Technical Paper
Technical Paper

Quantitative Interpretation (QI) – Facies-Based Seismic Inversion

Written by: Ehsan Zabihi Naeini and Russell Exley

Quantitative interpretation (QI) is an important part of successful exploration, appraisal, and development activities. Seismic amplitude variation with offset (AVO) provides the primary signal for the vast majority of QI studies allowing the determination of elastic properties from which facies can be determined. Unfortunately, many established AVO-based seismic inversion algorithms are hindered by not fully accounting for inherent subsurface facies variations and also by requiring the addition of a preconceived low-frequency model to supplement the limited bandwidth of the input seismic. We apply a novel joint impedance and facies inversion applied to a North Sea prospect using broadband seismic data. The focus was to demonstrate the significant advantages of inverting for each facies individually and iteratively determine an optimized low-frequency model from facies derived depth trends. The results generated several scenarios for potential facies distributions thereby providing guidance to future appraisal and development decisions.

Derisking via quantitative interpretation (QI) is an essential part of successful hydrocarbon exploration and appraisal. In ideal circumstances, QI using amplitude variation with offset (AVO) inversion can be used to identify lithologies, indicate pore fluid fill, and determine net rock volume. However, the detail that can be extracted from a conventional AVO inversion workflow is limited by the averaging effects of not taking into account facies variations and adopting a simplified and rigid low-frequency model. To overcome this, Kemper and Gunning (2014) introduced an inversion algorithm that iteratively updates the low-frequency model input and in doing so ultimately outputs an optimized facies model and the associated elastic properties. The detail provided by the described joint impedance and facies-based inversion allows operators to pursue reservoir targets with increased confidence by quantifying facies distributions, reservoir geometries, and volumetrics. An example is shown here using a broadband long-offset seismic data set, broadband well tie, and wavelet estimation, followed by the newly developed facies-based seismic inversion.

The case study in this paper centers on a Paleocene discovery, known as Avalon, in block 21/6b of the UK Central North Sea located at the northwestern edge of the Central Graben just south of the Buchan Field. The discovery consisted of an 85 ft column of oil in good quality sands and was initially defined using conventional simultaneous pre-stack inversion. The reservoir sands lie within the proximal part of the prolific northwest to southeast late Paleocene Forties and Cromarty depositional trend. This fairway includes the giant Forties Field. In general, Cromarty and Forties members have high porosities, high net-to-gross and, as a result of these rock properties, the reservoirs provide an ideal natural laboratory for applying AVO-based inversion techniques.

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