Microporous and tight limestones in the Urgonian Formation (late Hauterivian to early Aptian) of the French Jura Mountains: Focus on the factors controlling the formation of microporous facies Chadia Volery a, ⁎, Eric Davaud a , Christophe Durlet b , Bernard Clavel c , Jean Charollais a , Bruno Caline d a University of Geneva, Earth and Environmental Sciences, Department of Geology and Paleontology, Rue des Maraîchers 13, 1205 Genève, Switzerland b University of Bourgogne, UMR CNRS 5561 Biogeosciences, Blvd Gabriel 6, 21000 Dijon, France c Ch. des Champs d'Amot 24, 74140 Messery, France d Total Exploration and Production, CSTJF, Avenue Larribau, 64000 Pau, France abstract article info Article history: Received 2 March 2010 Received in revised form 22 June 2010 Accepted 24 June 2010 Available online 1 July 2010 Editor: B. Jones Keywords: Microporosity Micrite Carbonate Reservoir Urgonian Microporous and tight limestones, with contrasting porosity and permeability values directly related to the microfabric of the micritic matrix, outcrop in the Urgonian Formation of the French Jura Mountains. This study investigates the factors controlling the differentiation between the microporous and tight facies, and proposes a diagenetic model for the development and preservation of the microporosity in these limestones. The petrophysical properties are not related to the depositional texture, the petrographical content or the mineralogical composition. However, the tight layers contain indications of emersion (e.g.: bird eyes, keystone vugs, and desiccation cracks). The sedimentation in very shallow conditions up to emersion is confirmed by the covariant more positive values of oxygen and carbon isotopes. The microporous intervals systematically occur a few meters below the tight layers affected by emersion. This position strongly suggests the importance of meteoric water input rapidly after sedimentation in the differentiation between tight and microporous limestones. The diagenetic model proposed for the development and preservation of the microporous facies involves partial early cementation of the interstitial mud, mainly composed of low-Mg calcite crystals (sedimentation during a calcite sea period), inside a meteoric phreatic lens by in situ dissolution–reprecipitation processes (“hybrid Ostwald ripening”). This early cementation partly preserves the original microfabric and intercrystalline microporosity and allows the carbonate sediment to resist compaction during burial. The identification of the conditions favorable to the development of microporosity in these Urgonian limestones may improve the knowledge and modeling of some microporous carbonate reservoir rocks. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Limestones characterized by a microporous intercrystalline frame- work made up of sub-rhombic low-Mg calcite crystals with sizes generally smaller than 8 μm (micrites, lato sensu) account for many carbonate reservoirs, especially in the Middle East (Alsharhan and Nairn, 2003; Volery et al., 2009). The mean porosity and permeability of these reservoirs rate about 20% and 100 mD, respectively. However, despite a considerable economic interest, the genesis of microporous limestones remains poorly understood. Some studies have investi- gated these rocks (e.g.: Budd, 1989; Kaldi, 1989; Moshier, 1989; Perkins, 1989; Lambert et al., 2006; Richard et al., 2007), but little is known about the main factors involved in the development of the intercrystalline microporosity. Recently, an inventory of shallow-marine microporous carbonate formations in the Middle East revealed that microporous limestones developed during periods of calcite seas (Volery et al., 2009). A precursor mud mainly composed of low-Mg calcite crystals was necessary to form microporous limestones. This essential prerequisite was confirmed with the study of Late Miocene lacustrine microporous micrites from the Madrid Basin presenting identical matrix micro- fabric to the Cretaceous marine microporous limestones of the Mishrif Formation in the Middle East (Volery et al., 2010). In the Jura Mountains, the Urgonian Formation (late Hauterivian to early Aptian) constitutes thick limestones deposited during a period of calcite seas (Sandberg, 1983; Hardie, 1996; Lowenstein et al., 2001; Dickson, 2002) and characterized by several microporous horizons (Richard et al., 2007). One of them, recently exposed in the region of Sedimentary Geology 230 (2010) 21–34 ⁎ Corresponding author. University of Geneva, Earth and Environmental Sciences, Dept. of Geology and Palaeontology, Rue des Maraîchers 13, 1205 Genève, Switzerland. Tel.: +41 22 0 79 750 30 58 (mobile); fax: +41 0 22 379 32 10. E-mail addresses: Chadia.Volery@unige.ch, chadia@infomaniak.ch (C. Volery), Eric.Davaud@unige.ch (E. Davaud), b.clavel1@orange.fr (B. Clavel), jdcharollais@bluewin.ch (J. Charollais), bruno.caline@total.com (B. Caline). 0037-0738/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2010.06.017 Contents lists available at ScienceDirect Sedimentary Geology journal homepage: www.elsevier.com/locate/sedgeo