Evolution of electrical resistivity and NMR during early-stage maturation of an organic-rich chalk

Abstract

This paper studies the changes in the electrical resistivity of an organic-rich source rock chalk containing a Type-IIs kerogen during early-stage maturation of the kerogen. As bitumen is generated during maturation, the chalk changes from water wet to a complex mixed-wet pore system, with the micritic pore space remaining water wet while the bitumen expelled into the macro pores converts some pores to oil wet. This mixed-wet system results in non-Archie electrical behavior with pronounced curvature of the I-Sw curve. We find that this curvature is well fit by connectivity theory. The connectivity theory parameters are determined both on native state core and from artificial laboratory pyrolysis which generates bitumen. NMR is used to distinguish the brine in micritic, water-wet pore spaces from that in the intergranular, mixed-wet pore spaces, from which the observed connectivity parameters may be independently estimated. Comparison between electrical resistivity and dielectric logs show that the connectivity parameters match the laboratory results over a calcareous section more than 300 m thick.