鄂尔多斯盆地下古生界构造-沉积演化、古地理重建及有利成藏区带划分

Tectono-sedimentary evolution,paleo-geographic reconstruction and play fairway delineation of the Lower Paleozoic,Ordos Basin

鄂尔多斯盆地下古生界是重要的天然气勘探层位,相继发现了奥陶系顶部风化壳、古隆起周缘中组合和马家沟组盐下等多套含气层系,证明了盆地下古生界具有良好的勘探开发潜力,也揭示出深化古地理重建工作的急迫性。在分析鄂尔多斯盆地构造-沉积演化过程的基础上,对早古生代重点时期的古地理进行了重建,进而提出了成藏区带划分的新思路。研究表明,鄂尔多斯盆地经历了太古宙—元古宙裂陷沉积、寒武纪—奥陶纪沉积-剥蚀、石炭纪—三叠纪拗陷稳定沉积、侏罗纪—白垩纪挤压成藏和晚白垩纪—现今调整定型等5大阶段。盆地下古生界古地理格局经历了早-中寒武世的古陆消减、沉积增长、边缘裂陷阶段和早奥陶世的隆-坳相间阶段,并控制了优质成藏区带的分布。结合近期勘探研究进展,依据源-储组合和空间分布,划分出了基于主控因素的4类成藏区带:潮缘滩白云岩成藏区、奥陶系盐下台内丘滩成藏区、西部双滩带白云岩和页岩成藏区以及岩溶-滩相白云岩成藏区。不同区带显示出了不同的成藏条件和主控因素,需要选择针对性的勘探策略。

The Lower Paleozoic has been an important target for natural gas exploration in the Ordos Basin.Several suites of gas-bearing sequences,such as weathering crust reservoir on top of the Ordovician,middle assemblage dolomite and sub-salt dolomite,have been discovered successively,which proves that the Lower Paleozoic in the basin is of good exploration and development potentials,and thus paleo-geographic reconstruction there is in urgent need.The study proposes a new idea of play fairway delineation of the Lower Paleozoic in the Ordos Basin from the perspective of tectono-sedimentary evolution and paleo-geographic reconstruction therein.The research shows that Ordos Basin experienced five stages of evolution,namely,the Archean-Proterozoic fault depression-deposition stage,the Cambrian-Ordovician deposition-denudation stage,the Carboniferous-Triassic stable depression-deposition stage,the Jurassic-Cretaceous compressional hydrocarbon accumulation stage and the adjustment and finalization stage from the Late Cretaceous till present.The paleo-geographic pattern of the Lower Paleozoic Ordos Basin underwent the stages of paleo-land reduction,sediment filling,margin rifting and the alternated uplift-depression during the Early Ordovician,which controlled the distribution of quality play fairways.Besides,the latest exploration achievements help us identify four types of quality play fairways in terms of source-reservoir assemblage,spatial distribution and major controlling factors,that is,hydrocarbon accumulation within dolomite of marginal tidal-flat facies,hydrocarbon accumulation within the Ordovician pre-salt layer of intra-platform mound shoal facies,hydrocarbon accumulation within dolomite and shale of marginal tidal-flat and intra-platform mound shoal facies in the west,hydrocarbon accumulation within dolomite of karstification and shoal facies.The play fairways of different types feature separate hydrocarbon play elements and major controlling factors,and thereby targeted exploration strategies are in need.