Deep Water Depositional Facies and Reservoir Quality, Evaluation Based on Fourier Analysis of Gamma Ray Logs

By

FILLON, RICHARD H.

Earth Studies Associates, New Orleans, LA,

LAWLESS, PAUL N.

Dominion Oil and Gas, New Orleans, LA

Sequence stratigraphic well log analysis combines specific gamma ray/resistivity log relationships and biostratigraphic indices to identify depositional sequence and parasequence boundaries. Visual identification of these well log patterns relies on recognition of long wavelength (>100-1000 ft) variations in signal amplitude. Systems tracts and lithologic facies are distinguished based on more subjective “fining-upward,” “coarsening-upward,” “arcuate-convex,” and “blocky” log patterns. We believe systems tract and facies recognition can be improved by assessing variations in <100 ft wavelengths contained in well logs.

To test our thesis, several Gulf of Mexico deep-water siliciclastic gamma ray logs sampled at 0.5 ft intervals were subdivided using classical sequence stratigraphic analysis. The subdivided sections of the logs were processed using Fourier analysis to examine the importance of wavelengths <100 ft. Calculated this way wavelengths are comparable to sand-shale bed couplet thicknesses.

Wavelength spectra in the study record striking, sequence-related differences in <100 ft bed couplet thicknesses. For example, a strong representation of sand-shale couplets <3 ft to 9 ft thick in a fining-upward sequence clearly distinguishes it from an overlying coarsening-upward sequence containing fewer 3 ft to 9 ft couplets and more 9 ft to 15 ft couplets. Thus, bed thicknesses are interpreted to increase upward in a transition from probable overbank facies to prograding imbricate fan-lobe facies. Log intervals marked by “blocky” patterns characteristic of amalgamated fan-lobe sand facies, including reservoir and reservoir quality wet sands, contain both thinner (<4 ft) couplets and thicker (ca. 30 ft and >60 ft) couplets. Spectral analysis of lithologic well logs can significantly improve stratigraphic well log evaluations in deep-water settings.