Reservoir characterization of folded &
fractured Lisburne carbonates

Dr. Jerry Jensen, Assistant Professor

Graduate Students: Alexander Karpov (M.S.); Thang Bui (Ph.D.)

Abstract from December 2001 DOE report:

Lisburne Group Fracture Distribution and Flow Modeling

A.V. Karpov, Thang Bui, J. L. Jensen, Texas A&M University
and C. L. Hanks, University of Alaska Fairbanks

Fractures play a key role in the Lisburne Group fluid flow behavior. Field data from grainstone intervals of the Wahoo Limestone in the eastern Sadlerochit Mountains, a relatively undeformed area, have been analyzed to develop fracture distributions and models.

The models covered a range of possible cases. The "base" case assumed vertical fractures with either of two azimuths (to correspond to the two fracture sets observed in the field) and no effect of fracture intersection. Other models allowed more variability in fracture strike and interaction. These models were interrogated for wellbore placement and hydraulic connectivity behavior. The wellbore trajectory analysis showed:

• A positive correlation was observed for the "base" case between number and area of fractures connected to wellbores. This relationship, however, diminished for other cases.
• Optimal horizontal well azimuth orientations for the base case and the case with variable fracture strike and dip are in the range 0o to 30o. Wells with such orientation will be connected to the maximum number and area of fractures.
  
• Variability in strike and dip increases the number and area of fractures connected to the wellbore. However, it does not change the optimal wellbore orientation.
• ENE fracture termination increases the number and decreases the area of fractures connected to the wellbore. It also shifts the optimal wellbore trajectory from the bisector between the two sets (30o) towards the direction normal to the NNW set (60o).
  

The hydraulic connectivity analyses showed:

• The fracture network is weakly anisotropic if uniform fracture transmissivity is assumed.
• The system remains weakly anisotropic when the transmissivity of either set is diminished.
• ENE fractures provide critical connections for flow in any direction, as opposed to the NNW set, whose transmissivity is important only for the NNW flow.
  
• The fracture system is above the percolation threshold in all the cases.
• The smaller fractures become important if ENE fracture termination and strike and dip variability are included in the model. This case is less interconnected, closer to the percolation threshold, and more sensitive to removal of smaller fractures.
  

Analysis has also begun of data from five folds exposed in the Forth Range, North Sublik and South Sublik areas. These data are being assessed for fracture geometries and their relationship to position on the fold and fold tightness.

Abstract submitted to AAPG Pacific Section:

Evidence and implications for significant late and post-fold fracturing on detachment folds in the Lisburne Group of the northeastern Brooks Range

T. Bui, J. Brinton, C. Hanks, and J. Jensen

In exposed detachment-folded Lisburne Group carbonates, field evidence and statistical analysis suggest that a significant population of fractures post-date folding.
Both prefold fractures and penetrative strain associated with peak folding are overprinted by late-folding and post-folding fractures. The fractures in the first set strike EW parallel to the fold axis, are perpendicular to bedding, are often only partially filled with cement, and generally terminate at bedding planes. These and the earlier structures are consistently overprinted by pervasive late extension fractures. The later, NS fractures strike perpendicular to the fold axes, are vertically extensive, evenly spaced and unfilled. Both have similar average and median spacings.
Statistical analysis of fold angle and fracture spacing indicates that, as the folds tighten, both the EW and NS fracture spacing increases by a factor of two or three and becomes slightly more variable. This behavior is opposite from that expected if the fractures were closely related to folding. It suggests that the two sets are similar and are only weakly affected by the folding.
This weak genetic relationship between folding and formation of the most obvious and open fractures serves as an important example with major consequences for reservoir modelling. Complex genetic and timing relationships between fractures and folds may result in several fracture sets, each having different characteristics (e.g., size, amount of fill, and termination type). Unless recognised, genetically disparate fractures may be combined into one or a few sets to produce a reservoir model with fracture properties which do not apply to any of the sets. This could result in inappropriate wellbore placement or inaccurate productivity and recovery estimates.