COMPOSITIONAL LAWS
The unaltered, original, compositions of petroleums of increasing
maturity conform to the following series of relationships, based upon
the characteristics of a randomly chosen suite of 198 western Canadian oils.
The three exponential series are of universal occurrence.
(1) The modal methane concentration is 25- 30 mole percent (n= 198).
(2) The modal methane/ethane ratio (C1/C2) is 3.5 - 4.0 (n = 198), relatively
invariant with maturity (Thompson, 2010).
(3) Ethane, propane, pseudo-butane and pseudo-pentane form a decreasing
exponential series of slope given by SF(C2-P5)
(4) Propane, n-butane and n-pentane form a second, decreasing, exponential
series of slope given by SF(C3-nC5).
(5) P6-P29 form a third, decreasing, exponential series of slope best given by
SF(P15-P25), defined in the carbon number range least subject to alteration.
Reliable estimates of P30+ can be made by summing the series P30 to P100 at
the same slope.
(6) The mean value of SF(P15-P25) is 1.133 (n = 195).
(7) P5/P6 is modally 0.75 - 0.80 (n = 194), i.e., a concentration discontinuity
occurs such that P6 > P5.
(8) In a group of Canadian oils believed to be unaltered (certain Middle Devonian
oils in Rainbow, Virgo and associated fields in NW Alberta) the following
relationship links SF(C3-nC5) and SF(P15-P25):
SF(P15-P25) = 0.689 + 0.279.SF(C3-nC5) r = 0.87 ............ Eqn. 1
(9) A series of five pyrolyses of increasing severity employing a single Type II
asphaltene generated realistic synthetic petroleums of increasing maturity
(Thompson, 2004) The following relationship was observed:
SF(P15-P25) = 0.694 + 0.277.SF(C3-nC5) r = 0.87 ............ Eqn. 2
The equations are effectively identical, given the scatter in the data. (
Thompson, 2004, p 11).
Employing the original, numerical, asphaltene pyrolysis data, the inverse
regresssion can be calculated:
SF(C3-nC5) = 2.786.SF(P15-P25) - 1.615 r = 0.87 ............ Eqn. 3
(Thompson, 2010).
(10) Equation 3 yields SF(C3-nC5) = 1.54 for the average oil where
SF(P15-P25) = 1.133.
(11) Eqn. 4 relates SF(C2-P5) and SF(C3-nC5):
SF(C2-P5) = 0.256+ 0.596(SF(C3-nC5)) r = 0.95 (n = 78).... Eqn. 4
A MATHMATICAL MODEL OF PETROLEUM COMPOSITION
Employing the numerical relationships above, and a seed value of
30 mole percent methane, the model petroleum composition illustrated in
Figure 10
was developed. The figure compares the model with an actual oil, postulated to be unaltered,
from Shekilie field, Muskeg Formation (M. Devonian), northwest Alberta,
part of the Rainbow group. The model closely simulates an actual reservoir fluid which
proves to possess modal characteristics.
THE P5 - P6 DISCONTINUITY
It is implied above that the P6 > P5 inequality which generates a slope break
and secondary maximum at P6, is an original, as-generated feature. This is not
evident upon inspection of the sample suite of 198 reservoir fluids, derived from
both Type II and Type IIS kerogens. Reducing the suite to those which are
free of evidence of biodegradation, 146 cases, (criteria: Thompson, 2010, as discussed
in Section 6 below), the concentration discontinuity/slope break occurs at P5 in 16 cases,
at P6 in 45, at P7 in 50 and at P8 in 35. It is therefore necessary to seek confirmatory evidence
for a P5-P6 location.
PYROLYSIS OF MODEL COMPOUNDS
Normal-alkanes are the principal components of petroleums and there is widespread
evidence that, during generation, long chain alkane entities source all other
components except the minor quantity of those of direct biogenic origin (Thompson, 2006).
Petroleum composition requires a principal process of free radical fragmentation of
the precursors. Accordingly, investigators have employed thermal cracking of
compounds such as n-hexadecane and polyethylene.
Figure 11
depicts the products of the catalyst-free pyrolysis of n-hexadecane. Important petroleum
features are reproduced: two suites of compounds forming exponential progressions with
a discontinuity between.
Figure 12
illustrates pyrolysates of a petroleum asphaltene,
compared to an unaltered reservoir fluid oil. Nearly exact matching shows the validity
of asphaltene pyrolysis in petroleum simulation. Pyrolyses of model compounds and of
asphaltenes justify locating the initial discontinuity at P5-P6. Migration of the
secondary maximum to P7, P8 and higher carbon numbers, must be attributed to secondary
alteration, particularly, to evaporative fractionation.
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