Nucleation Controls Selective Anhydrite
Cementation In San
Andres Reservoirs of West Texas
BROWN, ALTON A.
In many West Texas Permian reservoirs unaffected by secondary anhydrite
dissolution, coarse-crystalline
anhydrite
occludes a greater fraction of large
pores than smaller pores in the same rock sample. Where
anhydrite
occurs in a
void, the pore is typically completely occluded by a single crystal. Most of the
anhydrite
crystals terminate at the edges of voids and conform to their shape.
Crystals do not extend through pore throats except where both pores and throats
are exceptionally large (millimeters and tens of microns, respectively).
It is proposed that this crystallization pattern results from nucleation
control of anhydrite
cementation during burial diagenesis. Low nucleation-site
density due to slight supersaturation of
anhydrite
in the fluid makes it
unlikely that more than a single
anhydrite
crystal is heterogeneously nucleated
in most pores. As pore surface area decreases with decreasing radius, pores are
less likely to have even a single nucleation site, so small pores are more
likely to remain open than large pores. Uncemented pore throats result from
surface energy effects, also indicative of a low degree of supersaturation.
To test this hypothesis, thin sections from Wasson field (TX) were point
counted to determine the frequency of anhydrite
cementation in pores of
different sizes. Results support the surface nucleation-control model.
Nucleation-site frequency was estimated by fitting probabilistic models to the
data. Sparsity of multicrystalline fill of very large pores indicates that
anhydrite
nucleation and growth continues during burial diagenesis rather than
being a short-lived phenomenon.