On the dissolution paths and formation mechanisms of paleokarst reservoirs: Constraints from reactive transport modeling

Abstract

Paleokarst is widespread in ancient carbonate rocks and is often related to hydrocarbon reservoirs, but the physico-chemical mechanism of forming such paleokarst reservoir remains poorly understood due to the complexity of meteoric water–rock interactions and a lack of geological research methods other than petrological analysis. Here, we implement the reactive transport modeling approach and reconstruct the dynamic evolution of the fluid–mineral–porosity system during paleokarst processes. In an eogenetic karst system, the early dissolution path is dominated by relatively uniform and diffuse dissolution in intergranular pores. The dissolution path then gradually transitions to preferential dissolution in local high-permeability zones, resulting in spongy dissolution. This is interpreted as the result of a combination of a heterogeneous initial pore distribution and the positive feedback between increasing flow and increasing dissolution. A higher degree of porosity heterogeneity and the presence of penecontemporaneous microfractures could facilitate the formation of preferential flow and spongy karst systems. Comparative simulations of eogenetic and telogenetic karst systems show different dissolution paths, formation mechanisms and distributions of hydrocarbon reservoirs, i.e., intergranular pore-dominated eogenetic karst with high mobility and high reactivity characteristics versus tectonic fault/fracture-dominated telogenetic karst with high mobility and low reactivity characteristics. The results provide a better general understanding of the water–rock interactions in meteoric diagenetic environments, and they indicate that hydrocarbon exploration should concentrate on carbonate shoals/mounds in the structural slope break zones associated with syngenetic faults, paleo-uplifts/highlands, and platform margins to identify high-quality eogenetic karst reservoirs and on tectonic fault/fracture systems and the lower parts of the high-permeability layers connected by these faults/fractures to identify high-quality telogenetic karst reservoirs.