Methyl phenanthrene Maturity index
Methylphenanthrene of aromatic hydrocarbon has five kinds of isomers with different positions of methyl group. Among them, 1 – and 9 – methylphenanthrene are structurally unstable than 2- and 3 – methylphenanthrene, so the proportion of methylphenanthrenes as a whole will decrease as maturity progresses. The maturity index introduced below is based on the
Hydrogen and methyl group of phenanthrene skeleton are in large positional and it tends to be not a dimethyl or methyl group transfer reaction. For detailed, see page 173 of Ishiwadi Yamamoto (2004). Note that the 4-methyl isotope is considered to be negligible due to its relatively small amount.
2. Methylphenanthrene is charged in F1b + F2 fraction because methylphenanthrene is an aromatic hydrocarbon.
The chromatogram was extracted mass fragments ion of m/z 192 and two sets of two peaks appearing about 2 minutes after the phenanthrene (m / z 178) peak, a total of 4 peaks (about 0.5 min in total width) were observed at a peak of methylphenanthrene respectively. The peak corresponds to 3-, 2-, 9- (4-containing) and 1-methylphenanthrene from the left. Below is an example of extracted chromatograms of the Proterozoic sample. In the sample of interest, measure the area value of each of the four peaks of methyl phenantrene.
Calculation of MPR and MPDF;
The most widely used index of maturity using methyl phenentrene is MPI-1 (Methylphenanthrene Index I; Radke and Welte, 1983). MPR (Methylphenanthrene Ratio: Radke et al., 1982a, 1986) and MPDF (Methylphenanthrene Distribution Fraction: Kvalheim et al., 1987) are recommended as alternative indicators. These indices are obtained as follows. The relative content (= area value) of n-methylphenanthrene is expressed by
MPR = 2 – MP/ 1 – MP
MPDF = 2 – MP + 3 – MP /1 – MP + 2 – MP + 3 – MP + 9-MP
The proportion of unstable 1- and 9-methylphenanthrene decreases, so these indicators will increase. Since the value of the index represents the relative maturity degree, in order to investigate to what extent maturity actually progresses, it is better to convert these indices into vitrinite reflectance * 2 (Radke, 1988; Kvalheim et al., 1987).
The formula is, Rc (MPR) = 1.1 log10 MPR + 0.95
or Rc (MPDF) = – 0.166 + 2.242 MPDF
Rc, means calculated vitrinite reflectance. For the relationship between Vitrinite reflectance and the aging stage, please refer to Ishikata / Yamamoto (2004) at page 257 etc etc. However, these conversion equations are not absolute (different values may be proposed in the future for different details), so it may be safe to capture them as “at least the initial wet gas band”.
MPI-1 is an index obtained by using relative amounts of 4 types of methylphenanthrene isotope and phenanthrene. Phenanthrene is incorporated because it is thought that unstable methylphenanthrene isotope undergoes demethylation reaction and the proportion of phenanthrene increases as maturity progresses. The correlation between the indicator and maturity is good at low maturity degree, but the correlation with phenanthrene is particularly bad at high maturity degree, so this indicator cannot be used.
Vitrinite (vitrinite) is one of the constituent units of coal that can be observed under a microscope and plant trees. It comes from the quality part. Vitrinite reflectance is a reflectance when colinite which is one type of vitrinite is rotated under the microscope and it is known that it increases as maturity progresses and it is widely used as a representative maturity degree index at the present time.