25th International Meeting on Organic Geochemistry IMOG 2011

O-17
Composition analysis of individual petroleum inclusions:
Preliminary application in the reservoir-filling history of theTahe
oil field, Tarim Basin, NW China
Weijun Shi 1,2 , Maowen Li 1,2 , Binbin Xi 1,2 , Jin Xu 1,2 , Zhirong Zhang 1,2 , Jianzhong Qin 1,2 ,
Hong Jiang 1,2
1 Sinopec Research Institute of Petroleum Exploration and Production, Beijing, China, 2 Wuxi Institute of
Petroleum Geology, Sinopec Research Institute of Petroleum Exploration and Production, Wuxi, China
(corresponding author:shiwj0082@yahoo.com.cn)
Individual hydrocarbon fluid inclusions can be
extracted and verified under microscopic observation,
different types of inclusions formed at different
diagenetic stages can be then analyzed with a variety
of techniques. This report presents the results of our
recent project that uses an online gas
chromatography-mass spectrometer (GC/MS) to
study the chemical compositions of hydrocarbons in
individual petroleum inclusions in carbonate
reservoirs. Because of the small volume and the low
concentration of hydrocarbons in individual inclusions,
this work represents a significant technical
breakthrough in the petroleum system study of the
Tarim Basin, NW China..
Tahe oil field is situated in southwest of Arkekule
Uplift, Tarim Basin. Crude oils are produced
dominantly from the Ordovician carbonate reservoirs,
overlain by Triassic-Lower Jurassic strata. Upper
Carboniferous-Devonian and Upper-Middle Jurassic
strata are absent in the study area. Ordovician
carbonate core samples were collected from eastern
Tahe oil field and analyzed for petroleum inclusions.
Petroleum inclusions are extremely abundant in these
samples, occurring mainly in calcite and quartz
cements and calcite vein in carbonate rock matrix.
Coeval aqueous and petroleum inclusions are readily
identified and distinguished under the microscope by
their fluorescence properties, occurrences and
homogenization temperatures. Two types of
petroleum inclusions are identified. The first and
dominant type displays blue fluorescence, and the
homogenization temperatures of the associated
aqueous inclusions vary from 74 to 92 ooC. The
second type of petroleum inclusions display yellow
fluoresce, iand variable homogenization temperatures
from 58℃ to 113℃ .
Laser ablation supports the GCMS analysis of small
samples (< 100 mm). It affords ultra-high sensitivity
and an ability to separately analyse in situ
microscopically discrete morphological entities. The
new laser ablation GCMS facilities equipped with
identical GC inlets have been recently established at
the Wuxi Institute of Petroleum Geology, where the
instrument uses a pulsed 193 nm excimer laser
targeting the analyses of oil bearing inclusions.
Excimer laser is the most suitable tool to open the
fluid inclusions hosted in quartz or calcite and the high
performance inlet ensures excellent products transfer
.
Results of our on-line GCMS analysis for individual
inclusions indicate that C4-C30 hydrocarbons are
present, and two types of petroleum inclusions bear
distinctly different chemical compositions. The blue
fluorescence inclusions have higher proportions of
saturated hydrocarbons and gaseous hydrocarbons,
but lower proportions of aromatic hydrocarbons than
the yellow fluorescencing inclusions.. Normal alkanes
in the blue fluorescencing inclusions show a unimodal
distribution, with the maximum at n-C17, and
those in the yellow fluorescencing inclusions have
dual modal distribution, maximized at n-C15 and C25.
In general, the yellow fluorescencing inclusions
contain much more water soluble aromatic
hydrocarbons than those in the blue ones, as
indicated by the ratio of 1,7-dimethylnaphthalene/n-
C14 alkane. TBecause the GC retention time of both
compounds are similar, their ratio is unlikely to be
affected by sample handling processes. Five samples
were analyzed, and the results show that 1,7-
DMN/nC14 ratios in the yellow fluorescencing
inclusions are around 3.37, while those in the blue
fluorescencing ones are only 0.46. It is reasonable to
infer that the composition and the formation time are
different for these two types of inclusions.
On the basis of homogenization temperatures and
fluorescence, in particular the composition,it can be
delineated that there were two main stages of the
petroleum-filling in the Tahe oil field. Compared with
the blue fluorescence petroleum inclusions, the yellow
ones have a lower maturity level, which may be
caused by an early hydrocarbon charge.
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