The Chemistry of Secondary Porosity
The development of secondary porosity (porosity enhancement) in many sandstones is the result of aluminosilicate and/or carbonate dissolution. The dissolution of aluminosilicate minerals and subsequent porosity enhancement is a problem of aluminum mobility. Our experimental data demonstrate that it is possible to increase significantly the mobility of aluminum and to transport it as an organic complex in carboxylic acid solutions. These same carboxylic acid solutions have the capability of destroying carbonate grains and cements.
Carothers and Kharaka have shown that concentrations of carboxylic acid anions range up to 5000 ppm over a temperature range of 80–200° C in some oil field formation waters. Our experiments show that acetic acid solutions at the same concentrations and over the same temperature range can increase the solubility of aluminum by one order of magnitude, whereas oxalic acid solutions increase the solubility of aluminum by three orders of magnitude. The textural relations observed in the experiments are identical to those observed in sandstones containing porosity enhancement as a result of aluminosilicate dissolution.
A natural consequence of the burial of sedimentary prisms is the maturation of organic material. These maturation reactions result in the evolution of significant amounts of organic acids and carbon dioxide. The experiments suggest that the enhancement of porosity in a sandstone as a result of aluminosilicate or carbonate dissolution is the natural consequence of the interaction of organic and inorganic reactions during progressive diagenesis. The degree to which porosity enhancement develops depends on the ratio of organic to inorganic matter, the initial composition of the organics, the sequences, rates and magnitude of diagenetic reactions, fluid flux, and sand/shale geometry.
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Clastic diagenesis has evolved from a very descriptive science to a much more process-oriented study. This evolution has been driven by the realization that many hydrocarbon reservoirs have significant diagenetic compotents directly affecting porosity and permeability characteristics. The prediction in time and space of reservoir characteristics affected by diagenesis can greatly reduce the risk in the search for hydrocarbon accumulations, particularly in subtle targets lacking pronounced structural expression. This publication contains three sections designed to increase understanding in the processes controlling clastic diagenesis: Conepts and Principles; Aspects of Porosity Modification; and Applications of Clastic Diagenesis in Exploration and Production. The first two sections deal with processes controlling various aspects of clastic diagenesis, and the third section applies these principles and observations to specific examples. Altogether, the three sections contain 22 chapters.