AN APPROACH TO EXPLORATION DIRECTION OF OIL-GAS IN THE MARINE RESIDUAL BASINS OF INDEPENDENT LOWER YANGTZE BLOCK
Ye Zhou1, Liang Xing2, Ma Li3, Zhang Tingshan1, Xu Keding2
1. School of Resource and Environment, Southwest Petroleum Institute, Chengdu 610500; 2. Zhejiang Oilfield Company, China National Petroleum Corp., Hangzhou 310013; 3. Jiangsu Oilfield Branch Company, China Petroleum & Chemical Corp., Yangzhou, Jiangsu 225009
The Paleozoic-Early Mesozoic marine prototype basin and Mesozoic foreland basin, once occurred on the independent Lower Yangtze Block, were reformed into a part of the East Asian Yanshanian Plateau by violent thrusting and erosion during the Yanshanian orogeny, then superimposed by extensional fault-depression resulted from the oblique subduction of the Pacific plate in the Late Yanshanian to Himalayan period. Nowadays, the area became a reformed residual superimposed basin with a two-layer structure, namely, which consisted of the Pre-Cretaceous compression-thrust system and the Upper Cretaceous-Neogene extension-depression system. So the petroleum geology of the marine Mesozoic and Paleozoic strata in Lower Yangtze area was of highly complexity, which could be summarized as, for the marine source rocks, secondly generation of hydrocarbon and later formation of oil-gas accumulation during Late Cretaceous to Neogene. Each Paleogene extensional fault-depression might be, generally, an independent oil-gas preservation unit for hydrocarbon from the marine source rocks. The foundations for later formation of oil-gas accumulation from marine source rocks should be two factors: one being a totally closed system reconstructed by sedimentation of regional covers of the Paleogene and Neogene, another being effective source rocks characterized with mixed origins in late period. An essential condition for later formation of oil-gas accumulation might be effective reservoir-cap association with a suitable time-space matching. The North Jiangsu-South Yellow Sea Basin should be the best structural unit of the totally closed systems reconstructed by the Cenozoic sedimentaton in Lower Yangtze area, and it should be, furthermore, a main field for later formation of oil-gas accumulation. The key beds for later formation of the marine source rocks might be unconformities of the Indosinian and Yanshanian movements, the Pukou Formation (K2p), the Taizhou Formation (K2t) and the first Member of Funing Formation (E1ƒ1), which made up the associations of Paleozoic sources and Mesozoic reservoirs, or Paleozoic sources and Cenozoic reservoirs. The preservation units such as the Yancheng, Funing, Hai’an, Gaoyou, Qintong and Huangqiao-Rugao fault depressions might be target areas in near future exploration.