Source rock geochemical study of shallow biogenic methane accumulations in
Crete (Greece) island
Nikos Pasadakisa,*, Emmanouil Manoutsogloua, Avraam Zelilidisb, Maowen Lic
a
Mineral Resources Engineering Department, Technical University of Crete, Greece
(* corresponding author: pasadaki@mred.tuc.gr)
b Department of Geology, University of Patras, Greece
c Geological Survey of Canada, Calgary, Alberta, Canada
Shallow methane gas accumulations generated
from microbial activity emerged as an increasingly
important energy resource. In such countries as
Greece with limited hydrocarbon resources, the use
of biogenic gas, even in low quantities for domestic
heating, appears as a viable and economically
accepted option.
In an extended area (30 km2) located in the central
part of Crete island (Heraclion-Messara) region (Fig.
1), numerous methane gas seepages have been
reported during the last decades, mainly associated
with water wells. The gases contain approximately
90% methane and 10% nitrogen (vol.), indicating a
possible biogenic origin. Drilling data show that the
gas accumulations are located in sandstone beds
together with water of high salinity. Sedimentary
succession consists of coarsening upwards cycles,
each up to 100 m in thickness, with marls at the base
and sandstone beds at the top of each cycle. This
work aims to investigate the origin of the shallow
methane gas shows and to reveal its possible source
formations.
methane gas, while separated layers of low
permeability are considered as the “caprocks” of the
reservoirs.
Rock samples from four wells (each with
approximately 400 m of penetration) were analyzed
using standard geochemical procedures (Rock-EvalTOC, solvent extraction, fractionation and GC-MS
analysis).The measured total organic carbon values
(TOC) are relatively low, but well within the same
magnitude as those reported low organic productivity
environments, commonly associated with biogenic
gas production [2]. The low ratios of extract to TOC
and non-polar to polar extract fractions are interpreted
as the indication of immature organic matter [3]. The
calculated hydrogen (HI) and oxygen (OI) indices,
from the Rock-Eval analysis, indicate that most of the
samples are related to type IV kerogen, with few
exceptions showing abnormally high OI which was
attributed to the presence of immature humic
material.
The n-alkanes showed a unimodal distribution with
a progressive enrichment of odd-numbered alkanes in
the C29-C33 range, which correlates positively with the
age of the sediments. The observed predominance of
nC29, nC31 and nC33 found to correlate positively with
the formation depth indicating an increased higher
plant terrestrial organic matter input. The biomarkers
data are used to constrain paleodepositional models,
suggesting that a lacustrine and a lagoonal–shallow
marine paleo-environment existed in a restricted
basin.
Fig. 1. Location of the study area.
References
The island of Crete in the eastern Mediterranean
represents an emerged part of the Hellenic Arc. The
Late Cenozoic evolution of Crete has been controlled
by the final stage of northward subduction of the
African plate beneath the Aegean lithosphere.
Extensional faulting in the Middle Miocene resulted in
the denudation and considerable thinning of the
thickened crust and in the formation of the Neogene
basins [1]. Depositional environments were lacustrine,
brackish lagoons and marginal marine environments.
Within these basins, thick (more than 1000 m),
organic-rich,
sedimentary
successions
were
accumulated in stacked, coarsening upward cycles.
These 80-100 m cycles consist of massive mudstone,
which passes upwards to channelized sandstones
with sufficient porosity and permeability. These
formations can serve as the source rock of the
[1] Meulenkamp, J.E., Dermitzakis, M., GeorgiadouDikeoulia, E., Jonkers, H.A., Böger, H. (1979)
Field Guide to the Neogene of Crete,
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Palaeontology, University of Athens, Athens, p.
32.
[2] Pang, X., Zhao, W., Su, A., Zhang, S., Li, M.,
Dang, Y., Xu, F., Zhou, R., Zhang, D., Xu, Z.,
Guan, Z., Chen, J., Li, S. (2005) Org. Geochem.,
36, 1636–1649.
[3] Curiale, J.A., Lin, R. (1991) Org. Geochem. 17,
785-803.