well log analysis and seismic data interpretation of the central part of
WELL LOG ANALYSIS AND SEISMIC DATA INTERPRETATION
OF THE CENTRAL PART OF CONCESSION-6 (SORRA FIELD),
MIDDLE EASTERN PART OF SIRT BASIN, LIBYA
Funding Agency: Sirt Oil Company Libya
*Adapted from extended abstract prepared in conjunction with poster
presentation at GEO 2014 University of Bucharest, Romania
Many geophysical surveys were carried out
in the study area which represent a small part
of the middle sector of concession 6 which is
located in the middle eastern part of Sirt
Basin. The thesis presents the geology of the
study area in particular and the analysis of
geophysical data carried out by the Sirt Oil
Company (SOC) in concession-6 in different
stages, with the aim of discovering
This research was carried out in order to
answer questions relating to why reservoir is
limited, dimension of the reservoir and
whether to confirm previous studies or not
relating to absence of oil in this field. All
data used are 2D seismic sections and well
log records which are the main geophysical
tools of this study. figure 1 Shows location of
the study area and base map. The seismic
data was correlated with nine boreholes,
including: gamma-ray, SP,
resistivity, induction, neutron and densit y
Fig 1 Location and Base Map of the Study Area.
Geological background from wells
Sorra field is Upper Cretaceous Waha and
Bahi Formations strata and early Palaeozoic
Gargaf quartzite sandstones. Bahi Formation
consists of coarse-grained sandstone;
conglomerate and quartzite, Bahi are present
in several wells. The maximum thickness of
these strata encountered is 185 ft in PP7.
Waha Formation limestone is conformably
overlained (capped) by usually Kalash
Limestone, which is the excellent mappable
seismic marker. Waha Formation forms the
most important reservoir in the Sorra Field.
from density log). Consequently, if the
density and neutron logs are run
simultaneously, zones where gas occurs will
show a large curve separation, where zones
of oil and water show similar curves tracks.
Figure 2 shows the gas effect.
Well logs analysis
There are nine wells drilled in the study
area with maximum depth 9075 ft in pp9
and minimum depth 8789 ft in pp3.
6FF40 induction log type is used in the study
area with both curves resistivity and
conductivity. The conductivity curve gives
approximately zero reading at the gas
zones and resistivity curve gives very high
resistivity value at the same zone. Shale
content was calculated using gamma ray logs
that showed the percentage of shale in
Bahi/Gargaf formation in the wells PP5, PP6,
PP7, PP9 and pp10. pp10 has a relatively
low shale where drilled on the top of the
target whereas PP7 has the highest shale
content in the five wells in the southeastern
part of the study area.
The readings of the Neutron, Sonic, and
Density logs depend not only on porosity but
also on formation lithology and fluid content.
Crossplot of two porosity logs are convenient
to display both porosity and lithology
information. In the study area, Neutron and
density porosities are crossplotted for making
determination of lithology and porosity. Gas
effect gives low neutron porosity and show a
high-density porosity (porosity calculated
Fig 2 Neutron and Density Gas Effect in PP5 on Study
Looking to the percentage of porosity in
Bahi/Gargaf formation from crossplot in
the wells PP9 16% PP6 12% PP7 10%
and PP5 9%, PP9 has the highest porosity
value of Bahi/Gargaf formation, which
may be due too high percentage of shale
content in this formation by increasing of
shale the porosity also increases but
the quality of the reservoir decreases
because permeability decreases. For
Waha formation PP9 has porosity of 14%
and PP6 has 19%. PP1 is a dry well this
may be because of increasing of shale
ratio the best quality of the reservoirs is
found in PP6. In other word porosity
trend increases in north west direction,
porosity trend represented only the area
that wells found in it in the central east of
the study area.
We try to drive from the seismic data as
much information as possible about
geology of the study area and about the
nature of the rocks as an effort to know
about the probability of encountering
petroleum in the structures that we map.
Seismic data have been interpreted in two
modes, with gradations between the
modes. The first is in areas of substantial
well control, in which the well
information is first tied to seismic
information and the seismic then supplies
the continuit y between the wells for the
zone of interest.
The structural trap is an anticline; time
maps were contoured for three horizons
Kalash Hagfa and Khalifa with contour
interval 16 ms. Time-structure maps are
constructed to display the geometry of
selected reflection. All the three time
maps show the minimum value in south
west whereas the maximum value in north
east and west. Average velocity calculated
from wells that are given in Table 1, used
to convert Kalash time map reflector (seal
rock) to depth map. Figue 3 shows 3D
Kalash depth model which is contoured
w i t h contour interval 100 ft. The
northwestern part of the area reach to
8400 ft and the shallower depth located in
the southwestern part of the map reach to
6500 ft. The regional dip of Kalash is
from the south to the north.
Final interpretation shows missing
(pinching out) of Waha formation in
PP10 and PP4 which can be interpreted
as a result of nondeposition palaeotopographic high of the basement. This
is also clear in the seismic data (line 6V242A-84 between PP3 and PP4) figure 4
shows lateral facies change from
limestone to sandstone.
Waha Formation is found in wells PP9,
PP6, and PP3. The formation become
shaley in wells PP9 and in the north west
of the study area. This is confirmed by the
study of shale content. The thickness of
the formation increases toward PP9 and
give the maximum thickness reach to 154
ft. Kalash Formation was found in all
wells and gives the minimum thickness in
well PP10. This formation formed the cap
of the reservoir (Waha Formation) and
it changes in lithology from limestone to
dolomitic limestone This is confirmed by
the study of crossplot in the study
Sirt basin owe thermal alteration. Sorra
gas pool is at a temperature gradient
much higher than the average for Sirt
basin. There is a small exception for the
well PP-2 because it consists of a very
small amount of oil, but it is not
economically available. The thermal
maturity is a major factor controlling oil
and gas generation from organic matter.
Table 1 Calculation of Kalash Depth from Well Data
Averaging of Actual
Fig 3 3D Surface For Kalash Depth Horizon.
Fig 4 Pinching out of Waha Formation between PP3 and PP4 in Line 6V-242A-84 in the Study Area.
I would like to express my sincere thanks
to the exploration division of Sirt Oil
Company (SOC) for releasing all data
needed for this study in addition special
thanks to prof. Mohammed B. Abdelmalik
and to university of Benghazi earth science
department for their help. I am grateful to
university of Bucharest for giving a chance
to present this study.
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