莺歌海盆地东方区块地层超压对成岩作用的影响

Effects of formation overpressure on diagensis in the Dongfang block of Yinggehai Basin

莺歌海盆地东方区块莺歌海组下部和黄流组发育地层超压体系,超压顶界面深度从盆地中心向边缘逐渐变深。在综合前人研究基础上,笔者应用热解分析、生物标志化合物研究及岩石显微镜和扫描电镜等技术,并与邻区的常压环境下的相关特征进行对比,深入分析了地层超压对成岩作用的影响。结果表明:1超压对反映有机质热演化特征的指标(镜质体反射率、热解峰温、生产指数等)未产生可识别的影响,但对黏土矿物转化和烃类结构演化这类产物浓度变化速率高、体积膨胀效应强的反应有明显的抑制作用;2超压可降低砂岩的机械压实速率,使孔隙度得以有效保存;3超压可以通过抑制黏土矿物的转化来减少胶结物生成的物质来源,同时通过增大碳酸盐在流体介质中的溶解度抑制碳酸盐胶结物的生成;4研究区储层普遍高含CO2,在超压环境下CO2在流体中溶解度增大,会大量生成H+,促进溶蚀作用的产生;5超压环境促进了烃源岩生烃作用生成的酸性流体的排出,增强了相邻砂岩储层的溶蚀作用,有利于次生孔隙生成,从而发育了异常高孔带。

An obvious overpressure system was found within shallow strata in the Dongfang block of Yinggehai Basin and the burial depth of its top interface increases gradually from the center of the basin to the edge. Based on pyrolysis analyses, biomarkers, petrographic microscopic identification and scanning electron microscopy, the impact of formation overpressure on diagensis was deeply discussed. The result showed that the overpressure does not bring on a recognizable effect on geochemical indicators which reflect the thermal evo- lution stage of organic matter （vitrinite reflectance, pyrolysis peak temperature, production index）, but obviously restricts the transfor- mation of clay minerals and the evolution of hydrocarbon structures because these products vary greatly in concentrations and have a strong effect of cubical expansion. Formation overpressure can delay the mechanical compaction of sandstones, thus effectively save the porosity. Formation overpressure can also decrease the formation of cements by suppressing the transformation of clay minerals, and meanwhile restrain the formation of cements mainly by increasing the solubility of carbonates in a fluid medium. Moreover, reser- voirs in the research region generally contain high CO2, and the solubility of CO2 in geological fluids will increase in an overpressure environment so as to promote corrosion by producing more H ＋ . Besides, the mudstone overpressure can accelerate the discharge of acidic fluids through hydrocarbon generation, which may promote the dissolution of adjacent sandstones in reservoirs and favor the generation of secondary porosity, thus an anomalously high porosity zone develops to form a favorable reservoir.