ADVECTION
The term advection implies migration and describes the addition of one petroleum fluid to another,
the first flowing from a deeper reservoir or source rock, generally over-pressured. Analysis of molecular
profiles of reservoir fluids facilitates the recognition of additions of whole oils, gas-
condensates, "intermediate" gases, dry gases and methane. The advection of methane, with or
without gas-liquids in solution, is the primary, causative, action in the generation of non-thermal
gas-condensates by evaporative fractionation (Thompson, 1987) or by total vaporization (refer to
RECENT RESEARCH, preceding Part 1).
EVAPORATIVE FRACTIONATION
The term evaporative fractionation (EF) was introduced to describe a major
process in the alteration of petroleum (Thompson, 1987). Three steps are
visualized: (a) the injection of gas into an oil reservoir; (b) development
of near or complete saturation at higher pressure; (c) pressure release through faulting resulting
in rapid degassing and migration of only a gas phase. Released gases are encountered in other
reservoirs as gas-condensates.
Oils having undergone EF are deficient in gasoline range components (transferred
to gas-condensates) and modified by increase in aromatics and naphthenes.
Examination of molecular profiles of reservoir fluid oils has also characterized EF in
broad compositional terms, as described in
Section 5.
Gasoline
compositional changes subsequent upon EF are detailed in
Section 10.
E3
The parameter E3 is defined in Thompson (2010) to provide a measure of gas-liquid
addition or depletion. A value of E3 of unity is interpreted as indicating that the degree
of cracking evidenced by SF(C3-nC5) is compatible with that evidenced by SF(P15-P25).
Values greater than unity indicate gas-liquid enrichment, values less than unity,
gas-liquid depletion.
E3 compares two slope factors (Eqn. 1), the observed and hypothetical values of
SF(C3-nC5). The hypothetical value is determined on the basis of a regression
(Eqn. 11, below) presenting the relationship between SF(C3-nC5) and SF(P10+) in five
pyrolyses of increasing severity of treatment of a petroleum-derived, Type II, clastic-
sourced asphaltene, employing the data of Table 1 of Thompson (2002).
E3 = SF(C3-nC5)[Observed] / SF(C3-nC5)[Hypothetical].........Eqn. 1
In reservoir fluid oils of western Canada (n = 198) the observed mean value of E3 is
1.15, standard deviation 0.23, modal range 1.00 to 1.05 (29 cases). This indicates that
the majority of western Canadian oils are enriched in light gas-liquids,
as demonstrated previously (Thompson,
2002) and discussed in detail for the abnormally pressured Brazeau River Cardium pool,
Alberta (Thompson, 2004).
E7#
E7# is recommended as a replacement of the parameter E7, as it was described in Edition 2,
2012, (following section). E7 compares two Slope Factors to assess addition or
depletion in the range P6 - P15. A more easily envisioned comparison is that between
the observed concentration of P7, [P7}, and the hypothetical original concentration as calculated from
SF(P15-P25), designated here [P7#], as indicated in Equation 2, a format analogous to that defining E3:
E7# = [P7] / [P7#] ........ Eqn. 2
The calculated hypothetical, original, value of P7, [P7#] is determined from Eqn. 3:
[P7#] = [P15].((SF(P15-P25)exp^8) ........ Eqn. 3
i.e., the actual concentration of P15 multiplied by SF(P5-p25) raised to the power of 8 (in view of
the eight carbon number step between 15 and 7).
Values of E7# greater than unity evidence enrichment in gasoline-range compounds in
an added gas condensate.
E7
E7 provides an alternative index of the degree of addition or depletion of gasoline range
compounds in the advection of gas-condensate or loss due to evaporative fractionation.
E7 is calculated in the P6+ fraction and is proportional to the extent of departure of
the concentration of the reference pseudo-component, [P7], from a hypothetical concentration
predicted on the basis of the slope of the P15-P25 suite, SF(P15-P25), and on the
concentration of P15.
In the evaluation of E7 two objectively-determined slopes are compared, that defined
by the vector joining P7 and P15 in the molar profile and that joining P15 and P25, the
latter yielding the slope factor SF(P15-P25). Ideally, in unaltered oils, these segments
form a single, continuous, vector, that is, the graphical projection of the log-linear vector passing
through the P15-P25 pseudo-components also passes through those in the P7 -P15 region. In this
case the inter-pseudo-component concentration ratios in the two intervals would be
statistically equal. P15-P25 data exhibit greater consistency than ranges such as P6-P29,
even P10-P29, because of distortion introduced by light end alteration.
E7, defined in Eqn. 4, utilizes the eight carbon-number step P7 to P15, necessitating
the use of the eighth root of the concentration ratio [P7]/[P15], to determine the mean
slope.
E7 = (([P7]/[P15])^1/8) / (SF(P15-P25)) ......... Eqn. 4.
It is assumed that the original distribution from P6 to beyond P30 was exponential.
Pseudo-components in the P7-P15 range often fail to form an exponential series and
exhibit a slope break because of advective addition of light gas-condensate.
