The effects of kerogen maturity on pore structure and wettability of organic-rich calcareous shales

Abstract

Organic-rich shales are considered important unconventional resources nowadays. The primary objective of this research is to evaluate the hydrocarbon potential in calcareous shale plugs by investigating their wettability and pore structure. We conducted spontaneous imbibition (SI) tests on calcareous shale plugs with low maturity level from 5 wells drilled in the early-oil and oil windows in East Shale Basin (ESB) of the Duvernay Formation. We compared the SI results with those of highly-mature siliceous plugs in the oil and gas windows in the West Shale Basin (WSB) of the Duvernay Formation as well as their petrophysical properties. We investigated the relationships between kerogen maturity and rock mineralogy on SI results and pore structure of organic-rich shales by analyzing SEM images and low-pressure gas sorption (LPGS) test results.The ESB plugs are categorized as calcareous shales that are rich in organic matter (average total organic carbon (TOC) of 5.5 wt%). The results of SI tests show that ESB plugs are preferentially water-wet with exceptions in plugs with higher kerogen maturity. The oil wettability index (WIo) of ESB plugs is positively correlated with production index (PI) and Tmax, but not with TOC content. The LPGS test results show that micro and fine mesopores (< 10 nm) within organic matter (OM) are more abundant in plugs with higher kerogen maturity. The lack of well-developed organic pores within OM of the early-mature ESB plugs is also confirmed with SEM images. Our results suggest that ESB plugs with low maturity level in the early-oil window have low oil potentials despite their TOC-rich characteristics. One may use this finding for selecting sweet spots for shale oil production by evaluating kerogen maturity level of the organic deposits.