济阳坳陷古近系页岩文石含量与烃源岩热演化的对应关系

The correspondence between the aragonite content of Palaeogene shale and the thermal evolution of source rocks in Jiyang depression

文石在济阳坳陷古近系湖相页岩中较为常见,多发育于富碳酸盐矿物的页岩中,属于化学成因。统计发现,济阳坳陷古近系湖相页岩文石含量的变化包络线与埋藏深度具有明显对应关系。在埋深2 300 m以上的地层中,文石含量变化不大;当埋深超过2 300 m时,文石含量包络线随埋藏深度的加大而快速降低;埋深达到约3 000 m,文石含量包络线降低的速率达到最大;当埋深超过约3 500 m时,页岩中的文石大部分消失。文石含量包络线的这种变化趋势与烃源岩热演化阶段基本一致,表明页岩地层中文石的含量变化明显受有机质热演化的控制。为进一步验证该解释,将研究区富文石和富方解石的页岩样品分别与有机酸(乙酸)和油层水在高温高压下进行多组溶解实验,测试实验前后碳酸盐矿物的溶解程度,结合不同埋藏深度烃源岩热演化的流体特征分析文石含量的变化趋势。分析认为:文石含量与有机质成熟演化过程中排出的有机酸浓度成反相关,有机酸生成的越多,文石的溶解越严重。当埋深大于3 500 m时,烃源岩进入高成熟—过成熟阶段,有机酸的生成逐渐停止,文石含量的减少也趋于停滞,并在页岩孔隙水中开始重新结晶出较稳定的方解石。

Aragonite is a common mineral in Palaeogene lacustrine sediments of Jiyang depression,mostly developed in carbonate-rich shale,and characterized with chemical genesis.Statistics reveal that the change envelop curve of aragonite content in Palaeogene lacustrine shalehas an obvious correspondence with the burial depth.In the strata above the buried depth of 2 300 m,the aragonite content changes little.When the buried depth is greater than 2300 m,the envelop curve of aragonite content is declining quickly with the increase of buried depth.When the buried depth reaches about 3 000 m,the envelop curve of aragonite content shows the maximum decline rate.When the buried depth exceeds 3 500 m,a majority of aragonite in shale disappears.This change trend of the envelop curve of aragonite contentis basically consistent with that in the thermal evolution stage of source rocks,indicating that the aragonite content change in shale formation is obviously controlled by thermal evolution of organic matters.To further validate such an explanation,multiple groups of dissolution experiment are carried out on the aragonite-rich and calcite-rich shale samples withthe addition of organic acids（acetic acid）and reservoir water under high temperature and high pressure,so as to test thedissolution degree of carbonate minerals prior and posterior to the experiment and analyze the change trends of aragonite content based onfluid characteristics in the thermal evolution of source rocks at different buried depths.The analysis shows that the aragonite content is negatively correlated to concentrationof the organic acid discharged in the maturity evolution process of organic matters.The more thegenerated organic acid is,the more intensified the dissolution of aragonite will be.When the buried depth is more than 3 500 m,source rocks enter the high mature–over mature stage;when the formation of organic acidis gradually terminated,the disappearance of aragonite also tends to stagnate,and the recrystalization of shale calcite occurs in shale pore water.