1-step inversion concept
seismic inversion; pcube+
PCube+ is a Bayesian seismic inversion method that inverts directly for litho-facies from pre-stack seismic data. PCube was developed by a consortium led by the Norwegian Computing Centre (Norsk Regnesentral https://www.nr.no/en/GIG) and was commercialized by Sharp Reflections in their PreStack Pro software suite.
Bayesian inversion for litho-facies classes (LFC)
The seismic data is used to constrain the allowable set of facies models by assessing the match of the prior model synthetic to the seismic.
*Acknowledgement to Norsk Regnesentral, Statoil (now Equinor) and Sharp Reflections for material.
Forward modelling and comparison to seismic
This illustrates the concept of generating facies models and associated elastic properties, and comparing the synthetic seismic model to the real seismic data. Only models that provide a 'good' match to the seismic are allowed.
Posterior litho-facies probabilities
From the set of allowable models the posterior probabilities for the various litho-facies can be calculated.
Additional constraints on forward modelling
Using additional geologic constraints on the LFC transitions produces more realistic, blocky-facies posterior models.
Example: Synthetic wedge model
A simple wedge model shows the improvement using a 1-step inversion approach directly to rock properties (LFCs) over the 2-step workflow (AvO inversion followed by transform to rock properties).
Comparison of 1-step and 2-step inversion results
The 1-step approach produces a more realistic blocky model of the wedge, without being biased by the prior low frequency model.
PCube+ inputs
Inputs to PCube+ include partial angle stacks and wavelets, horizons and uncertainties, litho-facies definitions with associated elastic property distributions and expected facies thicknesses, and prior LFC probabilities per layer.
PCube+ outputs
Outputs from PCube+ include most likely facies and individual facies probability volumes, volumes of elastic properties (AI, SI, Vp/Vs), and updated horizons.
PCube+ outputs contd.
This shows that the match of inverted elastic properties to well logs is good, and the distribution of most likely facies within the reservoir that could be used to constrain geologic and reservoir modelling.