Page 260 - Geology of Carbonate Reservoirs

FRACTURED RESERVOIRS 241

8.5.2.1 Quanah City Field

Location and General Information Quanah City Field is located in the Hardeman

Basin, at the town of Quanah, Texas (Figure 8.5 ). The one - well field with three offset
dry holes is named for the town of Quanah, and was developed by Sun Oil Company
during the 1960s. The discovery well, the Sun Carrie Minshew #1, initially produced
338 BOPD. Later, an independent operator drilled a sidetrack to the Sun Oil

Company Tabor Oil Unit #1 that produced 2016 BOPD. The target reservoir at
Quanah City Field is the Mississippian (Vis é an) Chappel Limestone, which also
produces in a few wells south of Quanah City Field. Those wells are not included
in this discussion.

Structural Setting Both present structure and paleostructure are important in this

case. Present structure at Quanah City Field was initially mapped on subsurface
geological data as a simple, faulted anticline. The Minshew #1 and the Quanah City
#1A wells are deviated holes; consequently, it was difficult to determine true stratal

thicknesses and structural attitudes from the vintage logs that were available at the
time of the original study. By today ’ s standards, the comparatively primitive 1960s
technology for making borehole surveys provided sets of coordinates for construct-
ing azimuth and depth plots of the borehole. There were no dipmeter logs, but by
good fortune, a seismic structure map was made available just before research on
the field was completed. This map (Figure 8.15 ) reveals that the bottom - hole loca-

tions of the Minshew #1 and Quanah City #1A wells are close to faults that are
interpreted to be older, reactivated faults, a common feature in the Hardeman Basin.
The faults are also interpreted to have been the primary cause for natural fractures
in the Chappel Limestone reservoir.
Paleostructure played an important role in localizing the objective “ reef mound ”

in this field. Although none of the four field wells penetrated the entire Carbonifer-

ous section, the location of the Quanah City mound juxtaposed between two large
faults suggested to Ahr and Ross ( 1982 ) that a fault - related paleostructural high
existed on the Ordovician dolomite (the Ellenburger Formation) that underlies the
Carboniferous section in this area. The paleo - high was interpreted to have been the
site on which the Chappel Limestone mound nucleated and, according to Ross (Ahr

and Ross, 1982 ), field closure was increased by both mound growth and postdepo-
sitional fault reactivation.

Depositional and Diagenetic Characteristics The Quanah City mound (Figure
8.16 ) is interpreted to have grown on an underlying paleo - high in a subtidal, open
marine setting. The mound facies consists largely of lime mud interpreted to have
been produced in situ, mainly by microbial activity. Fenestrate bryozoans and cri-
noids are common in the mudstones and wackestones of the mound center. A fl ank-
ing veneer of crinoidal grainstones and packstones is present. Early diagenetic
dolomite is common in the muddy facies and is interpreted to have replaced neo-
morphic microspar in mud - supported rocks, leaving larger skeletal allochems rela-
tively unaltered. Early in the burial history, spiculiferous zones in the mound facies
are interpreted to have been replaced by chert. Subsequent leaching dissolved much

of the previously unaltered carbonate allochems, “ chalkified ” some siliceous crusts,
and resulted in solution - collapse brecciation of limestone, dolomite, and “ chalky ”