American Oil and Gas Reporter - March 2018 - 67

SpecialReport: Advancing Shale Science
team to utilize well control and seismic data as a predictive tool
in preparing a "geoprognosis." Drilling geohazards, such as
faults and/or near-surface karsting identified by seismic, are
communicated to all parties responsible for well operations to
reduce drilling risk. Seismic structure maps are converted to
depth and used in both pre- and post-drilling analyses.
The Trees Ranch 3-D seismic data cover an area where
the Wolfcamp's lithology is highly variable and differing
pressure cells exist within the Wolfcamp section, as inferred
by the seismic stacking velocity. Both of these factors may
cause erroneous structural exaggeration to appear on a seismic
depth map.
To create seismic structure maps in depth, the time-interpreted
seismic horizon is converted to depth by incorporating the geologist's well markers to the seismic data. In the Trees Ranch 3-D
area, there are a limited number of deep well penetrations with
gamma, sonic and density data that are used to create synthetic
seismograms to tie well data to the seismic interpretation. After
tying vertical well subsurface marker data, a seismic time-todepth conversion velocity grid is computer-generated.
The velocity grid also is hand-edited to add any needed bias,
which may account for local depositional changes predicted by
existing well control. For example, carbonate content increases
with closer proximity to the Central Basin Platform. The geophysicist's depth structure map on the targeted horizon then is
integrated with the existing geologic prognosis to create a directional plan for the drilling engineer.
Additional preparation includes estimating the major formation
top depths, landing depths and trajectory of the desired horizontal
target. A series of subsurface structure and isopach maps are
utilized to create the well's initial prognosis, and then further
refined through integrating the seismic depth map of the
targeted horizon.
While drilling the vertical and curve sections of a well, the
geologist and geophysicist are in close communication to ensure
interpretations are correlated. The team uses MWD gamma ray
to pick major formation tops using a cross-section of nearby
wells, and compares them with the pre-drill seismic depth map
to see how they align with the prognosis tops. If discrepancies
exist between the predicted and actual tops, the geologist and
the geophysicist team works together to determine a projected
plan to safely land the lateral into the desired target window.
A new depth structure map is created after encountering the
top target zone and the directional plan is adjusted. The changing
stratigraphic nature of the Upper Wolfcamp can make MWD
gamma ray quite difficult to correlate. Having a series of type
logs across the length of the wellbore can be beneficial. In areas
where well control is sparse, seismic may be the only guide for
estimating formation dip.
Formation Dip
When Parsley Energy drilled its first horizontal well in the
Trees Ranch area, the Trees State No. 16-1H, the seismic depth

structure map (Figure 2) indicated the Wolfcamp formation
would rise 60 feet over the 5,000-foot lateral length. However,
the geosteering interpretation using MWD gamma ray yielded
a depth structure map that had a flat to slightly updip structure,
only rising approximately 10 feet from heel to toe (Figure 3).
Later, the MWD gamma ray depth map interpretation was
confirmed when Parsley drilled the Trees State No. 14-154301H lateral that came toe-to-toe with the Trees State 161H. The projected true vertical depth top of the Wolfcamp at
total depth came within 10 feet of the projected TVD top of
Wolfcamp at total depth on the Trees State 16-1H. In this instance, seismic data integrated with geology was used as a
tool to predict formation dip prior to drilling, while drilling
the lateral, and later was refined after both wells reached
total depth.
FIGURE 3
Trees State No. 16-1H Geosteering Plot
MWD Gamma Ray Curve

3rd Bone Spring
Wolfcamp

Actual Wellbore Path

Target Zone

% Shale
% Lime
Total Gas
ROP

FIGURE 4
Post-drill Seismic Depth Map for Trees State 16-1H
And Trees State 14-15-4301H
Trees State 14-15-4301H
SSTVD
(feet)
7,410
7,426
7,442
7,456
7,474
7,490
7,505
7,521
7,537
7,553
7,569
7,585
7,601
7,617
7,633
7,649
7,665
7,681
7,696
7,712
7,728
7,744
7,760
7,776
7,792
7,803
7,821
7,840
7,856
7,871
7,887
7,903
7,919
7,935
7,951
7,967
7,983
7,999
8,020

Geosteering Wolfcamp
Control Points

BHL geosteering control points
closely match

Trees State 16-1H

Trees Ranch 3-D Survey

Trees State 16-1H Post-Drill Wolfcamp Structure, SSTVD
Reeves and Pecos Co., Tx.

0
CI = 10 feet, Grid spacing = 500 feet

Scale = 124,000

2,000

4,000

6,000 ft
10/13/16

MARCH 2018 67

American Oil and Gas Reporter - March 2018

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