American Oil and Gas Reporter - April 2019 - 63

SpecialReport: Drilling Technology

Identifying Productive Zones
Based on the ratios from the pilot
hole, the upper Paleozoic formations were
interpreted to be in the potential productive
oil zone. In the deeper Pennsylvanian
and Barnett formations, the ratios indicate
a potential productive wet gas condensate
trend, while the deeper Woodford ratios
point to a potential productive condensate
or a high GOR fluid.
The gas quantity (normalized for rate
of drilling) indicates an increasing trend in
the upper Paleozoic. Below the Wolfcamp,
gas quantity decreases until the Pennsylvanian. From the Pennsylvanian to the
Woodford section, normalized gas quantity
increases significantly, with the highest
measured quantities in the Woodford.
The evaluation indicates a mixing
trend from bacterial to thermogenic gases
in the shallower upper Paleozoic formation,

FIGURE 2
Gas Isotopic Ratios Plotted with Mud Gas Compositional Data
On Geochemical Fingerprinting Models
Litho

TGAS Normalized

C1-C3

100,000

Bernard Chart

C1/(C2+C3)

Upper Paleozoic

10,000

1,000

100

1
-100
7,000 ft

-90

-80

-70
-60
-50
-40
d C13 C1_QC (per mil) [per mil]

-30

-20

-75

Schoell Fluid Associations

d C13 C1_QC (per mil) [per mil]

-70
Middle Paleozoic

ozoic using reservoir information collected
while drilling.
Several zones of interest were selected
for reservoir fluid and geochemical characterization using surface logging data
acquired while drilling. Zones with higher
hydrocarbon quantity were identified and
then characterized for gas composition
and ratio analysis to identify barriers,
compartmentalization and predict fluid
type and properties.
Figure 1 is a composite log showing
reservoir fluid characterization utilizing
light hydrocarbon gas data and mud gas
ratios plotted with bulk mineralogy, measurement-while-drilling gamma (first track)
and wireline neutron density and porosity
(last track on the right). Plotting the mud
gas isotopic ratios associated with zones
of interest alongside mud gas ratios on
specific geochemical fingerprinting models
helps define reservoir seal integrity, diffusion patterns, compartmentalization,
origin, fluid associations, fluid mixing,
gas type and source rock maturity.
The Haworth gas ratio curves clearly
show a fluid compositional transition
from green color-shaded higher wetness
zones in the upper Paleozoic formation
to the red color-shaded lower wetness
zones in the Pennsylvanian to the Woodford. Haworth ratios characterize whether
gas is rich or lean in higher molecular
weight alkanes (ethane to pentane) versus
a lighter molecular weight alkane such
as methane (gas ratios cam be calibrated
to local production data when available
to improve the accuracy of GOR and
fluid type predictions).

-65
-60
-55
-50
-45
-40
-35
-30
-25
-20

Lower Paleozoic

while the deeper Pennsylvanian to Woodford gases appear to be thermogenic
(upper right in Figure 2). The fluid associations plot at the bottom right of Figure
2 shows a clear delineation of gas ratios
versus isotopic ratios for each formation
and source rock.
The gas from shallower upper Paleozoic plots high on the boundary between
the "mixed bacterial/thermogenic" and
"mature gas co-formed with oil or oilassociated gas" windows. Gas from the
upper Paleozoic plots in the center of
the "oil-associated gas" window. Gas
from the Pennsylvanian and Barnett
shows much lower wetness, but still plots
within the oil-associated gas window.
Gas from the Woodford and Devonian
formations plots at the boundary of the
"oil-associated gas" and "post-mature
rich gas with light oil" windows. The
isotopic ratio curves show increasing
maturity trends (isotopically heavier
ratios) from shallow to deep, with a more
prominent trend in increased gas maturity
along the pilot hole's vertical column.
Hydrocarbon quantity (using weight
percentage of oil present in the rocks)
and quality were measured from drilled
cuttings using a flame ionization detector.
Mud circulation challenges and lost control
material saturation made it difficult to
obtain these organic measurements, but
the gas chromatography data indicate
quantity increasing in the upper Paleozoic
and all samples within the light hydrocarbon range. However, differences in
the character of the chromatography between the samples suggest fluid barriers
and compartmentalization.

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80
C2+%

Total organic content and pyrolysis
measurements performed on select samples
imply that the maturity trends for the
upper Paleozoic are in the oil generation
window and in the condensate gas window
for the Barnett and Woodford.
Elemental analysis was performed on
the cuttings to characterize the rock properties while drilling and provide input to
rock typing. The depth of the samples
was quality checked by comparing the
MWD gamma against the calculated gamma from XRF. The cuttings were representative of depth following the drilling
and MWD tool depths. There was a small
delay in cuttings arrival at the top of one
formation in the upper Paleozoic because
of mud losses. A shift in all cuttingsbased data had to be applied to compensate
for the delayed arrival.
In the Wolfcamp formation, the upper
part of the target shows higher organic
proxies and terrestrial material as indicated
by titanium and zirconium. The lower
part of the target shows a decline in these
terrestrial materials. Below the target
base, the rock is predominantly carbonate-rich, with low silica, titanium and
zirconium, but with a higher quantity of
formation gas.
Crucial Information
The elemental data provided crucial
information on the rock properties within
both targets. Figure 3 shows the various
chemostratigraphic layers around the
target. The first track plots XRF-calculated
gamma (red) and MWD gamma (green).
Calcium and silica identify the upper
silica-rich layers and the deeper carbonate
APRIL 2019 63

American Oil and Gas Reporter - April 2019

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