American Oil and Gas Reporter - January 2016 - 56

Unconventional Resource Science
ly propagate across less than half the vertical thickness of the competent layers.
Megascopic fractures that are observed
to cross multiple mechanical or lithological boundaries are nearly vertical and exhibit sinusoidal profiles on image logs. The
largest megafractures have been resolved
beyond the wellbore using borehole
acoustic reflection surveys in vertical
and horizontal wells. Because of the low
resolution of the acoustic data, the extents
and spacing of megafractures are not
well defined, but morphologies are likely in excess of 100 meters horizontal and
10 meters vertical, and spacing is on a similar scale to their lengths.
Although petrophysical data indicate
that carbonate- and silica-rich intervals
within the Marble Falls exhibit similar brittleness values, LBFs are concentrated in
the most silica-rich (spiculitic) lithofacies
and are relatively rare within more carbonate-rich intervals. Shale and mudstones are
nearly devoid of LBF. Silica content, rel-

ative to both clay and carbonate fractions,
therefore appears to have been a controlling factor in generating and preserving
LBF. In this regard, the Marble Falls is
unique in the Fort Worth Basin, in that it
contains ubiquitous fractures with relatively large apertures that constitute a prolific and widespread reservoir.
Multiple Natural Fracture Sets
Among other features, the orientations
of natural and drilling-induced fractures
were determined from visual analyses of
more than 90 microresistivity image
logs across the Marble Falls play and
were evaluated on rose diagrams (shown
in blue in Figure 5). The strike of the principal horizontal compressive stress
(sHmax) in the study area is defined by
drilling-induced fractures in a narrow
range from north 15 degrees east to
N35E (N29E mean) and the strike of the
minimum principal horizontal stress direction (sHmin) is defined by borehole

FIGURE 5
Polar Frequency (Rose) Diagram Plotting Strike Azimuths
Of Fractures in Marble Falls Siliceous Spiculite Facies
Natural LBF
J1 - Conjugate - N10E/N20W
J2 - Orthogonal - N35E/N45W
J3?
Drilling-Induced
Fractures

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56 THE AMERICAN OIL & GAS REPORTER

obliquity as N61W (mean), both of
which are consistent with published
measurements of the contemporary stress
field.
At least two generations of natural fractures in the Marble Falls were observed in
image logs. The dominant population of
fractures (J1) exhibits a bimodal distribution of strikes that may be interpreted as
defining a conjugate set, which has modal
means of N20W and N10E and combined
mean of N5W. Natural fractures with
similar orientations are observed within the
Barnett, but are not observed in younger
Pennsylvanian-aged rocks. The nearly
north-to-south orientation would be expected of Type II fractures associated with a
north-to-south oriented axis of flexure. We
suggest that J1 fractures were generated
by flexure along the north-to-south trending Bend Arch during deformation associated with Ouachita orogenesis.
A secondary population of fractures
(J2) defines an orthogonal set, the strike
of which has bimodal means of N35E and
N45W, each showing nearly equal distribution and frequency (green in Figure 5).
The J2 fracture orientations are nearly
identical to those in younger Pennsylvanian rocks and are almost certainly postPennsylvanian in age. Two generations of
fractures also are visible in thin sections, where older calcite-filled fractures
(veins) are cut subparallel by numerous
younger fractures. The sections were not
oriented, but it is inferred that the veins
represent J1 and the fracture swarms
represent J2.
In addition, a potential tertiary set of
fractures may be interpreted from the data
that have orientations varying between
N50E and S60E with a mean that is nearly east-to-west (yellow in Figure 5). This
set simply may reflect noise in the data
with semi-random orientations, possibly
owing to local stress field perturbations or
regional stress field changes over time.
However, the strikes indicated would be
expected of Type I fractures associated
with a north-to-south oriented flexural axis,
suggesting a genetic relationship to the J1
set.
Some Marble Falls natural fractures
may have been formed by other processes with more localized effects, which
may explain anomalous trends in fracture
data observed in some wells. Likely examples of alternative fracture-generating
processes include karst development in the

American Oil and Gas Reporter - January 2016

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