25th International Meeting on Organic Geochemistry IMOG 2011

O-05
Highly condensed, sulfur-rich hydrocarbon detected in oils
altered by thermochemical sulfate reduction: Precursors to TSRsolid
bitumen
Clifford Walters 1 , Kuangnan Qian 1 , Chunpingq Wu 1 , Anthony Mennito 1 , Zhibin Wei 2
1 ExxonMobil Research & Engineering, Annadale, NJ, United States of America, 2 ExxonMobil Exploration
Company, Houston, TX, United States of America (corresponding
author:clifford.c.walters@exxonmobil.com)
Thermochemical sulfate reduction (TSR) involves a
complex series of free radical and redox reactions
that occurs in hot (>120°C) carbonate reservoirs
whereby petroleum is oxidized by sulfate forming
primarily H2S, CO2, and an insoluble, highly sulfurenriched
and largely polynuclear aromatic organic
solid. The process negatively impacts the quality and
quantity of the petroleum reserves. Using ultrahigh
resolution mass spectrometry, we show that the TSRorganic
solid is formed via condensation and
sulfurization reactions that take place within the
petroleum fluid.
A suite of Smackover Fm. oils from several subbasins
along the northern rim of the Gulf of Mexico was
analyzed using APPI- (atmospheric pressure
photoionization) and NESI- (negative ion electrospray
ionization) ion cyclotron resonance Fourier transformmass
spectrometry (ICRFT-MS). The absence or
extent of TSR alteration had been assessed by prior
isotopic and molecular analyses. The selected oils
thus represent fluids that were exposed to a range of
thermal maturity (from early oil generation through the
oil window into gas generation and oil cracking) with
and without the influence of TSR alteration.
The composition of oils unaltered by TSR changes in
a manner consistent with thermal processes. With
increasing thermal stress, the distributions of aromatic
hydrocarbons (HC) and heteroatomic species
containing one and two sulfur atoms (1S and 2S,
respectively) shift toward species with smaller
polynuclear aromatic cores and lower degrees of
alkylation. In contrast, TSR-altered oils contain
relatively high concentrations of species with highly
condensed, polynuclear aromatic cores that are
enriched in sulfur. The strongest enrichments are
seen in condensates that have experienced the
greatest degree of TSR alteration. These fluids are
produced from reservoirs with high concentrations of
H2S and they possess relatively high concentrations
of sulfur-containing diamondoids and light
hydrocarbons with very heavy δ 13 C values –
characteristics known to indicate the degree of TSR-
alteration. The degree of alkylation is lower in the
TSR-altered oils than non-TSR altered oils of
equivalent maturity suggesting that TSR also
promotes the oxidative removal of alkyl sidechains.
These sulphur-enriched species are termed ―protosolid
bitumen‖ as they are marginally soluble in the
TSR-altered oils and could easily precipitate with
slight chemical alteration or changes in reservoir
conditions.
The presence of these species prove that sulfurenriched-solid
bitumen may arise from the TSR
process and do not require the prior occurrence of
asphaltenes in the unaltered oil.
H/C
HC 1S 2S HC 1S 2S 3S
2.02
1.75
1.5
H/C
1.25
1.01
.75
Unaltered
37.3° API
0 mol% H 2 S
Brantley-Jackson Chitsey
Min
0.5.5
0 .05 .1
.15
0
.05 .1 .15
0 0.5 1.0 0 0.5 1.0 Min
* Only HC,1S,2S,3S Shown.
S/C
S/C S/C
TSR-altered
51.3° API
16 mol% H 2 S
Max
Max
ccwalte - 10-Jan-2011 11:55
APPI-ICR-FTMS comparing the relative distribution of
aromatic hydrocarbons (HC) and sulfur-containing
(1S, 2S, & 3S) species in unaltered and TSR-altered
oil from the Mexia-Talco trend (Tx). The TSR-altered
oil is enriched in highly condensed HC and contains
highly condensed 1S, 2S and 3S species that are not
present in the unaltered oil.
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