Upper Permian (Zechstein) microbialites: Supratidal through deep subtidal deposition, source rock, and reservoir potential Mirosław Słowakiewicz, Maurice E. Tucker, Richard D. Pancost, Edoardo Perri, and Michael Mawson
ABSTRACT Zechstein 2 (Z2) carbonate microbialites flourished under arid paleoclimatic conditions in the Late Permian. Microbial carbonates from the Roker Formation outcrop in northeast England, with its subsurface equivalent being the Main Dolomite from northwest–central Poland. The Z2 carbonate deposits developed in supratidal through deep subtidal zones and consist of various stromatolites and thrombolites. Planar stromatolites and thrombolites characterize intertidal and supratidal facies, and biohermal stromatolites with oolitic grainstone and crinkled stromatolites typify shallow subtidal facies. The Z2 subtidal and/or intertidal microbialites with oolites form complexes more than 10 m (33 ft) thick and are important reservoir facies for hydrocarbons. Subtidal (slope) and intertidal (lagoonal) microbial mudstone and wackestone have poor reservoir properties but contain total organic carbon as much as 2 wt. % and are considered as potential source rocks. The thermal maturity assessed from C27 17a-trisnorhopane (Tm) and C27 18a-trisnorhopane (Ts) as the Ts/ (Ts + Tm) ratio, C30 moretane/hopane ratio, sterane ratio expressed as 20S/(20S + 20R), and bb/(bb + aa) ratio shows to indicates a mature character of organic matter with respect to oil generation.
INTRODUCTION Microbial carbonates (microbialites) resulting from organomineralization (microbially induced or microbially derived) processes are a major component of shallow-water Upper Permian Zechstein 2 (Z2) carbonates, having been reported from the Danish (Clark and Tallbacka, 1980; Stemmerik and Frykman, 1989), German (Mausfeld and Zankl, 1987; Strohmenger et al., 1996), and Polish (Słowakiewicz
Copyright ©2013. The American Association of Petroleum Geologists. All rights reserved. Manuscript received October 26, 2012; provisional acceptance February 6, 2013; revised manuscript received April 22, 2013; final acceptance June 18, 2013. DOI:10.1306/06181312179
AAPG Bulletin, v. 97, no. 11 (November 2013), pp. 1921–1936
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AUTHORS Mirosław Słowakiewicz Organic Geochemistry Unit, Bristol Biogeochemistry Research Centre, and the Cabot Institute, School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, United Kingdom; Polish Geological Institute, Polish Geological Survey, ul. Rakowiecka 4, 00-975 Warsaw, Poland;
[email protected] Mirosław Słowakiewicz holds a Ph.D. in geology from the AGH University of Science and Technology, Kraków, Poland. His work includes organic matter preservation and alteration, source rock formation in modern and ancient carbonateevaporite systems, carbonate sedimentology and petrography of microbial systems, and paleoclimatic changes of the ancient world. Maurice E. Tucker Department of Earth Sciences, University of Bristol, Bristol BS8 1RJ, United Kingdom; former address: Department of Earth Sciences, Durham University, Durham DH1 3LE, United Kingdom;
[email protected] Maurice Tucker is interested in the deposition and diagenesis of limestones and their sequence and cycle stratigraphy; dolomitization; microbialites, especially tufa; and, occasionally, evaporites. It does not matter what age or where in the world they occur as long as they fizz. Richard D. Pancost Organic Geochemistry Unit, Bristol Biogeochemistry Research Centre, and the Cabot Institute, School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, United Kingdom;
[email protected] Richard Pancost is a professor of biogeochemistry and obtained his Ph.D. in geosciences from Pennsylvania State University. His work focuses on the application of organic geochemical methodologies to the understanding of past climatic and environmental change, the cycling and preservation of organic matter, and methane cycling in modern and ancient settings. Edoardo Perri Dipartimento di Scienze della Terra, Università della Calabria, Via P. Bucci Cubo 15b, 87036 Rende, Italy;
[email protected] Edoardo Perri holds a Ph.D. in geology from the University of Calabria, where he is an assistant
professor of geology. His research and teaching interests include ancient and modern marine and continental carbonate sedimentary systems, with particular interest in the nano- to macroscale study of microbially mediated carbonate deposits. Michael Mawson Department of Earth Sciences, Durham University, Durham DH1 3LE, United Kingdom;
[email protected] Michael Mawson graduated in geology from Durham University (United Kingdom), where he later also undertook his Ph.D. research after a short spell working in the oil industry. His research interests are mostly in the field of carbonate sedimentology, particularly in Upper Permian Zechstein Group carbonates, the subject of his doctoral thesis. He also has a keen interest in evaporites and works as a geologic consultant.
