Wavelet analysis is a sensitive method for automatically detecting and distinguishing abrupt discontinuities (i.e., faults, unconformities), cyclicity, and gradual changes in sedimentation rate by transforming depth-related sedimentary signals (i.e., gamma-rays) into wavelengths at distinct depth intervals. We used wavelet analysis for evaluation of the spatio-temporal distribution of oil source rocks and for estimating accumulation rates in a sedimentary basin having high resolution.
The method was applied to 16 gamma-ray logs from the Jurassic Egret Member (an oil-source rock succession 55 m to 227 m in thickness), offshore eastern Canada. Dominant gamma-ray cycles having wavelengths varying from 2.8 m (western margin of the basin) to 24 m (eastern part of the basin) have been detected by wavelet analysis. The coincidence of the ratio of predominant gamma-ray cycles with the ratio of Milankovitch spectra (about 400, 100, 40, 20 k.y.) suggests that climatic cycles are an important factor controlling sedimentary cyclicity in the Egret Member. Dominant wavelengths likely represent ~100 k.y. eccentricity, giving accumulation times of ~1.9 m.y. for stratigraphically complete sections having 19 successive 2.8 m gamma-ray cycles and giving accumulation times of ~600 k.y. for incomplete successions having only 6 cycles.
Up to four discontinuities occur in gamma-ray log cyclicity and separate the Egret Member into subunits. We interpret the discontinuities as unconformities or faults and as related to sediments having low petroleum potential. The stratigraphic completeness of the Egret Member is correlated to total mass of organic carbon and decreasing thickness of non-source rock intervals, having correlation coefficients of r = 0.8 and r = -0.76, respectively.