Twenty crude oil samples were obtained
from the Gudong Oilfield
and their organic geochemical characteristics were analyzed. The oil
samples were classified into three families by hierarchical cluster
analysis and principal component analysis based on 13 source-related
and depositional environment-related biomarker parameters. Oils in
family I have low ratios of C
19
/C
23
tricyclic
terpanes and C
24
tetracyclic terpane/C
26
tricyclic
terpanes, and relatively high ratios of steranes/hopanes and C
30
4-methylsteranes/ααα20R C
29
sterane, thus indicating that microalgae were the dominant organic
matter input for the source rocks of family I. The gammacerane/C
30
hopane ratios are higher than that of family II and family
III, whereas the C
35
/C
34
homohopane ratios are
lower, thus indicating a suboxic, brackish water environment for the
source rocks. The inferred source rock is the first member of the
Shahejie Formation in the Huanghekou Sag. Family II is characterized
by high ratios of C
19
/C
23
tricyclic terpanes
and C
24
tetracyclic terpane/C
26
tricyclic terpanes
but relatively low ratios of steranes/hopanes and C
27
/C
29
αααR steranes. These findings suggest
that the original organic matter of the source rocks had a greater
contribution from terrigenous higher plants than from microalgal.
The relatively low ratios of gammacerane/C
30
hopane and
C
35
/C
34
homohopane suggest that the source rocks
were deposited in an oxic environment with a low salinity, thus corresponding
to the Dongying Formation in the Huanghekou Sag. Family III oils have
high C
27
/C
29
ααααR steranes
ratios and low C
30
4-methylsteranes/ααα20R
C
29
ratios, which indicate the contribution of microalgae
(especially zooplankton algae) to the source rocks. The relatively
high abundance of C
35
homohopane and low gammacerane/C
30
hopane ratios suggest a weakly reducing condition with low
salinity, which is in accordance with the third member of the Shahejie
Formation in the Gunan Sag. The C
31
S/(S + R) homohopane
ratios imply that oil samples in this study are in the mature stage,
although the ratios of C
29
20S/(20S + 20R) and C
29
ββ/(αα + ββ) steranes suggest
that the maturity of family II is higher than that of family I and
family III.