Reservoir Diagenesis and Convective Fluid Flow
A diagenetic model based on convective fluid flow has been analyzed for typical reservoir conditions. Calculations based on the model suggest that a significant fraction of the inorganic diagenesis observed in sandstone reservoirs can be attributed to the presence of slowly circulating aqueous fluids. Stability considerations indicate that static pore fluids do not exist in porous bodies of geologic dimensions and that pore fluids will convect at a rate of about 10−8 m per sec (~1 m per yr) in the presence of a normal geothermal gradient (25° C per km).
If it is assumed that the pore fluid maintains chemical equilibrium with the rock matrix, it follows that mass must be transferred as the fluid crosses isotherms. Minerals such as quartz, which have prograde solubilities under normal reservoir conditions, will move from hot source zones to cooler sinks. Minerals such as calcite, which have retrograde solubilities, will move from cool sources to hot sinks. The net effect is a continuous transfer of rock matrix in the reservoir for as long as the fluid circulates.
Because the temperature field can change sharply along a streamline, convection can localize precipitation and dissolution zones. In anticlinal structures, the fluid flow is most likely a modified torus in which warm fluid flows up the base of the ascending limb while cooler fluid flows down along the upper surface. The regions of most rapid heating and cooling of the fluid occur at the synclinal troughs and at the anticlinal crests. This flow pattern will produce zones of intense diagenesis at the crests and troughs of the structure. Zones of secondary porosity produced by the dissolution of framework grains or previously deposited cements are also predictable and the model provides explicit conditions for isomorphic replacement.
Since hydrocarbon solubilities are similar to quartz solubility in the temperature range 60–150° C, hydrocarbon transfer and accumulation should closely approximate that of quartz. Calculations suggest that convection can transfer significant quantities of hydrocarbons in molecular solution and exsolve them in traps in relatively short geologic times. The convection model thus links inorganic and organic diagenesis and provides reasonable explanations for such observed phenomena as secondary porosity and thermal anomalies.
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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.