Effective estimation of critical parameters in submarine gas reservoirs using P-wave velocity and density data for 2D/3D reservoir characterization

Efficient and accurate estimations of submarine gas reservoir porosity and gas saturation are essential for successful reservoir characterization. However, most classical rock-physics inversion methods for gas saturation (e.g., [1-2]) require pre-existing data, such as porosity, the mineral constituent ratio of the rock, or assumed empirical equations. These methods have limited applicability due to incomplete data and the unclear physical significance of the empirical equations, often yielding inversion results that deviate significantly from well log data and cannot be applied to non-well regions. Although an effective multi-parameter inversion method [3] exists that can estimate critical parameters from elastic impedance data, elastic impedance inversion requires high-resolution raw seismic gathers and complex procedures [4], making it more expensive for 2D/3D reservoir characterization.

To address this, a new method (US appl. 19/432,887) based on rock-physics theory for submarine gas reservoirs is proposed now. In this approach, gas saturation, porosity, the mineral composition of the rock matrix, and fluid-mixture properties are treated as independent variables, while density and P-wave velocity are treated as dependent variables. These four parameters are simultaneously inverted from density and P-wave velocity data.

Testing this method at Site 1245E on Hydrate Ridge along the Cascadia Margin [5] produced acceptable root-mean-square errors for gas saturation (0.0598) and porosity (0.0151), and the estimated mineral constituent proportions closely matched smear-slide analyses. Inversion tests at three additional gas-bearing sites [6-8] demonstrated that the proposed method outperforms previous approaches in accurately estimating porosity and gas saturation, with further validation using 2D density and P-wave velocity profiles from post-stack seismic inversion, which yielded porosity and gas-saturation profiles below the bottom-simulating reflector that align well with logging data.

[1] M., Collett, T.: Gas hydrate and free gas saturations estimated from velocity logs on Hydrate Ridge, offshore Oregon, USA. In: Proc Ocean Drill Prog Sci Results 204:1–25, 2006.

[2] Tinivella, U.: A method for estimating gas hydrate and free gas concentration in marine sediments, Boll Geofis Teor Appl, 40, 19–30, 1999.

[3] Yan, Y., Li, H., Hao, G., et al.: Simultaneous inversion of five physical parameters of submarine gas reservoir from synthetic elastic impedance for high-efficiency reserve evaluation, J Petrol Explor Prod Technol, 15, 68, 2025.

[4] Maurya, S. P.: Estimating elastic impedance from seismic inversion method: A case study from Nova Scotia field, Canada, Current Science, 116, 628–635, 2018.

[5] Tréhu, A. M., Bohrmann, G., Rack, F. R., Torres, M. E., et al., Proc ODP Init Repts, 204, 1–75, 2003.

[6] Riedel, M., Collett, T., Malone, M., Expedition 311 scientists: Site U1329, Proc IODP, 311, 107–2006, 2006.

[7] Collett, T., Riedel, M., Cochran, J., Boswell, R., Presley, J., Kumar, P., Sathe, A., Sethi, A., Lall, M., NGHP Expedition Scientists: National Gas Hydrate Program Expedition 01 Initial Report, Directorate General of Hydrocarbons, Ministry of Petroleum and Natural Gas: New Delhi, 2008.

[8] Paull C, Matsumoto R, Wallace P, et al. Proceedings of the Ocean Drilling Program, Initial Reports, 164, 1996.