Field work done in Pedernales state park, TX. Marball falls Fm. Statistical analysis and best fit scaling laws applied.

1. Fracture Length Analysis in Carbonate Rock:
Example from Marble Falls formation, Central Texas.
Study Area
The Early Pennsylvanian Marble Falls Formation in
North-Central Texas was deposited in a broad
carbonate ramp system during the initial stages of
Ouachita orogenesis.
Marble Falls Formation at certain parts of Texas, e.g.
northern part of the fort Worth Basin, has become a
fractured-driven, tight-oil resource play.
The studied outcrop of the Marble Falls shows
extraordinary exposure of fractured carbonate
pavements. Bedding ~ 10 degrees. Fractures are
striking ~WNW-ESE, and nearly perpendicular to the
bedding.
Take-home Message
Xiang Shana*, Qiqi Wangb, Stephen E. Laubachb
a PetroChina Hangzhou Research Institute of Geology, Hangzhou 310023, China
b Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin, Austin TX 78713, USA
*Author for correspondence: wangqiqi@utexas.edu
Outcrop Scanlines
A
B
Fig 2. Stereonet showing the strike
of fractures. Set all fractures
dip=90 degrees since real dip is
not measurable.
Fig 1. Outcrop location (left) and scanlines A and B setup,
orientated (right).
N
B
A
Fig 3. Scanline outcrop. Scanline set perpendicular to fracture set and measured from south to north. Scanline A is to the west of scanline B, the two are parallel.
Fig 4. Fracture occurrence versus distance (stick) plots. Shows
where fractures are along the scanlines.
- Length of A: 7.05m
- Length of B: 5.61m
- # of fractures from A: 50
- # of fractures from B: 44
Relative position and length of scanline stick plots represents real
relationships of the scanlines at the outcrop.
Fracture Spacing, Aperture and Length Analysis
Size of outcrop is larger than all measured fractures.
Rule of measuring length:
Length measured from fracture tip to tip, or ends at
where two fracs join.
Fig 5. Results of fracture aperture and length scaling, relationship between fracture aperture and length, and spacing analysis from scanline A (top 4) and scanline B (bottom 4). Power-law regression applied, with equation and
correlation coefficient reported. On both scanline A and B, aperture and length of fractures can be well described by power law distribution. Impressively, the relationship between length and aperture of the fractures also fits well
with power-law, even with a few outliers included. Truncation and censoring artifacts observed towards the upper and lower ends of size scaling curved. NCC plots indicates relatively weak regularly spaced fractal cluster pattern.
Intensity and Normalized correlation count plots. (by
CorrCount)
- 94 fractures measured on 2 parallel outcrop scanlines, across and perpendicular to one WNW-ESE fractures set in Marble Falls fm.
- Size of outcrop is larger than all measured fractures, with great fracture exposure. Great outcrop quality ensures reliable data quality, including length which is usually problematic.
- Simple rule is set up for length measurement: from fracture tip to tip, or ends at where two fracs join. Although this rule may not work as good at other outcrops.
- On both scanline A and B, aperture and length of fractures can be well described by power law distribution. Fracture length vs. aperture also fits well with power-law.
1.5m
Scanline A intensity (left) and NCC plots (right).
Scanline B intensity (left) and NCC plots (right).