Wavelet analysis of well-logging data from oil source rock, Egret Member, offshore eastern Canada

Wavelet analysis is a sensitive method for automatically detecting and dist inguishing abrupt discontinuities (i.e., faults, unconformities), cyclicity , and gradual changes in sedimentation rate by transforming depth-related s edimentary signals (i.e., gamma-rays) into wavelengths at distinct depth in tervals. We used wavelet analysis for evaluation of the spatio-temporal dis tribution of oil source rocks and fur estimating accumulation rates in a se dimentary 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 easter n 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 g amma ray cycles with die 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 rep resent similar to 100 k.y. eccentricity, giving accumulation times of simil ar to1.9 m.y. for stratigraphically complete sections having 19 successive 2.8 m gamma-ray cycles and giving accumulation times of similar to 600 k.y. for incomplete successions having only 6 cycles. Up to four discontinuities occur in gamma-ray log cyclicity and separate th e Egret Member into subunits. We interpret the discontinuities as unconform ities 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 interval s, having correlation coefficients of r = 0.8 and r = -0.76, respectively.