Role of Simultaneous Inversion of Seismic Angle Stacks in Reservoir Characterization

Samir Biswal
Oil & Gas, Reliance Industries Limited, Navi Mumbai, India

Mapping fluid distribution in a hydrocarbon reservoir away from wells is a challenging task. Finer resolution of well logs (~0.15m)with limited depth of investigations, can not predict the lateral variation in the rock properties directly. Correlation of logs between few wells does not serve prediction of reservoir properties laterally. 3D seismic data with vertical resolution of ~20m has better lateral coverage (25mx25m or less)

Variation of seismic amplitude with offset/angle has become an industry standard for estimating the lithology and fluid content. One widely used scheme that inverts seismic data divided into several angle stacks to estimate three rock properties, viz. P-impedance, density and S-Impedance or Vp/Vs or Poisson’s ratio. In band limited seismic data, absence of higher frequency (~ > 40 Hz) reduces the vertical resolution and the depletion of low frequencies (~ < 8 Hz) results in removing the compaction trend of the rock properties. Therefore inverted band pass P-impedance, Vp/Vs or density cannot be directly used to relate with the lithology or fluid.

A structural framework model within which the well log P-impedance, Vp/Vs and density, suitably interpolated, is used to fill up the low frequency hole in the inverted band pass elastic properties of the rocks. The full band elastic properties derived from superposing the inverted band pass results on the low frequency data are used to map reservoir properties, such as, lithology, pay sands, porosity, fluid saturation etc.

Discrimination of pay sands from non pay sands/ shale and other lithologies on the basis of elastic properties at resolution of well logs and of seismic data determines the effectiveness of inversion. We used a field example from East Coast basin of India to explain the utility of simultaneous inversion in reservoir characterization.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas