Page 180 - Geology of Carbonate Reservoirs

DIAGENETICALLY ENHANCED POROSITY 161

ate times of duration seem to have had the greatest positive effect on porosity
enhancement by dissolution. Intuitively, one would expect long duration, compara-
tively soluble rock, and persistent strong disequilibrium to produce karst features
with cavernous porosity. Mild disequilibrium, lower solubility, and shorter duration
may produce only minor enlargement of interparticle pores. Intermediate situations
include connected moldic and vuggy pores or solution - enlarged fracture porosity.
Dissolution commonly occurs during late burial diagenesis as well as at exposure
surfaces and during early burial. There is no single result from exposure to vadose
and meteoric water dissolution at unconformities or from reactive fluids in deep -

burial environments. Each situation must be evaluated on its characteristics in order
to diagnose the extent, type, and timing of alteration. It is relatively easy to identify
solution - enlarged pores in cuttings, cores, and especially in thin sections because the
boundaries of grains, cements, and matrix show truncation, corrosion, or otherwise
irregular and transgressive edges. In short, the boundaries of the enlarged pore
“ transgress ” original outlines of grains, cement crystals, and muds.

The gamma ray log can be a useful tool for indirect identification of dissolution
at exposure surfaces. Many carbonates contain insoluble residues rich in clays and
organic matter that emit natural radioactivity. As the carbonates are dissolved on
unconformities and the insoluble constituents are concentrated along the dissolu-
tion surface, the amount of natural radioactivity associated with the surface is also
increased. Simple contouring of API units from gamma ray logs on wells that pen-
etrated an unconformity may indicate high, intermediate, and low concentrations of
insolubles. It is common to find higher effective porosity in zones just below the

main surface of dissolution, because the unconformity surface per se may be plugged
with insolubles or cements. An example of this type of low to moderate dissolution -
enhanced porosity is characteristic of the reservoir at Lisbon Field, Utah. Fouret
(1996) found that high API gamma values correspond to zones with the greatest
amount of dissolution diagenesis found in cores. The greatest dissolution in the
Lisbon Field cores was found in wells that penetrated dissolution surfaces on the
crest of a large anticlinal structure that is clearly revealed on interval isopach maps.
Pore enlargement by dissolution during deep burial has been known for decades.
Moore and Druckman (1981) described deep - burial solution - enlarged pores in the
Jurassic Smackover Formation in Louisiana, as did Ahr and Hull (1983) along with
many later workers. Deep - burial dissolution is commonly followed by partial or
complete pore filling by minerals such as saddle dolomite, fluorite, sphalerite, and a

variety of metallic sulfides. These minerals provide clear evidence of burial diagen-

esis because they do not exist in ordinary sedimentary environments. Saddle dolo-
mite is a particularly useful mineral to aid in recognition of deep - burial diagenesis

because it can readily be identified in cuttings and it commonly occurs as vug or
fracture fillings that may in turn be correlated with dramatic changes on such logs

as the acoustic log (rapid scale change or “ cycle skipping ” ), the caliper log (washouts
at fracture and vuggy zones), the density – porosity log (very high porosity), and the
drilling time log (drilling breaks).
6.4.3 Large-Scale Dissolution -Related Porosity

Porosity at Lisbon Field is mainly the result of grain - scale dissolution on an
un conformity surface. Solution - enlarged pores and molds generally did not undergo