WELL LOG ANALYSIS AND SEISMIC DATA INTERPRETATION OF THE CENTRAL PART OF CONCESSION-6 (SORRA FIELD), MIDDLE EASTERN PART OF SIRT BASIN, LIBYA

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1 WELL LOG ANALYSIS AND SEISMIC DATA INTERPRETATION OF THE CENTRAL PART OF CONCESSION-6 (SORRA FIELD), MIDDLE EASTERN PART OF SIRT BASIN, LIBYA Elagieli Hamdi Funding Agency: Sirt Oil Company Libya Project: Dissertation *Adapted from extended abstract prepared in conjunction with poster presentation at GEO 2014 University of Bucharest, Romania Introduction 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 hydrocarbon. 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, temperature, resistivity, induction, neutron and density logs. Fig 1 Location and Base Map of the Study Area.

2 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. 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 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. Gas Gas zone zone effect effect Fig 2 Neutron and Density Gas Effect in PP5 on Study Area. 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 Neutron log Neutron log Density log

3 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. Seismic interpretation 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 continuity between the wells for the zone of interest. Structural Analysis 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 it h contour interval 100 ft. The maximum depth located in 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 6V- 242A-84 between PP3 and PP4) figure 4 shows lateral facies change from limestone to sandstone. Conclusion 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 area. 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.

4 Well Name Table 1 Calculation of Kalash Depth from Well Data Average Velocity Averaging of all Velocities Actual depth Calculatig Depth Error % PP % PP % PP % PP % Fig 3 3D Surface For Kalash Depth Horizon.

5 Waha formation Fig 4 Pinching out of Waha Formation between PP3 and PP4 in Line 6V-242A-84 in the Study Area. Acknowledgment 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. Reference Barr, F. T., and Weeger, A. A., (1972). Stratigraphic Nomenclature of the Sirt Basin, Libya, Petrol. Explore. Soc. Libya, Tripoli. Bateman, (1985). Log Guality Control. D. Reidel published company. Gadallah, M. R., (1994). Reservoir Seismology: Geophysics in Non Technical Language. Penn Well Publishing Company. Shaaban, F. F. and A. E. Ghoneimi (2001). "Implication of Seismic and Borehole Data for the Structure, Petrophysical and Oil Entrapment of Cretaceous-Palaeocene Reservoirs Northern Sirt Basin, Libya." Journal of African Earth Sciences 33: Sirt Oil Company, (1979). Geological Report of Sorra field (Concession 6). Geology and Hydrocarbon Potential. (SOC ) Exploration Department, al Bregah, Libya.