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The Attributes of a Wolfcamp “Reef” Play Pecos County, Texas*
By
Philip H. Carlisle1
Search and Discovery Article #10037 (2003)
*Online adaptation of presentation at AAPG Southwest Section Meeting, Fort Worth, TX, March, 2003 (www.southwestsection.org)
1Aspen Integrated Oil & Gas, L.L.C.
The presence of Upper Wolfcamp/Lower Leonard age stratigraphic reefs, or bioherms, as a reservoir target in Pecos County has been proven to be a viable exploration objective.
This objective has
been realized by utilizing the integration of geology and 3-D geophysics. A
simple geologic model was created to understand the geophysical response of the
play. Through this modeling, a focused exploration program for this reservoir
target has resulted in twelve new field discoveries, five development wells, and
five dry holes for an overall success rate of 77%. This reservoir objective has
produced in excess of 480MBO and 6,700 MMCFG since the first discovery 
well
was
completed in January 1998.
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INTRODUCTIONThe presence of Upper Wolfcamp/Lower Leonard age stratigraphic reefs, or bioherms, as a reservoir target in Pecos County has been proven to be a viable exploration objective. The Wolfcamp reservoir objective has been exploited through the integration of geology and 3-D geophysics. The program was initiated in 1994 after determining that the Wolfcamp would be a viable exploration target with significant potential. A geophysical model was developed based on a simple geologic concept from three key wells in the study area. A key geophysical response, as a result of this model, has been used to develop the play. Without seismic as the key tool to pick drilling locations, the development and addition of reserves would have been extremely difficult, evidenced by the number of historical wells which have penetrated the Wolfcamp, with limited success due usually to a lack in reservoir quality. The result
of the project has been a highly successful focused
exploration/exploitation program for Wolfcamp, which has resulted in
twelve new field discoveries, five development wells, and five dry holes
for an overall success rate of 77%. This reservoir objective has
produced in excess of 480MBO and 6,700 MMCFG since the first discovery
PROJECT LOCATION AND GENERAL GEOLOGY The project
area is located in Pecos County, roughly twenty miles east of Ft.
Stockton Texas, south of Interstate 10 (Figure 1). The area is
characterized by a complex series of deep-seated faults, on the southern
side of the Ft. Stockton high, which helped form the Tobosa basin of
late Simpson time. On the western edge of the project, more than
17,000feet of relief change can be detected as a result of the faulting
at the Ellenburger horizon. The fault trend is locally less than three
miles wide, and is situated regionally on the southern edge of the Ft.
Stockton high. The influences associated with this faulting were also
responsible for creating several large uplifts, one of which includes
the Puckett North anticline (Puckett North Ellenburger Field). The
Puckett North uplift was important to depositional sequences during
Pennsylvanian through early Leonard time, as evidenced by the carbonate
buildups found surrounding the uplift. A type The primary
focus for this project has been for the Wolfcamp age sediments.
Deposition of the Wolfcamp is interpreted to have occurred as a
shelf-margin reef and grainstone complex, at the shelf break. The shelf
break was set up by the deep-seated faults, seen mostly as
pre-Pennsylvanian faulting, but in some instances, reactivated up
through the Pennsylvanian and even into early Permian time. The Puckett
North anticline was probably an island during early Permian time, and
reefs and skeletal sand banks were deposited around the island. Paleo-environmental
work was completed for the Wolfcamp on several wells drilled in the
area, which helped confirm this concept. Figure
4 depicts the
generalized concepts for the area, based on sample Wolfcamp was
a known producer in this area, as the Hokit North, Hokit Northwest, and
Nuz fields had produced significant quantities of oil and gas from
Wolfcamp age reservoirs. Through December 1996, these reservoirs had
produced a total of 16.7 BCFG and 384 MBO. Of particular interest was
the Hokit North field, which had cumulative production of 8,032,750 MCF
and 159,830 BO, primarily from one Realizing
the potential of the Wolfcamp, the next step was to determine whether
the Wolfcamp was a feasible seismic target. A number of sonic logs were
available from wells in the area. The Sun Oil Co. No. 2-Jasper
County School Land (“JCSL”), the direct offset to the significant
