Effects of geochemical compositional heterogeneities on hydrocarbon expulsion and thermal maturation: An analog study of Maastrichtian source rocks from Jordan

Accurately identifying sweet spots remains a significant challenge for the petroleum industry despite the growing amount of information available for unconventional hydrocarbon resources. These challenges may stem from the inorganic geochemical heterogeneities in source rock composition that can vary within a given basin over time. This study investigates the relationship between source rock composition and the resulting hydrocarbon expulsion, retention, and thermal maturation behavior through artificial maturation experiments on Late Cretaceous (Maastrichtian) Jordanian source rocks (JSR). The JSR is a carbonate-rich Type IIS source rock, which is compositionally similar to major Arabian unconventional prospects (Tuwaiq Mtn, Hanifa, and Shilaif/Natih Fms) as well as other major carbonate source rocks (Eagle Ford & La Luna Fms). However, it is thermally immature and, therefore, can be considered as an immature analog to the mature unconventional Type IIS source rock prospects.

This study utilized a thick JSR interval in the Al-Lajjun area of western Jordan, by using core samples from a 72 m long vertical well. Initial characterization of the source rock interval using bulk organic and inorganic geochemical parameters revealed three distinct geochemical cycles. Representative homogeneous plug samples from each cycle underwent artificial maturation experiments and showed differences in hydrocarbon expulsion and retention trends along with a difference in thermal maturity. Samples with higher silica content exhibited an early hydrocarbon expulsion as compared to Ca-dominated samples. The Ca-rich samples demonstrated a higher hydrocarbon retention and delayed expulsion at corresponding maturity stages as compared to the Si-rich samples. Additionally, the silica-rich samples also displayed lower Tmax values than the calcium-rich samples of similar thermal maturity.

The findings of this study highlight the significance of inorganic compositional heterogeneities within a source rock interval that can lead to the formation of multiple play fairways with varying hydrocarbon expulsion and thermal maturity characteristics. These insights emphasize the need for a more comprehensive understanding of source rock composition when assessing thermal maturity and identifying sweet spots for unconventional hydrocarbon exploration and production.