东营凹陷超压顶封层及其附近砂岩中碳酸盐胶结物的成因

东营凹陷超压顶界面埋深2.200～2.800 m，在深部超压流体影响下，超压顶封层及其附近砂岩中碳酸盐矿化作用发育，碳酸盐含量一般在15%～40%。对53块砂岩样品共101个碳酸盐胶结物原位微区电子探针成分数据进行数理统计后发现，碳酸盐矿物主要可分为准同生白云石、方解石与铁白云石3种类型，综合X-衍射、阴极发光等判断其成岩序列为准同生白云石→方解石→铁白云石。根据碳酸盐胶结物成岩流体包裹体观测结果，超压顶封层及其附近砂岩中碳酸盐胶结物的沉淀伴随着超压流体环境，最小古压力系数为1.29～1.62，且沉淀温度明显高于背景温度值，指示与超压热流体侵入有关。结合前人研究成果认为,方解石、铁白云石胶结物的沉淀可能分别与东营组沉积末期、明化镇组沉积以来的超压流体活动有关；同时利用氧同位素地质温度计对上述认识进行了检验，其结果与实际观测值也较为吻合。方解石胶结物的沉淀温度90～120℃，其超压流体的δ¹⁸O_SMOW 值为零，方解石胶结物的δ¹⁸O_PDB 值为-16.86‰～-12.29‰；铁白云石胶结物的沉淀温度约110～135℃，其超压流体的δ¹⁸O_SMOW 值为0.25‰，铁白云石胶结物的δ¹⁸O_PDB 值介于−12.20‰～−10.20‰。晚成岩阶段碳酸盐胶结物的碳同位素值具有正漂移现象，δ¹³C_PDB 值介于−0.9‰～+3.58‰，说明其主要来源于沙四段湖相碳酸盐岩的溶解-再沉淀作用，有机酸的参与显得相对次要，或者是由于羧酸分子内碳同位素分馏作用的影响。

Origin of carbonate cements in the overpressured top seal and adjacent sandstones in Dongying depression

The burial depth of the overpressured top boundary ranges from 2 200～2 800 m in the Dongying Depression.Interacted by overpressured fluids that flow beneath,the overpressured top boundary is generally characterized by well-developed carbonate mineralization,commonly with carbonate content between 15%～40%.The statistical analysis on in-situ electron-microprobe compositional data of 101 carbonate cements from 53 sandstone samples indicated that carbonate minerals could be basically subdivided into three groups,i.e.penecontemporaneous dolomite,calcite and ankerite.Integrated with X-ray diffraction and cathodoluminescence data,their diagenetic sequence was determined as penecontemporaneous dolomite→calcite→ankerite.On the basis of observation of primary inclusions in carbonate cements,the precipitation of carbonate minerals in the overpressured top seal and adjacent sandstones was accompanied with overpressured fluids,with a minimum paleopressure coefficient ranging between 1.29～1.62 and a precipitation temperature obviously higher than the background temperature,which suggested a significant influence by overpressured thermal fluid invasion.Thus,based on previous study results,we proposed that the precipitation of calcite and ankerite cements might be related to activities of overpressured fluids since the deposition at the terminal Dongying stage and the Minghuazhen stage,respectively.Furthermore,this hypothesis was tested by measured oxygen isotopic values that fluctuate from-16.86‰ to-12.29‰ PDB for calcite and from-12.20‰ to-10.20‰ PDB for ankerite.Further investigations suggested that the calcite should precipitate at 90～120℃ for the δ18OSMOW of its homochronous overpressured fluid is around 0.00‰ and the ankerite should precipitate at 110～135℃ for the δ18OSMOW of its homochronous overpressures fluid is 0.25‰.δ13C values of carbonate cements formed at the late diagenesis show a positive shift,ranging from-0.9‰ to +3.58‰,which indicates an origin mainly from the dissolution-reprecipitation process of the Es4 lacustrine carbonate,or an influence of intramolecular carbon isotopic fractionation within carboxylic acids,though the latter seems to play a relatively minor role.