1.3.1

Introduction to Fracturing in Response to Folding

Pages 1.3.1–1.3.5 will interest anyone working on: reservoirs with permeable fractures; fault seal or fault leakage; reservoir compartmentalization.

Is the structural model important or unimportant?

The structural model is not important.  This website is primarily concerned with sub-seismic scale fractures of all types, especially faults and joints. Section 1.3 is NOT intended to provide a careful description and comparison of the many different types of quantitative structural models that are applicable to understanding small-scale deformation.

Section 1.3 has two main goals:

  1. 1.Illustrating how large-scale deformation can localize and generate minor structures.
  2. 2.Describing how structural models are used to:
  • Optimize exploration and development drilling.
  • Select wells for detailed (=expensive) logging and testing programs.
  • Build quantitative geologic and engineering reservoir models.

A secondary goal is providing non-specialists with an introduction to fault-bend folding, which is an important aspect of both industrial and academic structural geology.

The limitations of models used in the examples are not important because the general concepts are not tied to any specific structural model or any particular type of faulting. The discussions in section 1.3 are not intended to provoke experts by championing structural models that they may not like. Rather, the purpose is to stimulate your thinking about how to apply structural models in general to solve the problem that you're working on.

The structural model is very important.  Different models can provide very different predictions for a given structure. For example, the dip-angles of the axial planes and the strain predictions in section 1.3.2 would be different if we used a flexural-slip instead of a Coulomb shear model.

The models described in pages 1.3.2–1.3.4 are widely used in industry and are proven to give useful results in a wide range of geologic environments. Also, these models are simple and easy to apply. Each page points out that even a simple, qualitative analysis can yield valuable, practical results. However, if you need very detailed and accurate results, then you should consult an expert familiar with all of the available models. Eventually this website will have a separate and more complete section on quantitative structural analysis.

About pages 1.3.2–1.3.5

Pages 1.3.2–1.3.4 provides some guidelines and warnings about applying these methods. Each page covers a particular faulting regime and includes real world examples and outcrop photos. Each page illustrates basic principals that apply to all types of faulting and each page covers at least some material that is not covered on the others. Consequently, each page is relevant regardless of the style of faulting that you are most interested in.

Specific material covered on the pages includes:

  • The process of Coulomb-shear folding at flattening and steepening bends of extensional (normal) faults.
  • How fault-bend folding produces structural domains, which are packages of rock with homogenous deformational histories.
  • How this type of folding produces extensional strain.
  • Complexities due to multiple fault bends and 3-dimensional fault bends.
  • Systems of normal faults.
  • The process of flexural-slip folding at thrust ramps.
  • Types of strain produced by this folding process.
  • How quantitative structural analysis is used to define structural domains.
  • How structural domains equate to fracture domains.
  • Page 1.3.4 covers some aspects of fracturing during regional faulting, with emphasis on wrench faulting.
  • Fault stepovers.
  • Secondary structures formed during regional faulting.
  • Statistical vs. deterministic models of fractured reservoirs.
  • When a rock mass is deformed enough to need quantitative structural analysis.
  • Warning that strain estimates from models do not predict fracture type or density.
  • Warning that strain estimates do not predict fracture type.
  • Using fracture strain predictions derived from structural models to plan wells, work programs, and build reservoir models.
  • How to use these data in conjunction with other data sets such as seismic anisotropy and borehole data to build more accurate geological models of your reservoir.