In this study, we conducted rock/fluid experiments to investigate wettability of calcareous shale plugs from a well drilled in the early oil-window (EOW) of East Duvernay. The wettability of EOW plugs was compared with that of highly-mature and quartz-rich plugs in the oil and gas windows (OGW) of the Duvernay Formation to investigate the effects of kerogen maturity and mineralogy on pore morphology and wettability of shales. We investigated the effects of organic-pore connectivity and fractures on wettability of the EOW plugs. By using CT scan images, we divided EOW plugs into highly-fractured (HF), slightly-fractured (SF), and non-fractured (NF) plugs. We used reservoir oil and brine and conducted comparative imbibition tests on the core plugs to investigate effects of fracture intensity on imbibition profiles. The core plugs were characterized by analyzing the results of tight-rock analysis (TRA), x-ray diffraction (XRD), and rock-eval pyrolysis.

Compared with the quartz-rich OGW plugs, the EOW plugs are categorized as calcite-rich shale (calcareous shale) with high average calcite content of 60%. The EOW plugs are rich in organic matter (average total organic carbon (TOC) of 7.3 wt%) with significantly high value of Hydrogen Index (HI > 500). Surprisingly, the results of wettability tests show higher normalized imbibed volume of brine compared with that of oil, suggesting that the EOW plugs are preferentially water-wet. This trend is opposite to what we previously observed for the oil-wet OGW plugs with significantly high organic porosity, positively correlated to TOC content. We did not observe well-developed organic pores within organic matter of less-mature EOW plugs. We also observed that the normalized imbibed volume of oil is much higher in the HF and SF plugs compared with that in the NF plugs. The results suggest that the fractures enhance accessibility of isolated pores, leading to more connected pore network for oil imbibition. This observation suggests that fracture porosity plays a significant role in wetting behavior of the EOW plugs. The results show that the porosity measured by Boyle’s law helium-porosimetry using crushed EOW samples is significantly higher than their effective porosity. This is because crushing the samples enhances accessibility of isolated pores considered as ineffective porosity under intact conditions. Combined analyses of imbibition profiles and core images of the fractured plugs show that oil rapidly imbibes into the fracture system, and then gradually imbibes from fractures into rock matrix.

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