Distribution Models of Feldspar Dissolution Products in Different Diagenetic Environments in Clastic Reservoir

It is still debatable whether feldspar dissolution products can be carried effectively out of clastic reservoir, which lead to puzzle of feldspar dissolution’s effect on reservoir property. This study focused on the Paleogene clastic reservoir in Dongying Sag, East China. Characteristics of feldspar dissolution products (kaolinite, illite and quarts overgrowths) in different diagenetic environments were analyzed, with application of thin sections, SEM observation and X-ray analysis. The author proposed three feldspar dissolution products distribution models in different diagenetic environments (relatively open or closed system) and discussed its effects on reservoir property. Model 1: In buried relatively closed diagenetic environment, inefficient reservoir grid-pore pathways cause extremely slow pore-water flow. When pore-water is in near-equilibrium with nearby minerals, despite feldspars dissolve, the leaching products such as Al and silica have to re-accumulate in authigenic minerals quickly to reach new balance between fluids and minerals. By image analysis, we found almost isovolumetric authigenic minerals precipitated in situ, which means the leaching of feldspars transforms primary pores just to secondary pores and clay intercrystal pores with poor pore structure. Thus dissolution enhances little porosity, and permeability is even lower (especially when illite dominates). Model 2: In buried diagenetic environment with fault system as efficient fluid pathways, acid fluids with relative low salinity can dissolve feldspars largely and transport cations in long distance. In this case, leaching of feldspars can enhance porosity and permeability to some extent, and vice versa. Model 3: In epidiagenesis environments where unconformity developes, effect of feldspar dissolution varies from vertical division. In vadose zone, with fractures and pores developed, meteroric water can dissolve feldspars largely and transport cations downwards, enhancing reservoir property. In upper part of phreatic zone, as pore-water salinity rises with intrusion of cations from vadose zone and fluids slow down due to change of flow mode, dissolution destroy pore structure due to cements precipitation; in deeper part of phreatic zone, pore water’s solubility is limited, and weak feldspar dissolution change little of reservoir property. Thus single view on feldspar dissolution’s effect may put the prediction of favorable reservoirs originated from leaching of feldspars at risk.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California