琼东南盆地古近纪成煤沉积体系类型及特征

Types and characteristics of Paleogene coal-forming sedimentary systems in Qiongdongnan Basin

琼东南盆地天然气勘探开发事实证明,煤系及煤层是最重要的生烃源岩。基于琼东南盆地地震资料、钻井岩心及测井资料,在对古近系渐新统含煤沉积特征分析的基础上,建立了沉积相与地震相的对应关系,识别出琼东南盆地渐新统发育冲积扇-潮坪-潟湖组合、冲积扇-潮坪-滨海组合、冲积扇-海沼沙岭平原-滨海组合等14种沉积组合;依据取心段煤的宏观与微观物理化学性质,判断崖城组煤层形成过程中受海水影响且沉积环境还原性较强,说明煤层形成于近海沼泽环境。基于成煤特征及成煤环境分析,划分出4类成煤沉积体系,分别为扇三角洲体系、辫状河三角洲体系、潮坪-潟湖体系和障壁海岸-海沼沙岭平原体系。扇三角洲体系煤层主要发育在扇三角洲平原、废弃朵体、朵体间洼地,煤层较薄,且易发生分叉;辫状河三角洲体系煤层主要发育在辫状河三角洲平原、分流间湾和泛滥平原等地区,煤层薄且不稳定;潮坪-潟湖体系中煤层多发育在潮上坪的沼泽及潮间坪上部的潮道间湾,潮汐三角洲平原以及淤化的潟湖等地区,在多潮道地区,煤层层数多,但厚度较薄,煤层间距大;而在潮道不发育地区,含煤层数较少,但厚度较大,煤层间距较小,煤层结构复杂;障壁海岸-海沼沙岭平原体系煤层主要发育在海岸平原沼泽地区,煤层层数多、但单层厚度小和横向变化较快。

The practice for exploring natural gas in Qiongdongnan Basin proves that coal measures and coal seams are the most important hydrocarbon source rocks.Based on the characteristic analysis of coal-bearing deposits in the Oligocene Paleogene by the seismic data,drilling core and logging data in Qiongdongnan Basin,the corresponding relationships between sedimentary facies and seismic facies are established,and 14 kinds of sedimentary assemblages are identified in the Oligocene of Qiongdongnan Basin,including the alluvial fan-tidal flat-lagoon,alluvial fan-tidal flat-coast,alluvial fan-sea bog sand ridge plain-coast assemblages.According to the micro-and macro-physicochemical properties of cored coal intervals,it is made certain that the Yacheng Formation coal seam was affected by seawater during its formative process and its sedimentary environment tended to be strong in reduction,indicating that the coal seam is developed in the offshore marsh environment.In view of the characteristic and environmental analysis for coal forming,four types of coal-forming sedimentary systems were classified,namely fan delta,braided river delta,tidal flat-lagoon and barrier-coast-sea marsh sand ridge plain systems.Among them,the thin coal seam in the fan delta system,prone to bifurcation,is mainly developed in the fan delta plains,abandoned lobes,and interlobe depressions;the coal seam in the braided river delta system,featuring thinness and instability,mainly grows in the braided river delta plains,diversion bays,and floodplains.The coal seam in the tidal-lagoon system is mostly developed in the swamps of the supratidal flats and the inter-tidal-channel bays of the upper intertidal flats,the tidal delta plains and the silted lagoons,and in areas of multiple tidal channels,the coal seam layers are relatively large in number,and characterized by thinness and larger spacing,while in areas of single tidal channels,the number of coal-bearing seams,complex in structure,is relatively small,but their thickness is relatively large,and their spacing is small.At last,the coal seam in the barrier coast-sea marsh sand plain system is mainly developed in coastal plain marsh areas,characterized by multiple layers and faster change in lateral,but thinness for each layer.