SEISMIC AVO RESPONSE TO VARIATIONS IN SANDSTONE RESERVOIR PROPERTIES
AND PREDICTION OF RESERVOIR QUALITY
by
Sharon L. Goehring
Accurate prediction of hydrocarbon reservoir properties, including reservoir sand quality and fluid content, plays a vital role in identification of prospective drilling locations, especially in regions with little well data available. As exploration for hydrocarbons reaches into deepwater environs and other shale-rich basins, lithologic heterogeneities create variations that may result in faulty analysis of seismic response. Modeling seismic reflection amplitude variation with offset (AVO) for differing shaly sandstone geometries may be a useful tool in estimating reservoir properties.
This thesis examines the most
effective strategy for prediction of reservoir properties using eight different
reservoir models with five clay or shale geometries. An empirical model
relating compressional-wave and shear-wave velocities
to porosity and shale volume represents shale in volumetric proportions. Several models using the Hashin-Shtrikman
effective medium equations test geometries where clay occurs within the pore
space or as a component of the dry rock frame.
Lastly, a Backus averaging of sand and shale laminae estimates
properties of a thinly-layered reservoir. All models use the Biot-Gassmann-Geertsma equations for fluid substitution and
Shuey’s approximations to the Zoeppritz
equations for AVO parameters. Three borehole
suites of downhole logs, 15.75 square miles (40.79
square kilometers) of 3D seismic data including common depth point (CDP)
gathers in the deepwater
Analysis of results proves two different modeling schemes sufficient for determining reservoir properties and prediction of reservoir quality. Each model was compared to subsurface data for prediction accuracy at data target horizons and for the significance of results between them. The Hashin-Shtrikman equations, which model pore-space clays acting as bridges between sand grains, fit the observed seismic data best. As this clay-fill geometry would affect permeability in the reservoir, it suggests an inferior reservoir quality. Lower than expected flow rates in the producing wells of this field support this suggestion. The Backus-averaged laminae model provides statistically significant different results and if it had fit the data more accurately, it would indicate better reservoir quality. For sandstone reservoirs with varying shale volume, these two AVO models should produce results important for reservoir characterization in a prospective hydrocarbon play.