Abstract
The estimation of the hydrocarbon generation potential of shales in unconventional and conventional resources depends on an accurate estimation of the original total organic carbon (TOCo) content and original hydrogen index (HIo). TOCo is partially lost during thermal alteration whereas Mo/Al is hardly altered, suggesting that a linear correlation between them in immature shales can be used to estimate TOCo in mature shales. However, this correlation method is not applicable when immature and mature shales have different kerogen types. Herein, a refined method to calculate TOCo for mature shales using a linear correlation between two major fractions of organic matter (generative organic carbon (GOC), and non-generative organic carbon (NGOC)) and Mo/Al is proposed. If thermal maturity increases, only GOC that generates hydrocarbon is reduced without change in NGOC associated with kerogen type. However, the linear correlation between TOCo and Mo/Al of immature shales can change the NGOC as well as GOC in the mature shales. To estimate TOCo of mature shales, original GOC (GOCo) in the mature shale is calculated using linear correlation between the GOC and Mo/Al of the immature shale and subsequently, the calculated GOCo is combined with the NGOC unconverted from thermal alteration in the mature shales. We applied the refined method to the Devonian Muskwa Formation where mature and immature shales have different kerogen types. The HIo was calculated from the ratio of GOCo to TOCo using the refined method. The calculated HIo values are similar to those roughly deduced from the cross-plot of Tmax vs. HI values. The result indicates that the refined method is useful for accurate estimation of TOCo and GOCo in the shales. The refined method improves the estimation of original characteristics of organic matter in a shale gas reservoir where the original kerogen type cannot be identified due to an increase in thermal maturity.
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