Research paper
Characteristics of molecular nitrogen generation from overmature black shales in South China: Preliminary implications from pyrolysis experiments

https://doi.org/10.1016/j.marpetgeo.2020.104527Get rights and content

Highlights

  • N2 generation from shales and kerogens was experimentally investigated.

  • N2 formation is closely related to thermal maturity.

  • Inorganic nitrogen is released differently from organic nitrogen.

  • EqVRo >3.4% and high content of organic nitrogen indicate high N2 risk.

Abstract

High content of molecular nitrogen (N2) is one of the natural gas exploration risks in petroliferous basins where black shales act as source rocks of either hydrocarbon gas or molecular nitrogen. In this study, two overmature and one low-maturity shale samples and their kerogens were investigated to determine the generation characteristics of molecular nitrogen as well as methane and the differences in the release processes of inorganic nitrogen fixed in ammonium-bearing minerals and organic nitrogen bound in kerogen. The results illustrate that with increasing pyrolysis temperature, the yield of methane first increases and then decreases with an inflection temperature of 650 °C (EqVRo = 3.4%), whereas the yield of molecular nitrogen shows a continuous increase throughout the pyrolysis experiment. The molecular nitrogen during the stage of methane generation (i.e., EqVRo < 3.4%) is more preferentially derived from the inorganic nitrogen in ammonium-bearing minerals, whereas the significant generation of molecular nitrogen from organic nitrogen in kerogen commences only after the methane generation potential is exhausted (EqVRo > 3.4%). The results also reveal that at the experimental maxima of thermal maturity (EqVRo = 4.9%), the generation potential of inorganic molecular nitrogen (mg/g Ninorg) is much lower than that of organic molecular nitrogen (mg/g Norg), indicating that in our experiments the organic nitrogen in kerogen is more easily to be converted into molecular nitrogen than the inorganic nitrogen in ammonium-bearing minerals. All these results indicate that molecular nitrogen content in shale gas may change dramatically during thermal evolution, and source rocks with exceptionally high maturity and high abundance of organic nitrogen likely lead to a high molecular nitrogen risk, particularly in regions of poor preservation conditions of shale gas, which may be the main reasons for the high molecular nitrogen content in the Lower Cambrian shale gas in South China.

Introduction

During past decades, various studies have investigated the geological carbon and nitrogen cycle from the atmosphere to deep strata (Falkowski, 1997; Kump and Arthur, 1999; Marty and Dauphas, 2003; Deutsch et al., 2007; Lam et al., 2009; Canfield et al., 2010; Maloof et al., 2010; Busigny et al., 2013; Stüeken et al., 2016; Wang et al., 2018; Mettam et al., 2019). Sedimentary rocks are an important carbon and nitrogen pool (Brandes et al., 2007; Algeo et al., 2008; Stüeken et al., 2016; Wang et al., 2015, 2018), and the carbon and nitrogen elements are derived from both organic and inorganic contributions (Boyd, 2001; Leithold et al., 2016; Stüeken et al., 2017). The mineralization of C and N during early diagenesis is controlled by the nature of biomass, microbial activity, oxygen availability, temperature/thermal stress, sedimentation rate, and fluid-rock interactions; thus, the contents of carbon and nitrogen in sediments may be highly variable (Boyd, 2001; Jurisch et al., 2012; Leithold et al., 2016; Stüeken et al., 2017). Thermal and chemical degradation processes lead to the loss of carbon and nitrogen in sedimentary rocks as diagenesis and catagenesis proceed (Behar et al., 2000; Kelemen et al., 1998, Kelemen et al., 2006; Boudou et al., 2008; Jurisch et al., 2012). One of the main ways to release carbon and nitrogen from sedimentary rocks is by the formation of gases, which is also one of the important processes in carbon and nitrogen cycle (Falkowski, 1997; Deutsch et al., 2007; Canfield et al., 2010; Stüeken et al., 2016; Wang et al., 2018).

Molecular nitrogen is one of the most common non-hydrocarbons in natural gas (Jenden et al., 1988; Dai, 1992; Littke et al., 1995; Zhu et al., 2000; Kotarba and Nagao, 2008, 2014; Liu et al., 2012). Although most conventional natural gases contain only a few percent of molecular nitrogen (N2), N2-rich natural gas in reservoir rocks is also known in many basins around the world (Krooss et al., 1995; Littke et al., 1995; Zhu et al., 2000; Chen and Zhu, 2003; Dai et al., 2005; Mingram et al., 2005; Liu et al., 2012). Previous studies have suggested several possible origins of molecular nitrogen in natural gas, including decomposition of sedimentary organic matter and/or minerals, primordial nitrogen from deep mantle, and radiogenic, atmospheric and magmatic processes (e.g. Maksimov, 1975; Allègre et al., 1987; Gold and Held, 1987; Andersen et al., 1993; Baxby et al., 1994; Krooss et al., 1995, 2005; Zhu et al., 2000; Dai et al., 2005; Mingram et al., 2005; Liu et al., 2012; Kotarba et al., 2014). Among the above sources, thermal degradation is one of the most common process for N2 accumulation in natural gas pools, which has been experimentally confirmed by many authors (Boudou and Espitalié, 1995, 2008; Krooss et al., 1995; Littke et al., 1995; Behar et al., 2000; Kelemen et al., 2006; Heim et al., 2012; Jurisch and Krooss, 2008, 2012). In addition, some post-genetic processes may also lead to the relative enrichment of molecular nitrogen in natural gas. They include the thermochemcial sulfate reduction (TSR) reactions (Liu et al., 2012; Jenden et al., 2015), the loss of early generated methane due to tectonic activity (Su et al., 2019; Wu et al., 2019), and the competitive adsorption/desorption between molecular nitrogen via reducing the partial pressure of methane in shales or coals (Bustin et al., 2016; Zhang et al., 2017; Li and Elsworth, 2019; Ghalandari et al., 2020).

