The Carboniferous Lisburne Group is an important naturally-fractured reservoir in the subsurface of the North Slope of Alaska. The closest exposed section of Lisburne is in the northern part of the northeastern Brooks Range fold-and-thrust belt, approximately 75 miles to the southeast. Lisburne carbonates of the 'Sunset Pass' section of the eastern Sadlerochit Mountains have two donlinant sets of fractures: an early, through-going set striking NNW and a later, more restricted set striking ENE. Both sets of fractures are calcite filled. In general, fracture density increases in calcareous mudstones and/or dolon~itic horizons. There is no obvioiis correlation between fracture density ancl bed thickness.Although the stnlct~lral setting of the Lisburne Group exposed in the northeastern Brooks Range and that of the Lisburne in the subsu~face to the northwest are very different, they both share the NNW striiking fracture set. This suggests that both sections of Lisbilrne fractiired under siillilar (possibly the same) NNW compressional stresses, probably related to NNW-directed thrusting. In both the surface and subs~rface, the ENE stsiking set is most likely related to local structures. This similarity in fracture pattern between the Lisburne of exposed in the Sadlerochit Mountains and the Lisburne in the subsurface suggests that fracture patterns in surface exposures of the Lisburne Group can be used to study subsurface fracture characteristics.

The distribution and orientation of naturally-occurring fractures in the Lisburne Group exposed in the northeastern Brooks Range fold-and-thrust belt can provide a clearer understanding of the origin of and geologic controls on fractures in the Lisburne Group of the North Slope subsurface. The Lisburne Group is involved in a wide variety of map-scale structures in the exposed parts of the range, providing a series of natural experiments where various models for the formation and distribution of fractures in the Lisburne Group can be tested. Structural settings and associated fracture patterns investigated in during the 1994 field season included: high-angle transverse faulting in the eastern Sadlerochit Mountains; moderate folding in the Clarence River syncline; and tight folding with minor thrust faulting in the Fourth Range.Preliminary analysis of field results c o n f m that early regional fractures that form in front of a fold-and-thrust belt are overprinted or reactivated by subsequent deformation. New fractures that form in response to local structures either utilize the preexisting fracture set and/or obscure it. In the Sadlerochit Mountains, for example, young fracture sets do not necessarily developed parallel to the high angle faults (strike -20°), but appear to reactivate and form new fractures parallel to the regional fracture set (strike -340'). Increase in the intensity of the fault-related fractures is limited, however, to within 50-100 m of the fault itself.In examples of folded Lisburne Group, the intensity of the folding, the position with respect to the fold hinge, and the intensity of bedding-parallel slip govern what fracture set dominates. For example, while the early regional fracture set is well-preserved on the limbs of the Clarence River syncline, extensional and shear fractures related to folding are dominant near the core of the fold. However, because the fold is relatively open, there is not abundant evidence of fracturing related to bedding-parallel slip. In contrast, in the Fourth Range where the Lisburne is deformed into tight detachment folds, the early regional fracture set can not be reliably identified, and there is abundant evidence of bedding-parallel slip and related fracturing of individual beds throughout the Lisburne Group. Large, through going NS extensional fractures are present, but appear to be relatively young and are probably related to the local fold.