Large Volume Reconnaissance AVO and Anisotropy Prediction: OPL 256 Nigeria

By

Maurice Gidlow¹, Chris Thomas², Raj Dhillon², Tom Ziegler², Steve Pitman²

(1) PGS Geophysical, Walton on Thames, United Kingdom (2) PGS Geophysical,

 In 2000 a reconnaissance 1850sq km 3D survey was acquired in OPL 256/7 offshore Nigeria. The purpose of this 3D seismic survey was to image and accurately map tectonic features of the area. In addition to identifying hydrocarbon accumulations, the aim was to obtain high-resolution data for the mapping of the fault systems and deltaic packages. Since multiples pose a significant problem in this area, one of the main challenges was to effectively attenuate the multiples present in the data.

The OPL 256/7 area, offshore Nigeria, is characterised by a thick Neogene section deposited in a deepwater setting in the outer compressional zone of the Niger Delta. Target structures within the sand/shale Agbada Formation consist primarily of a series of shoreward-dipping toe-thrusts that sole out in the over-pressured shale of the Akata formation, together with large turtle-back anticlines. Proven reservoir facies within the Agbada Formation were deposited in channel and turbidite lobe environments.

In this paper we outline the acquisition parameters and discuss in detail the pre-stack imaging sequence, paying particular attention to the subjects of AVO analysis, as well as lithology prediction from transverse anisotropy measurements. We present a selection of AVO attributes which include the fluid factor, and show how, using a 3D visualization system, our ability to interpret these volumes is enhanced. We describe also our methods of predicting bulk anisotropy parameters from pre-stack analysis, and show how these can be used to describe large scale shale distributions.