A methodology for estimating the organic porosity of the source rocks at the mature stage: example from the marlstone in the Shulu Sag, Bohai Bay Basin

Abstract

In recent years, organic-matter pores have been identified as an important storage space type in unconventional reservoirs. Accurate estimation of the organic porosity is critical for unconventional petroleum evaluation and exploration. Organic-matter pores account for an important proportion of the pore network in the Shulu organic matter-rich marlstone. This study determined the characteristics of organic pores in this tight reservoir and provided a method to restore the original hydrogen index (HIo) of samples within the oil window zone, based on the relationship between the hydrogen index (HI) and temperature of the maximum yield of pyrolysate (Tmax). The original total organic carbon (TOCo) and the organic porosity (Φo) were estimated based on data from the pyrolysis experiment using a mass-balance relation. The results show that the value of HIo is in the range of 64 to 1048 mg HC/g TOC, with an average of 605 mg HC/g TOC. The value of TOCo is between 0.12 and 9.7 %,with an average of 1.9 %. The value of Φo is mainly from 0 to 2.5 %, with an average of 0.67 %,which is approximately 16.8 % of the total porosity. The calculated value of Φo is consistent with that obtained by SEM image-based organic porosity measurements. The approach outlined in this article involves a relatively simple methodology for estimating the organic porosity of kerogen-rich source rocks within the oil window zone, which is useful in the exploration and development of unconventional petroleum resources.