T274 Paying Attention to Mudstones: Priceless!

Wednesday, 12 October 2011
1:30 PM - 5:30 PM
Room 211CD

THE “HOLE” TRUTH—POROSITY IN ORGANIC-RICH SHALES OF THE UPPER JURASSIC KIMMERIDGE CLAY FORMATION, OFFSHORE UNITED KINGDOM

Paper number: T274-6
Program: Technical Sessions
Time: 2:45 PM – 3:00 PM

Authors

Abstract

The mechanisms by which oil and(or) gas are stored in “unconventional” shale reservoirs remains uncertain, although recent studies suggest that a significant amount of secondary porosity develops in kerogen concomitant with petroleum generation. To better understand porosity development in organic-rich shales, an investigation was undertaken on cores from 6 wells that penetrated the Upper Jurassic Kimmeridge Clay Formation (KCF), offshore United Kingdom, where it is progressively buried to depths of ~6,100 ft to ~15,300 ft (subsea). In the shallowest core, KCF shales contain TOC contents up to 10 wt %, and the rocks are immature to marginally mature based on relatively high (>400 mg hydrocarbon/g rock) hydrogen indices (HI) and vitrinite reflectance (Ro) values of ~ 0.6% of the mostly marine algal organic material. In the deepest core, TOC contents of KCF shales are still high (as much as 8 wt %), but the shales are thermally mature based on low HI (<30 mg hydrocarbon/g rock) and high Ro values (~1.2%). The KCF demonstrates intermediate HI and Ro values where it is buried between 6,100 and 15,300 ft.

Preliminary petrographic and SEM observations of rock chips, polished thin sections, and ion-milled samples reveal both primary and secondary pores. Primary, interparticle porosity exists between detrital grains, particularly illite flakes or platelets in all cores; however, with increasing depths, compaction resulted in increased alignment of clay platelets and a commensurate reduction in pore sizes, with the smallest pores (<<1 µm across) in the deepest samples. Secondary, intraparticle pores between pyrite crystallites in framboids are common at all depths, but pores are typically small (<0.5 µm across). Additional secondary porosity resulted from partial dissolution of detrital potassium feldspar, with dissolution most extensive where the KCF is more deeply buried. Some micropores and nanopores were observed in organic material, but a systematic increase in organic porosity as a function of depth and thermal maturation has not yet been documented for the KCF.

Although much of the porosity in KCF shales is secondary, the volume of organic porosity remains uncertain. Nevertheless, secondary porosity likely plays a critical role in the overall storage capacity for hydrocarbons in KCF shales.