Reservoir Characterization of a Basin Floor Fan System Using Rock Physics to Integrate Seismic Amplitude and Well Data, Offshore South Africa.

Abstract

An extensive technical review in recent years involving well and seismic data on development areas in the southern coast of south Africa that cover the late Jurassic to early Cretaceous, fluvial to shallow marine rift fill to a succession of deep marine channel complex drift Albian sequence has been carried out. The aim of this study is to identify potential infill targets, which will ultimately improve the reservoir sweep efficiency and ultimate recovery. The focus of this paper is the 14A deposits, described as a third order sequence of Albian age and consist of a submarine fan complex that includes channel and fanlobe sandstones and overbank fines encased in deep-marine shales. The 14A sand has proved to be the most productive and economic oil play in the basin to date. The reservoir characterization is executed by integrating seismic amplitude and well data using rock physics modelling to demonstrate the reliability on pre-stack data to resolve hydrocarbon presence. The workflow implemented is based on two phases; (a) A forward model (AVO modelling) to investigate changes in seismic response due to reservoir quality facies (fluid content, porosity, shaliness) away from well control, and (b) to conduct an AVO analysis (I-G attributes cross-plot) to characterize rock properties based on seismic response. This workflow seeks to assist in identifying other hydrocarbon accumulations in the area of interest (upswept or prospective), and to mitigate the uncertainty that affects expected reservoir performance. The integration process show that AVO modelling is good in discrimination between oil and brine in the reservoir, especially when the porosity is high, because the fluid occupies a higher percentage of the bulk rock. Similarly, AVO Gradient attribute in the interval of interest shows a strong response that coincide with the discovered hydrocarbon and new potential upswept area or production accumulations. On the other hand, porosity models show a very strong effect on the acoustic impedance response than fluid. In addition, an increase in clay content leads to an extreme decrease in acoustic impedance and a slight increase in Poisson’s ratio.

AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018