Estimation of organic and inorganic porosity in shale by NMR method, insights from marine shales with different maturities

Abstract

The porosity system of shale reservoir contains organic pore and inorganic pore, of which the gas adsorption characteristics and wettability properties differ dramatically. In order to conduct estimation of organic pore and inorganic pore in shale reservoir, six marine shale samples were selected to quantify the proportion (vol%) of different pores by using the new method, in which the NMR T2 spectra of samples saturated with deionized water and kerosene were respectively acquired, representing pore size distribution of hydrophilic pore and lipophilic pore correspondingly. The NMR T2 signals of different saturating fluids were introduced to calculate the proportion (vol%) of organic pore and inorganic pore. Further, the observation of argon milled image of shale samples which can give statistical analysis is also conducted accordingly. The results from the two methods are almost in agreement, indicating feasibility of the new method. By using the method in shales with varying maturities, the proportions of organic and inorganic pores of shales in different evolution stages were figured out. Generally, the TOC is positively correlated with organic porosity that almost an organic porosity of 1.51–1.76% (vol.) is provided for per unit TOC (1.0%, wt.) in low-mature shale. And the porosity increase caused by per unit TOC (1.0%, wt.) during organic matter maturation is 2.53–3.41%, while the proportion of inorganic porosity continuously decreases with thermal maturity. From the study, it is significant that the organic porosity and inorganic porosity of shale with given TOC and thermal maturity can be roughly estimated.