Seismic Avo Inversion Techniques for Exploration of The Cooper Basin Unconventional Plays
Doctor of Philosophy
University of Adelaide
This thesis is comprised of six topics, each of which complements the knowledge of exploration challenges and applicable seismic AVO inversion techniques for Cooper Basin unconventional reservoirs. The first topic discusses unexpected fracture stimulation treatment behaviors that may relate to the Cooper Basin’s high tectonic stress and overpressure. This study analyzes the Cooper Basin in-situ stress, rock strength and pore pressure, which are compared with image logs interpretation of natural fractures and borehole breakouts.
Topic two presents a sensitivity analysis of three-term AVO inversion and provides knowledge on the reliability of the inversion results. This analysis uses stochastic forward modelling of random noise and various source-receiver offsets followed by AVO inversion. Bayesian probability is used to quantify how increases in signal-to-noise ratio and far offset distances influence the reliability of inversion results. This example used a single set of rock properties, where measurements of seismic signal have been ‘normalized’ in attempts to make these results applicable to all reservoirs. This analysis presents some limitations with performing AVO inversion that are helpful for deciding suitable inversion types for the available data set.
A major challenge in the Cooper Basin is mapping thin fluvial tight gas sand bodies that are difficult to interpret on seismic data due to strong reflections from adjacent Permian coals. This is not the common AVO problem of distinguishing between coal and gas sand, but a more difficult class-I AVO problem of mapping fluvial sands beneath a sheet coal that varies in thickness. Topic three provides a solution of using Poisson’s ratio attribute calculated from extended elastic impedance (EEI) technique and a rotation of near and far
partial stacks. Noise sensitivity and tuning analyses on these techniques also show advantages and disadvantages associated with each technique.
Seismic data from the Cooper Basin exhibits azimuthal anisotropy in both AVO and HTI interval velocity (derived from migration velocity analysis). Topic four investigates if anisotropy is caused by fractures or by the Cooper Basin’s large difference between minimum and maximum horizontal stress. This study compares both migration velocity anisotropy and AVO anisotropy extracted from a high-quality 3D survey to a ‘ground truth’ of dipole sonic logs, borehole breakout and fractures interpreted from image logs. The results suggest that stress is the dominant cause of the HTI anisotropy observed in the seismic data.
Topic five evaluates alternative (non-AVO) techniques for mapping natural fractures; diffraction imaging and common seismic attributes such as incoherence and curvature. These techniques are applied to a Cooper Basin data set and the results are analyzed on their ability in detecting subtle features (small faults, fractures and channel-edges) and their resolution (vertical and spatial).
The final topic in this thesis investigates the ability of rock properties to detect the presence of overpressure, a topic that adds further complexity to advances in shale gas exploration in the Cooper basin. Results show that while overpressure can and does impact rock properties in the Cooper Basin, variations in gas saturation have a similar and stronger impact. Separation of the overpressure signature and the saturation signature is problematic.