渤海海域沙一二段混合沉积分布特征与勘探实践

Distribution Characteristics and Exploration Practice of Mixed SiliciclasticCarbonate Sediments in Bohai Sea Area

渤海海域广泛发育陆源碎屑与碳酸盐矿物的混合沉积,随着近年来混积型储层的油气勘探相继获得重要发现,混合沉积分布规律亟待深入研究.基于大量基础资料,对渤海海域古近系混合沉积进行了系统性的分析,明确了其时空分布特征,完善了混合沉积相关地质认识.渤海海域古近系沙一二段(E2s1-2)主要发育以陆源碎屑为主的混积岩、以生物成因碳酸盐矿物为主的混积岩以及以化学成因碳酸盐矿物为主的混积岩三大类岩石类型.受控于干旱的气候与较高盐度的水体条件,混合沉积主要发育于沙一二段沉积时期,在空间上远离区域物源体系,主要围绕盆内局部凸起分布,低凸起、凸起倾末端、近岸隆起、远岸隆起等正向的水下古地貌单元是混合沉积发育的有利部位.根据发育位置、古地貌背景、与陆源碎屑供给的关系等古地理要素,混合沉积成因样式可分为近岸混积扇三角洲、近岸扇体侧翼混积滩坝、近岸隆起混积滩坝、远岸隆起混积滩坝4种类型及六亚类.近岸混积扇三角洲、近岸扇体侧翼混积生屑滩坝、近岸隆起混积碎屑滩坝、远岸潜山隆起混积生屑滩坝4种具体类型的混合沉积物产出规模与厚度较大,易形成粒度粗、生屑含量高、杂基含量少、白云石化作用发育、物性条件较好、产能较高的优质储层,是较为有利的勘探类型.上述地质认识有效地指导了一批混积型储层的重大勘探发现,揭示了混合沉积领域巨大的勘探潜力,对渤海海域中深层勘探具有重要的推动意义.

Mixed siliciclastic-carbonate sediments are widely developed in Bohai Sea area.With successive discoveries of mixed reservoir in oil exploration recently,the distribution characteristics of mixed sediments need to be addressed.Based on abundant geological data and comprehensive analyses,the time and spatial distribution characteristics of Palaeogene sediments are figured out in this study,promoting geological cognition about mixed sedimentation.Three types of E2 s1-2 mixed rock in Bohai Sea area are identified as siliciclastic-dominated mixed rock,biological-carbonate-dominated mixed rock and chemical-carbonate-dominated mixed rock.Controlled by arid climate and saline water environment,the mixed sedimentation mainly develops in the first to second members of Shahejie Formation(E2 s1-2)and is spatially distributed around uplift inside the basin,being far away from outside regional provenance.The favorable background of underwater positive paleotopography includes low uplift,pitching end of uplift,near-shore uplift and offshore uplift.According to development position,paleotopography setting and relationship with siliciclastic supply,the genetic pattern of mixed sedimentation could be grouped into four categories:(1)near-shore mixed fan delta;(2)near-shore mixed beach bar between fan deltas;(3)mixed beach bar on near-shore uplift;and(4)mixed beach bar on offshore uplift.Synthetic evaluation indicates that the near-shore mixed fan delta,near-shore mixed biological beach bar between fan deltas,mixed siliciclastic beach bar on near-shore uplift and mixed biological beach bar on offshore buried hill are specific favorable types of exploration since the mixed reservoir possesses large thickness and scale,coarse grains,high content of carbonate particles,less matrix and dolomite,which results in good physical properties and significant productivities.These geological cognitions have led to several discoveries of mixed reservoirs and reveal great exploration potential of mixed sediments field.The above theoretical understanding and exploration practices obtained are of great significance for promoting deep exploration in Bohai Sea area.