows that the emphasis of future hydrocarbon exploration should be placed upon the Early Palaeozoic marine strata, especially the target strata in the lower part of the gypsum horizons in the middle-upper Yangtze area.