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Understanding Atypical Background Trends in AVO Analysis: A Case Study on a Shaly Sand Gas Reservoir in the Magdalena Valley, Colombia


AVO interpretation is facilitated by crossplotting AVO intercept (A) and gradient (B). Under a variety of reasonable geologic circumstances, brine-saturated sandstones and shales follow a well-defined “background” trend in the A vs B cross-plot. “AVO anomalies” are properly viewed as deviations from this background and may be related to hydrocarbons or lithological factors (Castagna & Swam, 1997). In this paper we show a case study where two unusual geologic conditions produce the presence of an atypical background trend in the A vs. B crossplot. The first condition is the encasing of the reservoir between shales of great thickness which have been identified as a source rock, as well as a potential reservoir (Barreto et. al, 2007). The second is the absence of wet sands at the reservoir level, which, in most cases, could be used as a guide to identify ‘conventional’ trends in the A vs. B cross-plot. In order to be able to identify AVO anomalies related to gas bearing sands in this geological context, detailed log conditioning and rock physics analyses were performed in order to have a robust set of well logs. Next, rock physics modeling and fluid substitution were performed in order to understand the difference between the AVO anomalies related to gas bearing sands and those related to the atypical trend produced by the organic shales. These results along with AVO attribute volumes calculated from seismic data, after rigorous pre-stack conditioning (Singleton, 2009), were used to create stratigraphically guided detected volumes in order to isolate possible and different kinds of pay areas. The resulting volumes were used to reduce the uncertainty during volumetric calculations and to optimize the exploitation plan of the reservoir.