埋藏历史对碳酸盐溶解-沉淀的影响--以四川盆地东北部三叠系飞仙关组和塔里木盆地北部奥陶系为例

The Influence of Burial History on Carbonate Dissolution and Precipitation——A Case Study from Feixianguan Formation of Triassic,NE Sichuan and Ordovician Carbonate of Northern Tarim Basin

埋藏成岩过程中碳酸盐的溶解与沉淀强烈控制着深埋藏地层中碳酸盐岩储层质量,与之有关的地质过程是人们多年来不懈研究的热点领域。热流体沿断裂向上运移所造成的碳酸盐溶解成为近年人们普遍认可的模式,但这不能圆满解释大量存在的不与断裂伴生的深埋藏碳酸盐的溶解机制。基于四川盆地东北部和塔里木盆地北部深埋藏碳酸盐溶解与沉淀机制的研究,认为埋藏过程中盆地的沉降与抬升是碳酸盐溶解与沉淀以及次生孔隙发育的另一重要机制,在改变流体对碳酸盐饱和状况方面具有和热流体沿断裂向上运移相同的重要性。研究表明:与川东北飞仙关组类似的埋藏历史更有利于深埋藏成岩过程中碳酸盐的溶解,主要特点是当前埋藏深度不是最大埋藏深度,在最大埋藏深度附近发生热化学硫酸盐还原作用,其后盆地持续抬升造成温度降低和碳酸盐溶解,与之有关的次生孔隙形成所造成的岩石力学性质的变化与上覆载荷减小可以得到平衡,次生孔隙得以保存,岩石储层质量改善;塔里木盆地北部的奥陶系埋藏历史不利于深埋藏成岩过程碳酸盐的溶解,主要特点是当前埋藏深度是最大埋藏深度,古喀斯特发生之后盆地持续沉降造成温度升高和碳酸盐沉淀,并与上覆载荷的增加同步发生,结果是孔隙的封堵、岩石的致密化和储层质量的变差。

The dissolution and precipitation of carbonate in buried diagenesis extensively control the quality of carbonate reservoir.The related geological process has been the research focus for years.The hydrothermal fluids migrating upwards from deeper strata along faults can cause the dissolution of carbonate.However,this model can not fully explain the dissolution mechanisms of extensively existing carbonate which are not associated with fault.Based on the study of the dissolution and precipitation mechanisms of deep buried carbonate in Northeast Sichuan Basin and Northern Tarim Basin,we propose that the subsidence and uplift of basin during buried process is another important mechanism for the dissolution and precipitation of carbonate and the formation of secondary porosity.The changing of the fluids saturation to carbonate is as important as the upward migration of hydrothermal fluids along fault.Our study reveals that the burial history of Feixianguan Formation,NE Sichuan is more favorable to the dissolution of carbonate during burial diagenesis.The main feature is that the present burial depth is not the deepest buried depth.TSR occurs at the deepest buried depth.Then,the continuous uplift of the basin causes the decrease of the temperature and the dissolution of carbonate.The formation of related secondary porosity results in the changing of rock mechanics,which can be balanced out with the decreasing overlying load.Therefore,the secondary porosity can be preserved and the reservoir quality can be improved.On the contrary,Ordovician burial history in Northern Tarim Basin is not favorable to the dissolution of carbonate in buried diagenesis.The main feature is that the present buried depth is the deepest burial depth.The continuous subsidence of the basin after paleokarst could lead to the decrease of the temperature and the precipitation of carbonate,combined with the increase of overlying load that result in the compacting and severe degradation of the quality of reservoir.