Selective dissolution of feldspars in the presence of carbonates: The way to generate secondary pores in buried sandstones by organic CO2

Abstract

Carbonates are suggested to dissolve rapidly than feldspars by laboratory experiments. Petrography texture of selective dissolution of feldspars in the presence of carbonates, however, is widespread in buried sandstones and even shales, inspiring a revisit to the chemistry of burial secondary pores.Feldspar dissolution, precipitation of secondary minerals (quartz, clays), and carbonate cementation are common chemical reactions in the Eocene sandstones in the northern Dongying Sag. Petrography evidence demonstrates the selective dissolution of feldspars in the presence of carbonate minerals (both detrital and authigenic minerals) in these buried sandstones. The equilibrium constant of calcite leaching reactions is much smaller than that of K-feldspar leaching reactions. Numerical simulations of chemical reactions in K-feldspar-calcite–CO2–H2O systems utilizing the Geochemist's Workbench 9.0 (GWB) indicate that only a small amount of calcite was dissolved at the onset of simulation processes, while much K-feldspar was dissolved with precipitation of quartz, clays and some calcite for extended periods of time. Precipitation of secondary calcite could also promote feldspar dissolution. Simulation of reactions in a simplified sandstone system with constraints of present-day pore water and partical pressure of carbon dioxide (pCO2) in the northern Dongying Sag indicates that the pore waters are close to equilibrium with calcite. Petrography evidence and modeling results share consistence in confirming that only feldspar could be dissolved extensively, with precipitation of quartz, clays and some carbonate minerals.The organic CO2-leaching theory (Schmidt and McDonald, 1979) regarding the dissolution of mainly carbonates and, to a lesser degree, feldspars to form secondary pores has been questioned due to supposed extensive internal consumption of organic CO2 in source rocks by carbonate minerals. The CO2 in sandstones and shales and the isotopic composition of CO2 and carbonate cements in sandstones, however, suggest migration of organic CO2 from mudstones (even with large amounts of carbonate) to sandstones. Calculations in this study indicate that, with suppression of carbonate dissolution in sedimentary rocks, many secondary pores can be generated in sandstones through selective dissolution of feldspar by organic CO2 from thermal evolution of organic matter. Selective dissolution of feldspars, rather than carbonates, in sedimentary rocks is then likely a general mechanism to decipher the chemistry of secondary porosity.Other secondary porosity generation mechanisms were also reevaluated with numerical simulations. Results demonstrate that reverse weathering reactions of silicates may take place with increasing of temperatures, but cannot provide enough H+ to dissolve carbonates at elevated temperatures. Additionally, cooling of hot fluids can dissolve only insignificant amounts of carbonate unless significant amounts of water are provided.