Natural Fractures

Contractional fault-bend folding:

Implications for localized fracturing

Contents

Introduction
Initial state
Folding processes and strain
Anticlinal growth stage
End of anticlinal growth
Anticlinal broadening
Implications for reservoir geology
2D Example: Fault bend fold in outcrop, Neuquen Basin, Argentina
The third dimension
3D Example: The Nan Yi Shan field, Qaidam Basin, China

Introduction

Background. This section assumes that you have read the introduction to Section 1.3 and are familiar with:

Natural fracture nomenclature.

Fracture orientations relative to the orientations of the principal stresses.

Regional fracture orientations.

Structural model. The discussion in this section is based on a conceptual model of fault-bend folding described in:

J. Suppe, 1983, Geometry and kinematics of fault-bend folding: American Journal of Science, v. 283, p. 684-721

V. S. Mount; J. Suppe; S.C. Hook, 1990, A forward modeling strategy for balancing cross-sections: Bulletin of the American Association of Petroleum Geologists, v. 74, no. 5 (May), p. 521-531.

However, the general concepts of structural domains and structurally-controlled fracturing are independent of any structural model.

Constraints and assumptions of the model.

Bed length is conserved.
Bed thickness is conserved.
Flexural-slip deformation only. Flexural-slip is simple-shear deformation by slip along bedding planes combined with flexure of the strata. Flexural-slip is what happens when you bend a deck of cards.
No out of plane movement. This model of fault-bend folding can be applied in either two or three dimensions. Two-dimensional models assume conservation of material in the transport plane. In other words, rocks are assumed to move in a vertical plane parallel to the regional transport vector.

Background material for non-geologists

CONTENTS

	Initial state
	Thrust faults typically run parallel to bedding in weak horizons (such as shales) and then ramp steeply through competent stratigraphic packages (such as carbonates) until they encounter another weak horizon, as shown in this figure. Folding begins at the active axial planes (green) as soon as the hangingwall block begins moving.

Figure 1

	Folding processes and strain
	Folding at the base of the ramp is easy to visualize. As flat-lying strata are pushed onto the ramp they fold to match the ramp angle at axial plane 2. Some slip is consumed by this folding process.

Figure 2

The folding process at the top of the ramp is less obvious. Clearly, strata above the ramp cut-off do not fold as long as they are above the ramp.

At the top of the ramp the strata must collapse along axial plane 1 to fill the void.
Strata within the red triangle fold to form the dashed yellow triangle, which has the same area as the red one.
Substantial slip is consumed by this folding process.

Strain due to folding. Folding requires shear strain of the individual strata that lie between slip surfaces as well as of the entire stratigraphic column. The type of strain required by this model is termed simple shear. When you push a deck of cards so that it slides to form a stack with slanted edges, you have deformed the deck of cards by simple shear. The type and amount of shear at each active axial plane can be calculated directly from the model. Unfortunately, this model does not accurately represent all aspects of natural folding. This model works well for analyzing large structures because the entire stack of strata does appear to deform by flexural-slip in nature. However, individual strata between slip-surfaces do not deform internally by simple shear so strain predictions from this model are not a good predictor of fracture system properties. The actual strain mechanisms of individual strata are beyond the scope of this page. A page on the subject will be added to this website, eventually.

CONTENTS

Anticlinal growth stage

Active axial planes (green) do not move because they are pinned to the fault bends.
Inactive axial planes (red) move with the hangingwall block.
Rocks that have been folded are more likely to be fractured (gray).
Most of the fault slip coming into the structure is consumed by anticlinal growth. Consequently, the slip-rate of the fault on the upper flat (above/in front of the ramp) is substantially lower than the slip-rate on the lower flat (below/behind the ramp). The slip-rate on the ramp above axial plane 2 is somewhat lower than the slip-rate on the lower flat.
Strictly speaking, fracture development invalidates the modeling assumptions of constant bed-length and bed-thickness because fracture development allows non-recoverable rock strain. However, in many cases this strain is not significant.
If slip-rate is a significant control on fault zone and fault-damage zone development, then slip-rate variations may affect fault permeability and leakage.

Figure 3.

CONTENTS

	End of anticlinal growth
	Axial plane 3 has reached the top of the ramp. Most of the strata that overlie the ramp have been folded, but the triangle between axial planes 1 and 3 still has not been folded. Inactive axial planes (red) move with the hangingwall block.

Figure 4.

CONTENTS

Anticlinal broadening stage.

