25th International Meeting on Organic Geochemistry IMOG 2011

P-110
Geochemical characteristics of the mixed marine oils in the
Tazhong Uplift of Tarim Basin, NW China
Yankuan Tian 1 , Chupeng Yang 2 , Zewen Liao 1 , Haizu Zhang 3
1 State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of
Sciences, Guangzhou, China, 2 Guangzhou Marine Geological Survey, Ministry of Land and Resources,
Guangzhou, China, 3 Institute of Petroleum Exploration and Development, Tarim Oilfield Company, Korla,
China (corresponding author:liaozw@gig.ac.cn)
Tazhong oil reservoirs are located in the central
Tarim Basin, NW China, which have been charged
mainly by two sets of marine source rocks, namely
the Cambrian-Lower Ordovician (∈-O1) and the
Middle-Upper Ordovician (O2+3). Due to the complex
post-depositional alterations such as the multiple
hydrocarbon generation, migration and accumulation
episodes, most oils in this area existed as the mixed
oils. The contribution proportions from each source
rocks are significant for petroleum exploration and the
evaluation of hydrocarbon resource in this area.
1. Geochemical studies
Geochemical study of three oils from different
subunits and strata were performed, exemplified by
the oils of Tz104, Tz122 and Tz4-7-38.
The terpane distributions of crude oils were
characterized by higher concentrations of tricyclic
terpanes, C29 norhopane/C30 hopane > 1, reflecting
the last charge from the Middle-Upper Ordovician
(O2+3) source rocks. Adsorbed compounds inside
asphaltenes had similar terpane distribution patterns.
However, terpanes occluded inside asphaltenes,
which represented the origin oils directly derived from
kerogens (Liao et al., 2006), suggested contributions
from the Cambrian-Lower Ordovician (∈-O1) source
rocks. Likely, the sterane distributions implied the
mixed oils deriveded from both source rocks.
2. Quantifying the contribution proportions
In Tarim Basin, the oils sourced from ∈-O1 are
more enriched in 13 C than those from O2+3. Oil Tz62
with δ 13 C -28.606‰ sourced from ∈-O1 and YM2 with
δ 13 C -33.462‰ from O2+3 were widely accepted as
two end member oils (Li et al., 2010). The bulk stable
carbon isotope values of mixed oils are restricted by
the following formula:
δ 13 Cmixed = δ 13 C∈ -O1 * f+δ 13 CO2+3 * (1-f),
where δ 13 Cmixed , δ 13 C∈ -O1 and δ 13 CO2+3 stand for the
bulk stable carbon isotopes of the mixed oils, Tz62
and YM2 oils, f stands for the contribution proportions
from the ∈-O1 source rocks to the mixed oils.
According to the formula, the relative contribution
proportions for 108 oils collected from different
subunits in the Tazhong area were quantified and
discussed. The results (Fig.1) indicated that on the
whole the O2+3 source rocks contributed more to most
oil reservoirs in this area. However, the relative
contribution of both source rocks varied a lot, without
evident regularity for the different oil reservoirs in
different subunits of the Tazhong area.
Fig. 1. Diagram of the compositions of the mixed oils
from different subunits in the Tazhong area, Tarim
Basin.
References
Li, S., Pang, X., Jin, Z., Yang, H., Xiao, Z., Gu, Q.,
Zhang, B., 2010. Organic Geochemistry 41, 531-
553.
Liao, Z., Graciaa, A., Chrostowska, A., Creux, P.,
Geng, A., 2006. Organic Geochemistry 37, 291-
303.
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