Origin of Porosity in Neogene Carbonate Rocks of Akita, Japan

Abstract

Primary porosity in carbonate rocks originates from primary pore-spaces between and also within particles during deposition. Secondary porosity is a modification of primary porosity influenced by subsequent physical and chemical reactions after deposition. These reactions are compaction, fracturing, solution, cementation and inversions such as dolomitization, calcitization and so on.Oil-bearing Neogene carbonate strata of Akita, Japan, of bathyal to inner-neritic origin, are composed mainly of small skeletal debris. During syn-diagenesis, calcium, magnesium and carbon dioxide from both the sea-water and also from basin volcanic detritus altered the originally siliceous sediments to dolostone and dolomitic limestone.The most fundamental and significant pore-space reducing reaction in these carbonates was compaction. However, this was resisted by the presence of authigenic clay minerals. The next most important reaction tending to reduce porosity was cementation. Factors effective in increasing pore-space were inversione, such as dolomitization and transformations of opal to calcite, dolomite and quartz.