柴北缘腹部砂岩中碳酸盐胶结物特征及成因探讨

Characteristics and genesis of carbonate cement in abdomen sandstone in northern margin of Qaidam Basin

利用岩心观察和描述、镜下鉴定、碳氧同位素分析等手段,结合研究区沉积环境和构造背景,对研究区侏罗系—新近系储层砂岩中碳酸盐胶结物的类型、期次、分布形态、地球化学特征、成因机制等方面进行了详细的研究。结果显示:柴北缘腹部地区中新生界砂岩主要胶结物类型为碳酸盐胶结,胶结物分为早、中、晚3期,早期碳酸盐胶结物包括方解石、白云石、菱铁矿,中期主要是方解石和含铁方解石,晚期为含铁白云石。侏罗纪柴北缘腹部地区经历了由封闭咸湖向开放型湖盆的转变过程,δ13C值为-16.77‰~8.01‰和δ18O值为-18.52‰~-8.34‰,古盐度值(Z)和古温度分别为86.50~133.72℃和67.81~142.19℃,成岩阶段处于早成岩阶段B期—中成岩阶段B期。古近系(E1+2、E31、E32)形成于开放水体环境,δ13C值为-8.71‰~-2.40‰,δ18O值为-16.62‰~-8.77‰,古盐度和古温度分别为102.85~116.16℃和63.03~117.28℃,成岩阶段为早成岩阶段B期—中成岩阶段A期。新近系上干柴沟组(N1)和下油砂山组(N21)也形成于开放水体环境,δ13C值为-6.81‰~-3.80‰和δ18O值为-12.73‰~-6.13‰,古盐度和古温度值分别为107.65~114.89℃和25.54~88.93℃,成岩演化阶段处于早成岩阶段A期—早成岩阶段B期。不同层位碳、氧同位素含量基本呈现出相同的变化趋势,但是仍存在个别点呈现相反的趋势,可能与大气淡水有关。其碳酸盐胶结物成因机制既包括有机成因也包括无机成因,早期主要受大气淡水淋滤作用,中期主要是有机质热解脱羧作用提供碳源,晚期主要为生物成因碳酸盐,同时也含有少量有机碳源。

Based on core observation and description,microscopic identification,analysis of carbon and oxygen isotope analysis,etc.,combined with the sedimentary environments and tectonic background,this paper makes a detailed study on the genesis and mechanism of the type,sub-distribution,morphology and geochemical characteristics of carbonate cements from Jurassic to Neogene reservoir sandstone in the study area.The results show that the main cementation types of Meso-Cenozoic sandstone in the abdomen area of the northern margin of Qaidam is carbonate cementation,which can be divided into early,middle and late three stages.The early carbonate cementation includes calcite,dolomite and siderite,the middle stage are mainly calcite and iron-bearing calcite,and the late stage is iron-bearing dolomite.The abdomen of the northern margin of Qaidam Basin in Jurassic experienced a transition from closed salty lake to open lake,withδ13C values ranging from-16.77‰to 8.01‰andδ18O values ranging from-18.52‰to-8.34‰,paleosalinity(Z)values ranging from 86.50 to 133.72 and paleo-temperature from 67.81 to 142.19℃,respectively.And the diagenesis stage was in the early diagenesis stage B to middle diagenesis stage B.In Paleogene(E1+2,E31,E32),the northern margin of Qaidam Basin was in an open water environment,and the carbon and oxygen isotopes were from-8.71‰to-2.40‰and from-16.62‰to-8.77‰,respectively.The paleosalinity and paleotemperature were 102.85-116.16℃and 63.03-117.28℃,with the diagenesis stage of the middle diagenetic stage A to the early diagenetic stage B.In Neogene(N1 and N21),it was also in an open water environment,withδ13C values of-6.81‰--3.80‰andδ18O values of-12.73‰--6.13‰,The paleosalinity and paleotemperature were 107.65-114.89℃and 25.54-88.93℃,whose diagenetic evolution stage was in the early diagenesis stage A to middle diagenetic stage B.The content of carbon and oxygen isotopes in different layers shows the same trend,but there are still some points showing the opposite trend,which may be related to atmospheric fresh water.The genesis mechanism of the carbonate cement includes both organic and inorganic origin.In the early stage,it was mainly affected by the leaching of fresh water.In the middle stage,it was mainly provided by the thermal release carboxylation of organic matter.And in the late stage,it was mainly biogenic carbonate,and also contained a small amount of organic carbon sources.