Novel maturity parameters for mature to over-mature source rocks and oils based on the distribution of phenanthrene series compounds

Pyrolysis experiments of a low-mature bitumen sample originated from Cambrian was conducted in gold capsules. Abundance and distribution of phenanthrene series compounds in pyrolysis products were measured by GC-MS to investigate their changes with thermal maturity. Several maturity parameters based on the distribution of phenanthrene series compounds have been discussed. The results indicate that the distribution changes of phenanthrene series compounds are complex, and cannot be explained by individual reaction process during thermal evolution. The dealkylation cannot explain the increase of phenanthrene within the EasyRo range of 0.9% ∼ 2.1%. Adding of phenanthrene into maturity parameters based on the methylphenanthrene isomerization is unreasonable, even though MPI 1 and MPI 2 could be used to some extent. Two additional novel and an optimized maturation parameters based on the distribution of phenanthrene series compounds are proposed and their relationships to EasyRo% (x) are established: log(MPs/P) = 0.19x + 0.08 (0.9% < EasyRo% < 2.1%); log(MPs/P) = 0.64x − 0.86 (2.1% < EasyRo% < 3.4%); log(DMPs/TMPs) = 0.71x − 0.55 (0.9% < EasyRo% < 3.4%); log(MTR) = 0.84x − 0.75 (0.9% < EasyRo% < 3.4%). These significant positive correlations are strong argument for using log(MPs/P), log(DMPs/TMPs) and log(MTR) as maturity parameters, especially for mature to over-mature source rocks.


Introduction
Exploration for hydrocarbons in highly mature to over mature thermal stages is an important field in current global oil and gas exploration (Tissot and Welte, 1984;Liang and Chen, 2005;Walters et al., 2011;Wang and Han, 2011;Wei et al., 2010;Dutta et al., 2013). Recent breakthroughs have shown the significance of this research, including studies of the Sinian Dengying and lower Cambrian Longwangmiao reservoirs in the Sichuan Basin, South China (Huang and Wang, 2008;Zou et al., 2014a;Zou et al., 2014b) and the deep Lower Paleozoic reservoirs in the Tazhong Uplift, Tarim Basin, northwestern China (Li et al., 2015). However, the Sinian-Lower Paleozoic source rocks are generally of very high thermal maturity, and characterized by lacking of vitrinite (Dai et al., 2008;Dai et al., 2014;Zhang et al., 2007;Zhang et al., 2008;Leng et al., 2013;Wu et al., 2014). As a consequence, the vitrinite reflectance and most biomarker maturity parameters are infeasible to assess the maturity of Sinian-Lower Paleozoic source rocks and oils (Zhang et al., 2007;Zhang et al., 2008;Leng et al., 2013;Wu et al., 2014).
Aromatic hydrocarbons are important fraction in crude oil and source rock bitumen. The pathways of organic reactions could lead to the change in abundance and distribution of aromatic hydrocarbons in oils and ancient sediment extracts (Strachan et al., 1988;Radke et al., 1982a;Radke et al., 1982b;Alexander et al., 1985;Alexander et al., 1994). Due to the higher thermal stability of aromatic hydrocarbons, many aromatic hydrocarbons maturity parameters, which were based on the isomerization or alkylation-dealkylation processes of substituted phenanthrenes containing alkyl groups, had been hired to assess the maturity of source rock bitumens or crude oils with higher maturity (Stojanović et al., 2001;Stojanović et al., 2007;Wilhelms et al., 1998). However, some aromatic hydrocarbons maturity parameters only can be applied in a certain maturity range. For instance, the classical aromatic hydrocarbon maturity parameters -methyl phenanthrene index MPI 1 and MPI 2 show a reversal at higher thermal maturity (Radke et al., 1982a).

