In this paper we show a case study where a gas bearing sand reservoir encased with shale of great thickness and rich in organic matter, which are considered as source rock and potential reservoir (Barrero et. al, 2007), occasioned the presence of unconventional background trend in the AVO intercept (A) and gradient (B) crossplot. This condition, change the expected AVO response of the shales, altering the direction of the typical background trend, in which coincide the AVO response of the shales and brinesaturated sandstones. In consequence, to be able to identify AVO anomalies related to gas bearing sands in this geological context, a detailed log conditioning process and rock physics analysis were performed to have a robust set of well logs. Next, rock physics modeling, fluid substitution and thickness modeling was performed in order to understand the difference between the AVO anomalies related to gas bearing sands regarding to the 'non-conventional' trend produced by the organic shales. The results obtained along with AVO attribute volumes calculated from real seismic data, after a rigorous conditioning process in pre-stack domain, were used to identify potential AVO anomalies related to gas bearing sands.