25th International Meeting on Organic Geochemistry IMOG 2011

P-325
Origin of abnormal sonic response in the shale interval of
Northern Songliao Basin, China
Huo Qiuli, Zeng Huasen, Fu Li, Ren Zhigao, Fan Qinghua, Zhang Xiaochang
Exploration and Development Research Institute, Daqing Oilfield Company Ltd., PetroChina, Daqing, China
(corresponding author:yjydh206@yahoo.com.cn)
The deviation of sonic transit time log from normal
trend in the shale interval has been widely interpreted
as overpressure since Magara (1978). The
prerequisite for the relationship between sonic transit
time and overpressure is that overpressure is induced
by undercompaction or disequilibrium. The origin of
overpressure is still in controversial, however. Aplin et
al.(1995) suggested that the relationship of porosity
vs. depth was influenced by lithology heterogeneity.
Another example is that overpressure can exist in the
shale interval of normal pressure (e.g. Teige et al.,
1999, Hao et al., 2007). It seems that overpressure
and the deviation of sonic transit time log from normal
trend are not necessary closely correlated. The
deviation of sonic transit time log from normal trend is
widespread in the shale interval of Qingshankou
formation at the basin scale, especially on the bottom
of Qingshankou 1st member (K2qn1) which is the
main source rock in the Songliao Basin. In most of
past studies (e.g. Wang, 1984, Chi et al., 2000, Xiang
et al., 2006), those deviations of sonic transit time are
considered to be induced by overpressure of shale
based on undercompaction or disequilibrium
mechanisms, and as the main force that migrated the
oil generated from K2qn1 downward to the underlying
Fuyang reservoirs. However, our recent studies
showed that the deviations of sonic transit time may
be more closely correlated to the organic carbon
content in the shale interval which has never been
noticed before. Profiles of sonic log and total organic
carbon content (TOC) of three representable wells
with different organic maturities, which excludes the
hydrocarbon generation origin of overpressure, show
that the sonic log nearly traces the TOC profile in
each well (see Fig. 1). It is accordingly believed that
the deviation of sonic log is induced by organic
content heterogeneity in the shale interval (i.e.
lithology heterogeneity). The sonic transit time of
shale in K2qn1 with different burial depths was then
plotted by depth in Fig. 2 to decrease the effect of
lithology heterogeneity. The result is obvious and the
sonic transit time decreases as the depth increases
without any deviation, which corroborated that the
sonic transit time deviation in the shale interval is
most likely the result of lithology heterogeneity
(organic content) instead of overpressure induced by
undercompaction or disequilibrium.
Fig. 1 Sonic transit time and TOC profiles by depth in the
Songliao Basin. TOC log were calculated using method
described by Passey et al.(1990) while calibrated by
measured data (solid circle). SK-I is a scientific drilling well
from ICDP project. Ro is the reflectance of vitrinite.
Depth(m)
0 60 120 180
0
Du 401
500
1000
1500
2000
2500
Jin 37
Gu 302
SonicTransit Time(us/ft)
Shengshen 2
Si 103
Shuang 11
Fig. 2 Sonic transit time (of shale) profile by depth in the
K2qn1. Short horizontal lines are the sonic transit time
ranges.
References
1.Aplin, A.C., Yang, Y., Hansen, S., 1995. Assessment of
[beta] the compression coefficient of mudstones and its
relationship with detailed lithology. Marine and Petroleum
Geology 12, 955-963.
2.Magara, K., 1978. Compaction and Fluid Migration.
Elsevier Science & Technology.
458