Active folding commences at the top of the ramp at axial plane 5 which forms when axial plane 3 is transported past the top of the ramp and out along the flat. Axial plane 1 becomes inactive when this happens.
Folding at axial plane 5 refolds strata that were previously folded at axial plane 2.
The triangular area between axial planes 1 and 3 is never folded. Rocks forward (to the left in this case) of axial plane 5 are transported passively.
Slip is not consumed by folding during anticlinal broadening, so the slip-rate is the same along the entire fault.

Figure 5.

CONTENTS

Implications for reservoir geology.

These concepts have several implications for reservoir geology:

At least seven different structure/fracture domains (labeled D1-D7) are present in this simple, idealized cross-section.
The top of this anticline is composed of two very different structural domains even though the structure and stratigraphy appear identical in maps and cross-sections. This point is important because structural experts working in the oil industry normally do not differentiate domain D4 from D3 in maps and cross-sections because quantitative structural analysis is usually done only to define structural traps, not the flexure history of packages of rock.

Figure 6.

Fracturing of the forelimb and backlimb occurred under different conditions so that D2 and D5 may be very different geologically.
In theory, D1, D4, D6 and D7 are undeformed so that on average the types and density of fractures and other minor structures should be the same in each domain. In nature, D1, D4 and D6 were transported in different relative positions so each domain may have suffered some fracturing and each domain may have somewhat different fluid-transport properties. However, given that the packages were passively transported, they should be much less fractured than the other domains.
Fracturing and other reservoir-scale deformation features observed in outcrops may closely resemble reservoir rock deformation because structural domains can extend to the surface.

CONTENTS

2D Example: Fault-bend fold in outcrop, Neuquen Basin, Argentina

Photograph showing the forelimb of a fault-bend fold in an outcrop of limestones and shales. The fold is in the early stages of anticlinal growth (compare with Figure 3). Fractures are much more abundant in the folded panel (D7 in Figure 6). The fault began as a sharp break forming a planar ramp and flat joined at a sharp corner as illustrated in Figures 1 through 7. The corner broke off and the fault progressively developed a damage zone with continued slip. The complex fault zone shape and several minor faults produced additional fractured and folded panels in the hangingwall block that are visible in an unannotated, detailed (340kb) version of this photo.

Figure 7 (111kb)

P: Pen for scale. Solid yellow line: Original trace of fault. Dashed green line: Active axial plane. Dashed red line: Inactive axial plane. Solid blue: Fault zone and fault strands. Dashed blue line: Minor backthrust. Half-arrows show the sense of motion across the faults. Click in the rectangles to bring up detailed photos of these areas in a new browser window (Left-23kb, Center-109kb). Left-hand rectangle: Slightly fractured rocks in the never-folded but transported domains above the upper bedding-parallel fault segment (domain D6 in Figure 6). Right-hand rectangle: Heavily fractured rocks in the once-folded domain above the fault (domain D7 in Figure 6) and almost unfractured rocks in the never folded or transported domain below the fault (domain D5 in Figure 6).

CONTENTS

	The third dimension.
	Figure 7 is a three-dimensional view of a fault-bend fold anticline. The plunging nose of the anticline results from decreasing displacement along the fault.

Figure 8. After Medwedeff(198_).

Each plunging nose has an additional structural domain that was folded by transverse flexural-slip, which is slip parallel to the fold axis and perpendicular to the transport direction.
Development of the plunging fold nose requires transverse extension of the folded rocks, which violates the constant bed-length constraint used in 2-D models. This extension is accomodated by fractures.
In theory, transverse flexural-slip can distribute the strain across the entire fold. In nature, fractures are often concentrated in the plunging fold noses where the transverse extension is concentrated.
The amount of fold-axis parallel extension as estimated from quantitative structural analysis of seismic data provides a direct estimate of the fracture strain, but not of the fracture porosity.
The flat-lying rocks ahead of and behind the fold (Domains D1 and D6 in Figure 6) have also been transversely extended due to the change in displacement along the fault and should be fractured.

CONTENTS

3D Example: Nan Yi Shan field, Qaidam Basin, China

Time slice from 3D seismic survey across Nan Yi Shan field showing localization of gas sag on the northern plunge-panel of the fold. The best production test results from the fractured reservoir were from wells in the gas sag zone. Wells on the structurally higher fold crest had poorer tests. This suggests that localized fracturing in the plunge panel domain improved reservoir porosity and permeability. The fractures probably formed by stretching of the plunging nose.

Figure 9. After: R.J. Paul, 1993, Seismic detection of overpressuring and fracturing. An example from the Qaidam Basin, People's Republic of China, Geophysics, v.58, no. 10, p. 1532-1543.

CONTENTS

	Continue to page 1.3.4: Fracturing during wrench faulting