American Oil and Gas Reporter - September 2015 - 58

SpecialReport: Horizontal & Innovative Drilling
In central and eastern La Salle County, normal faults have a northeast-tosouthwest regional trend with throws of
50-150 feet. The faults segment portions
of the productive Eagle Ford trend into distinct fault blocks. Mechanical stratigraphy
and disproportionate fault propagation
create varying stress and fracture orientations, especially within the transfer zones
where relay ramp structures have developed. Relay ramps are a common feature.
They occur between overlapping normal
faults and accommodate displacement
transfer between adjacent structures.
In normal fault regimes, fracture orientation is related not only to fault geometry, but also to a region's far-field stress
ratio. In areas where the ratio of maximum-to-minimum horizontal stress
(SHmax/SHmin) is greater than 1.0, fractures
will tend to develop parallel to normal

faults. Conversely, fractures will tend to
develop perpendicular to normal faults in
areas with SHmax/SHmin stress ratios equal
to 1.0. Stress perturbations may occur
around relay ramps, allowing a natural
fracture (joint) set to form parallel to the
orthogonal (SHmin) stress direction (Figure 1).
Given that these fractures are relatively recent, the regional far-field stress
state is likely similar to conditions at the
time of fracture formation. Therefore,
these features are more likely to be open
and conductive, compared with the regional SHmax parallel fracture sets. In addition,
a low stress bias (stress ratio equal to 1.0)
favors fracture complexity during stimulation, which can increase dramatically the
stimulated reservoir volume (SRV), and
more specifically, the stimulated surface
area.

FIGURE 1
Schematic of Relay Ramp Structures in Eagle Ford Study Area

Faulted
Monocline

Relay Ramp

FIGURE 2
Top of Buda Dip Azimuth (Left) and Dip Angle (Right)
Dip Angle (deg)
2.00

Dip Azimuth (deg)
360

Source: Three-D seismic data owned by Global Geophysical

58 THE AMERICAN OIL & GAS REPORTER

High-resolution 3-D seismic was used
to interpret all major horizons above and
below the Eagle Ford. Horizons were
depth converted and utilized in a 3-D structural geomodel. Within the geomodeling
software, numerous surface attributes
were computed and compared. Through
various iterations, surface-derivative mapping utilizing dip azimuth and dip angle
were found to be the most useful attributes
in discerning major structural features, and
in linking regional and field-specific
structural features.
Regional faults, developed parallel to
SHmax, may be discriminated more effectively from features developed orthogonal
to regional fault trends using these methods (relay ramps and associated fault
normal fracture sets). Figure 2 shows dip
azimuth (left) and dip angle (right) to the
top of the Buda formation immediately below the Eagle Ford. The numbers denote
examples of the main structural features
that are evident:
* A relay ramp transfer zone with a
southwest-dipping ramp with orthogonal
(SHmin parallel) fault/fracture features (1);
* Regional, through-going (SHmax
parallel), large-displacement normal faults
marked by the dashed black lines (2);
* A transfer zone with a regional
southeast dip (3); and
* A steeply dipping monocline (4).
Because initial Eagle Ford development
was based largely on lease retention and
drilling obligations, wells were drilled
through all types of identified structural
features, creating an appreciable dataset to
evaluate the positive and negative productivity impacts of such features. Along with
production metrics, many wells have C1C5 mud gas logging, completion flowback
water salinity, buried array microseismic,
and produced gas isotope data. These
datasets were integrated with vertical (pilot) well data, including core and FMI logs.
Mud Gas Analysis

Structural Framework

Drilling reports and mud logs were reviewed to identify major gas kicks or related well control issues, which are known
to be an initial indicator of potential natural fracture systems in low-permeability reservoirs. The study also reviewed gas
chemistry variations using gas wetness ratios, and assessed normalized total mud gas

American Oil and Gas Reporter - September 2015

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