Porosity in Shales of the Organic-Rich Kimmeridge Clay Formation (Upper Jurassic), Offshore United Kingdom

Petrographic, SEM, and RockEval pyrolysis analyses of organic-rich shale samples from 6 wells that penetrated the Upper Jurassic Kimmeridge Clay Formation (KCF), offshore United Kingdom, were performed to evaluate the nature (physical and chemical) of the organic material and to document changes in organic porosity as a function of increasing thermal maturity. The formation is at depths ranging from ~6,100 ft to ~15,300 ft (subsea). It is thermally immature to marginally mature in the shallowest core samples, where total organic carbon (TOC) contents are as high as 10 wt%, vitrinite reflectance (Ro) values are ~0.6%, and hydrogen indices (HI ) are high (>400 mg hydrocarbon/g rock). In contrast, it is thermally mature in the deepest core (Ro values ~1.2%), with high TOC contents (as much as 8 wt%) but low HI values (<30 mg hydrocarbon/g rock). In addition, the KCF has intermediate HI and Ro values in other core samples.

At least four distinct types of organic components were observed in petrographic and SEM analyses, which are, in decreasing abundance: 1) amorphous organic material admixed with clay platelets (as much as 20 μm long); 2) elongate (up to 300 μm) mat-like masses (micro-algal mat?) with small (<0.5 μm) quartz, feldspar, and clay entrained within it; 3) discrete particles (possibly alginate?); and 4) Tasmanites microfossils. Regardless of depth and thermal maturity, the following observations were made of porosity in shales of the KCF. On ion-milled surfaces, there are irregular-shaped micropores and nanopores (0.1-0.01 μm across) in some mat-like masses, whereas regularly shaped micropores (up to 1 μm across) are in the discrete organic particles. Other types of pores, particularly interparticle (i.e., between illite flakes or platelets as well as between authigenic quartz euhedra), and intraparticle (i.e., between crystallites in framboidal pyrite) are also present and are noteworthy because they compose much of the observable porosity in the KCF shales.

No systematic increase in organic porosity was observed in any organic material within the KCF with increasing depth and thermal maturity. As such, organic porosity does not contribute significantly to overall pore volume in the KCF, even in organic-rich shales that are thermally mature. Therefore, the petroleum storage potential in the formation appears to reside largely within interparticle and intraparticle pores between or within inorganic components of the shales, respectively.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California