Molecular nitrogen, though worthless from a commercial point of view, is probably a key to an improved understanding of the formation mechanisms, longevity and compositional evolution of natural gas accumulations on the geologic time scale, and therefore is a valuable study object for geoscientists (Krooss et al., 1995; 2005; Mingram et al., 2005; Kotarba et al., 2014). Moreover, high molecular nitrogen content reduces the methane concentration in natural gas and is bound to pose a serious risk in natural gas exploration (Littke et al., 1995). Recently, a high content of molecular nitrogen has been encountered in some overmature Lower Cambrian shale gas in southeastern Chongqing and northern Guizhou, China (Liu et al., 2016); Jiao et al. (2017b). Liu et al., 2016 reported that these high contents of molecular nitrogen is mainly of thermal origin in terms of their nitrogen isotopic composition. However, the Lower Silurian shales, another set of overmature Lower Paleozoic shale strata in the same area, has not exhibited a high content of molecular nitrogen (Dai et al., 2014). Therefore, the mechanism, timing, and potential of molecular nitrogen generation in Lower Paleozoic shale gas in the Upper Yangtze region still requires further investigation.

In this study, one Lower Cambrian shale sample and one Lower Silurian shale sample that display similar thermal maturity, organic matter abundance, and kerogen type but different nitrogen content and species were selected from two wells in the Upper Yangtze region to investigate their release characteristics of organic nitrogen in kerogen and inorganic nitrogen in ammonium-bearing minerals. In addition, nitrogen release from a low-maturity shale sample from the Mesoproterozoic was also studied for comparison with the two overmature Lower Paleozoic shales. This study may help to evaluate the molecular nitrogen generation potential of organic-rich shale during thermal maturation and to understand the risk of high content of molecular nitrogen in overmature shale gas.

Section snippets

Geological setting

The Sichuan Basin, which is located in the Yangtze platform, is one of the main natural gas province in China. Marine shales were widely developed during the early Paleozoic in Sichuan Basin and its peripheral areas, including the Lower Silurian to Upper Ordovician and the Lower Cambrian shales. The Lower Silurian Longmaxi Formation has become the most important marine shale gas-producing layer in China (Zou et al., 2010, 2015; Zeng et al., 2012; Tian et al., 2013; Dai et al., 2014) and

Samples

Sixteen core samples of Lower Silurian Longmaxi shale and sixteen Low Cambrian Shuijingtuo shale were collected from wells YC4 and YC2 in southeastern and northeastern Chongqing, respectively (Fig. 1; Table 1). Based on the quantity of core samples and their geochemical characteristics, one typical Longmaxi shale sample at the depth of 745.8 m of Well YC4 and one Low Cambrian Shuijingtuo shale sample at the depth of 1101.9 m of YC2 were selected for pyrolysis experiments and named YC4R and

Organic geochemistry and mineralogy

In general, Longmaxi shales collected from well YC4 had a TOC range of 1.2–4.5% with total nitrogen content (Ntot) ranging from 1.0 to 1.9 mg/g shale, whereas the range of TOC and Ntot contents of Lower Cambrian shales collected from well YC2 are 1.4–3.8% and 1.6–2.8 mg/g shale, respectively (Table 1; Fig. 3). The black shales from the two wells have similar TOC contents but distinct Ntot contents, which is beneficial to their comparative study of nitrogen release during pyrolysis.

The two

Conclusions

Two overmature Lower Paleozoic black shales and one low-maturity shale with similar kerogen type and TOC content but different nitrogen compositions were used to investigate the nitrogen release during pyrolysis experiments. Based on the experimental results, the characteristics of molecular nitrogen generation from different nitrogen species and the molecular nitrogen risk of overmature Lower Paleozoic black shales were tentatively discussed. The main conclusions are summarized below.

  • (1)

    The

CRediT authorship contribution statement

Haifeng Gai: Conceptualization, Validation, Formal analysis, Writing - original draft. Hui Tian: Conceptualization, Writing - review & editing, Resources, Supervision. Peng Cheng: Validation, Visualization. Chunmin He: Investigation. Zijin Wu: Visualization. Sui Ji: Data curation. Xianming Xiao: Resources.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This study was jointly supported by the Natural Science Foundation of China (Grant No. 41925014, 4160304, and 41522302), the National Science and Technology Major Project (2017zx05008-002-004) and GIG “135” project (135TP201602). This is contribution No.IS-2875 from GIGCAS.

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