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Book Chapter

Reservoir Diagenesis and Convective Fluid Flow

By
J. R. Wood
J. R. Wood
Chevron Oil Field Research Company La Habra, California
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T. A. Hewett
T. A. Hewett
Chevron Oil Field Research Company La Habra, California
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Published:
January 01, 1984

Abstract

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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Contents

AAPG Memoir

Clastic Diagenesis

David A. McDonald
David A. McDonald
Petro-Canada Calgary, Alberta
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Ronald C. Surdam
Ronald C. Surdam
University of Wyoming Laramie, Wyoming
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American Association of Petroleum Geologists
Volume
37
ISBN electronic:
9781629811598
Publication date:
January 01, 1984

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