莺歌海盆地天然气成藏条件探讨

DISCUSSION ON THE FORMATION CONDITIONS OF GAS RESERVOIRS IN YINGGEHAI BASIN

北北西—南南东向狭长形莺歌海走滑盆地于始新世开始发育，经历渐新世崖城期海陆过渡相沉积后，海相环境持续至今。快速沉降配以来自红河三角洲等３个主要方向的充足物源供给，以碎屑岩为主的地层已厚约２００００ｍ，并形成高温高压场。盆地长期处于右旋扭动应力场中，中部有大型走滑断裂活动，派生东西向张力，扩展盆地空间，造成地壳减薄，盆地快速沉降；向南南东加强的张扭作用使沉积中心向南南东渐次转移，诱发岩体侵入斜列式排布。早第三纪末和中中新世的两次明显左旋扭动，前者使西北部地层回返抬升，沉积中心进一步向南南东转移，产生南北向构造脊；后者造成强烈反转，产生南北向构造脊和长轴背斜。有以生气为主的３套上第三系烃源岩，渐新统和始新统烃源岩因深埋、高成熟而对天然气聚集有着重要意义。从盆地边缘到中心，有勘探意义的储层依次有渐新统、中新统、上新统和第四系，岩性由粗变细。岩相纵、横向配置构成三种不同的运移方式。按成藏机制预测有４种不同的成藏模式。

The north-northwest-south-southeast long and narrow Yinggehai strike slip basin began to be grown in Eocene and the marine enviroment in the basin is kept up to now after undergoing the transitional deposits in Yacheng age of Oligocene.Because of rapid deposition and sufficient material supply coming from three major directions as Red river delta,etc.,the thickness of the strata being mainly composed of clastic rocks is up to about 20 000 m ,a high temperature and high pressure field being formed.The basin has been sat in a dextrally torsional stress field for a long time and there are active large-sized strike slip faults in its central part,which incluces east-west tension and enlarges the basin's space,making the earth crust be thinned and the basin be rapidly subsided.The transtensional action enhanced toward south-southeast caused the depocenter to be gradually moved toward the same direction,inducing the intrusive bodies to form an echelon arrangement.There were two obviously sinistral torsional movements at the end of Paleogene and Middle Miocene.The former made the strata in the north-west part rising and caused the depocenter to be further moved toward south-southeast to form north-south structural crests;the latter made up a strong overturn,causing the north-south structural crests and long axis anticlines to be formed.Three sets of hydrocabon source rocks in the Neogene,Oligocene and Eocene,in which the gas was mainly generated,are of important significance for natural gas accumulation because of their deep burial and high maturity.From boundary to centre of the basin,the reservoir beds for exploring for gas are Oligocene,Miocene,Pliocene and Quaternary and the lithology changes from coarseness to fineness.Three different migration patterns were constituted due to the vertical and lateral matching of lithofacies and there are four reservoir formation modes there according to the forecast of reservoir formation mechanism.