What happens to the petrophysical properties of a dual-porosity organic-rich chalk during early-stage organic maturation?

We observe a set of significant petrophysical changes at a critical stage of organic maturation in samples measured from an organic-rich (up to ~18 wt% TOC) source rock. The observations are from the same Late Cretaceous, Ghareb-Mishash formation in Israel that display various stages of organic maturation. The Ghareb-Mishash samples were cored from three wells from two basins at depths from ~300 to 1200 m. Among the observed changes at this critical stage of organic maturation: electrical resistivity increases sharply and displays non-Archie behavior; permeability decreases sharply with a steeper Klinkenberg-corrected gas permeability-porosity relationship; and NMR T2 distributions display two well-separated peaks rather than one. To explain these observations, we analyze the maturity of the organic-rich chalk using Rock-Eval and LECO analysis of the samples before and after bitumen extraction to determine bitumen content and maturity (measured as Tmax). We find that at a critical Tmax for this Type-IIS kerogen (S/C>0.04), the rate of bitumen and light hydrocarbon generation increases sharply. SEM studies show that the chalk contains micritic pores as well as intergranular macropores, and the macropores contain the organic matter. The occurrence of the double peak in the NMR data indicates the shift from strong to weak diffusional coupling and thus the isolation of the micritic pore space from the macropore space for both molecular diffusion and fluid flow. In this dual-porosity Type-IIS self-sourcing chalk, enough bitumen is generated during maturation so that the macropore surfaces become more oil-wet and bitumen blocks the entrances to the micritic pore space. This interpretation of bitumen and light hydrocarbons in the macropore system represents a consistent understanding of the different petrophysical responses, with important exploration and production implications.