2D Seismic Interpretation and Well Log Analysis of Missakaswal Area, Upper Indus Basin, Pakistan Urooj Shakir 1, Muyyassar Hussain 2, Rida Azhar 1, Anum Zafar 1, Snodia Asghar 1, Muhammad Raiees Amjad 1 1 Department of Earth and Environmental Sciences, Bahria University, Islamabad 2 LMK Resources, Islamabad Abstract Missakaswal area is located ten kilometers from Gujar Khan, Rawalpindi district of Punjab. Geographically the study area lies between 33 11' 06" and 33 11' 18.24" North and from 73 20' 42" to 73 21' 00" East. Stratigraphically, it lies in the Upper Indus which is characterized by large numbers of thrust and normal faults producing asymmetrical structures (anticlines/ synclines). The paper highlights the 2D structural interpretation and well log integration in order to delineate the subsurface structure and hydrocarbon potential. Two way time structure and depth contour maps have been prepared using surfer software. Time to depth conversion is done using the available stacking velocities. The depth structure map shows high relief anticlinal structure in the subsurface bounded to west and east by reverse faults. According to the petroleum geology, source rock in the study area is Patala Formation of Paleocene age; Chorgali Formation and Sakesar Limestone of Eocene age are acting as reservoir rocks while Murree Formation of Miocene age is acting as a seal. The petrophysical analysis is done to confirm the potential hydrocarbons bearing zone of the well Missakaswal-02. The total thickness of Chorgali Formation encountered in the well is 48 meters, but log data is only available of the starting 25 meters. The lower package of Chorgali Formation is still in production so well log data of this package is not available and as a result, log interpretation has not been carried out at complete package. Log analysis of 8 meters show average volume of shale 34%, average effective porosity 7% with average saturation of water < 40% and high hydrocarbon saturation > 60%. This makes the Chorgali Formation a potential zone. Key Words: Seismic, Potwar, Hydrocarbons, Petrophysics, Structural Trap. I. INTRODUCTION The area under study is Missakaswal oil field which is located ten kilometers from Gujar Khan. Geographically the study area lies between 33 11' 06" and 33 11' 18.24" North and from 73 20' 42" to 73 21' 00" East (Figure 1). Geologically, it lies in the Upper Indus Basin of Pakistan. Structurally, Potwar Sub-basin is a highly complex area and mostly surface features do not reflect subsurface structures due to presence of decollement at different levels (Moghal et al., 2007). The Potwar area is one of the oldest regions of oil production in Pakistan (Khalid et al, 2015). The first commercial discovery, made in Potwar sub-basin in 1914, was the Khaur Field by Attock Oil Company. Since then, this area has been viewed as an area of great interest for hydrocarbon exploration (Kemal et al. 1991). The main purpose of this study is to understand the tectonic and structural trends present in the subsurface. The structural analysis of the subsurface is done using seismic interpretation, which helped alot in getting a better understanding of the geological and stratigraphic nature of the area. Petrophysical analysis has enabled to picture the Hydrocarbon potential of the interested reservoir zones. The data provided for study includes a base map (Figure 2a), five 2D seismic lines (hard sections) (Table 1) and well log data/well tops of the Missakaswal-02 well. The well is located at some offset of the strike line GNA-11( Figure 6b). The Las file of the well 02 was not available so synthetic seismogram could not be prepared. The horizons were marked and confirmed through the T-D chart. Borehole stratigraphy of the well is shown in the figure 6c. The geological data was collected from the available literature. Fig. 1 Location map of Missakaswal. (Google earth) Table 1. Seismic lines provided for interpretation Line name Line type Line orientation 994-GNA-09 Dip Line SE-NW 994-GNA-10 Dip Line SE-NW 994-GNA-11 Strike Line SE-NW 994-GNA-14 Dip Line SE-NW 994-GNA-16 Dip Line SE-NW II. TECTONICS AND STRUCTURE STYLE Upper Indus Basin is located in northern Pakistan and is separated from central Indus Basin by Sargodha high. It consists of fold and thrust belt to the south of MBT. It forms the southern part of the Himalayan fold belt. The thrusting is terminated to the west and east by longitudinal transcurrent fault systems. The Indus river further divides the Indus Basin into Kohat sub-basin (west) and Potwar sub-basin (east) (Kadri, 1995). Page 1
