Lithology and Fluid Prediction in Lightly Explored Basins

Abstract

In this paper we present a methodology for performing lithology and fluid prediction in lightly explored basins. We use the deep water area of the Orange basin, offshore South Africa to illustrate the methodology. In the Orange basin, there are numerous wells drilled on the shelf, however, there have been no wells drilled in the deep water. Firstly, petrophysical analysis of the shelf wells is performed to determine end member properties of the sands and shales. From the analysis, rock property trends such as Vp versus Vs and Vp versus density are determined. In addition, the uncertainties associated with the trends are also calculated. A critical step in extrapolating from the shelf to the deep water is to use seismic derived interval velocities to improve the estimate of Vp as a function of depth. Using seismic interval velocities and well data, we derive expressions for Vp of the sands and the shales that are functions both of depth and seismic interval velocity. The rock property trends are used to perform stochastic AVO modelling for single interfaces. The AVO modelling gives an estimate of the average response of interfaces such as shale-to-brine sand and shale-to-gas sand interfaces, as well as a measure of the uncertainty of the estimate. Therefore, a range of AVO responses is provided. Lastly, the AVO modelling is compared with AVO anomalies observed on the seismic data.