控制储层中异常高孔带发育的成岩作用——以琼东南盆地西部崖城区为例

Digenesis controlling development of abnormal high porosity zones:A case from Yacheng area in the western Qiongdongnan basin,South China Sea

基于大量物性数据和沉积构造背景,利用铸体薄片、有机地球化学和流体包裹体技术,研究了控制琼东南盆地西部崖城区储层异常高孔带发育的成岩作用.认为崖城区三段异常高孔带的成因不同,控制其发育的主要成岩作用分别为有机酸溶蚀与大气淡水淋滤作用,热流体溶蚀作用和有机质生成天然气引起的超压保持作用.研究结果表明:崖城区共发育3段异常高孔带,分布于2 400~3 100m,3 600~4 100m和4 500~4 800m.第1段异常高孔带属于有机酸和大气淡水混合溶蚀型,即有机质早期大量释放的有机酸和大气淡水共同溶解了长石、碳酸盐胶结物等产生次生孔,早期绿泥石包壳未起有效作用.第2段异常高孔带属于热流溶蚀型,热流体使烃源岩加快熟化从而延长有机酸的产出期,而且促使蒙脱石迅速转化为伊利石从而释放更多的H^+;深部流体携带大量酸性组分且具有较强的流体性,可以对储层产生淋滤,也加快了Al^(3+)的迁移;更高的温压可以促进溶蚀反应的发生,提高溶解度.第3段异常高孔带属于超压保持性型,强超压有效抑制了压实作用和自生矿物发育,保持了孔隙度.超压体系向上集中排放形成热流体使得该段超压储层硅质流失,致使自生石英未规模发育.伊蒙转化反应被提前而且超压对该反应也有抑制作用,使得自生伊利石未大量形成.该区天然气充注期为晚期,烃类侵位未能抑制石英等自生矿物的生长,但盆地晚期快速沉降使得源岩大量生成天然气促使超压形成,间接贡献了第3段异常高孔带的发育.

Based on abundant petrophysical property data of rocks and sedimentary and tectonic background,digenesis controlling development of abnormal high porosity zones in Yacheng ar-ea in the western Qiongdongnan basin were comprehensively investigated using cast thin sections,geochemistry data and fluid inclusion technology.It is found that digenesis controlling the development of the three abnormal high porosity zones in Yacheng area are different,which are organic acid corrosion and meteoric water leaching,thermal fluid corrosion and overpressure maintenance caused by gas generation,respectively.The results indicate that there are three abnormal high porosity zones,distributing in 2 400—3 100 m,3 600—4 100 m and4 500—4 800 mof Yacheng area.The genesis of the first abnormal high porosity zone stems from the corrosion from organic acid and meteoric water.Feldspars and carbonate cements are corroded by both organic acid released from organic minerals in the early stage and meteoric water to generate secondary porosity.The chlorite cladding almost has no contribution to the development of the abnormal zone.Thermal fluid gives rise to the formation of the second abnormal high porosity zone.The thermal fluid activities make source rocks mature rapidly,therefore the organic acid generation period is extended,and more hydrogen ions are released due to smectite transferring into illite rapidly.Deep fluids carrying acidic components with strong flow lead to not only leaching of reservoir but also the rapid migration of the aluminum ions.The higher temperature and pressure promote erosion effects of reservoir,and the solubility is increased.The genesis of the third abnormal high porosity zone belongs to the type of overpressure maintenance.The compaction and development of authigenic minerals are effectively inhibited by strong overpressure,thus the reservoir porosity is preserved.Besides,the thermal fluids discharged upward by overpressure system cause the loss of silica which give rise to little development of authigenic quartz.The development of illite is also prohibited by the smectite transferring into illite ahead of time and overpressure.Natural gas charging in the study area occurs late.The development of authigenic minerals including quartz are not restrained by hydrocarbons emplacement.However,strong overpressure caused by abundant gas generation due to the rapid subsidence in Qiongdongnan basin indirectly contributes to the development of the third abnormal high porosity zone.