The nature of porosity in organic-rich mudstones of the Upper Jurassic Kimmeridge Clay Formation, North Sea, offshore United Kingdom Neil S. Fishman a, ⁎ , 1 , Paul C. Hackley b , Heather A. Lowers a , Ronald J. Hill c , Sven O. Egenhoff d , Dennis D. Eberl e , Alex E. Blum e a U.S. Geological Survey, Denver, CO, USA b U.S. Geological Survey, Reston, VA, USA c Noble Energy, Denver, CO, USA d Colorado State University, Fort Collins, CO, USA e U.S. Geological Survey, Boulder, CO, USA abstract article info Article history: Received 1 February 2012 Received in revised form 24 July 2012 Accepted 27 July 2012 Available online 3 August 2012 Keywords: Kimmeridge Organic porosity Microporosity Nanoporosity Ar-mill Maceral Maturation Analyses of organic-rich mudstones from wells that penetrated the Upper Jurassic Kimmeridge Clay Forma- tion, offshore United Kingdom, were performed to evaluate the nature of both organic and inorganic rock constituents and their relation to porosity in this world-class source rock. The formation is at varying levels of thermal maturity, ranging from immature in the shallowest core samples to mature in the deepest core samples. The intent of this study was to evaluate porosity as a function of both organic macerals and thermal maturity. At least four distinct types of organic macerals were observed in petrographic and SEM analyses and they all were present across the study area. The macerals include, in decreasing abundance: 1) bituminite admixed with clays; 2) elongate lamellar masses (alginite or bituminite) with small quartz, feldspar, and clay entrained within it; 3) terrestrial (vitrinite, fusinite, semifusinite) grains; and 4) Tasmanites microfossils. Al- though pores in all maceral types were observed on ion-milled surfaces of all samples, the pores (largely nanopores with some micropores) vary as a function of maceral type. Importantly, pores in the macerals do not vary systematically as a function of thermal maturity, insofar as organic pores are of similar size and shape in both the immature and mature Kimmeridge rocks. If any organic pores developed during the generation of hydrocarbons, they were apparently not preserved, possibly because of the highly ductile na- ture of much of the rock constituents of Kimmeridge mudstones (clays and organic material). Inorganic pores (largely micropores with some nanopores) have been observed in all Kimmeridge mud- stones. These pores, particularly interparticle (i.e., between clay platelets), and intraparticle (i.e., in framboidal pyrite, in partially dissolved detrital K-feldspar, and in both detrital and authigenic dolomite) are 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 ma- turity indicates that such porosity was probably unrelated to hydrocarbon generation. Instead, much of the porosity within mudstones of the Kimmeridge appears to be largely intraparticle and interparticle (adjacent to inorganic constituents), so the petroleum storage potential in these organic-rich mudstones largely resides in inorganic pores. Published by Elsevier B.V. 1. Introduction Intense research on mudstones (shales) in recent years, particularly those containing abundant organic carbon, has been driven by the de- sire to better characterize these fine-grained rocks in order to gain a more comprehensive understanding of them as unconventional petroleum (gas, oil, or natural gas liquids) reservoirs. Technological ad- vances have resulted in the successful completion of thousands of wells in the last few years that now produce petroleum from these low po- rosity/low permeability unconventional reservoirs, also referred to as shale gas or shale oil reservoirs. In fact, in the United State, the volume of proven reserves of natural gas increased 11% between 2008 and 2009, largely due to the development of shale gas (Energy Information Administration, 2010). Furthermore, oil and natural gas liquids (e.g., propane, butane, etc.) are increasingly being produced from mudstones. Overall, petroleum exploration in these International Journal of Coal Geology 103 (2012) 32–50 ⁎ Corresponding author. E-mail address: nfishman@hess.com (N.S. Fishman). 1 Current address: Hess Corp., Houston, TX, USA. 0166-5162/$ – see front matter. Published by Elsevier B.V. http://dx.doi.org/10.1016/j.coal.2012.07.012 Contents lists available at SciVerse ScienceDirect International Journal of Coal Geology journal homepage: www.elsevier.com/locate/ijcoalgeo