Page 262 - Geology of Carbonate Reservoirs

FRACTURED RESERVOIRS 243

chert. Early dissolution (prefracturing) also formed moldic and vuggy porosity in
the skeletal “ hash ” zones in both mound and fl ank facies.
Further mound growth occurred accompanied by slumping and soft - sediment

deformation of grainy flank facies. Finally, a capping facies of brachiopods and
bryozoans stabilized the upper surface of the flank deposits. Peloidal, silty lime mud

filled intermound areas, elevating the surrounding seabed into fair - weather wave

base. Oolite shoals formed over the mound and adjacent flanks. Acicular isopachous

rim cement formed on ooids in the marine phreatic environment. A later burial
stage of chalcedonic quartz replaced allochems, grains, early cement, and neomor-
phic microspar matrix. Burial compaction produced tangential, concavo - convex,
sutured grain contacts and stylolites. Two episodes of fracturing are evident. The
earlier fractures and remaining interparticle pores were filled with coarse, blocky

calcite typical of burial cementation. A later generation of fractures was partially

filled with saddle dolomite and bitumen. Saddle dolomite crystals are excellent
indicators of fractured reservoirs because the crystals are obvious even in rotary
cuttings and their occurrences are generally limited to fractures or connected vugs
(Ahr, 1982 ).

Geological Concept The Quanah City Field reservoir rock is a “ mud mound ” of

the type commonly associated with Mississippian mound facies around the globe.
The Quanah City buildup is interpreted to have formed in a shallower setting than
the typical Waulsortian mounds of Europe. It is more similar to the Late Mississip-
pian mounds of Derbyshire, in central England. Coincidentally, many of those
mounds in the area around Castleton, England are also fractured and are host rocks

to sulfide ores similar to MVT ores, along with especially colorful fluorite known as

Blue John. Without the extensive diagenesis and fracturing that dramatically altered
the Quanah City mound, it is unlikely that enough porosity and permeability would
have existed to make a reservoir. Although oolitic grainstones were deposited on
top of the mound, they became cemented to the extent that they provided the seal
for the reservoir. The extensive leaching that produced vuggy and moldic porosity
enhanced the storage capacity of the Quanah City reservoir, but many of the molds

and vugs are not touching vugs. They provide a significant amount of porosity but
only limited permeability. Porosity and permeability from routine core analyses
were plotted against depth and depth - shifted core lithofacies to reveal that high
porosity and permeability do not correspond to depositional lithofacies and perme-
ability does not correlate well with porosity. The dominant lithologies are early -
replacement dolomite and saddle dolomite, which do not correlate with depositional
lithofacies except to the extent that early dolomitization appears to be more abun-
dant in mud - supported rocks. High permeability corresponds to highly fractured
zones in cores. Fractures provide essential permeability by connecting the molds,
vugs, and other matrix porosity, making this a Type II fracture system. The fractures
at Quanah City Field are interpreted to have been formed in conjunction with fault-
ing. Fractures also appear to be oriented parallel to the fault system because produc-
tive fractures were penetrated by the Minshew #1 borehole but missed by the
Quanah City #1A well. Only the Minshew #1 made a well.
Cores from the Minshew #1 well are replete with moldic and vuggy porosity,
natural fractures, and fracture - filling saddle dolomite. The abundant dissolution

porosity is interpreted to provide the main storage capacity for this well, but the