Application of Amplitude Variation with Offset to Limestones, Offshore Colombia
Seismic attributes have the potential to characterize reservoir petrophysical properties such as porosity and permeability, and have evolved as mainly applicable to siliciclastic rocks. Properties of carbonate rocks at seismic scales, however, are significantly different from properties of siliciclastic rocks, due to spatial heterogeneity in porosity, permeability and rock fabric. In addition, limestones are susceptible of undergoing post-lithification physical changes due to geochemical processes and diagenesis. In recent years, great efforts have been made to improve the seismic characterization of carbonates. At the forefront of these developments is Amplitude Variation with Offset (AVO) analysis of carbonate rocks.
In our ongoing study, we are evaluating the sensitivity of AVO analysis to detecting gas presence in a calcareous rock unit from a producing field offshore Guajira, Colombia. Production in that field comes predominately from a Miocene carbonate reservoir, often assigned to the Jimol Formation. A 1999 640 channel, 36 km-long 2D seismic line from the Colombian offshore was selected for this purpose. Noise in the seismic data is widespread, and include direct, guided, and refracted waves, side scattering and random noise, short-period multiples, reverberations and phantoms. Two dry holes are intersected by this line. The sequence of our methodology consisted of (1) relative amplitude-preserving processing of the seismic field records; (2) modeling well data in order to extract the normal (wet) trend; (3) performing fluid substitution to derive the gas trend; (4) inverting both real and modeled data to estimate the shear modulus-to-density and bulk modulus-to-density ratios as well as Poison’s ratio; and (5) cross-plotting and analyzing these parameters versus porosity.
Should our study demonstrate sensitivity of AVO analysis to gas charge in noisy seismic data, future efforts will apply this analysis for derisking new prospects in the Guajira offshore region. Otherwise, additional laboratory tests will be performed to further characterize physical properties of these reservoirs, for fine-tuning the sensitivity of AVO in this area.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009