下扬子独立地块海相残留盆地油气勘探方向探讨

强烈的燕山期造山运动使下扬子独立地块海相原型盆地和前陆盆地被改造成冲断、剥蚀严重的燕山高原,晚燕山期—喜马拉雅期太平洋板块斜向俯冲形成张裂断陷盆地叠置,最终使其成为具有由前白垩系挤压冲断体系和上白垩统—新近系伸展断-坳陷体系组成双层结构的改造型残留叠合盆地。这既造成下扬子海相古生-中生界油气地质高复杂性,又造就了残存于晚期上白垩统—新近系断-坳陷盆地下伏的海相烃源岩“二次生烃”和晚期成藏优势。各个古近纪伸展断陷往往是独立的海相烃源含油气保存单元,古近-新近系区域盖层共同构筑的沉积重建型整体封闭保存体系和具晚期混源特色的海相有效烃源是海相含油气保存单元晚期成藏的基础,时空匹配的有效成藏组合是晚期成藏的必要条件。南黄海、苏北盆地是下扬子区沉积重建型整体封闭体系最好的构造单元,也是海相烃源晚期成藏的主要场所,印支和燕山运动不整合面上下、浦口组、泰州组和阜宁组一段是海相烃源古生中储或古生新储晚期成藏的主要层位,盐城、阜宁、海安、高邮、溱潼和黄桥—如皋断陷保存单位是首选勘探靶区。

AN APPROACH TO EXPLORATION DIRECTION OF OIL GAS IN THE MARINE RESIDUAL BASINS OF INDEPENDENT LOWER YANGTZE BLOCK

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 (K_2p), the Taizhou Formation (K_ 2t) and the first Member of Funing Formation (E_ 1f 1), 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.