Formation of Secondary Porosity in Sandstones: How Important is It and What are Controlling Factors?: ABSTRACT

Abstract

Secondary porosity in sandstones may form by chemical dissolution of grains or cements. Care must be taken to distinguish between local dissolution and reprecipitation which do not increase the net porosity, and leaching which produces a net increase in the porosity, requiring a throughflow of large volumes of undersaturated pore water to remove the reaction products in solution. Such undersaturated pore water may be derived from: (1) meteoric water driven by a hydrostatic head; (2) subsurface pore water made acid by the release of CO2 from maturing kerogen; and (3) clay mineral reactions involving transformation of kaolinite and smectite to illite and dissolution of feldspar and carbonate--(Al2Si2O5(OH)4 + KAlSiO8 = KAl3Si3O10(OH)2 + 2SiO2 + H2O). In the Jurassic sandstones of the North Sea (Statfjord field), secondary porosity is formed during early diagenesis from meteoric water dissolving feldspar. However, the kaolinite formed as a product of the leaching may form aggregates of pore-filling cement, reducing porosity and especially permeability. Petrographic examination and microprobe analyses show that clastic feldspar is rimmed by authigenic feldspar indicating that leaching was not important in creating secondary porosity End_Page 549------------------------------ during deeper burial. Calculations of the total amounts of CO2 released by kerogen in sedimentary basins suggest that secondary porosity is favored by high contents of humic kerogen, but that the CO2 produced and/or pore-water flow in many types of basins are too small to create significant amounts of secondary porosity. Reactions with clay minerals may, however, provide additional sources of acid. End_of_Article - Last_Page 550------------