--> --> Rock Property Prediction from Seismic Inversion: F-A Gas Field, Bredasdorp Basin, Offshore South Africa

2018 AAPG International Conference and Exhibition

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Rock Property Prediction from Seismic Inversion: F-A Gas Field, Bredasdorp Basin, Offshore South Africa

Abstract

The F-A field which was first discovered in 1970 is one of the oldest gas fields in the Bredasdorp Basin, Offshore South Africa. To better manage the field’s decline and the remaining life span, a new static and dynamic reservoir model was required. The previous model for reservoir history match and forecast was built 20 years ago in 1998. The objective of this paper is to describe how acoustic rock properties through Acoustic Impedance maps obtained from seismic inversion may assist in populating the distribution of volume of clay, effective porosity and hydrocarbon saturation in the static reservoir model. The inversion project is based on the 1997/8 acquired F-A 3D seismic data, which was processed in 1998 to zero phase (1998 F-A 3D). Although there is no new seismic data acquired over the F-A gas field, the reservoir knowledge of the area has improved significantly through regional seismic interpretation and integration with production and pressure data. The final acoustic impedance volumes result was obtained after several iterations of testing different Acoustic inversion methods. This inversion method used a model-based inversion algorithm to convert the 1998 F-A 3D seismic data to 3D Acoustic Impedance volumes. It consisted of building a low frequency background Acoustic Impedance model, wavelet estimation using wells and seismic data, inversion quality analysis at the well locations and 3D Acoustic Impedance inversion. In order to evaluate the potential of Acoustic Impedance to predict reservoir properties like volume of clay, effective porosity and water saturation, Acoustic Impedance was cross plotted against each of these properties, respectively, at log as well as at seismic scale. It will be shown that volume of clay and water saturation do not show any correlation to Acoustic Impedance, but a good relationship to porosity in clean sandstones. While the obtained results cannot be generalized, such results suggested that this workflow can be applied successfully in characterizing the porosity trends in the F-A upper shallow marine sandstone. This led to converting the Acoustic impedance volume to a pseudo-porosity volume. The results also indicated some potential for this workflow to be applied to the interbedded fluvial reservoir sandstone underlying the more compact shallow marine sandstone. Keywords: Seismic Inversion, Characterization, Acoustic Impedance, Low Frequency Initial Acoustic Model, Interpolation, Blocking