下扬子地区晚奥陶世凯迪期—早志留世鲁丹期岩相古地理及其油气地质意义

下扬子地区奥陶—志留纪之交岩相分异明显,发育了多种类型的岩石地层单位,其沉积岩的颜色、岩性、厚度、赋存化石组合等方面存在显著的差别。笔者通过对研究区内野外露头及钻井剖面的考察,总结前人研究成果,利用岩石学、古生物学、生态学及室内分析方法将其划分为滨岸相、孤立台地相、浅海陆棚相、陆棚坡折相、欠补偿盆地相和盆地浊积岩相6种沉积相类型。岩相古地理研究表明,晚奥陶世凯迪晚期,受加里东构造运动影响,华南板内碰撞挤压作用显著,"江南古陆"不断抬升和扩张,直接导致了旧的"台-坡-盆"格局的消失和新的古地理格局的形成,之前发育的自西北向东南方向水体变深的古地理格局彻底转变为北西、南西、南东三面被古陆包围,向东北方向水体加深的古地理格局。晚奥陶世赫南特期,由于南极冰盖凝聚导致的全球海平面下降和气候变冷,扬子地台海域变浅,致使海底盆地上升到最低含氧深度面之上处于充氧水层中,富氧环境代替了缺氧环境,岩相由五峰组黑色页岩转变为更浅水的观音桥层泥灰岩或新开岭层泥岩;江南沉积区长坞组—于潜组的复理石沉积组合转变为更浅水的的文昌组—堰口组近磨拉石沉积组合。早志留世鲁丹期,随着全球海平面重新上升,隆起面积继续扩大,构造围限作用加剧,黑色页岩重新出现。尤其是安徽南部的安庆—铜陵—巢湖一线,江苏中部的南京—句容一线五峰组—高家边组底部的黑色硅质页岩和炭质页岩,笔石带与中上扬子的五峰—龙马溪组具有很好的一致性,厚度均在40～60 m,TOC含量1.2%～4%,具很好的生烃潜力,应视为下一步烃源岩及页岩气勘探开发的重点研究区域。

Lithofacies palaeogeography of the Late Ordovician Kaitian Stage-the early Silurian Rhuddanian Stage in Lower Yangtze region and its petroleum geological significance

The lithofacies differentiation of the Ordovician-Silurian deposits in the Lower Yangtze region is obvious, with the development of many types of lithostratigraphic units. There are significant differences in color, lithology, thickness and occurrence of fossil assemblages of the sedimentary rocks. Based on the investigation of outcrops and drilling profiles in the study area, the authors summarized the achievements of previous researches and used the methods of petrology, paleontology, ecology and indoor analysis to divide the lithofacies into six sedimentary facies, i.e., coastal facies, isolated platform facies, shallow continental shelf facies, shelf break facies, under-compensated basin facies and turbidite basin facies. The lithofacies palaeogeography research shows that, in the Late Ordovician Kaidi period, due to the Caledonian tectonic movement, the collision and extrusion of the South China Plate were significant. The "Southern Yangtze Plateau" continued to rise and expand, which led directly to the disappearance of the old "platform-slope-basin" pattern and the formation of a new paleogeographic pattern, i.e., the paleogeographical pattern in which the water body which had been developed from the northwest to the southeast was completely transformed into the pattern that the all three sides of northwest, southwest and southeast directions were surrounded by the ancient land and got deepened toward the northeast in paleogeography. During the Late Ordovician Hernannian period, due to the declining of the global sea level and the climatic cooling caused by the Antarctic ice sheet condensation, the sea surface of the Yangtze platform got shallower so that the submarine basin rose above the lowest oxygen-containing depth in the oxygenated water layer, and the oxygen-rich environment replaced the anoxic environment. The Changwu-Yuqian Formation dolomitic sedimentary assemblage in the Jiangnan sedimentary area was transformed into shallower Wenchang Formation-Yankou Formation close to molasse combination. In the early Silurian period, with the rising of global sea level, the uplifted area continued to expand, the tectonic confinement effect was intensified and the black shale reappeared. Especially along the Anqing-Tongling-Chaohu line in the south of Anhui Province, the Wufeng Formation of Nanjing-Jurongin area in the middle part of Jiangsu and the black siliceous shale and carbonaceous shale at the bottom of Gaojiabian Formation, the Longmaxi Formation has good consistency with thickness of 40-60 m and TOC content of 1.2%-4%. These areas have good potential for hydrocarbon generation and should be regarded as the key research areas for the exploration of source rock and shale gas in further exploration.