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

SU Ao; CHEN Honghan; HE Cong; LEI Chuan; LEI Mingzhu; LIU Yanhua

Abstract：Based on abundant petrophysical property data of rocks and sedimentary and tectonic background,digenesis controlling development of abnormal high porosity zones in Yacheng area 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 and 4 500-4 800 m of 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.