producer, had a sonic Additional cross section work was then completed to determine the validity of the model, and especially, the continuity of the draping shale, which appeared to be critical to imaging the “reef” interval. Regional cross-section work confirmed that although the depositional environments in the Wolfcamp section change, the draping shale was present over a fairly large area. Several regional cross-sections help to confirm the depositional environments and show the changes in the Wolfcamp section. Figure 9, cross-section A-West to A’-East and Figure 10, cross-section A-North to A’-South both demonstrate the complexity of the Wolfcamp section. The Nuz field, which has also produced significant hydrocarbons, shows that a geologic environment was similar to that found at the Hokit North field area. Although the thickness of the carbonate in producing wells was not considerably different from non-producing wells, the producing wells certainly had developed high porosity grainstone and reefal zones, which are excellent reservoirs. Figure 11 is a cross-section through several wells in the Nuz field area. It was also important to note that the Wolfcamp drape shale was also present across the Nuz field. Having completed field studies, regional cross section work and modeling, a knowledge base had been developed, and was the basis for all future Wolfcamp exploration. As was noted previously, significant production had been established by previous operators in a focused area north of the Puckett North anticline. A relative production bubble map is utilized in Figure 12 to illustrate pre-1996 activity for the area, having cumulative production from the Wolfcamp of 384 MBO and 16.7 BCFG, from a total of fifteen wells. Discovery of these Wolfcamp reserves was primarily from drilling for deeper objectives. 3-D seismic
was first acquired in the study area in early 1996, which included data
across the No. 2-JCSL “A” Based on the
early modeling work and subsequent characteristics seen on the seismic
shot across the No. 2-JCSL “A” The Wolfcamp
is an intriguing reservoir target. Of the sixteen successful wells
drilled in the program, few wells have similar reservoir performance. In
general, it can be said that reservoir has high permeability. Tests
indicate two darcies of permeability are not uncommon. Vertical barriers
are not often found, as perforations low in the section will often drain
the reservoir pressure throughout the entire Wolfcamp “reef” section.
Most of the reservoirs are pressure depletion drives, although water
movement and pressure support is present in the Habanero Field. This
field has produced the greatest quantity of oil from any single
reservoir development, having a cumulative in excess of 245 MBO,
primarily from one Lateral
heterogeneity is somewhat unpredictable, as would be expected in a
carbonate environment. The 1-Ancho, an early In the same area, but on a larger scale, lateral heterogeneity is quite prevalent. As more wells in this particular area are included in a review, it becomes clear that multiple reservoirs are present, confirmed by different reservoir pressures. Cross-section HokitNW-HokitNW’ (Figure 17) illustrates a larger area that shows the heterogeneity of the reservoir. These wells are near the Ancho feature, but show quite different reservoir characteristics. Of particular interest is the 1-Arbol, which was completed in September of 2000, and has cumulative production of over 93 MBO and 302 MMCFG. An original reservoir pressure of 2,833 psi was encountered on drillstem test in the Wolfcamp at this location. Several Nuz
field producers accentuate another example of the heterogeneity found in
the Wolfcamp reservoirs. Cross-section Nuz-Nuz’ (Figure
18) demonstrates
that several carbonate reservoirs are present over a short distance.
Reservoir pressures are noted on the cross-section, with a low of 152 psi (abandoned in 1999) to a current original bottom hole pressure of
2637 psi in a One of the
major obstacles of the play has been in acquiring and processing
reliable seismic data. This is largely due to two factors. The first is
the severe topographic changes that occur as a result of mesas being
present in the project area. The second is the surface geology.
Cretaceous age limestones are present, including the Edwards, which
makes data acquisition difficult due to the fact that the source tends
to ring, and high frequencies are lost. Data acquired at the objective
depth is typically between 15 and 25 Hz. Resolution is therefore reduced
considerably due to the low frequencies recovered. Processing data is
difficult as Drilling
and operations have been difficult for the area as Significant
hydrocarbons have been produced from the Wolfcamp interval, which has
proven to be an excellent carbonate reservoir target in Pecos County.
Early development of Wolfcamp reserves was primarily found drilling
deeper objectives. Without the application of geophysics, and
specifically 3-D, the development of the recent reserves would have been
difficult. The overall success rate has been high, even though the
geophysical data quality has been negatively affected by acquisition and
processing factors. Geophysical problems have been minimized through
careful data REFERENCESWright, W. Floyd, 1979, Petroleum Geology of the Permian Basin, West Texas Geological Society, Publication No. 79-71. Scholle, Peter A., Bebout, Don G., and Moore, Clyde H., 1983, Carbonate Deposition Environments, AAPG Memoir 33. Peppard & Associates, 1982, The Wolfcamp and Pennsylvanian Sediments of the Ozona Platform, Project 20230. ACKNOWLEDGMENTSThe author wishes to thank R.E. Glasscock and John W. Ruwwe, Jr. for their believing in the author and the project. These individuals have been supportive in many ways throughout the development of this project, making this not only a successful endeavor, but also a personally rewarding venture. |
