American Oil and Gas Reporter - September 2017 - 45

SpecialReport: Geophysics & Reservoir Evaluation
Testing this hypothesis further, an
evaluation was performed of the gas-tooil ratios from hydrocarbons produced
from the Wolfcamp across the Midland
Basin. This regional information was incorporated with the results obtained from
the source rock properties collected within
the study wells to create a GOR trend
map of the basin. As depicted in Figure
3, the map shows higher GOR variabilities
for hydrocarbons produced from the Wolfcamp across the basin, ranging from black
oil to wet and dry gas. Within the study
area, a trend of mixed hydrocarbons of
varying GOR was observed, with most
corresponding to volatile oil to wet gas
and dry gas on the flanks of the Eastern
Shelf.
GOR variability recorded within the
study area cannot be explained by the
current maturity levels of the Wolfcamp
formation seen in study wells. The GOR
data, therefore, could represent evidence
of possible mixing of hydrocarbons produced from source rocks with different
maturity levels. It is proposed that gas
condensate and wet/dry gas was generated
from deeper source rocks such as the
Woodford formation, and migrated laterally or vertically and accumulated within

reservoir facies of the Wolfcamp along
the Eastern Shelf Platform.
Additionally, it is likely that hydrocarbons (volatile and black oils) generated
from basinal facies of the Wolfcamp formation (which host good source rock
quality with higher maturity) migrated
laterally within the source rock interval
(primary migration) and mixed with the
condensate and gas generated from the
more mature source rocks. Some geochemical studies in the Permian Basin
have reported many oils that exhibit intermediate geochemical characteristics
indicative of mixing. Moreover, hydrocarbon migration from deeper source
rocks also has been referred in the Permian
Basin. Under this hypothesis, the faults
and fractures would play an important
role in hydrocarbon migration.
Petrophysical Analysis
To further evaluate the play's presence,
quality and hydrocarbon potential, and
to test the basin center hydrocarbon migration hypothesis, six wells were selected
from across the study area for petrophysical analysis. All six wells were used to
derive reservoir properties, including
lithology, porosity, water saturation (Sw)

FIGURE 3
GOR Map of Hydrocarbons Produced from
Wolfcamp Formation in Midland Basin
GOR (2nd Month)
Color by:

Avg (Gas Oil Ratio...

30,000.00 - Max (205...
3,500.00 - 30,000.00
900.00 - 3,500.00
200.00 - 900.00

Min (0.00 - 200.00

Shape by:

Petroleum Fluids type
Wet and Dry Gas
Gas Condensates
Volatile Oils
Black Oils

(None)

All Values

Size by:

Avg (Gas Oil Ratio...

≥ 30,000.00

≤ 3,500.00

High GOR (Wet and
Dry Gas)

GOR (Volatile, Black
and Heavy Oils)

Maturity Level (Vitrinite Reflectance (%Ro)
Well Location

Color by:

Avg (Gas Oil Ratio...
1.00 - 1.30
0.70 - 1.00
0.55 - 0.70
Min (0.45) - 0.55

Shape by:
(None)

Maturity Level
Late Mature
Peak Mature
Early Mature
Immature

Oil Window

aging 1.25 and the Wolfcamp D averaging
1.35 wt%. The remaining hydrocarbon
potential and hydrogen index (HI) values
indicate a decrease from the north to the
south (Figure 2).
Vitrinite reflectance (%RO) measurements suggest that source rock intervals
experienced a low thermal stress. The
maturity in the entire Wolfcamp formation
within the study area was measured as
immature to early mature, with source
rocks showing a slight increase in maturity
toward the southern and southeastern
parts of the study area, with values of
~0.7-0.8 %RO.
Taking into account the tectonic evolution of the Midland Basin and the information available from the wells (stratigraphic column, maturity, etc.) a 1-D
model was built to simulate maturity
through geological time. The model indicated that the maturation process started
at different geological times within the
study wells (dependent on well locations
and thermal properties). For the most
southeastern well, this process began during the Late Permian. In other parts of
the study area, the maturation process
started later, within the Late Triassic to
Middle Jurassic, and continued through
geological time.
At present-day thermal conditions
recorded within the study area, wells indicate low bottom-hole temperatures (<80
degrees Celsius for corrected values),
which are insufficient to continue the
maturation process of the source rocks.
Because of the low level of maturity and
poor to fair source rock potential of the
Wolfcamp formation in the study wells,
it is unlikely that significant hydrocarbon
volumes were generated in situ. Notwithstanding, the current wells in the study
area are producing hydrocarbons from
the Wolfcamp formation, suggesting that
accumulated hydrocarbons may have been
sourced from areas of higher maturity
(such as areas that have reached the gas
window) away from the Sterling County
study area and/or from deeper, mature
source rocks.

All Values

Size by:
Avg (Ro_Measured)
≥ 0.66

≤ 0.45

SEPTEMBER 2017 45



American Oil and Gas Reporter - September 2017

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