Page 263 - Geology of Carbonate Reservoirs

244 SUMMARY: GEOLOGY OF CARBONATE RESERVOIRS

fractures provide essential permeability, indicating that the reservoir is a Type II
fractured reservoir. Another important line of evidence to support the interpreta-
tion that the reservoir is fractured is the acoustic log. Although no dipmeter logs,
image logs, or pressure tests were available for study, the acoustic log from the
Minshew #1 exhibits several distinctive “ cycle skips ” in the productive zone just
above the cored interval. Cycle skips are nearly instantaneous scale changes that
must be made to keep up with shifts in acoustic transit time so the tool can continue
to record accurately in zones of rapidly changing porosity. Characteristics similar to
those in the Minshew #1 well were observed by the author in nearby fi elds, where
in addition to cycle skips in the acoustic log, caliper logs indicated “ washouts ” in

the zone of greatest fracturing, difficulty was encountered in maintaining steady

mud circulation because the mud would infiltrate the fractures, drilling time was
dramatically decreased as compared to nonfractured zones above and below the
fracture horizon, and saddle dolomite crystals were recovered in lagged cuttings
known to be from fractured zones. In severely fractured zones it is possible for large
pieces of borehole wall to “ cave ” into the hole, causing the drill bit to jam. Taken
together, these lines of evidence are good general indicators that fractures have
been encountered during drilling. Direct observations of fractures in borehole core
provided confi rmation.
The original concept for establishing the drilling location in Quanah City Field
was simply to drill a structural anomaly that was detected by seismic surveys. The
anomaly turned out to be the Quanah City mound, a buildup that accentuated
bathymetric relief by growing on top of a preexisting high — a fault block. After the
discovery well was drilled, it was clear that the reservoir was strongly infl uenced by
fractures. The lesson from this experience is that the identification of natural frac-

tures in the subsurface is largely dependent on the ability of geophysicists to fi nd
structures that are likely to be fractured or to extract seismic attributes that are
known to indicate fractures. In some cases where faults or folds can be mapped on
surface outcrops and extrapolated to the nearby subsurface or where structures can
be mapped in the subsurface with the seismograph, it may be possible to predict
where fractures will occur based on structural geometry, inferred loci of stress con-
centrations, and knowledge about the brittle characteristics of the host rocks. If

fracture orientation can be identified with borehole imaging logs or other direc-
tional devices, opportunities may exist to drill horizontal wells to encounter the
greatest number of fractures per unit lateral distance in the subsurface.

8.5.2.2 Dickinson Field

Location and General Information The Dickinson Lodgepole Unit (DLU) is
located in the Williston Basin of North Dakota and consists of nine individual fi elds
(Figure 8.17 ). The discovery well, the Conoco Dickinson State (DLU) #74 well was
completed in 1993 in the Carboniferous Lodgepole Formation (Young et al., 1998 ).
The drilling location was chosen to test a structural anomaly identified on seismic

lines, not to test for fractured carbonates. The well flowed at an initial stabilized rate

of 1150 BOPD and 0 barrels of water per day and produced at rates in excess of
1500 BOPD for six months until curtailment. Two years later Duncan Oil Company

drilled a second Lodgepole discovery nearby. These Lodgepole Formation wells are
particularly attractive because initial production rates are commonly about