25th International Meeting on Organic Geochemistry IMOG 2011

P-074
Discussion on appliance of 25-norhopanoids compounds in
organic geochemical research
Chunhua Ni 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:nichunhua.syky@sinopec.com)
Abundant of crude oil which suffers from different
biodegradation degrees occurs widely in the world. At
present, the biodegradation degrees of crude oil are
mainly under qualitative identification based on some
biomarkers detected in the crude oil which stand for
different antibiodegradation abilities.
19 oil samples collected from the primary oil and gas
structures of Bohai Bay Basin, North China have been
investigated for their physical properties, molecular
geochemistry and carbon stable isotope. According to
the comparison of geochemical parameters, 4 oil
samples from the same oil and gas structure show the
similar characteristics which prove they have the
same genetic type, although there are some
differences in their physical properties. The theory
research indicates that there is a relative positive
relationship between the abundance or concentration
of 25-norhopnoids and the biodegradation degree of
crude oil. Therefore, the two ratios which are C29nor(25-norhopane)/C30-17
α -hopane ratio and C28nor(25-norhopane)/C29-17
α -norhopane ratio
respectively are applied to semi-quantitatively
evaluate the biodegradation degree of the four crude
oil. They distribute obviously in the different areas
from the figure which reveals there are four
biodegradation degrees. The results of our study
coincide well with the conventional knowledge, the
higher the biodegradation degree of crude oil is, the
heavier its density is, that is to say, the oil sample with
the highest biodegradation degree has the heaviest
density. In addition, the crude oil with lower burial
depth is more likely to suffer from biodegradation, so
the No. 4 oil sample whose burial depth is only 1131.6
meters encounters the highest biodegradation degree
among them.
In contrast with foreign petroliferous basins, Chinese
basins are famous for their more complex evolution
history of tectonic movements and most of them have
two or more phases of hydrocarbon generation,
migration and accumulation.
Geochemical characteristics of the crude oil from Well
B1 with underproductive oil flows in Southern North
China Basin are studied and they show the crude oil
is characterized by higher abundance of C16
homodrimane, C23 tricyclic terpanes, pentacyclic
terpanes, C31 homohopane and C27 sterane. Oilsource
correlation shows it is from the source rock of
the Lower Cretaceous, which belongs to an
indigenously generating and accumulating oil
reservoir. According to the biodegradation sequences
of different biomarkers, normal alkanes are prior to be
destroyed by the bacteria and 25-norhopanoids series
usually appear when the crude oil undergoes severe
biodegradation. Interestingly, normal alkanes and 25norhopanoids
series coexist in the oil sample from
Well B1. How to explain this strange phenomenon?
With the rapid development of numerical simulation
technology, more and more professional software are
used to complete the inversion of the evolution history
of the petroliferous basins, including thermal history,
hydrocarbon generating history and hydrocarbon
charging history. Correspondence with the analysis of
burial-thermal evolution history, there were at least
two phrases of hydrocarbon charging of the crude oil
from Lower Cretaceous of Well B1, the first charging
phrase was the Early Cretaceous when it was
dominated by low-matured oil with 0.50% Ro and
suffered from biodegradation, the second charging
phrase was from late Tertiary to Quaternary
predominant with high-matured oil with 0.70-1.30%
Ro. So the crude oil from Well B1 in Southern North
China Basin is the mixture of the two-phase charging.
It has been noticed that different biomarkers have
their specific prerequisite and they should be
comprehensively analyzed in combination with other
research methods to promote their effectiveness. 25norhopanoids
compounds is no exception.
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