25th International Meeting on Organic Geochemistry IMOG 2011

P-038
Organic geochemistry of coals and carbonaceous shales from
the palaeocene Los Cuervos formation in San Pedro Del Río
(Táchira State, Venezuela)
Marcos Escobar 1 , Manuel Martinez 2 , José R. Gallego 3 , Gonzalo Marquez 4 , Azucena
Lara-Gonzalo 3
1 University of Zulia & CARBOZULIA, Maracaibo, Venezuela (Bolivarian Republic of), 2 Central University,
Caracas, Venezuela (Bolivarian Republic of), 3 University of Oviedo, Mieres, Spain, 4 University of Huelva,
Palos de la Fra., Spain (corresponding author:escomar24@gmail.com)
Several authors [1] have previously postulated good-
to moderate-quality continental source rocks in the
Palaeocene Los Cuervos Formation (Venezuela).
This possibility makes it feasible to undertake an
organic geochemical and petrographical investigation
of the carbonaceous shales and coals in a geological
section of this formation opposite San Pedro del Río
village (Táchira State). It should be noted that the
heterogeneous character of sedimentary depositional
environments of continental facies, a common
occurrence in a fluviodeltaic system such as the one
that gave rise to the aforementioned unit [1], involves
the need to perform a detailed evaluation of the study
section since possible transgressive or regressive
pulses may alter the quality of the source rocks
throughout any given stratigraphic column.
Twelve samples, six of carbonaceous shales and the
rest of coals, were collected and analysed. These
samples (TLC-1 to TLC-12) were numbered from the
bottom of the stratigraphic column of the Los Cuervos
Formation in San Pedro del Río. The following
analyses were performed: shales (COT and Rock-
Eval pyrolysis), coals (vitrinite reflectance, organic
petrographic and Rock-Eval pyrolysis), and bitumens
(SARA method and gas chromatography coupled to
mass spectrometry for the saturate fraction: acyclic
isoprenoids, n-alkanes, steranes, and terpanes).
Results clearly indicate that vitrinite reflectance, Rock-
Eval, %22S, %20S, and COT values are
characteristic of moderate to good source rocks that
have reached an early oil window maturation level
(�0.6) [2]. Likewise, the n-alkane distribution patterns,
pristane to phytane ratios (>3), presence of 18�(H)oleanane
and predominance of ���C29 steranes over
C27 homologues suggest a type III kerogen [2].
The increase in the total sulfur content in these coals
from bottom (�0.8%) to top (�1.4%) of the sequence
suggests the existence of a transgressive pulse [3],
whose maximum corresponds with the TLC-10 and
TLC-11 samples, within a regressive event, typical of
the rest of the Los Cuervos Fm. There is also an
evolution towards a fluvial depositional environment
that characterizes the overlying formation: El Mirador.
The predominant maceral group in these coals,
except TLA-6, is the vitrinite type (80%). The upward
increase in desmocollinite and corpocollinite
concentrations, as well as of exinites, may be
attributed to a change in the dominant vegetation in
the swamp or peatbog, from woody plants at the base
to mangroves or ferns, with slight gymnosperm input
and a marine influence at the top of the sequence [4].
The facies diagram based on the tissue preservation
index (TPI) and gelification index (GI) [5] (Figure 1)
denotes that the study section represents a
progressive transition between a limnotelmatic facies
in the lower part of the Los Cuervos Fm., associated
with a meandering fluvial channel system, and a
limnic facies in the central and upper part deposited
in a barrier-protected swamp in a low coastal area.
Fig. 1. TPI vs GI diagram for the study coals.
References
[1] Boesi, T., et al. (1988). Stratigraphic study of the
Northandean Flank. In: Memoirs of III Bolivarian Symposium
on Petroleum Exploration of the Subandean Basin, 613-637.
[2] Hunt, M. (1996). Petroleum Geochemistry and Geology,
2nd Edition. Freeman, New York.
[3] Caruccio, F.T., et al. (1977). Paleoenvironment of Coal and
its Relation to Drainage Quality. US EPA Agency, Cincinnaty.
[4] Teichmüller, M. (1989). Int. J. Coal Geol., 12, 1-87.
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