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


    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.


    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.


    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.