25th International Meeting on Organic Geochemistry IMOG 2011

P-340
Atypical fluids in offshore shallow buried reservoirs
Denis Levaché, Christine Lafaurie, Yannick Poirier, Gérard Ségalini
TOTAL Exploration & Production, Pau, France (corresponding author:denis.levache@total.com)
In (deep)offshore exploration, shallow buried
reservoirs are usually avoided due to the strong risk
of biodegradation and consequently highly viscous
fluids. Indeed the maximum risk of biodegradation
being situated towards 40-50°C, reservoirs around
900-1000m of depth are the riskiest.
However recent discoveries of light fluids in reservoir
at shallow depths have triggered questions regarding
the charge and preservation of the HC in these
reservoirs.
Typically these fluids have an apparent low
biodegradation level and good physical properties (i.e.
low in-situ viscosities and API above 30°). Strikingly
the molecular distribution of the oil is very unusual as
seen in Figure 1. Almost no n-alkane is present after
n-C17/C18 where Pristane and Phytane dominate.
The geochemical study on liquids by GC and GC-MS,
detailed analysis of associated gases (composition
and isotope ratio measurement � 13 C and � 2 H) and
mud gas analyses allow to propose a complex filling
history of these reservoirs involving multiple steps:
� A first phase of filling by an early charge that
is quickly biodegraded in reservoirs
subjected to temperatures lower than 60°C.
Such fluids are usually encountered in the
shallow reservoirs of these formations.
� The second phase of filling by retrograde
condensation of a light fluid and also
possibly a stripping by methane. The
reservoirs cannot hold the gas phase thus
generated as shown by the isotopic analysis
of mud gases that indicates continuous gas
fluxes from deeper origin.
The remaining fluid consists of the retrograde
condensate overprinting the initial biodegraded
charge.
The use of the geochemical interpretation helps to
build the proper thermodynamic model that match the
behavior of these fluids in these reservoirs. The
model used here involves two source rocks that have
expelled at various stages of maturities. The predicted
expelled fluids placed at the reservoir conditions have
a behavior in good agreement with the observed fluid
composition.
Norm.
Finally 1400 the understanding of the presence of these
gas fluxes is the main driver to predict the HC fluid
quality and thus the success of a prospect in these
1200
shallow reservoirs.
1000
800
600
400
200
0
*FID1 A, (L:\POLE MOLECULAIRE\GC-ID\ANALYSES\CAMEROUN 2008\NJOM001B\165171ID.D - NOVEMBRE\BL081118.D)
nC6
nC5
nC7
nC8
nC9
nC10
nC11
nC12
nC13
nC14
nC15
0 25 50 75 100 125 150 175 200 225
nC16
Pr
nC17
nC18
Ph
N-alkanes depleted from nC18
Fig.1 C6+ fraction Gas Chromatography of a selected
atypical oil.
471
min