Tectonic Controls on the Formation and Saturation of Conjugate Shear Fracture Networks: Combining Outcrop Analogues With Modelling Studies

Abstract

Natural fracture networks within subsurface rocks often form a system connected discontinuities which affects rock strength, effective permeability and local stress fields. If open, these connected systems can greatly enhance fluid flow and therefore productivity within tight reservoir rocks. However, if cemented, these features can also form significant fluid flow barriers. One of the most commonly observed arrangements of these natural systems are the conjugate fracture networks, which form as a result of a tectonic compressive stress, and show complex hierarchal patterns. Previous mechanical modelling studies have shown that natural mode I arrangements show predictable geometrical patterns. These characteristic features are believed to be the product of fracture interaction and resulting deviations in the tensile stress field. However, to what degree shear fracture interaction affects the resulting network geometry is still poorly understood.

In this study, we expand on to key lessons learned from modelling mode I fractures, and apply them to model and analyse the tectonic stress conditions responsible for the development of natural conjugate fracture system. This is done by combining a 2D finite element approach, a slowly increasing tectonic stress and the assmumption that micro fractures which can grow under sub critical stress conditions. The acquired results are then compared to interpreted outcrop patterns acquired from a natural examples of fractured carbonates (Salitre Fm, NE Brazil).

First of all, our results show that no fracture propagation will occur until a material dependent sub critical stress value is reached. However, once this critical value is reached, the initial micro fractures quickly develop into a developed system, at which all initial flaws are either fully developed or don’t show any sign of propagation. The models also show that the number of fully developed fractures is dependent on the magnitude of the applied tectonic stress. Furthermore, our results show that mode II fracture interaction results in driving stress localization and rotation, and therefore, areas of continued localized fracturing at different angles. Another interesting observation regards the number of open fractures in the model, which increases quadratically with applied tectonic stress. Finally, our modelling observations can explain the geometry relatively complex network patterns observed on outcrops using relatively simple geological settings.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018