Acoustic Impedance as a Lithological and Hydrocarbon Indicator- A Case Study in Nandasan Field of Mehsana Block, Cambay Basin, India

Apoorva Sahoo and Soumitri S. Dash
Sub Surface Team, ONGC, Mehsana, India

Nandasan field was discovered in 1975 & was put on production since 1986.It is producing from Kalol and Mehsana pays ranging in age from lower to middle Eocene. Mehsana member is deposited as multilayered sand bodies separated by shale and coal partings deposited in a lower delta plain.In order to understand the low resistivity gas bearing sand layers in Mehsana member of Nandasan field of Mehsana Block,an integrated study was carried out considering the effect of geological depositional environment.In this study,around 20 wells from both the fields were taken up.The P-wave acoustic impedance for different sand layers were computed using sonic and density logs and then plotted against appropriate reservoir parameters mainly shalinesss and porosity. Subsequently these were analyzed with the composite log suite to identify the zones associated with different clusters of points on the cross plots.It is observed that hydrocarbon bearing sands (especially gas bearing sands) have relatively low impedance, water bearing layers and shales have roughly equal and higher impedance and hence can be very easily discriminated .It also indicates that low impedance corresponds to clean reservoirs and high impedance correlates with silty and tight layers. Hence it is possible to identify hydrocarbon bearing sands even though its resistivity is highly subdued as is the case in some layers of Nandasan and Linch fields of Mehsana.

The present study aims at highlighting the fact that it is possible to identify gas bearing reservoirs using P-wave acoustic impedance from logs and may be fine tuned to derive acoustic impedance values from 3D seismic inversion volumes.Such integrated approach will increase the confidence of understanding reservoir delineation and characterization while attempting exploration for hydrocarbons in the other parts of the basin.