Analyses Of The Early Paleozoic Foreland Basin And Hydrocarbon Prospect In The Lower Yangtze Region

Great changes of lithofacies and paleogeography occurred in the Lower Yangtze region during late Ordovician and Silurian, and thick siliciclastic was deposited and covered on the previous carbonate platform. The deposition was coeval with the orogenic event in South china, so study the type of the basin and its evolution has great importance to understand the relationship between the orogenic and basin, as well as the whole tectonic setting. Complex structures were formed in the study area due to multiple phases of compression and extension during Mesozoic and Cenozoic. To analyze the feature and extent of each tectonic event is the key to understand the structure features and also beneficial for the hydrocarbon exploration.The thesis is focused on the basin formation and evolution. First of all, the method of zircon dating was used to analyze the timing of the orogenic event, in order to study the tectonic setting of the basin; second, comprehensive analyses of sediment thickness, subsidence process and detrital zircon provenance were carried out to study the development and evolution of the foreland basin; and then influence of Mesozoic and Cenozoic tectonic events on the foreland basin was studied in detail; finally, the elements of hydrocarbon and shale gas were evaluated from the aspects of source rocks, reservoir, seal and plays.Zircon U-Pb dating indicates that the Badu and Chencai group were metamorphosed at about453-442Ma, and the syntectonic granites of Jinxi and Cizu were emplaced at about450Ma and430Ma respectively. Combined with available data of other metamorphic rocks and syntectonic granites as well as information about basin evolution, it is suggested that orogenic ativity mainly took place during455-425Ma, which was nearly coeval with the Caledonian orogeny. Based on above information and absent Pan-Africa orogeny in South China,1tentatively proposed that South China was near the Caledonian orogeny in Silurian, and place it to the north of the Greenland or the north of Scandinavian.According to the analyses of strata correlation, thickness, sedimentary facies, subsidence and detrital zircon provenance, it is confirmed that the basin in the lower Yangtze region was a foreland basin during late Ordovician to Silurian, which was controlled by the early Paleozoic orogeny in South China. The foredeep was located in North Zhejiang and South Anhui, the forebulge along the Yangtze River, and the backbulge in the north of Jiangsu Provenance. Detrital zircon dating indicates the provenance of the foreland basin has five age populations,2500Ma,1800-2100Ma,900-1200Ma,40-860Ma and425-460Ma. The former three populations were derived from the recycling of the Proterozoic and early Paleozoic rocks in the orogenic belt, and the later two correlated well with the Neoproterozoic and early Paleozoic tectonic events, respectively. The youngest age of425Ma provided an age constrain for the timing of the foreland basin development. Combined with sedimentary features and provenance changes, it is proposed that the foreland basin was in underfilled stage during late Ordovician, filled stage in Llandovery, and overfilled stage in Wenlock.It is revealed that the foreland basin has experienced two phases of compression during late Triassic and middle to late Jurassic and one phase of extension in late Cretaceous and Paleogene since it formed, according to the distribution of unconformity, the contact relationship of strata and rift basin development. The first compression was from NW toward SE and mainly influenced the area of north Yangtze River, while the late compression was from SE toward NW and mainly influenced the area in the south of Yangtze river. The formation of the basin can be divided into seven tectonic sequences, and from bottom to top they are Sinian-middle Ordovician, upper Ordovician-Silurian, upper Paleozoic-middle Triassic, upper Triassic-middle Jurassic, upper Jurassic-lower Cretaceous, upper Cretaceous-Paleogene and Neogene-Quaternary; according to deformation style and timing, ten tectonic units are divided, including Jiangnan belt, Jiangyin-Wuxi detachment folds, Changzhou-Xuancheng rift, Maoshan thrust belts, Nanling-Jurong rift, Niangzhen thrust belts, Tongling-Fanchang thrust belt, Chuzhou-Susong thrust belts and Subei rift basin.Studies of source rocks, reservoir, seals and accumulation conditions of hydrocarbon reveal that the black shales of lower Cambrian and upper Ordovician-lower Silurian are thick, wide distribution, high in total organic carbon, and matured enough, which are favourable for shale gas exploration, and the Jiangyin-Wuxi area is the prospect play for the relatively simple structures.