# Phase Decomposition Applications as a Reconnaissance Tool

In the February 2022 installment of Geophysical Corner, we discussed phase decomposition and its applications. It is a novel technique that can decompose a composite seismic signal into different phase components, which in turn can help with the characterization of thin target sandstone or carbonate reservoirs. We followed it up with another article in May 2022 in which we extended that discussion to the application of phase decomposition as a reservoir management tool, with the odd phase component (sum of 90-degree and minus-90-degree phase components) showing better correlation with the wells that control the injection and withdrawal of a natural gas storage reservoir in Denmark.

Phase decomposition can improve reservoir characterization and the technique is particularly useful in those areas where thin-bed interference causes the phase of the input seismic response to differ from the phase of the embedded wavelet in the data.

For a zero-phase wavelet in the data and thin low-impedance layers below tuning thickness, the waveform phase response generated after carrying out phase decomposition is found to be minus 90 degrees, which stands out as an anomaly. On the contrary, a corresponding high-impedance thin layer exhibits a similar (plus) 90-degree phase waveform response. By generating a synthetic response with use of well data and a zero-phase wavelet, such observations for thin reservoir layers can be understood with confidence and correlated with real seismic data. Phase decomposition can help immensely in direct interpretation of seismic data in terms of reservoir and non-reservoir zones, among other applications.

## Defining ‘Phase’

We have noted that the term “phase” can carry multiple meanings within the geoscientific community. To some, the term implies a simple phase rotation applied to the seismic data, usually carried out to improve the well-to-seismic tie correlation. To another section, it refers to the phase attribute that was introduced more than four decades ago and computed using complex trace analysis. Such folks assume that the phase decomposition discussed here is some repackaged version of complex trace analysis, which is just not true.

It may be clearly stated that the phase decomposition process described here is very different from the phase information determined from complex trace analysis. While in the latter, amplitude and phase are independent, and the determined phase attribute conveys no amplitude information, and is thus not interpretable. In the “phase decomposition” described here, amplitude is computed as a function of phase. It is this nature of the attribute which makes it amenable to a number of different and accurate interpretations.

## Two Areas of Application

In this article we share the applications of phase decomposition to seismic data from two different areas. The first being from the Green Canyon of the Gulf of Mexico where the first well intersects a gas-saturated sandstone reservoir, but the second well drilled not very far from the first, targeting another high-amplitude anomaly in the same stratigraphic interval encounters low gas-saturated sandstone with no commercial value.

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