中国中西部大型湖盆沉积体系域研究进展及页岩油气勘探*

中国陆相大型湖盆碎屑岩—碳酸盐岩沉积区分布广泛,含油气盆地内新发现了多个油气田,特别是近年来在致密油/页岩油勘探方面获得了突破。根据沉积体系域演化和可容纳空间演变解释层序地层单元,建立层序演化模式,预测油气储集体,适合于研究有成因联系的异旋回地层序列。深水体系的沉积层序体系域研究进展表现为引入强制海退(湖退)的概念,从经典的层序地层学格架三分体系域发展为四分体系域和二分体系域,不同尺度的相对整合序列和体系域分布与命名具有紧密联系。强制海(湖)退条件下形成的下降期体系域(FSST)和强制海(湖)退楔体系域(FRW)有内涵细节上的差异。采用A-P-D(加积-进积-降积)准层序叠加分析方法,分析体系域格架下薄层细粒沉积砂体的沉积搬运机理,提高了体系域识别和油气储集体预测的准确度。柴达木盆地古近系和四川盆地侏罗系研究实例表明,湖侵体系域随湖平面上升,浅湖和半深湖范围扩大,沉积中心地区发育三角洲前缘薄层砂体和深水浊积扇及多旋回泥页岩—泥灰岩组合,有利于页岩油气储集体的发育,成为“甜点”勘探目标区。展望未来,与页岩油气勘探有关的沉积层序体系域领域研究在以下几个方面将取得重要进展: (1)咸化湖盆深水沉积体系的高频旋回体系域研究; (2)与页岩型和混积型页岩油类型相关的湖侵体系域混积体系研究; (3)体系域级别的层序—古地理研究; (4)体系域格架下的源-汇系统及其古湖泊分布特征研究; (5)湖泊古水深、古气候和古环境的地球化学指标分析; (6)富有机质页岩发育的微纳米级孔—缝体系储集能力研究; (7)煤系环境纯湖相泥岩和沼泽相(湖沼或河沼)泥岩对规模天然气生烃贡献率的沉积基础研究。

Advances on depositional systems tracts in large lake basins and shale oil and gas exploration in mid-western China

Detrital siliciclastics and carbonate deposits are widely distributed in China,and many new oil and gas fields have been discovered in petroliferous basins,especially in tight oil/shale oil exploration in recent years. It is suitable for studies of allo-cyclic sequences with genetic connections to explain the sequence stratigraphic unit according to the evolution of the sedimentary systems tract and accommodation changes. Through this approach,sequence evolution models could be built and oil and gas reservoirs could be predicted. The advances on systems tracts of deep-water systems are manifested in the introduction of the concept of forced regression(sea or lake),developing from the classical three-division systems tracts to the four systems tract and/or two systems tracts,and the distribution of internal relative comformity sequence and systems tract at different scales are closely related to naming. The falling stage systems tract(FSST)and forced regression wedge systems tract(FRW)are formed under different conditions. A-P-D parasequence stacking pattern analysis is used to analyze the Hyperpycnal and the sediment mechanism of normal fluid(Homopycnal and/or Hypopycnal),which improves the accuracy of systems tract identification and oil and gas reservoir prediction. Research examples of the Qaidam Basin and Sichuan Basin confirm that lacustrine transgressive systems tract develops as as lake level rises,shallow lake and semi-deep lake areas expand,the deltaic thin-bedded sands and deep-water turbidite fans and multi-cycle muddy shale-marl combinations are developed in depocenters. It is conducive to the development of shale oil and gas reservoirs and become the “sweet spots” of exploration target areas. Looking forward to the future,important progress will be made in the field of shale oil and gas exploration in the following aspects: (1)research on high-frequency cycle systems tract of deep-water depositional systems in salt lake basins; (2)study on mixed depositional systems of TST that are related to shale type and mixed sediment shale oil types; (3)sequence-paleogeographic studies at the scale of systems tract; (4)studies on source-sink systems and the distribution characteristics of ancient lakes under the framework of systems tract; (5)geochemical indexes analysis of paleo-water depth,paleo-climate and paleo-environment in lakes;(6)reservoir capacity of micro-nanoscale pore-fracture systems in organic-rich shales;(7)sedimentological research on the contribution rate of pure lacustrine mudstones and swamp mudstones to large-scale natural gas hydrocarbon resources.