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.