Resolved stress analysis, failure mode, and fault-controlled fluid conduits

Alan P Morris,Kevin J Smart,David A Ferrill

doi:10.5194/se-11-899-2020

Abstract

Abstract. Failure behaviors can strongly influence deformation-related changes in volume, which are critical in the formation of fault and fracture porosity and conduit development in low-permeability rocks. This paper explores the failure modes and deformation behavior of faults within the mechanically layered Eagle Ford Formation, an ultra-low permeability self-sourced oil and gas reservoir and aquitard exposed in natural outcrop in southwest Texas, USA. Particular emphasis is placed on analysis of the relationship between slip versus opening along fault segments and the associated variation in dilation tendency versus slip tendency. Results show that the failure mode and deformation behavior (dilation versus slip) relate in predictable ways to the mechanical stratigraphy, stress field, and specifically the dilation tendency and slip tendency. We conclude that dilation tendency versus slip tendency patterns on faults and other fractures can be analyzed using detailed orientation or structural geometry data and stress information and employed predictively to interpret deformation modes and infer volume change and fluid conduit versus barrier behavior of structures.

Highlights

IntroductionFaults and fractures often serve as conduits for fluid in lowpermeability rock (Barton et al, 1995; Caine et al, 1996; Zoback et al, 1996; Evans et al, 1997; Sibson and Scott, 1998; Ferrill and Morris, 2003; Faulkner et al, 2010; Alves and Elliott, 2014; Mattos et al, 2016; Mattos and Alves, 2018; Roelofse et al, 2020), including self-sourced oil and gas reservoirs (Ferrill et al, 2014a, b, 2020; Gale et al, 2014)or CO2 reservoirs (Trippetta et al, 2013; Ward et al, 2016; Miocic et al, 2020), and reservoir cap-rock seals (e.g., Petrie et al, 2014; Roelofse et al, 2019)
We explore the variability in resolved stress patterns along well-exposed and preserved, smalldisplacement normal faults in the Eagle Ford Formation and the relationship between dilation tendency, slip tendency, and deformation behavior at various positions along faults following the approach presented by Ferrill et al (2019a)
This study provides a clear example of how faults can serve as fluid conduits in mechanically layered low-permeability strata

Summary

Introduction

Faults and fractures often serve as conduits for fluid in lowpermeability rock (Barton et al, 1995; Caine et al, 1996; Zoback et al, 1996; Evans et al, 1997; Sibson and Scott, 1998; Ferrill and Morris, 2003; Faulkner et al, 2010; Alves and Elliott, 2014; Mattos et al, 2016; Mattos and Alves, 2018; Roelofse et al, 2020), including self-sourced oil and gas reservoirs (Ferrill et al, 2014a, b, 2020; Gale et al, 2014)or CO2 reservoirs (Trippetta et al, 2013; Ward et al, 2016; Miocic et al, 2020), and reservoir cap-rock seals (e.g., Petrie et al, 2014; Roelofse et al, 2019). Deformation behavior, in particular positive or negative dilation versus shear, is closely related to the orientation of the failure plane or zone with respect to the stress field at the time of deformation (e.g., Ramsey and Chester, 2004; Ferrill et al, 2017b). Recent work has shown that failure or reactivation mode along faults can be directly related to the dilation tendency versus slip tendency on the fault in the stress field at the time of deformation (Fig. 1; Ferrill et al, 2012, 2017a, 2019b; Ward et al, 2016; Meng et al, 2020, Miocic et al, 2020; Roelofse et al, 2020).

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