Distinct-element Stress Modeling in the Penola Trough, Otway Basin, South Australia
The Penola Trough of the Otway Basin, South Australia, is host to five economic gas fields containing an estimated 120 bcf of original gas in place in fault-related traps. However, throughout this trough, many other fault-dependent traps contain paleocolumns or partial paleocolumns. In 2001, the Balnaves 1 well discovered a semibreached structure. This structure was originally thought to be low risk because its associated fault was optimally oriented to seal with respect to the interpreted present-day maximum horizontal stress direction. On subsequent analysis of the wellbore image data, an open conductive fracture network was observed in the seal around the main bounding fault.
We propose that perturbations of the regional stress field around preexisting faults may open a fracture network in the seal. This hypothesis is tested for the Laira Formation (cap seal) using the finite-difference distinct-element method (DEM). To our knowledge, this technique has not previously been used to assess seal integrity.
The DEM has been used before for estimating perturbations around faults. The current work first summarizes and expands previous investigations of the perturbations developed in the two-dimensional (2-D) (horizontal) local maximum (σ1) and minimum (σ3) stress magnitudes produced around a single fault, it then uses this understanding to create and assess a 2-D DEM Penola Trough model.
For a single fault, the magnitude of perturbations were examined as a function of k = σ1/σ3, θ (the angle between σ1 direction and the fault strike), friction angle Φ, and fault stiffness jkn and jks. The magnitude of stress perturbations are highly sensitive to k, θ, and Φ, but less sensitive to fault stiffness. This insight is applied to horizontal 2-D models to identify areas of potential cap rock failure.
In a 2-D study of the Penola Trough areas of high shear stress are modeled where breached hydrocarbon columns are known to occur. We interpret areas of high shear stress to be zones of fractured rock and possible cap rock failure. Predicting zones of cap rock failure using DEM models could prove to be a very useful exploration tool in locations where cap rocks are known to be brittle and have suffered recent tectonic strain.
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This volume constitutes the proceedings of the AAPG Hedberg conference on seals held in Barossa Valley, South Australia, in 2002. The key driver for both the Hedberg conference and this publication was the recognition that knowledge of risk in the estimation of sealing capacity and fault-seal potential is important in making judgments at the exploration, appraisal, and development stages of the petroleum business. In addition, incorporating seal risk in the overall assessment of hydrocarbons in place can affect decisions to drill prospects and the location of appraisal and development wells, as well as reserve estimation. Improved methods to estimate seal capacity and fault integrity can lead to savings in well costs, improved recoveries through optimum placement of wells, and improved estimates of hydrocarbon in place. This volume contains 18 chapters that reflect the spectrum of presentations at the conference. The knowledge imparted by these chapters will be a window on the state of seal knowledge at this juncture of time and includes topics such as seal failure related to basin-scale processes, the role of geomechanics in seals, and the economic evaluation of prospects with a top seal risk.