Evidence and Implications for Significant Late and Post-Fold Fracturing on Detachment Folds in the Lisburne Group of the Northeastern Brooks Range
- T.D. Bui (Texas A&M U.) | J. Brinton (U. of Alaska) | A.V. Karpov (Texas A&M U.) | C.L. Hanks (U. of Alaska) | J.L. Jensen (Texas A&M U.)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- June 2003
- Document Type
- Journal Paper
- 197 - 202
- 2003. Society of Petroleum Engineers
- 4.1.2 Separation and Treating, 5.1.5 Geologic Modeling, 5.5.2 Core Analysis, 4.3.4 Scale, 4.1.5 Processing Equipment
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In exposed detachment-folded Lisburne Group carbonates, field evidence andstatistical analysis suggest that a significant population of fracturespostdate folding. Both prefold fractures and penetrative strain associated withpeak folding are overprinted by late-folding and post-folding fractures.Late-folding fractures strike east/west, parallel to the fold axes. These, andthe earlier structures, are consistently overprinted by pervasive late north/south extension fractures. Both east/west and north/south fracture sets havesimilar average and median spacings.
Statistical analysis of fold angle and fracture spacing indicates that, asthe folds tighten, both the east/west and north/south fracture spacingsincrease by a factor of two or three and become slightly more variable. Thisbehavior is opposite from that expected if the fractures were closely relatedto folding. It suggests that the two sets are similar to each other and areonly weakly affected by the folding.
This weak genetic relationship between folding and formation of the mostobvious fractures serves as an important example with major consequences forreservoir modeling. Complex genetic and timing relationships between fracturesand folds may result in several fracture sets, each having differentcharacteristics (e.g., size, amount of fill, and termination type). Unlessrecognized, this could result in inappropriate wellbore placement or inaccurateproductivity and recovery estimates. Modeling of this fractured system, forexample, showed permeability changes greater than 80%, depending on fracturefill, timing, and flow direction.
Understanding and quantifying the fracture distribution can be critical forthe exploitation of a fractured reservoir. Unfortunately, surveying andmeasuring fractures in the subsurface is usually difficult and very costly.Examination of fractures in outcrops of the equivalent formation may be auseful means of studying the fracture properties and their distribution underthe influence of different parameters. This research examines the fracturedistributions within the Carboniferous Lisburne Group carbonates, exposed inthe northeastern Brooks Range of Alaska.
The Carboniferous Lisburne Group in the northeastern Brooks Range is theclosest exposed stratigraphic equivalent to the reservoir of the Lisburne oilfield, located approximately 120 km to the northwest ( Fig. 1). A thrustfront separates the subsurface Lisburne field from the Lisburne Group exposedin the northeastern Brooks Range fold and thrust belt.1 The LisburneGroup has deformed primarily by detachment folding and thrusting throughoutmost of the northeastern Brooks Range.2 In the Fourth Range and theShublik Mountains, the Lisburne Group deformed into detachment folds overregional anticlinora (Fig. 2). Detailed structural and stratigraphicalaspects of the Lisburne Group and the northeastern Brooks Range are discussedby Homza and Wallace,3 Wallace and Hanks,2 and Hankset al.1 The exposed detachment folds in this area serve as atarget of the fracture study in this work.
Numerous studies have investigated the relationship between fractureproperties and mechanical/stratigraphical parameters. Many show that theaverage fracture spacing is directly proportional to the formation bedthickness.4-10 Fracture spacing also appears to be a function ofrheology, with more competent rocks having more closely spacedfractures.6 McQuillan4 investigated the Asmari limestoneoutcrops over an extensive area of the Zagros Mountains and proposed thatfracture density has an inverse logarithmic relation to bed thickness but isindependent of structural setting.
Others have investigated the effect of the lithology on the fracturedistribution. Hanks et al. 1 showed that lithology is theprimary controlling factor on fracture properties and characteristics inrelatively undeformed sections of the upper Lisburne Group in the easternSadlerochit Mountains. In these undeformed carbonates, grainstones are theleast fractured, with wider and more throughgoing individual fractures.Dolomitic mudstones are the most fractured, but they have narrower fractures oflimited vertical extent that generally terminate at bed boundaries.
Other researchers have investigated the effects of folding on the fracturedistribution. Murray11 assumed that fracture aperture increased asthe strain or the degree of the curvature of the bed increased. Severalstudies12-15 suggest that fracture density increases with increasingbed curvature. Because a fold can be developed under different scenarios, thefracture distribution within it can be highly variable. Jamison14suggested that the highest fracture density in a detachment fold could be foundin the midlimb region.
Thus, previous studies of the fracture distribution as a function ofmechanical stratigraphy in folded bedded rocks have generally focused on threemajor parameters: bed thickness, degree of deformation, and lithology(including the contrast between layers in layered formations). The observationshave been that the average fracture spacing is linearly proportional to bedthickness, fracturing is enhancing by the degree of strata bending, andlithology controls the difference in fracture spacing in beds of equalthickness.
In contrast to these observations, our analysis of fracture data fromdetachment-folded Lisburne Group carbonates in the northeastern Brooks Rangesuggests that:
Formation bed thickness does not have a consistent effect on fracturespacing.
Fracture density does not increase with increasing curvature. These andother field observations indicate a complex relationship between fractures andfolds. Our observations suggest that bedded rocks in folded systems mayexperience multiple fracture-generation episodes before, during, and afterfolding. Complex genetic and timing relationships between fractures and foldsmay result in several fracture sets, each having different characteristics(e.g., size, amount of fill, and termination type). Unless recognized, thegenetically distinct fractures may be combined into one or a few sets toproduce a reservoir model with fracture properties that do not apply to any ofthe sets.
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