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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 66 (1982)

Issue: 5. (May)

First Page: 644

Last Page: 644

Title: Reservoir Diagenesis and Convective Fluid Flow: ABSTRACT

Author(s): J. R. Wood

Article Type: Meeting abstract

Abstract:

Pore fluids in reservoir rocks are unstable under normal geologic conditions and, in the absence of forced flow, convective fluid flow can be expected as a general rule. Calculations suggest that fluid velocities on the order of 10-8 m-sec-1 should be typical scale velocities for convective rolls due to normal geothermal gradients (25°C - km-1). Velocities of this magnitude are shown to be sufficient to reduce porosity significantly in less than 5 million years if quartz is the pore-filling cement. If exsolved hydrocarbons are pore-filling materials, the time to complete fill decreases to about 2 million years, while pore filling with both exsolved hydrocarbons and quartz (72% HC) requires about 1.43 million years.

Mass transfer by convective fluid flow alone appears to be sufficient to account for the bulk of diagenesis in the deep subsurface. However, it can also be argued that many diagenetic reactions occur solely as a consequence of moving fluids maintaining chemical equilibrium with their dissolved load as the fluids cycle through temperature and pressure gradients. Phases are precipitated or dissolved during the cycle depending on the sign of the solubility coefficients of a mineral with respect to temperature or pressure. Quartz solubility, for example, increases with T and P under normal reservoir conditions and can be expected to move from hot to cold zones while calcite would be expected to show the opposite behavior. In particular, it is shown that the diagenesis in a convecting syst m is not a function of mineral solubilities, but rather the temperature and pressure coefficients only. This observation may be of considerable importance in assessing the significance of hydrocarbon transport and accumulation by molecular solution in the aqueous phase because it appears that the components of a petroleum phase exhibit similar temperature and pressure behavior.

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