Fizz gas

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Fizz Gas

"Fizz Gas" is a false Direct Hydrocarbon Indicator (DHI) representing a Low Saturation Gas reservoir.The seismic anomaly is a function of Reflection Amplitude and its controls of Pore fluid and Lithology. A Fizz Gas amplitude anomaly is created when there is a change in the pore fluid. The remanence of gas molecules in a reservoir changes the P-wave velocity to create a false bright spot commonly referred to as Fizz Gas.

Fizz Gas is often seen where Gas has migrated through rock layers but has not been trapped. The ‘Fizz Gas” or low saturations of gas remain in the rock layers and brighten up the Amplitude in a DHI. The amplitude anomalies or bright spots caused by the Fizz Gas can be misinterpreted as a high gas saturated reservoir. For this article, Seismological interpretations of Acoustic (p – wave) propagation is assumed. Thus the following explanation is applicable to vertical Component or near-offset Traces.

The effects of P wave Velocities in terms of Elastic Constraints

Seismic variation can be caused by many factors—Velocity variations, lithology compressibility contrasts, pore fluid densities etc. Fizz Gas is caused by a bulk modulus miscalculation which cause P wave Velocities attenuations. P Velocities can be described in terms of “Elastic” parameters, which define the effects on a solid when it interacts with Seismic Waves. Compressibility of the solid and Bulk Density is a contributing factor to P wave velocities. This is because P-wave Velocity is a function of the Bulk Density. Changes in pore space fluid saturation will cause a change in the P-Wave Velocity.

Bulk Modulus effects on Fizz Gas

Kb or the Bulk Modulus measures the incompressibility of the lithology (Solid). This incorporates the bulk modulus of the formation minerology (Kmineral), density of the pore fluid (Kfluid), and the porosity ϕ. Kb units are the same as pressure, force per unit area. Lithology Components in reservoir can be highly compressible, for example unconsolidated sands, or highly incompressible (Granite) which have a large effect on acoustic impedance. It is imperative to have the correct Kmineral when calculating P-Velocity.[1] This however is not the problem when encountering Fizz Gas. Fizz Gas causes amplitude attenuations in DHI because the Kfluid is miscalculated. In order to calculate a coherent Kb one must also incorporate the compressibility of the Reservoir Fluids; this includes the compressibility of pore fluid (Kfluid), Salinity is often a factor, and the compressibility of the Reservoir Gas. Bulk Density is greatly affected by the Bulk Modulus of all three components. Bulk Density varies a function of mineral composition, porosity and density of the pore fluid. Increasing Gas saturation within the pore space causes the density and bulk modulus to rapidly decreases. [2]

Fizz Gas in Seismic Interpretations

Figure 3 [3] illustrates how Gas Attenuates Seismic Amplitudes below it. The bulk modulus in the formation is highly dependent on pore fluid densities in the reservoir. A DHI will display a bright spot even when the Gas saturation levels are Low. The Amplitude Anomalies can be due to Multiple Factors. Change in lithology can cause Amplitude anomalies. Over Pressure or over burdening also cause significant Amplitude Anomalies. Amplitude anomalies are also a great indicator of the presents of Hydrocarbons, but be careful it may just be “Fizz Gas”.

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  1. Bruce S. Hart (2011) An Introduction to Seismic Interpretation. AAPG Discovery Series No. 16: Chapter 2, Principles of Seismic Methods pp. 1-33.
  2. Batzle, M., and Wang, Z., 1992, Seismic properties of porefluids: Geophysics 57, 1396-1408.

[1] [2]

External Links Bo Zhao, Hua‐wei Zhou, and Fred Hilterman (2005) Fizz and gas separation with SVM classification. SEG Technical Program Expanded Abstracts 2005: pp. 297-300. De-Hua Han and M. Batzle (2002). ”Fizz water and low gas-saturated reservoirs.” The Leading Edge, 21(4), 395-398.

  1. Batzle, M., and Wang, Z., 1992, Seismic properties of porefluids: Geophysics 57, 1396-1408.
  2. Xiaoyang Wu, Mark Chapman,, Xiang-Yang Li1 and Patrick Boston 1Edinburgh Anisotropy Project, British Geological Survey, Edinburgh EH9 3LA, U.K., and 2School of Geosciences, The University of Edinburgh, Edinburgh EH9 3JW, U.K.