The Potwar sub-basin is present in northern Pakistan at the western foothills of the Himalayas. It includes the Jhelum Plain, the Potwar Plateau and the Salt Range. The MBT and salt range thrust mark the northern and southern boundaries of Potwar sub-basin. While sinistral Jhelum fault, the Indus river and the Kalabagh dextral fault mark the eastern and western boundaries respectively (Moghal et al., 2007). The structure of the Kohat and Potwar sub-basin is shown in map view in figure 4. The sediments varying in age from Precambrian to quaternary are preserved in Potwar sub-basin (Kadri, 1995). The sediments that fill the Potwar sub-basin include Cambrian evaporates. These evaporates are overlain by the platform deposits of Cambrian to Eocene age which are relatively thin. Thick Miocene Pliocene molasses further overly these deposits. The Himalayan orogeny during Pliocene to Middle Pleistocene has resulted in the deformation of this whole section (Moghal et al., 2007).Various unconformities are responsible for interrupting the whole depositional sequence of Potwar subbasin out of which, the unconformity between Cambrian and Permian, and between the Eocene and Miocene are considered as the most significant ones (Khan et al., 1986 ). The generalized stratigraphy of Potwar sub-basin is shown in figure 3. Fig. 3 Stratigraphy of Potwarsub-basin, Pakistan(Aamir and siddiqui, 2006). Fig. 2a Base map showing seismic lines of Missakaswal Fig. 4 Tectonic map of Kohat-Potwar Plateau with highlighted box showing study area. (Kemal, 1992). Fig. 2b Time Depth chart prepared by stacking velocity III. METHODOLOGY A. Seismic Data Interpretation There are two main methodologies for analyzing seismic data. These methods include stratigraphic and structural analysis. The Structural analysis methodology is used for the seismic interpretation of this study. The seismic lines of Missakaswal area are old but the clarity of reflectors is good enough even at basement level. The overall quality of data is fair to good. Two horizons are selected and marked. These two horizons are Chorgali Formation and Sakesar Limestone from Eocene age. The correlation of data was done using Time Depth T-D chart manipulation technique as shown in Page 2
figure 2b. The control line chosen for TD chart is GNA-11 which is a strike line and the well is not exactly located on the seismic line rather it is at some offset rom it. The discontinuity in the reflector represents the faults. The main faults F1 and F2 are marked on the seismic sections which are detachment faults starting from Salt range Formation of Precambrian age and truncating in to Murree Formation of Miocene age. The scanned seismic sections of Missakaswal along with the marked horizons and faults are shown in figure 5 and 6. Table 2 Borehole Stratigraphy of Missakaswal well-02 Fig. 5 Horizons and faults on dip line 994-GNA-14. B. Time and Depth Sections Time section is a plot of the reflectors and the faults in the time frame against the shot points. The time section clarifies the subsurface picture as obtained on the seismic section. Depth sections have been produced after conversion of seismic data from time to depth using seismic stacking velocities (Average velocity). The velocity panels are provided with the seismic sections. The relationship between time to depth conversion using the available velocities is: S= V*T/2 Where: S=depth, V=velocity and T is time Depth contour maps are also prepared using the above relation. Fig. 6a Control line having well point-gna-11. C. Time and Depth Contour Mapping Time contours represent contour lines having the same time values. These values represent the time consumed by the generated seismic wave, to travel in the subsurface, strike a surface, reflect back and travel to the receiver. These values are plotted on a base map where latitude/longitude values of each shot point are given. Time and depth contour maps are generated by using the surfer software program. The Depth contour map represents the horizon in units of depth i.e. meters. This gives a more accurate structure style of the horizon in the subsurface. Figure 7 and 8 shows the depth contour maps for Chorgali Formation and Sakesar Limestone. Fig. 6b Horizons and faults on dip line 994-GNA-16 D. Interpretation of Depth contour map of Chorgali Formation The contour interval is 100m and the variations in the color represent changes in the depth. The interpretation of the map suggests that there is an anticlinal pop-up structure at Center and as we move towards periphery, the depth values increase progressively. The anticlinal structure is Page 3
bounded by two main faults F1 and F2. These are reverse faults that are dipping towards each other from east and west. This dipping of the faults towards each other while bounding the anticlinal structure is the reason why our area of interest is called a 2-way fault bounded Anticlinal pop-up structure and is shown in figure 7. There are no major misties present between the lines. However there are a few minor misties that have been removed during the interpretation process. E. Interpretation of Depth contour map of Sakesar Limestone The contour interval is 200m for Sakesar Formation and the variations in the color represent changes in the depth. The structure of the formation is fault bounded anticline pop-up structure because away from the faults the depth values are increasing. While the faults F1 and F2 are dipping towards each other from east and west. The crestal location is at around 1900m. The structure of the formation makes it a very suitable trap for hydrocarbons. F. Petrophysical Analysis The purpose of the petrophysical analysis is to obtain information from the of Missakaswal-02 well area. The raw log data obtained from LMKR includes the data from density log, gamma ray log, sonic log, SP log, neutron log and resistivity log. Chorgali Formation is marked as the zone of interest(table 2). Table 2 Zone of interest. Zone Formation Starting Depth (m) Ending Depth (m) Total Thickness