Detection of Hydrocarbons in Reefs Using AVO: Case History from Alberta, Canada

Deborah Miles, Gary Gassaway, Richard Brown, Laurie Bennett, Robert Bainer

Amplitude offset analysis (AVO) has been used since the late 1970s to find hydrocarbon accumulations. The method is usually used in clastic sediments, although some geophysicists have maintained that amplitude vs. offset inversion works in carbonate reservoirs that have a reflector associated with them. Lama and Vutukuri, in 1978, compiled laboratory data from carbonate cores collected throughout the world demonstrating the change in Poisson's ratio with a change in pore fluids or mineralogy. Since Poisson's ratio reflects changes in pore fluids or mineralogies in carbonates, amplitude vs. offset inversion can be used to determine the lithology and pore fluid changes.

Four common depth points for seismic lines on a Mesozoic reef complex in Alberta, Canada, were analyzed using SAMPLE, an amplitude vs. offset inversion. The locations varied from fore reef facies to back reef facies including two locations in the productive porosity of the main reef. SAMPLE analysis of the first location showed a tight shaly limestone of the Waterways Formation corresponding to the lime mud in front of the reef complex. In the reef complex, both the analyses showed hydrocarbons in the Swan Hills Formation. In the back reef location, the AVO inversion showed tight limestones with no hydrocarbons present. Thus, amplitude vs. offset inversion, SAMPLE, can determine whether the zone of interest is a tight limestone or shaly limestone, and that it is wet. SAMPLE also indic tes the presence of hydrocarbons in limestones at the second and third locations on the productive reef. Thus, amplitude vs. offset inversion can distinguish between hydrocarbon-bearing and nonhydrocarbon-bearing zones on the basis of interval velocity and Poisson's ratio.

AAPG Search and Discovery Article #91024©1989 AAPG Pacific Section, May 10-12, 1989, Palm Springs, California.