Four truths about holes in sandstone underlie current research and unsolved problems of sandstone diagenesis. First, primary intergranular porosity and permeability of sand are greatly reduced and subject to total destruction in the early stages of burial diagenesis. Compaction, cementation, recrystallization, and replacement are widely recognized porosity-reducing mechanisms. Rate of porosity loss with depth is related mainly to original sand composition.
Second, at later stages of diagenesis, secondary porosity can be produced by dissolution, of detrital and authigenic minerals. Porosity can be restored and enhanced at depth. Porosity of many major hydrocarbon reservoirs is mainly secondary, a fact just recently documented in the literature. Secondary porosity must be formed before hydrocarbon migration if it is to serve as reservoir porosity. Secondary porosity can be destroyed diagenetically, but it will persist to greater depths than will primary porosity.
Third, chemical diagenesis of sandstones is a kinetic process; mineral matter is dissolved, transferred, and precipitated by aqueous solutions moving through sandstones. The main source of water is from dewatering of shales interbedded with sandstones. Reconstruction of the chemical evolution of moving water, of its flow paths through a basin, and of the time of migration are the keys to predicting subsurface distribution of sandstone porosity. Mathematical modeling of hydrodynamics and mineral reactions by means of computer simulation is a promising approach to porosity prediction.
Fourth, the course of sandstone diagenesis in a given basin is programmed by the preburial, prediagenetic factors of provenance, depositional environments, and tectonic setting. These interrelated factors influence sand composition and texture, which in turn govern mineral reactions and fluid-flow rates.
Diagenetic processes determine porosity in terms of origin, amount, subsurface distribution, pore-size distribution, pore shape, surface area, and attendant permeability. Thus, diagenetic history must be taken into account by geologists and engineers in the petroleum industry. Rewards from studies of diagenesis will be sharper porosity prediction in exploration and more efficient management of rock-fluid interactions in producing reservoirs.
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Aspects of Diagenesis
There are a number of gaping holes in accumulated knowledge within the discipline of sedimentology. Perhaps one of the largest holes has been the general subject of diagenesis in clastic rocks. It was therefore fortuitous that two symposia covering various aspects of diagenesis (mainly in clastics) were presented a year apart in different parts of the country but with the same motivation – to contribute to the closing of that knowledge gap. Sedimentologists now have a fairly good idea of the what and the how of sediment deposition. What happens after the sediments are lithified has frequently been ignored. It was the aim of both editors of this publication to approach the subject from two different viewpoints. Schluger directed a symposium which looked mainly at clastic reservoirs, and Scholle presented a symposium which examined various aspects of paleotemperature control of diagenesis.