Imaging and Quantifying Turbidite Sand Systems in Deep-water Angola using Extended Elastic Impedance
Authors: Connolly Patrick A. , Schurter Greg J. , Davenport Mike M. 
Published: October 2008

The deep-water province offshore Angola has become an important focus for exploration in recent years (e.g. Hungo, Kizomba, Girassol). The reservoirs for the major discoveries are Miocene and Oligocene mid-slope turbidites, often formed into complex channel systems. As in all deep-water environments, wells are expensive and so to appraise these reservoirs we must make extensive use of the high quality seismic data. The information we require from the seismic data changes, however, as we progress from lead screening at the exploration stage through to detailed reservoir modelling for appraisal and development.

We demonstrate how extended elastic impedance theory, based on the two-term AVO equation, can be used to determine optimal projections of the seismic data for each of these applications. We then propose a simplified rock property model for this environment and show how it leads to a method to estimate net to gross directly from the optimised seismic data.

Introduction

Deep-water wells are expensive so exploration and appraisal programmes offshore Angola make great demand of the seismic data. The seismic must be of a quality that allows direct detection of the hydrocarbons such that the risk is sufficiently reduced to justify the drilling of the exploration well. Low risks can usually only be achieved by combining the appropriate seismic attribute with an appropriate spatial pattern. In other words the attribute pattern should fit a sensible structural and depositional model and it should be possible to interpret the presence of a fluid contact seen either as an attribute conforming to structure or a flat spot.

At the late exploration/appraisal stage detailed mapping of the reservoir will be required to assess the hydrocarbon volumes. Mapping the gross rock volume is a minimum requirement. However, the net to gross ratio in the channelised mid slope sands of deepwater Angola (Kolla et al, 2001) is very variable, so some method must be found to also estimate the net to gross of the reservoir from the seismic data. Reservoir models also require information about the size of aquifers so estimates of net sand volume may also be required.

Each of these activities requires different information to be extracted from the seismic data. Direct detection may need oil sand reflections to be maximised relative to brine sand reflections. Risk reduction will need fluid contacts to be accentuated, reservoir mapping will require shale/oil sand impedance contrasts to be maximised, and reservoir modelling may require fluid effects to be removed to allow the sands to be mapped through the contact.

The deep-water Tertiary offshore Angola, as with many such clastic successions consisting of unconsolidated or partially consolidated rocks, typically has small velocity and density contrasts across lithologic interfaces. In such circumstances the two-term AVO equation, R = A+Bsin2?, provides a useful model for the seismic response out to fairly large offsets. We can exploit the AVO response of the seismic data to design optimum datasets for each of these activities.