Clay minerals in carbonate reservoir rocks and their significance in porosity studies

Abstract

The writers have been studying the origin of porosity in the Neogene calcareous rocks of Akita, Japan, and in the Mississippian Windsor Group carbonate rocks of Nova Scotia, Canada. The former consist mainly of dolomitic clayey-siliceous marls, derived mainly from opaline skeletal debris of microscopic organisms. These marls were deposited in bathyal to inner-neritic environments. The carbonate rocks of Windsor Group, which include both dolostones and limestones, originated from biogenic carbonate debris and were deposited in inner-shelf to shoal environments. Study of the relationship between clay-mineral assemblages present and petrophysical properties of the carbonate rocks reveals that grain density is: ( 1) high in carbonate rocks containing kaolinite; ( 2) relatively low in montmorillonite-bearing rocks; and ( 3) low in carbonates poor in clay minerals content. Generally, absolute porosity of montmorillonite-bearing carbonate rocks is much higher than that of carbonate rocks in kaolinite and carbonate rocks poor in clay minerals. Kaolinite-containing carbonate rocks exhibit much higher grain density than rocks containing montmorillonite clay or those devoid of clay minerals. This could be explained by possible reaction between calcite (s.g. = 2.71) and some magnesium-bearing clays (e.g., montmorillonite, chlorite, vermiculite), which would give rise to dense dolomite (s.g. = 2.87) and kaolinite. Widely dispersed clay minerals (especially montmorillonite) present as cement within the pore spaces of lithified sediments could hold relatively large amounts of interlayer and adsorbed water and thus resist compaction. This may explain why montmorillonite-bearing carbonates commonly exhibit higher porosity.