Characteristics and implication of multilayer dawsonite in heterogeneity reservoir of the Honggang anticline, southern Songliao Basin, NE China

Abstract

Reservoir heterogeneity is one of the key factors regarding the long-term fate of the injected CO2. Natural CO2 reservoir and ancient CO2-bearing reservoir is a natural analog to investigate the influence of reservoir heterogeneity on the migration of CO2 and CO2-rock interaction with geological timescale. Dawsonite cements are detected in both sandstone and mudstone of the Upper Cretaceous Qingshankou reservoir in Honggang anticline of the southern Songliao Basin, China. Here, we present results of a petrographic characterization of this reservoir based on polarizing microscope, X-ray diffraction, and fluid inclusion data. These data were used to identify the vertical distribution characteristic of dawsonite and to identify the migration characteristics for the supercritical CO2 in heterogeneity reservoir. Our analytical results show that as the “CO2 trace mineral,” dawsonite appears as multilayered zones of cementation that are separated by mudstone interlayers. These multilayered dawsonite could be one of the geological produces for supercritical CO2 flooding through the heterogeneous rock, acting as the lateral migration and upward diffusion. Supercritical CO2 could move through the thinner mudstone interlayer, with the product of dawsonite developing in the mudstone as its “footprint,” although the vertical distance of diffusion in low-permeability caprock is limited. Combined with the truth that most of the present CO2 gas reservoirs (in K1q4) are in the deeper layers than the dawsonite-bearing sandstone (K1q4-K2y1) (developed in non-CO2 reservoir), we can deduce that CO2 was at some time abundant in the Honggang anticline but had now part of CO2 been migrated, with the other part consumed by the reaction with the primary rock and captured as new carbonate minerals. The dawsonite-bearing sandstones also record a sequence of hydrocarbon filling events. Combined with the truth that the injection time of CO2 is later than that of hydrocarbon, the early hydrocarbon should be deasphalted by the injection and migration of mantle CO2. Therefore, CO2 could also be stored as long-term carbonate minerals after the termination of a CO2-EOR project.