SLOPE FACTORS
Lohrenz and Bray (SPE No. 792, 1964) discovered the exponential distribution by carbon
number of P7+ pseudo-components in unaltered petroleums. Kissin (Geochimica et Cosmochimica Acta,
1987, p2445) extended the concept to describe an identical distribution exhibited by
normal-alkanes, the principal components.
Exponential series exhibit a constant inter-member ratio. In any unaltered
petroleum (reservoir fluid), commencing at P6, the concentration ratio P6/P7 equals
P7/P8 which equals P8/P9, and so on, to beyond P30/P31. Values of such ratios increase
with the maturity of the oil. Representative values range from 1.08 to 1.17. The ratio
value is termed a
Slope Factor .
Slope Factors, (SF), can be conveniently determined by curve fitting, employing a
statistical-plotting package such as "Kaleidagraph" (Synergy.com). Tabulating molar
percent, y, and the associated carbon number, x, for the reservoir fluid analysis of
interest, selecting "Curve Fitting, Exponential", an equation of the following form is
returned:
y = A.e^(-a.x) ......... Eqn. 5
where A defines an intercept, e is the base of natural logarithms raised to
the power of (-a.x). Solutions yield mole percent of the pseudo-component y, of carbon
number x.
The relevant Slope Factor is extracted from Eqn. 5 by evaluating e to the power of
a, dropping the negative sign. In this way Slope Factors are rendered as values
greater than unity, rather than as decimal fractions, characteristic of
an increasing
series. The conceptual reason for doing this involves two factors. Firstly, low carbon
number components are generated in increasing concentrations by the thermal
cracking of those of higher carbon number, suggestive of an increasing series.
Secondly, visualizing a reservoir fluid composition from P30 to lower carbon numbers, the
following equations obtain:
P29 = P30.(SFP15-P25)) …… Eqn. 6
P28 = P30.(SFP15-P25))^2 …… Eqn. 7
P27 = P30.(SFP15-P25))^3 …… Eqn. 8.
Thus, given any reference concentration, here P30, any other can be calculated by a
multiplication involving the appropriate power of the Slope Factor. Relationships of
his type can be employed in calculating amounts of components removed by alteration.
SLOPE FACTOR RANGES
An unaltered oil exhibits several exponential concentration series possessing differing
SF values.
Commencing at C1, the concentration of methane behaves independently, is not included in
an exponential series, and commonly exhibits values between 20 and 40 mole percent. The
concentrations of ethane, propane, P4 (equaling the sum of n- and isobutane) and P5
(comprising the pentanes) form an exponential series represented by the slope factor (Thompson, 2002):
SF(C2-P5)
Values range from 1.05 to 2.89 (n=87).
A second series in the same range comprises propane, n-butane and n-pentane represented
by
SF(C3-nC5).
Values here range from 1.40 to 4.06 (n=87).
The third, and most extensive, series comprises P6 through P29, limited by available
analytical data. This range may represented by the following slope calculations which, of
necessity in unaltered oils only, all yield identical SF values:
SF(P6-P29)
SF(P10+)
SF(P15-P25)
Values of SF(P15-P25) range from 1.08 to 1.70 (n=190).
SF(C3-nC5) is invariably greater than SF(C2-P5), and the latter greater than
SF(P15-P25).
SLOPE FACTOR RELATIONSHIPS IN UNALTERED
RESERVOIR FLUIDS
SF(C3-nC5) and SF(C2-P5) are related as follows:
SF(C3-nC5) = -0.88 + 1.443(SF(C2-P5)), r = 0.94 ......... Eqn. 9.
and
SF(C2-P5) = 0.226 + 0.617(SF(C3-nC5)), r = 0.94 ......... Eqn. 10.
As a result of the laws governing thermal cracking and the behavior of short-chain
free radicals, a concentration discontinuity occurs between P5 and P6 such
that P6 unexpectedly exceeds P5. P5/P6 commonly equals 0.85 in unaltered oils.
In data representing the pyrolysis of asphaltenes, which yields realistic
synthetic petroleums, SF(C3-nC5) and SF(P10+) increase simultaneously and are highly
correlated (r = 0.87). However, as discussed below, no correlation whatsoever is observed
in the great majority of reservoir fluid oils from western Canada because of secondary
alteration.
The relationship of SF(C3-nC5) and SF(P15-25) in the pyrolysates is almost identical with
that found in a group of oils which are evidently little altered, those of the
Rainbow-Virgo fields, NW Alberta (illustrated in Fig. 4, Thompson, 2004). The pyrolysis
data yields the following relationship:
SF(C3-nC5) = 2.786.SF(P15-P25) - 1.615 ........ Eqn. 11
SLOPE FACTOR RELATIONSHIPS IN ALTERED RESERVOIR FLUIDS
In the PVT data examined here SF(C3-nC5) and SF(P15-P25) are not correlated (r = 0.08).
This is due to a variety of commonplace alteration processes, particularly gas advection
and evaporative fractionation, defined above, as well as biodegradation
(see Section 6).
These additions or depletions in the gas-liquid and gasoline ranges frequently leave
the heavier liquid components unaltered, still characterized by SF(P10+) or
SF(P15-P25).
RETURN TO INDEX