川东北铁山—双家坝地区长兴组白云岩成因

Genesis of dolomite of Changxing Formation in Tieshan-Shuangjiaba area,northeast Sichuan Basin

礁滩相白云岩是川东北地区长兴组的主要储集层,研究其成因对促进天然气勘探与开发具有重要意义。通过对川东北铁山—双家坝地区上二叠统长兴组白云岩的结构、成因和成岩作用方式进行分析,并结合白云岩有序度、阴极发光、扫描电镜,以及铁、锰、锶微量元素和碳、氧同位素等资料,判断出长兴组微晶白云岩的白云岩化作用发生在原始沉积环境为相对封闭且强烈蒸发的台缘礁后局限海盆中;残余生屑白云岩和晶粒白云岩的原岩为礁灰岩和生屑灰岩,其原始沉积环境属台缘或台内礁滩相沉积,基于台内礁滩相带相对闭塞、蒸发较强,台缘礁滩相带更易暴露并接受大气水淋滤的影响,较好地继承了原岩的地球化学特征,并在不同相带之间存在明显差异。同时,分析了长兴组白云岩化过程中不同类型的白云岩化流体的水-岩反应过程和关系,以白云岩成岩环境和白云岩化流体性质及来源为依据,结合水文地质学模式,对白云岩成因进行了研究,并认为微晶白云岩属近地表准同生期的潮坪环境,由蒸发泵白云岩化形成,白云岩化流体来自准同生期蒸发海源卤水;残余生屑白云岩和晶粒白云岩属浅—中埋藏成岩环境,由热对流白云岩化形成,白云岩化流体来自晚二叠世沉积期的海源卤水。

As the main reservoir rock of Changxing Formation in the northeast Sichuan, the dolomite of reef flat facies has received more attention. Therefore, it is of great significance to comprehensively analyze its genesis in order to promote the natural gas exploration and development. The dolomite of Changxing Formation in Tieshan-Shuangjiaba area was analyzed from the aspects of structure, genesis and diagenesis. Based on the analysis of the dolomite order degree, cathodeluminescence, SEM, trace elements and stable isotopes, it is demonstrated that the dolomitization of dolomicrite of Changxing Formation took place in the relatively closed and strongly evaporated restricted sea basin behind the platform margin reef. The initial rocks of biogenic dolomite and crystalline dolomite are identified to be reef limestone and bioclastic limestone, which are originally deposited in the platform margin reef flat or intra platform reef flat. Geochemical characteristics of the dolomite inheriting properties of initial rocks significantly varied in different facies, which can be attributed to different sedimentary characteristics, namely the isolation and evaporation of the intra platform reef as well as the exposure and meteoric water leaching of the platform margin reef. Moreover, waterrock interaction in different types of dolomitization fluids was analyzed. Based on the analyses of the diagenetic environment of dolomite as well as dolomitization fluid properties and sources, combined with the hydrogeological model, the genesis of dolomite was studied. It is considered that the dolomicrite was formed by evaporating pump dolomitization during the penecontemporaneous period at near-surface with the dolomitization fluid of penecontemporaneous high salinity evaporated seawater, while the biogenic dolomite and crystalline dolomite were formed by thermal convection dolomitization in the shallow to middle-buried diagenetic environments with the dolomitization fluid of synsedimentary high salinity evaporated seawater sealed in the stratum.