Reservoir characterisation and seismic inversion in Karewa 3D, Northern Taranaki Basin

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Sahoo, T.R.; Thrasher, G.P.; Hill, M.G.; Arnot, M.J.; Griffin, A.G. 2015 Reservoir characterisation and seismic inversion in Karewa 3D, Northern Taranaki Basin. Lower Hutt, N.Z.: GNS Science. GNS Science report 2015/34 31 p.

Abstract: Reservoir characterisation and seismic inversion methodologies are widely used in the petroleum industry to identify and delineate hydrocarbon bearing reservoirs. This project was aimed at developing initial capability on reservoir characterisation and seismic inversion within GNS Science. A pilot project has been carried out on the Karewa 3D in the Northern Taranaki Basin to prepare a seismic inversion model, and to identify and delineate the Mangaa C1 gas bearing sand in the Pliocene section. In addition, this model was used for quantitative interpretation such as porosity estimation and Gas in-Place (GIP) calculation. The seismic inversion process involves well to seismic tie, extraction of a suitable wavelet, synthetic seismic generation, and integration of well log data and geological interpretation of the seismic data to prepare a seismic inversion model (Hampson-Russell, 2007). In this study, we have used post-stack Karewa 3D seismic data and Karewa-1 well information and prepared a P-Impedance seismic inversion model. Delineation of the gas sand and quantitative interpretation of porosity and GIP requires a rock physics analysis at the well location. Rock physics analysis was carried out at Karewa-1 well to characterise gas sands based on computed P-Impedance using acoustic (P-wave velocity (Vp)) and density logs. This analysis was also used to define relationships between the measured rock property (total porosity) and acoustic property (P-Impedance). Mangaa C1 gas sand shows a low P-Impedance character and it can be clearly delineated in the P-Impedance model using a P-Impedance cut-off value of 5570 ((m/s)*(g/cc)). This impedance model was then converted to porosity models (P90, mean and P10 cases) using linear relationships derived from rock physics analysis. Net Pore Volume (NPV), Net Hydrocarbon Pore Volume (NHPV) and Gas in-Place (GIP) were then calculated for the Mangaa C1 gas sand and the mean case GIP was compared with a previous study done by Todd Petroleum Mining Company. The volumetric estimates are encouraging as the results are consistent with the previous study by Todd, which followed a different seismic inversion approach (elastic inversion) using angle stacks of Karewa 3D seismic. (auth)