CONTROLLING FACTORS ON THE MESOZOIC-PALEOZOIC MARINE SOURCE ROCKS IN THE LOWER YANGTZE PLATFORM

In the thick sequence of the marine Mesozoic-Paleozoic in the Lower Yangtze Area, there are six sets of hydrocarbon source rocks, i.e. the Lower Ordovician, the Wufeng Formation of the Upper Ordovician, the Gaojiabian Formation of the Lower Silurian, the Qixia Formation of the Lower Permian, the lower section of the Upper Permian, and the Lower Qinglong Formation of the Lower Triassic. Therefore, there is great potential for oil and gas accumulation in the Area. The hydrocarbon source rocks are mainly controlled by the types of sedimentary facies, ancient landform patterns and tectonic evolution. The facies types favorable for deposition of hydrocarbon source rocks mainly include the continental shelf facies, carbonate platform facies and swamp facies. The deep shelf facies is the most favorable for hydrocarbon generation. In the time range from Sinian to Middle Ordovician, the North Trough and the South Trough were the most favorable areas for the deposition of source rocks. Influenced by the pulling-apart effect of the regional tectonism, several times of large-scale transgressions happened in the Lower Yangtze Area. Deposition of hydrocarbon source rocks were thus happened during the transgressive phase, for examples, in the Hetang Formation of Early Cambrian, the lower Ordovician and the lower part of the middle Ordovician, the Qixia Formation of Lower Permian and the Dalong Formation of the Upper Permian, the lower part of the Lower Triassic. The Late Caledonian movement drove the South China plate drifting toward the north, the Lower Yangtze plate had suffered under a compression stress, and the north of the lower Yangtze plate subsided, while the south was uplifting. As the result, the northern part of the area, which was subsiding, became a favorable area for hydrocarbon source rocks of the Wufeng Formation of upper Ordovician and the Gaojiabian Formation of the lower Silurian to form.