Article No~e00085
Naphthalene and phenanthrene series compounds, which are the most common component in aromatic hydrocarbons, are the major objects for aromatic hydrocarbons maturation parameters research (Stojanović et al., 2007[ 4

_ T D $ D I F F ] ).
Phenanthrene series compounds (Ps) have greater thermal stability than naphthalene series compounds. Examination of a great number of crude oils and source rock bitumen, as well as the corresponding geosynthetic reactions, has shown that the abundance and distribution of Ps were controlled by isomerization and alkylation-dealkylation processes, generation and cracking processes of Ps ([ 4 5 _ T D $ D I F F ] Chen et al., 2010). However, the abundance and local distribution of individual phenanthrene series compounds are susceptible to the origin of the organic matter (Fan et al., 1990), the presence of mineral catalysts in the source or reservoir rocks (Jovančićević et al., 1992;Jovančićević et al., 1993) and the mineral composition of the carrier system rocks. The whole distribution characteristics of Ps could be more effective to reflect the maturity. Therefore, it is necessary to study the relationship between the distribution characteristics of Ps and the thermal maturation.
In this paper, the distribution of Ps and their evolution with maturity have been investigated using a series of pyrolysis experiments for low-maturity bitumen originated from the Cambrian source rock, NW Sichuan Basin. The study aims to provide experimental evidence that will facilitate the development of novel aromatic hydrocarbons maturation indices, as well as to clarify limitations in the use of currently available indices, for application in studies of petroleum maturation.

Sample
A sample of bituminous dike that originated from the Lower Cambrian black shale was collected from an outcrop in Kuangshanliang, northwest Sichuan Basin. The outcrop is located in a destroyed paleo-reservoir, and its organic geochemical properties and sources of bitumen have been reported (Zhang et al., 2014;Zhou et al., 2013). The abundance of extracted organic matter (EOM) is 8.3%, and the abundances of saturates, aromatics, resins and asphaltenes are 0.04%, 1.15%, 14.90% and 83.90%, respectively. The bitumen dike is depleted in low molecular weight hydrocarbons due to evaporation and biodegradation during exposure to the atmosphere (Huang and Wang, 2008). The maturity parameters Ts/Tm, MPI-1 and MPI-2 are 0.52, 0.47 and 0.57, respectively, suggesting that the bitumen sample has not undergone serious thermal degradation (Zhang et al., 2014;Zhou et al., 2013[ 4 6 _ T D $ D I F F ] ) (Fig. 1). The geochemical analysis of this bitumen shows that it is organic-rich, with a total organic carbon (TOC) of 58.0%, S1 of 2.42 mg HC/g sample, S2 of 36.96 mg HC/g sample,  (e) m/z 178 + 192 + 206 + 220 mass chromatograms of aromatic fraction, displaying phenanthrene series compounds distribution.

Article No~e00085
temperature and time scales ranging from those in the laboratory to geological.
In this study, the EasyRo% was hired as a calibration of thermal maturation for extrapolating the results of the high heating rate pyrolysis experiments to extremely low heating rate geological conditions. The calculated EasyRo% corresponding to heating temperature is shown in Table [ 3

Distribution of phenanthrene series compounds
For each temperature point of the pyrolysis experiment, the corresponding EasyRo%, yields and relative abundance of different series compounds were obtained and discussed below.[ 5 2 _ T D $ D I F F ] The relationships between calculated EasyRo%, and yields of aromatic hydrocarbons and the relative abundance of Ps in the Table 1. [ 5 9 _ T D $ D I F F ] The yields of aromatic hydrocarbons and relative abundances of Ps, P, MPs, DMPs and TMPs.   EasyRo range of 1.0% ∼ 3.4%. Given that Ps are the main compounds in aromatic hydrocarbons (Fig. 2), Ps may be mainly generated within the EasyRo range of 0.7% ∼1.0% and are destroyed within the EasyRo range of 1.0% ∼ 3.4%.
The [Ps] have little change within the EasyRo range 0.7% ∼ 2.1% (Fig. 3b), despite great changes that have taken place in the yields of aromatic hydrocarbons within the same EasyRo range (Fig. 3a) [ ( F i g . _ 3 ) T D $ F I G ] [DMPs] firstly increase below the EasyRo% range of 1.1%, and then decrease within the EasyRo% range 1.1% ∼ 2.1%, and finally increase above the EasyRo% range of 2.1% (Fig. 3c).
[TMPs] rapidly decrease with maturity above the EasyRo% of 0.9% (Fig. 3c). Apparently, these changes are complex, and cannot be explained by individual reaction process during thermal evolution.
In summary, in addition to the differences among the thermal stabilities of isomers and alkylation-dealkylation, the relative generation and decomposition rates of different Ps also influence their distribution during thermal maturation.

