AAPG Bulletin, V. 84, No. 8 (August 2000), P. 1152-1172.
Geochemical Characterization of Natural Gas: A Physical
Multivariable Approach and its Applications in Maturity and Migration Estimates1
Alain Prinzhofer,2 Marcio Rocha Mello,3 and Tikae Takaki3
©Copyright 2000. The American Association of Petroleum
Geologists. All rights reserved.
1Manuscript received January 12, 1998; revised manuscript received December 21,
1998; final acceptance January 15, 2000.
2Division of Geology-Geochemistry, Institut Français du Pétrole, 1 and 4
Avenue de Bois Préau, 92852 Rueil Malmaison Cedex, France; e-mail: [email protected]
3Petrobras Research and Development Center - Cenpes, Cidade Universitária,
Quadra 7, Ilha do Fundão, 21949-900, Rio de Janeiro, RJ, Brazil.
We would like to thank Petrobras for the authorization to publish these results. In the
field, sampling and useful discussions were made possible by all the colleagues of
Petrobras-Salvador and Petrobras-Saõ Mateus, and in particular Fernando Taboada Fontes
and Roberto d'Avila. Gilberto Pereira da Silva did a lot for the best and most efficient
gas sampling. We would like also to thank Caroline Sulzer for performing all the chemical
and isotopic analyses, and Caroline Magnier and Elvyn Marshall for careful editing. John
Curtis, Moses Chung, and Martin Schoell made critical reviews that improved the logic and
the presentation of this paper.
ABSTRACT
Gas geochemistry has recently been shown to enhance information regarding the
geological history of hydrocarbons. In this paper, graphical representations of
physico-chemical processes affecting the chemical and isotopic signatures of natural gases
are exemplified. These diagrams are based on experimental studies and the use of basic
statistics to extract significant and synthetic parameters from the geochemical data. From
11 chemical and isotopic ratios, a statistical analysis (PCA) yields two very important
parameters. The first parameter, using mainly the C2+ fraction of the gas,
relates to maturity and the second parameter, involving the proportions and d13C values of methane, indicates mainly segregative
migration. Positive values of the second parameter indicate that gases accumulated far
from their source, whereas negative values correspond to residual gas pools after leakage
of a part of the fluids. A tentative reconstruction of the gas history has been performed
in two Brazilian basins: the Espirito Santo basin and part of the Recôncavo basin. The
Espirito Santo basin is located on the passive continental margin of the Atlantic Ocean,
and the Recôncavo basin corresponds to an intracontinental aborted rift. In both cases,
the source rocks are mainly lacustrine, with thermal maturities ranging between the oil
window and the beginning of the gas window. Results show that in the Recôncavo basin, a
major fault (the Mata Catu fault) acts as a drain for hydrocarbon migration at the basin
scale, associating a major isotopic fractionation to the gas migration with a clear
correlation between isotope fractionation and the distance of migration. In the Espirito
Santo basin, this segregation appears in the platform sediments to a lesser extent and is
absent in the gas pools located in the paleocanyons filled with turbidites. This
long-distance migration in the platform sediments suggests that a hydrocarbon kitchen is
located offshore.