陆相石膏赋存状态及沉积过程——以库车坳陷牙哈地区古近系苏维依组底砂岩段为例

Occurrence State and Deposition of Terrestrial Gypsum:A case studyof the Paleogene Suweiyi Formation,Yaha area,Kuqa Depression

库车坳陷牙哈气藏是我国开发时间最早、开发技术最为成熟的凝析气藏之一,其主要产层段为古近系苏维依组底砂岩段,岩性以含石膏团块的细砂岩为主。石膏的赋存状态多种多样,而不同赋存状态的石膏发育时期、水动力条件、沉积环境和对储层物性的影响均有不同。基于牙哈气田古近系苏维依组底砂岩段钻井取心、野外剖面、岩石薄片等资料,探讨石膏赋存状态及其形成过程。结果表明:(1)研究区古近系底砂岩段沉积成因石膏可分为异地沉积石膏和原地沉积石膏;(2)异地沉积石膏团块经流水搬运,搬运距离从近到远依次为:①撕裂状,粒径8~10 cm的不规则状石膏团块;②云雾状,粒径4~8cm的云雾状石膏团块,具棱角;③团砾状,粒径2~5 cm,近圆状石膏团块,磨圆度较好;④定向排列石膏,粒径0.5~2 cm,石膏团块定向排列,围岩见平行层理构造;原地沉积石膏包括蒸发浓缩形成均匀分布的点状石膏,以及流水搅动形成的具核心、不具同心圈层结构的鲕状石膏;(3)干旱气候背景下湖泊蒸发环境沉积时期可分为丰水期与枯水期,枯水期为盐湖蒸发沉积模式,石膏等盐类矿物蒸发沉积,丰水期为河流—三角洲—湖泊沉积,沉积砂体。不同赋存状态的石膏代表了不同的古环境、古气候、古水动力条件,该研究为苏维依组干旱气候条件下季节性河流—盐湖三角洲的沉积模式研究提供了新的思路。

The Yaha gas reservoir in the Kuqa Depression is a condensate gas reservoir with the earliest development time and most mature development technology in China.Its main producing interval is the bottom sandstone interval of the Paleogene Suweiyi Formation.The block is dominated by fine sandstone.The occurrence states of gypsum in sandstone are varied,and the development period,hydrodynamic conditions,depositional environment and physical properties of gypsum of different occurrence states are different.Based on the data of drilling cores,field profiles,and rock thin sections in the bottom sandstone section of the Paleogene Suweiyi Formation in the Yaha gas field,the occurrence state and deposition of gypsum were discussed.The results show that:(1)The different occurrence states of gypsum in the study area are caused by differences in the depositional environment,depositional process and dia⁃genetic environment.The sedimentary gypsum of the Paleogene bottom sandstone in the study area can be divided in⁃to allochthonous deposit and in-situ deposit gypsum.(2)The gypsum masses deposited in different places are primari⁃ly transported by flowing water,and the transport distances from near to far are as follows:①irregular gypsum mass⁃es with a particle size of 8⁃10 cm;②cloud shape gypsum masses with a particle size of 4⁃8 cm,angular;③Granu⁃lar gypsum,with a particle size of 2⁃5 cm;it is a nearly round gypsum mass,and the roundness is good;④direction⁃al arranged gypsum,with a diameter of 0.5⁃2 cm and parallel bedding structure.(3)The gypsum deposited in situ in⁃cludes the uniformly distributed spot-shaped gypsum formed by evaporation concentration and the oolitic gypsum with core and no concentric ring structure formed by flowing water agitation.Gypsum in different occurrence states repre⁃sents different paleoenvironment,paleoclimate,and paleohydrodynamic conditions and can provide new ideas for the study of the sedimentary process of the Suweiyi Formation in the seasonal river salt lake delta under an arid climate.