Evaluation of phenanthrene maturation parameters
In several earlier papers, crude oils and sedimentary bitumens have already been classified according to maturity parameters based on alkylphenanthrene alkylation-dealkylation reactions. Namely, some organic geochemical investigations (Stojanović et al., 2001;Stojanović et al., 2007;Simons et al., 2003;Singh et al., 1994), as well as geosynthetic modelling studies (Smith et al., 1995), have shown that during catagenetic evolution, depending on temperature and proton-donor type clay minerals, alkylation of phenanthrenes into methylphenanthrenes may have occurred, and also their dealkylation, particularly at higher degrees of maturity.
This suggest that the isomerization of MPs mainly occurs within the EasyRo  (Fig. 4b). Hence, adding of phenanthrene into the methylphenanthrene isomerization maturity indicators within the EasyRo range of 0.9% ∼ 2.1% is unreasonable, even though MPI 1 and MPI 2 could be used to some extent (Bao et al., 1992). Stojanović et al. (2001) suggested that together with methylphenanthrene isomerization in reservoir rocks, dealkylation also occurs, and the percentage of phenanthrene in the tricyclic aromatic fraction ([P1]) increase with maturity. Fig. 4b shows that [P1] increase within the EasyRo range of 0.9% ∼ 2.1%, while PAI 1 (MPs/P) slowly increase at the same EasyRo range (Fig. 4c).
Apparently, the dealkylation cannot explain the increase of phenanthrene within the EasyRo range of 0.9% ∼ 2.1%.
As shown in Fig. 4c, PAI 1 slowly increases within the EasyRo range of 0.7-2.1%, and then rapidly increases above EasyRo of 2.1%. This contradicts the view that PAI 1 will decrease with the increase of thermal maturity (Ishiwatari and Fukushima, 1979;Stojanović et al., 2007). This conflicting result suggest that the change of PAI 1 for natural geological sample is different from pyrolysis experiment result, and may be affected by some other factors.
Therefore, we need to be more careful, when we use the PAI 1 as a maturity parameter.
[ 7 5 _ T D $ D I F F ] Table 3. Values of phenanthrene maturity parameters at each temperature point.  EasyRo%.
Article No~e00085 Stojanović et al. (2007) suggest that alkyl-phenanthrenes ratio MDR and MTR, which are based on demethylation of DMPs and TMPs into corresponding MPs, could be applied to assess the maturity of oils. Fig. 5

Novel maturation parameters based on the distribution of Ps
In fact, isomerization and alkylation-dealkylation, generation and decomposition of aromatic hydrocarbons may occur simultaneously and influence each other, and the effects degree of individual reaction process on the distribution of Ps is variable at different thermal stress. As a consequence, it is difficult to distinguish the dominant factor that affects the distribution of Ps. However, it is feasible to treat each alkyl series compounds as a whole and to investigate the influences of maturity process on the distribution of Ps, and to define new maturity parameters based on the evolution of distribution characteristics of Ps.
The relationship between log(MPs/P) and EasyRo% could be divided into two stages, which are a slight slope of 0.19 within the EasyRo% range of 0.9% ∼ 2.1%, and a severe slope of 0.64 within the EasyRo% range of 2.1% ∼ 3.4% (Fig. 6a).
This suggests that the log(MPs/P) is more sensitive at EasyRo% above 2.1%. Although, higher R 2 and slopes for log(DMPs/TMPs) and log(MTR) than for log (MsP/P) were observed (Fig. 6), the abundance of DMPs and TMPs are commonly very low in over-mature samples, especially for TMPs (Fig. 2), so that it was very difficult to get the values of log(DMPs/TMPs) and log(MTR). On the contrary, relative high abundance of P and MPs in over-mature samples (Fig. 2) provides a possible mean for assessing over-mature source rocks.
Unfortunately, all four proposed equations have not been examined by natural geological maturity sequence. Concerning this fact and the difference between EasyRo% and natural maturity level, all above four equations and its application could be optimized by more pyrolysis experiments and investigation of natural samples.

Conclusions
A simulation pyrolysis experiment of bitumen in the vessels was carried out.