Zone 1 Chorgali 1775 1800 25 Fig. 7 Chorgali formation depth contour The structure is a northeast-southwest trending surface expression of 19 x 3.2 km, eroded down to Chinjis (Mio- Pliocene) with vertical closure 700-800 m. The fault throw is approximately 150 m and the crestal location is 1600 m AMSL. The contour value around well point is 1800 m which also confirms the depth obtained through time depth chart. The inter seismic line distance is 100 approximately. The gas water contact GWC is at about 1898 m. Calculation of volume of shale (VSH) The volume of shale is calculated with the help of gamma ray log. The graph of depth vs. shale volume is shown in figure 9. Fig. 9 Volume of shale calculated in zone of interest. Fig. 8 Sakesar Limestone depth contour Fig. 10 Depth vs. Effective porosity Page 4
Fig. 11 Depth vs Sw (LLd) Fig. 13 Depth vs. Sh (LLd) Fig. 12 Depth vs Sw (Msfl) Fig. 14 Depth vs Sh (Msfl). Porosity calculation Porosity values at different depths is computed using Neutron and Density Logs. Then the average porosities are calculated by combining Neutron and Density values. The final product is called effective porosity. It is calculated by using the following formula: Effective Porosity= Vsand * Porosity avg Where, Volume of Sand (Vsand) = 1-Vshale The graph of depth vs. effective porosity is shown in figure 10. Average Porosity (Porosity avg) = (Density porosity + Neutron porosity) /2 Saturation of hydrocarbons (Sh) The determination of saturation of hydrocarbon is very important because this will help us depict the reservoir potential to produce hydrocarbons. The formula for calculating the hydrocarbon saturation is : Sh = 1-Sw Where, Sh = Saturation of Hydrocarbon Sw = Saturation of Water The graphs of depth vs. water saturation is shown in figure 11 and 12.While the graphs of depth vs. saturation of hydrocarbon is shown in figure 13 and 14. Fig. 15 Depth vs. Moveable Hydrocarbons Cut Off The Cut Off value provides information about the volume of shale present in the rock. If the Formation porosity is greater than 6%, Vsh less than 30% and saturation of water is less than 30%, the Formation is generally accepted as a reservoir. IV. DISCUSSION Missakaswal area lies in the thrust regime with faulted anticlinal structure. The area is important because of its structural style. It has repeated packages of Eocene strata in the sub surface which is acting as a potential reservoir. Petrophysical analysis is performed at the upper portion of Chorgali Formation because the well log data of whole package of Chorgali and Sakesar formations is not available. Petrophysical results proved the Chorgali Formation a potential reservoir. Page 5
V. CONCLUSIONS Missakaswal area lies in a compressional regime characterized by thrust faulting and anticline structures. There is repetition of Eocene strata in Missakaswal area due to over thrusting. This is also evident on the seismic section. However due to unavailability of complete log data and lack of control on the seismic data, the repeated portion of Eocene age is not marked and confirmed. An anticlinal pop up structure is marked on the seismic sections with two thrust faults F1 and F2 dipping towards each other from east and west. Two way fault bounded closure is clearly observed on the depth Contour maps which confirms the presence of pop up structure in the study area. The closure values range from 1600 m to 1800 m for Chorgali Formation while it is 1800 m to 2000 m for Sakesar Formation. The pertrophysical analysis of Missakaswal-02 shows average volume of shale 34%, average effective porosity 7% and average hydrocarbon saturation >60% which makes ChorgaliFormation a potential hydrocarbon producing zone. The moveable hydrocarbon calculated is about 12% in the reservoir zone (Chorgali Formation). ACKNOWLEDGEMENTS We are thankful to Department of Earth and Environmental Sciences, Bahria University Islamabad, for the capable direction and scholastic feedback throughout the whole study. REFERENCES [1] Aamir, M. and Siddiqui, M.M., 2006. Interpretation and visualization of thrust sheets in a triangle zone in eastern Potwar, Pakistan. The Leading Edge, vol25, 24-37. [2] Kemal, A. (1992). Geology and new trends for petroleum exploration in Pakistan. In Proc. International Petroleum seminar on new directions and strategies for accelerating petroleum exploration and production in Pakistan (pp. 16-57). [3] Moghal, M. A., Hameed, A., Saqi, M. I., & Bugti, M. N. (2007).Subsurface geometry of Potwar sub-basin in relation to structuration and entrapment. Pak J Hydroc Res, 17, 61-72. [4] Kadri, I. B. (1995).Petroleum geology of Pakistan. Pakistan Petroleum Limited. [5] Khalid, P., Yasin, Q., Sohail, G. M. D., &Kashif, J. M. (2015). Integrating core and wireline log data to evaluate porosity of Jurassic Formations of injra-1 and nuryal-2 wells, western Potwar, pakistan. Journal of the Geological Society of India, 86(5), 553-562. [6] Kemal, A., Balkwill, H. R., Stoakes, F. A.(1991). Indus Basin HydrocarbonPlays. International Petroleum Seminar on New directions and strategies for accelerating petroleum exploration and production in Pakistan, 16-57. [7] Khan, M. A., Ahmed, R., Raza, H. A., & Kemal, A. (1986).Geology of petroleum in Kohat-Potwar depression, Pakistan. AAPG Bulletin, 70(4), 396-414. Page 6