3D seismic data is inverted directly for porosity by implementing a non-linear, globally optimised seismic inversion scheme. The technique takes into account the lithology dependent relationship between porosity and acoustic impedance without requiring the user to make a detailed interpretation of the seismic data and specifying the correlation between acoustic impedance and porosity for each lithological unit. For comparison seismic inversion for acoustic impedance is also carried out.
Seismic inversion directly for porosity is implemented by utilising that the correlation between acoustic impedance and porosity has a negative slope which is often approximately constant and not lithology dependent.
A wavelet is derived that inverts directly for porosity reflectivity and the absolute level of porosity is defined by constraining to a low frequency porosity model which is specified from available well logs.
To invert the seismic data a non-linear globally optimised seismic inversion technique is used. This full 3D multi trace technique does not directly include well logs or seismic horizons and therefore unbiased results are generated from post-stack, migrated seismic data. The seismic inversion is performed by utilising an estimated wavelet, a low frequency model and by specifying 5 inversion parameters.
The seismic inversion for both acoustic impedance and porosity is illustrated by a 3D seismic survey. The seismic data was acquired in the Danish part of the Central Trough with the main target being the Chalk Group which includes Upper Cretaceous-Danien chalk reservoirs. 7 wells are used in the study in order to invert a time window from 1.5 to 2.5 sec TWT, that includes a total of 456,000 traces covering an area of 71 sq km.
The results show that the technique is a fast and reliable method for estimating porosity directly from seismic data. However, the match between the porosity log and the inversion result is not as good as the correlation between the acoustic impedance log and the corresponding inversion result. This is attributed to the non-unique correlation between acoustic impedance and porosity.