New AVO Classification Helps Extract Information from Hard Rocks - an Example from the Green River Basin
Young, Roger A.1, Robert D. LoPiccolo1, Gordon Van
Swearingen1, Peter G. Smith2
1 eSeis, Inc, Houston, TX
2 Encana Inc, Denver, CO
The analysis of amplitude variation with offset (AVO) has, for years, been a mainstay tool for the explorationist in the less well-lithified rocks of the Gulf Coast and younger sedimentary sections elsewhere in the world. In the harder rocks characteristic of most of the Rocky Mountain basins the technique has been less useful. This is partly because the defined AVO classes were limited to gas sands underlying shales.
A new classification scheme has been proposed which promises to make this valuable tool useful in all sedimentary basins. In the new scheme the range of AVO types is extended to include all possible combinations of gradient and intercept, as opposed to the limited range implied by the traditional class 1, 2, and 3 AVO types. Additionally, all non-shale rock types, underlying a shale, are admitted to the scheme.
Lithologic and porosity information was extracted from a 2D seismic line from the Green River Basin. After the non-shale lithologies (including both sands and coals) were identified the data were analyzed for AVO types applying the proposed classification.
The section of interest is of Cretaceous age and includes fluvial sands, shales, and coals. Two source rocks are present in the area: subjacent coals appear to source retrograde reservoirs and are less desirable than deeper shales. The more economically attractive reservoirs will be further from the coal-rich intervals.
From well-ties in the area it was determined that coals typically appear as high-porosity zones with a type four or five AVO response. Record sections were prepared which represented lithology (shale vs. non-shale), porosity and AVO type. By screening for non-shale lithologies which had high porosity, and AVO Type four or five, both coals and sands could be displayed and more desirable sand packages identified.