ACKNOWLEDGEMENTS The outcrop data from northeast England presented in this paper were obtained by Michael Mawson for his Ph.D. thesis for which he thanks his family for help as well as the late Denys Smith for kind assistance. This research was also supported by a Mobility Plus Programme postdoctoral fellowship (MS) of the Ministry of Science and Higher Education of Poland and the Royal Dutch Shell. We thank William C. Parcell, Raymond W. Mitchell, and an anonymous reviewer for their useful remarks that improved the final manuscript. Richard D. Pancost thanks the Royal Society Wolfson Research Merit Award. We also thank Katarzyna Sobień for help in the field. The AAPG Editor thanks the following reviewers for their work on this paper: Raymond W. Mitchell, William C. Parcell, and an anonymous reviewer.
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and Mikołajewski, 2011) parts of the southern Permian Basin (SPB). The Z2 carbonates are well exposed in northeast England (Roker Formation) and, locally, in Germany (Stassfurt Carbonate), but most of them occur in the subsurface where they form important oil- and gas-bearing successions in the Netherlands, Germany, Denmark, and Poland (Doorneball and Stevenson, 2010). The Late Permian had a greenhouse climate, with minimum summer temperatures on land in mid-latitudes of the northern hemisphere more than 15°C higher than the present day (Khiel and Shields, 2005). This favored the deposition of carbonates, as well as evaporites, in the epicontinental Zechstein sea under the warm and arid paleoclimatic conditions (e.g., Słowakiewicz et al., 2009), which would have resembled the modern Trucial Coast of Abu Dhabi with its sabkhas and adjoining Persian Gulf (Purser, 1973; Kendall and Alsharhan, 2011). The climate also promoted the deposition of petroleum reservoir and source rocks in the Z2 carbonates, which are particularly important in the Polish part of the SPB (Doorneball and Stevenson, 2010). The Late Permian climate and related nutrient supply controlled primary productivity and the rate of organic matter generation, with the arid areas of the Z2 time generally being more productive (Słowakiewicz and Gąsiewicz, 2013). The pioneering work of Gerling et al. (1996) on Z2 carbonates from the German part of the SPB demonstrated the occurrence of both reservoir and source rocks within the same Z2 carbonate succession. They distinguished two types of source rocks: (1) dolomitic and calcareous microbially derived mudstone deposited on the lower slope and in lagoons, and (2) basinal calcareous mudstone. Additionally, based on organofacies types and the stable carbon isotopic composition of kerogen, they deduced the occurrence of a chemocline (below the middle slope) and three sources of organic matter: (1) terrigenous organic matter, dominating in basinal facies; (2) phytoplanktonand zooplankton-derived organic matter, dominating in slope facies; and (3) microbial-algal organic matter in lagoonal facies. The classification of the Z2 source rocks proposed by Gerling et al. (1996) was also adopted by Kotarba et al. (2000, 2003) and Kotarba and Wagner (2007), although the latter noted the importance of microbial-algal source rocks. However, Słowakiewicz and Mikołajewski (2011) suggested that microbially derived facies are effective source rocks mainly produced by cyanobacteria. Słowakiewicz and Gąsiewicz (2013) and Gerling et al. (1996) ruled out basinal facies as significant source rocks in view of their low total organic carbon (TOC) values (
