Secondary porosity in sandstones is created by subsurface dissolution of grains or cement by pore water that is undersaturated with respect to one or more of the major mineral phases. Such undersaturated pore water may be derived from: (1) meteoric water driven by a hydrostatic head; (2) compactional pore water containing CO2 released from maturing kerogen; (3) clay minerals reactions including the transformation of kaolinite and smectite to illite; and (4) reactions between clay minerals and carbonate releasing CO2. Calculations of the CO2 generated from different types of kerogen suggest that few basins will generate enough CO2 to produce large-scale leaching in thicker sandstones. Dissolution of minerals and removal of aluminum and silica in solution requires that very large volumes of pore water flow through the sandstone. Because leaching often enlarges primary pore space, it is very difficult to estimate the percentage of the pore space that is secondary. Leaching and formation of secondary pore space may also be accompanied by reprecipitation of other minerals so that the net gain in porosity is less than the observed secondary pore space.
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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.