Quantification of mixed hydrate and free gas concentrations by the joint integration of velocity and conductivity information

Medientyp: E-Artikel

Titel: Quantification of mixed hydrate and free gas concentrations by the joint integration of velocity and conductivity information

Beteiligte: Reeck, Konstantin [Verfasser:in]; Jegen, Marion [Verfasser:in]; Freudenthal, Tim [Verfasser:in]; Elger, Judith [Verfasser:in]; Hölz, Sebastian [Verfasser:in]; Deusner, Christian [Verfasser:in]; Wallmann, Klaus [Verfasser:in]; Lin, Saulwood [Verfasser:in]; Chi, Wu-Cheng [Verfasser:in]; Bohrmann, Gerhard [Verfasser:in]; Berndt, Christian [Verfasser:in]

Erschienen: Elsevier, 2023-12

Sprache: Deutsch

DOI: https://doi.org/10.1016/j.marpetgeo.2023.106569

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Beschreibung: Highlights • The developed joint inversion quantifies both free gas and hydrate concentration. • The robust method uses sonic and conductivity logs as main input parameters. • For the test site it reveals two hydrate accumulations with very different characteristics. • The whole range of concentrations is shown that can explain the observed data. • The method is applicable to most continental margins when there is borehole control. Abstract Quantification of gas hydrates in marine sediments is crucial for understanding gas hydrate systems. By empirical relationships or effective medium modelling, gas hydrate concentrations can be derived from velocity and/or conductivity logs. However, these approaches do not take the co-occurrence of free gas and gas hydrate into account leading to large uncertainties in the calculated free gas and gas hydrate concentrations. To overcome this issue we adopt a joint elastic and electric self-consistent/differential effective medium model as the basis for a new joint inversion scheme that distinguishes between both phases. We apply this scheme to p-wave velocity and electric induction data measured by downhole-logging of boreholes at Formosa Ridge off Taiwan - a known hydrate province with an active gas conduit. Gaussian Mixture Modeling separates the background signal of the host medium from anomalies and allows to determine a background porosity as a probability density function of depth. We use this derived porosity to jointly invert electrical conductivity and velocity data for hydrate and free gas concentrations. At Formosa Ridge, we find two resistive anomalies, one in the shallow and another in the deep part of the borehole. Only the deep anomaly in conductivity coincides with a high-velocity anomaly. This is consistent with ∼30% hydrate with ∼1% free gas concentration. For the shallow anomaly, increased velocities due to hydrate concentrations of ∼15% are compensated by a decrease in velocity due to ∼1% of free gas. The method reconciles the different sensitivities of the ...

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