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The "Hole" Story: Organic and Inorganic Pores in Organic-Rich Shales of the Kimmeridge Clay Formation (Upper Jurassic), Offshore United Kingdom

Fishman, Neil1; Hackley, Paul2; Lowers, Heather1; Hill, Ronald3; Egenhoff, Sven4
1USGS, Denver, CO.
2USGS, Reston, VA.
3Noble Energy, Denver, CO.
4Colorado State University, Fort Collins, CO.

Analyses of organic-rich shales from 6 wells that penetrated the Upper Jurassic Kimmeridge Clay Formation (KCF), offshore United Kingdom, were performed to evaluate the nature (physical & chemical) of the organic material and to document changes in organic porosity as a function of thermal maturity. The KCF, which is at depths ranging from ~6,100 ft to ~15,300 ft (subsea), is thermally immature in the shallowest core samples, where TOC contents are as high as 10 wt%, Ro values are ~0.35%, and hydrogen indices (HI ) are high (>400). 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).

At least four distinct types of organic macerals were observed in petrographic and SEM analyses. A key element of this study was to determine the areal distribution of macerals prior to evaluating organic porosity to ensure that macerals were present across the study area. The macerals include, in decreasing abundance: 1) bituminite admixed with clays; 2) elongate (< 500 μm) lamellar masses (alginite or bituminite) with small (<0.5 μm) quartz, feldspar, and clay entrained within it; 3) discrete terrestrial (vitrinite) grains; and 4) Tasmanites microfossils.

Although organic pores have been observed on ion-milled surfaces of all samples, they vary as a function of maceral type, but, importantly, do not vary systematically as a function of thermal maturity. Pores in lamellar masses are irregularly-shaped and small (<0.1 μm across), whereas regularly shaped pores (<1 μm across) occur in terrestrial macerals. Irregularly-shaped pores (<0.3μm) exist in bituminite admixed with clay. Other pores (inorganic pores), particularly interparticle (i.e., between clay platelets (~1-2μm in length)), and intraparticle (i.e., in framboidal pyrite (<0.1μm across) and partially dissolved K-feldspar (<2μm across)) are present and noteworthy because they compose much of the observable porosity in the shales in both immature and mature samples.

The absence of a systematic increase in organic porosity as a function of either maceral type or thermal maturity and the relatively small size of organic pores indicates that such porosity was unrelated to hydrocarbon generation. Instead, much of the porosity within the KCF must be largely interparticle or intraparticle so the petroleum storage potential in these organic-rich shales largely resides in inorganic pores.


AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012