We revealed detailed depositional environments and a controlling factor of CO₂ injectivitiy in terms of grain-size and pore-throat size distributions of the CO₂ storage aquifer of the Nagaoka CO₂ storage site, Japan. The target reservoir rock consists of six sedimentary facies, which is interpreted as prodelta to deltafront deposits. The high injectivity zones correspond to the distributary mouth bar deposits in a deltafront (Facies 3), whereas the low injectivitiy zones correspond to the distributary mouth channel deposits in a deltafront (Facies 2) and prodelta deposits (Facies 5). On the basis of scatter plots of median size versus sorting value of grain-size distributions, Facies 3 is characterized by well-sorted coarser grains. Moreover, pore-throat size distribution of Facies 3 shows larger pore than about 10 phi, indicating a high-permeability networks, whereas those of Facies 2 and 5 show dominant smaller pore-throat sizes. These grain-size and pore-throat features associated with depositional environments appear to be a controlling factor of CO₂ injectivity.

The bed-thickness of sedimentary beds and its frequency distribution in a geological body have been treated to show a lognormal distribution. In this paper it is discussed on a basis of a model presented by a great Russian mathematician, Prof. Kolmogorov, A.N. who said that a sedimentary bed has a thickness of so-and-so cm, but in some cases, it has a negative thickness. Actually, the presence of a washed -out phenomena will erode the pre-existing sedimentary layer, and sometimes the pre-existing will completely disappear. For such a washed-out bed, Kolmogorov described that it has a negative thickness. In this paper, an example for the Neogene Kakegawa Formation and its bed-thickness is presented, and additionally the bed-thickness of varved diatomite in the Hiruzenbara Formation of Okayama Prefecture is noted as an example of rather accumulative type of sedimentation.