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Avo Response as a Function of Pressure and Lithology in the Bay of Bengal, India

Nur Zahira WAHID
Honours - Bachelor of Science (Petroleum Geology & Geophysics)- 2009
Australian School of Petroleum
The University of Adelaide


Elastic properties of rocks are strongly influenced by local geologic trends and can be related to depositional environment or burial history. Knowledge about seismic response as a function of compaction trends will increase the ability to predict hydrocarbons, especially in areas with little well log information (Avseth et al., 2003b). Therefore, it is important to understand the rock physics trends and how changes in seismic amplitude with offset (AVO affects) are affected by those trends.

To begin with, the lithologies of the study area, the Bay of Bengal, South Asia, were separated into three end members, SHALE, WET and GAS sand. The Gassman equation was used to compute the elastic parameters of an 80% gas saturated sand from the WET sand properties. Depth trends for the elastic properties (P and S wave velocities and density) of these lithologies were generated and represented on crossplots. A simple earth model was created to represent SHALE overlying either WET or GAS reservoirs. The resultant seismic amplitude versus offset responses were examined as a function of depth. It was found that the AVO response in this region is categorised as Class IV. This is affected by factors such as uncertainty about the ‘wetness' of the original WET sand, mixed lithologies, and high porosity.

Apart from pore fluid saturations, the Gassmann calculations and resulting AVO effects can also be influenced by other factors such as mixed lithology, Vp-Vs transform and residual gas saturations. Flags were used to differentiate between sand and shale and care was taken to avoid other lithologies to ensure that the end member data was ‘pure'. This greatly improved the depth trends for each end-member. Shear velocity was calculated using the Greenburg-Castagna relation instead of Castagna's mudrock line, and it was found that the change in predicted Vs is significant at shallow depth but decreases in velocity as depth increases.

Another aspect that was looked into was the effect of the WET sands actually having a residual gas saturation, which affects the calculation of the gas sand properties. It is widely known that the effect of water saturation is greatest on velocity as it drops from 100% to 90% and the predicted AVO behaviour is affected if a completely water saturated sand is assumed when there is a residual gas saturation.

This project also provides a detailed analysis of the uncertainties in the rock properties and the effect on the AVO responses. Most of the results generated in this modeling study are strongly dependent on the trend interpretation and assumptions made about the region.

Australian School of Petroleum



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