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AAPG Rocky Mountain Section Meeting

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The Importance of Kerogen Quality to Landing Zone Determination and Geosteering

Abstract

This study proposes the Kerogen Quality Index (KQ), an index to define relative kerogen quality in source rocks for more accurate maturation assessment. KQ is defined as Hydrogen Index (HI) minus Oxygen Index (OI). Given that HI = S2/TOC*100 and that OI = S3/TOC*100, the KQ index essentially subtracts the non-convertible, oxygen associated organic matter from the convertible, hydrogen associated kerogen for a more accurate hydrocarbon generating potential utilizing only simple, cost-effective pyrolysis results. Higher KQ kerogen generally requires higher temps and pressures to initiate conversion and will move through maturation slower than lower KQ kerogen subjected to the same conditions. Plots of Tmax and KQ from lateral wellbores demonstrate significant variability of both KQ and Tmax from wellbore to wellbore, some with high KQ and some with low KQ. Empirically, samples with varying KQ values can generate the same API gravity oil across a range of Tmax values: lower KQ samples typically generate at lower Tmax values (lower temps and pressures required) where higher KQ samples typically generate similar API at higher Tmax (higher temps and pressures required). Conversely, samples with equivalent Tmax values can produce a range of API gravity oils. Production data from Hereford and Briggsdale Fields, Weld County, Colorado, show that similar API gravity oils are generated across a 12 degree Tmax range, and similar Tmax values associated with a 32-39 degree range in API values. Marrying the KQ index with stratigraphy, this study examines the possible correlation between KQ and Tmax (as a function of KQ) across relative stratigraphic depth in the Niobrara chalks and marls of the Hereford Field, DJ Basin, Colorado. Correlations of KQ with stratigraphy could help address questions of where best to land horizontal wells, augmenting other chemostratigraphic methods, and improve production predictions on a well by well basis.