25th International Meeting on Organic Geochemistry IMOG 2011

P-313
Calibration of absolute maturity for terrestrial-sourced oils and
gas condensates using PLS regression
Richard Sykes, Klaus-Gerhard Zink
GNS Science, Lower Hutt, New Zealand (corresponding author:r.sykes@gns.cri.nz)
The absolute thermal maturity of petroleum fluids can
be used to help constrain the depth and timing of
expulsion from source rocks and to clarify genetic
relationships. For example:
� Do suspected oil shows represent indigenous or
migrated oil?
� Are various oils from the same or sequential
charges?
� Did gas condensates form via phase separation
from oils or directly from source rocks at higher
maturity?
Absolute maturities are usually determined by
calibrating the fluids‘ molecular maturity parameters
against equivalent parameters for bitumens extracted
from source rocks of known maturity. Use of individual
parameters can be problematic if the selected
parameters reach equilibrium partway through the oil
window or are adversely affected by variation in
source organofacies. In this paper, we present and
apply a method for predicting absolute maturities of
terrestrial-sourced oils and gas condensates based
on Partial Least Squares (PLS) regression. The use
of factor-based regression allows multiple maturity
parameters to be used collectively, thus minimising
potential errors associated with individual parameters.
Initially, 28 molecular maturity parameters were
determined for 129 New Zealand coaly source rocks
with known Suggate coal rank [Rank(Sr)] and Tmax
values from throughout the oil window. Then, using
bivariate correlations, Principal Component Analysis
and PLS regression, two PLS regression models were
developed for prediction of Rank(Sr) and Tmax. The
first, optimised for Rank(Sr), uses a training set of 58
source rocks and 5 methylnaphthalene maturity
parameters; the second, optimised for Tmax, uses 52
source rocks, the same 5 methylnaphthalene
parameters and 3 methylphenanthrene parameters.
Internal cross-validation shows that the models
predict known Rank(Sr) and Tmax values with standard
errors of 0.40 rank units and 2.7°C, respectively,
which are comparable to those of measured values.
Application of the PLS models to a set of 132 New
Zealand terrestrial-sourced oils and gas condensates
predicts maturity values that are largely consistent
with Rank(Sr) and Tmax limits for the oil and gas
windows determined independently from source rock
pyrolysis studies (Fig. 1) [1, 2]. One notable exception
is the Galleon-1 condensate, which is thought to have
been generated via rapid, intrusion-related heating.
Tmax (°C)
475
470
465
460
455
450
445
440
435
430
onset oil expulsion for coals
near top of NZ Coal Band
Oils
Gas conds
Oil shows
onset oil expulsion for coals
near base of NZ Coal Band
Ga1
~Ro 0.90% ~Ro 1.15%
onset gas generation
end of oil window
~Ro 1.6%
11 12 13 14 15 16 17
Rank(S r)
Fig. 1. Calibrated Rank(Sr) and Tmax values for New
Zealand petroleum fluids. Ga1 = Galleon-1.
The gas condensates are overall more mature than
the oils, but with significant overlap in their maturity
ranges. Variations in maturity between petroleum
fluids within individual families and fields provide
useful insights relevant to understanding migration
pathways, charge histories and possible causes of
well failure.
[1] Sykes, R., Snowdon, L.R. (2002) Org.
Geochem. 33, 1441–1455.
[2] Sykes, R., Johansen, P.E. (2007)
Proceedings, 23rd IMOG, Torquay, P226-WE.
446