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.
The second application of phase decomposition is from the Smeaheia area, which lies about 30 kilometers east of the Troll gas field in the Norwegian North Sea. Two exploration wells drilled into three-way closure structures in the area, where the reservoir was expected to be good turned out to be dry. We describe these applications below.
Green Canyon Area
The King Kong field lies in the Green Canyon blocks 472 and 473 in deepwater (more than 4,000 feet of water depth) Gulf of Mexico (figure 1). It is a gas reservoir of Plio-Pleistocene age consisting of two sands associated with P-wave velocities in the range 6,000 – 6,800 feet per second compared with brine-saturated sands (with velocities ranging between 7,400 – 8,200 feet per second). Their density exhibits a dependence on the pore fluid but is significantly less pronounced. Consequently, these sands are associated with strong seismic amplitudes and stand out on seismic sections from a 3-D seismic volume over the area. Well KK-1 (in block 473) drilled into the King Kong reservoir encountered the two gas sands (thickness 26 meters) as seen on the seismic section in figure 2.
To the right in the same stratigraphic interval (figure 2) another high amplitude anomaly is seen, which represents the Lisa Anne prospect. Both anomalies look similar, in terms of amplitude.
The Lisa Anne prospect was tested by the drilling of well LA-1 (in block 474) drilled in water depth of 4,136 feet. The well encountered three good quality sands with good porosities (32 – 35 percent) in the interval of interest, but no commercial hydrocarbons were found.
Phase decomposition was run on the preconditioned input seismic volume and the minus-90-degree component examined. Figure 3 shows an inline section equivalent to the one shown in figure 2 and extracted from the minus-90-degree phase component volume. A bright phase anomaly is seen at the King Kong reservoir (green dashed ellipse), whereas no equivalent anomaly is seen for the Lisa Anne prospect (brown-dashed ellipse). Smaller anomalies (black-dashed ellipses) are seen on both sides of the section, which have not been analyzed, perhaps due to their size. The phase decomposition technique clearly discriminates the King Kong gas reservoir from the Lisa Anne prospect.
Smeaheia Area
The Smeahiea area lies about 30 kilometers east of the Troll gas field (figure 4), within the Norwegian continental shelf. It is located in a fault block bounded by the Vette Fault to the west and the Øygarden Fault to the east and is raised about 300 meters relative to the Troll field. The Late Jurassic Sognefjord, Fensfjord, and Krossfjord formations form the producing reservoir zones in the Troll gas field.
In the Smeaheia block, there are two closure structures, the Alpha structure to the west and the Beta structure to the east. Two exploration wells, namely 32/4-1 and 32/2-1 have been drilled into these structures, and although the reservoir is good, both wells turned out to be dry, indicating that the Smeaheia area is not charged with hydrocarbons.
In the Smeaheia area, the Sognefjord Formation is the primary reservoir consisting of medium to coarse-grain, well-sorted, micaceous, and minor argillaceous sandstone. Below this formation lies the Fensfjord Formation consisting of medium-grained, well-sorted sandstone with shale intercalations. Underlying the Fensfjord Formation is the Krossfjord Formation with medium to coarse-grained, well-sorted sandstone.
Overlying the Sognefjord Formation are the Heather and Draupne formation shales. While the Heather formation comprised of silty claystone with thin streaks of limestone interfingering the Sognefjord, Fensfjord and Krossfjord sandstones. The Draupne Formation consists of dark grey to brown/black shale that is non-calcareous, carbonaceous, and fissile claystone. Both the Heather and Draupne formations serve as primary seals for the proposed CO2 storage reservoir sandstones of the Sognefjord, Fensfjord and Krossfjord formations.
Figure 5a shows a segment of an inline seismic section extracted from the available seismic data volume (which has a bandpass filter applied), which passes through well 32/4-1 and exhibits the different markers overlaid. The gamma ray curve for well 32/4-1 is shown overlaid on the section. The ‘Base Quaternary’ (in yellow) represents an angular unconformity, and the ‘Peak Draupne’ (in dark blue) the top of the shale formation. The ‘Sognefjord’ (in green), ‘Fensfjord (in cyan) and ‘Krossfjord’ (in bluish green) are the markers of interest representing sandstone formations as described above. Two prominent faults in solid black (Vette fault to the west and Øygarden fault to the east) are also shown overlaid on the section.
We run phase decomposition on the preconditioned input seismic volume and focus on the reservoir intervals as shown in the stratal section shown in figure 6 extracted from the minus-90-degree phase component volume. As both wells encountered brine sands no anomalous values are seen on this section.
Conclusion
The application of phase decomposition to seismic data can provide a set of phase components wherein the thin low-impedance layers stand out as an anomaly on the minus-90-degree phase component data. We have demonstrated the application of phase decomposition on two different areas, one from the Gulf of Mexico where one well encounters a gas-saturated reservoir, but another well drilled not very far from the first intersects a low gas-saturated sandstone with no commercial hydrocarbon. The second application on seismic data from Smeaheia area in the Norwegian North Sea has two wells drilled into three-way closure structures but encountered brine sands. Phase decomposition application in both cases turned out to be very helpful, exhibiting anomalies on the minus-90-degree phase component data. Consequently, phase decomposition can be used as a reconnaissance tool for exploration or frontier areas, in lieu of detailed inversion work.
Acknowledgements
We would like to acknowledge US Geological Survey National Archive of Marine Seismic Surveys for access to the Green Canyon Phase II 3-D seismic survey (G3D201407-02), and Equinor ASA and Gassnova SF for access to the Smeaheia dataset.
The first author would also like to thank the Geomodeling Technology Corporation for making the Attribute Studio software available, which has been used for some visualization displays shown in this article.
The phase decomposition module is owned by Lumina Geophysical, Houston.
(Editors Note: The Geophysical Corner is a regular column in the EXPLORER, edited by Satinder Chopra, founder and president of SamiGeo, Calgary, Canada, and a past AAPG-SEG Joint Distinguished Lecturer.)