Constraining the genetic relationships of 25-norhopanes, hopanoic and 25-norhopanoic acids in onshore Niger Delta oils using a temperature-dependent material balance Umar A. Lamorde a,b,⇑ , John Parnell b , Stephen A. Bowden b a Exploration Section of Research and Development Division of Nigerian National Petroleum Corporation, 4-9 Moscow Road, Port Harcourt, Nigeria b Geofluids Research Group, Department of Geology and Petroleum Geology, University of Aberdeen, Meston Building, Aberdeen AB24 3UE, UK article info Article history: Received 13 June 2014 Received in revised form 4 December 2014 Accepted 8 December 2014 Available online 13 December 2014 Keywords: Biodegradation Crude oil Hopanes 25-Norhopanes Hopanoic acids 25-Norhopanoic acids Mass balance Niger Delta abstract Analysis of oil samples from the Niger Delta (Nigeria) revealed a range of structurally related hopanes, including 25-norhopanes, and hopanoic and 25-norhopanoic acids. 25-Norhopanes were detected in all medium and heavily biodegraded oils and were most abundant in the heavily degraded oils. Hopanoic acids (C 30 -C 33 ) and 25-norhopanoic acids (C 30 -C 31 ) were most abundant in moderately degraded oils and occurred in reduced concentration in heavily degraded oils but were absent from, or in trace concen- tration in, slightly degraded oils. Consideration of the structures suggests that 25-norhopanoic acids form via carboxylation of 25-norhopanes or demethylation of hopanoic acids. Mass balance for the onshore Niger Delta oils suggests that formation of 25-norhopanes operates independently of 25-norhopanoic acid formation and that 25-norhopanoic acids are likely transient intermediates for only a small propor- tion of the 25-norhopanes. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction The geochemical derivatives of hopanoids are abundant in many crude oils and rock extracts (e.g. Ourisson et al., 1987; Ourisson and Albrecht, 1992; Kannenberg and Poralla, 1999). The extensive biosynthesis of hopanoids by bacteria (Ourisson et al., 1979; Rohmer et al., 1984), coupled with their geochemically sta- ble skeleton, causes them to be ubiquitous in sediments (Ourisson et al., 1982, 1987). Their presence in both crude oils and rock extracts has made their product pentacyclic terpanes use- ful in petroleum geochemistry. Although hopanes are geologically stable under many conditions, numerous studies have shown that the concentrations of hopane and extended hopanes may decrease during biodegradation, whereas 25-norhopanes increase (Seifert and Moldowan, 1979; Volkman et al., 1983; Peters and Moldowan, 1991; Moldowan and McCaffrey, 1995; Bennett et al., 2006; Wang et al., 2013). Thus, 25-norhopanes are generally regarded as indicators of, if not products originating from, the biodegradation of oil and bitumen. However, the latter idea remains contested and three hypotheses may still be postulated to explain the origin of 25- norhopanes in crude oil: (i) microbial demethylation of hopanes via removal of methyl from the A/B ring C-10 position (Seifert and Moldowan, 1979; Peters and Moldowan, 1991; Peters et al., 2005); (ii) small quantities of 25-norhopanes derived from source rocks and nonbiodegraded oils become enriched during biodegra- dation due to their greater resistance to biodegradation than hopanes (Chosson et al., 1992; Blanc and Connan, 1992), and (iii) the microorganisms responsible for the severe degradation of petroleum produce 25-norhpanes independently of hopane degradation. Many authors have supported the formation of 25-norhopane through demethylation of hopane during petroleum biodegrada- tion and the process has been well described (Seifert and Moldowan, 1979; Rullkötter and Wendisch, 1982; Volkman et al., 1983; Requejo and Halpern, 1989; Peters and Moldowan, 1991; Moldowan and McCaffrey, 1995; Peters et al., 1996; Tocco and Alberdi, 2002; Bennett et al., 2006; Wang et al., 2013). However, the occurrence of 25-norhopanes in source rocks and non- degraded oil is not disputed (Noble et al., 1985; Blanc and Connan, 1992; Chosson et al., 1992). They have not been shown to be covalently bound to kerogen or asphaltenes (Peters et al., http://dx.doi.org/10.1016/j.orggeochem.2014.12.004 0146-6380/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author at: Research and Development Division, Nigerian National Petroleum Corporation, 4-9 Moscow Road, Port Harcourt, Nigeria. E-mail address: ulamorde@yahoo.com (U.A. Lamorde). Organic Geochemistry 79 (2015) 31–43 Contents lists available at ScienceDirect Organic Geochemistry journal homepage: www.elsevier.com/locate/orggeochem