Correlating Porosity with Acoustic Impedance in Sandstone Gas Reservoirs: Examples from the Atokan Sandstones of the Arkoma Basin, Southeastern Oklahoma
Cemen, Ibrahim; Fuchs, Jeff; Gertson, Rodney; Hager, Christine
It has been suggested by several researchers that seismic impedance values determined from the 3-D seismic inversion data can be used to locate the zones of low and high porosities in sandstone reservoirs. We have applied this concept to three well known sandstone gas reservoirs in the Arkoma foreland basin, located in southeast Oklahoma. The reservoirs are the Lower Atokan Spiro; and Middle Atokan Brazil and Red Oak Sandstones. We used 3-D seismic acoustic inversion volume calibrated to well control to map porosity changes in these three gas reservoirs. Every well with a sonic log within the survey area has been analyzed to understand velocity changes in these reservoirs along strike and dip of the main direction of thrusting. Structural control plays a major role in the productivity of these sandstone reservoirs. Therefore we also determined the structural control on the porosity of these reservoirs with constructing several balanced structural cross-sections in the area of seismic inversion data since understanding the structure over an area is just as important as identifying changes in rock properties.
Our interpretation of the seismic inversion data suggest that the areas of tighter anticlinal folds in our cross-sections correlate reasonably well with the lower acoustic impedance values due to fracture porosity in the Spiro Sandstone. In areas where the Spiro experienced facies changes the acoustic impedance value is also a good predictor of porosity Therefore, acoustic impedance shows a reliable correlation with higher porosities as seen in areas of lower acoustic impedance in the Spiro reservoirs. However, we have not found a very reliable correlation between the porosity and acoustic impedance in the Brazil and Red Oak Sandstones. We suggest that this is due to the variable shale content together with the tuning thickness of intervals. The two sandstone units contain thick shale sections because of the nature of their depositional environment. Therefore, seismic inversion data seem not to be a reliable predictor of porosity when thick shale sections are present. When we isolate the homogeneous sandstone parts of the two units, however, there is a linear correlation between the acoustic impedance and porosity. Therefore, we suggest that determining the linear relationship between the acoustic impedance and porosity may be used only inhomogeneous relatively thick sandstone reservoirs.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013