Mechanism for Framework Grain Dissolution (Secondary Porosity in Sandstones): ABSTRACT

Abstract

We propose that organic and clay maturation in concert are responsible for much framework grain dissolution (secondary porosity). Petrographic observations indicate a pulse of porosity formation near the top of the oil-generation window and that there is often not enough authigenic clay to account for the aluminum removed from the dissolved grains. Geochemical considerations indicate that H+ ions are required for aluminosilicate dissolution and that the aluminum must be complexed to concentrations greater than 100 ppm in order to transport aluminum out of the sandstone using water volumes available in most basins. The smectite to illite conversion, which is coincident with early organic maturation, produces additional pore water and can desorb organic molecules from the s ectite interlayers. The early stages of organic-matter maturation generates H+ (as carbon dioxide), volume-change pressures to move fluids, and water-soluble organic matter. The soluble organic matter can contain ligand compounds (e.g., short-chain fatty acids) which complex aluminum. The organic ligands in the shale complex aluminum at relatively low concentrations because aqueous aluminum activity is depressed by the formation of illite from smectite. The H+ and organic ligand-bearing solution is expelled into sandstones where the aluminum activity is buffered at higher levels by feldspar, thus allowing higher levels of complexed aluminum. The solution dissolves the feldspars and other aluminosilicate components and complexes much of the resulting aluminum for transport out of the sand tone. End_of_Article - Last_Page 630------------