Structural interpretation and petrophysical approach for subsurface fracture identification of Joya Mair area, NW-Himalayas, Pakistan

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Identification of fracture zone is a challenging task without the image log data. There are many brownfields around the world where the image log has not been acquired; therefore, there must be an alternative way of fracture identification. In this paper, a conventional log response technique for fracture delineation has been discussed. The study area lies in the Upper Indus Basin of Pakistan, which is sub-divided into the Kohat and Potwar basins. Minwal-X-1 of Joya Mair area, which lies in the Potwar Basin, is selected for this purpose. Eocene limestone units are formed due to fracture porosity.

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Nội dung Text: Structural interpretation and petrophysical approach for subsurface fracture identification of Joya Mair area, NW-Himalayas, Pakistan

Turkish Journal of Earth Sciences Turkish J Earth Sci (2022) 31: 34-48 http://journals.tubitak.gov.tr/earth/ © TÜBİTAK Research Article doi:10.3906/yer-2103-10 Structural interpretation and petrophysical approach for subsurface fracture identification of Joya Mair area, NW-Himalayas, Pakistan 1 1, 2 Pal Washa Shahzad RATHORE , Muhammad Armaghan Faisal MIRAJ *, Muhammad Bilal MALIK , 1 1 1 1 Sher AFGAN , Rana Faizan SALEEM , Shan SHAHZAD , Irza AKHTAR  1 Institute of Geology, University of the Punjab, Lahore, Pakistan 2 LMK Resources, Pvt. Ltd., Punjab, Pakistan Received: 18.03.2021 Accepted/Published Online: 21.10.2021 Final Version: 28.01.2022 Abstract: Identification of fracture zone is a challenging task without the image log data. There are many brownfields around the world where the image log has not been acquired; therefore, there must be an alternative way of fracture identification. In this paper, a conventional log response technique for fracture delineation has been discussed. The study area lies in the Upper Indus Basin of Pakistan, which is sub-divided into the Kohat and Potwar basins. Minwal-X-1 of Joya Mair area, which lies in the Potwar Basin, is selected for this purpose. Eocene limestone units are formed due to fracture porosity. The objective of the current research is to interpret the subsurface structure and to identify the fractured zones with the help of conventional well log responses. Subsurface seismic interpretation reveals that the area has a triangular zone structure formed as a result of compressional tectonics. Well log responses indicate the presence of the fracture zones in the Chorgali Formation and Sakesar Limestone of Eocene age in Minwal-X-1 well. Few zones, which can be possible fractured zones, in these formations were marked by analyzing the conventional log responses and secondary porosity index (SPI). All the conventional log responses and cross plots support the presence of fracturing in Eocene limestones. The technique of identifying fractured zones with the help of conventional log responses is beneficial for researchers as FMI, and core data are very expensive and not available to everyone. Therefore, the convention log-based approach can be helpful where image log data (FMI) is not available. Key words: Fracture delineation, seismic interpretation, petrophysical analysis, triangular zone, compressional tectonics, FMI 1. Introductıon respectively. The well Minwal-X-1 in Joya Mair area was Upper Indus Basin of Pakistan is sub-divided into Kohat discovered in 1991 (Figure 1). The purpose of the current and Potwar basins. Among these, the Potwar Basin is study is to interpret the subsurface structure as well as to a hydrocarbon prolific area, and, due to compressional mark the fractured zones in the tight Eocene limestones tectonics, the subsurface structural configuration allows based on conventional wireline log data. This technique is the hydrocarbons to trap in a significant amount, and the helpful for the identification of fractured zones in the wells samples have been taken from the Rajian, Adhi, Meyal where the image log data had not been acquired. structures, etc. (Jadoon et al., 2015). For delineating the subsurface structures and stratigraphy, different 2. Geological setting and stratigraphy geophysical techniques, including seismic survey, gravity Indian Plate, a fragmented part of Gondwana motherland, survey, and well logging are usually used (Telford et lies between Australia, Africa, and Antarctica during al., 1990). To understand the structural complexity of the Permian to Middle Jurassic time (Powell, 1979). The the Potwar basin, consideration of the underlying Pre- rapid rate of increase in the northward movement of the Cambrian salt is very important because it acts as a Indian Plate resulted in the shrinkage of the Neo-Tethys decollement, and it is the source of regional thrusting that (Tahirkheli et al., 1977; Jaeger et al., 1989; Chaudhry et al., resulted into the formation of pop-up structures, triangular 1994; Ahsan and Chaudhry, 2008). This rate of increase zones, snake head structures, and so on (Pilbeam et al, was facilitated by the Ninety East Ridge and strike slip 1977; Shah et al., 2012; Sato et al., 2020). movement (Kazmi and Jan, 1997). During the Cretaceous Joya Mair area lies in the eastern part of the Potwar time, intra-oceanic subduction formed a series of volcanic basin which is bounded by Soan Syncline (SS) and Salt arcs that were accreted to the Eurasian Plate around 102 Range Thrust (SRT) in the north and south direction, Ma, while a separation of Indian Plate from Australia and * Correspondence: armghan.geo@pu.edu.pk 34 This work is licensed under a Creative Commons Attribution 4.0 International License.
RATHORE et al. / Turkish J Earth Sci Figure 1. Location map of the Joya Mair area (study area). Antarctica was documented as 95 Ma (Treloar and Izatt, is not well defined due to the shallow drilled targets, but 1993; Kearey et al., 2009). Around 55 to 50 Ma, the upheaval the outcrop data indicates same stratigraphic sequence as of Himalayas resulted from the collision of Eurasian Plate encountered in rest of the Potwar basin (Jaswal et al., 1997). and Indian Plate (McKenzie and Sclater, 1971; Searle et Pre-Cambrian to Quaternary rocks were documented al., 1987; Smith et al., 1994; Beck et al., 1995; White and with three significant unconformities at different time Lister, 2012; Miraj et al., 2021). This upheaval accounts for span, i.e. Ordovician-Carboniferous, Cretaceous-Tertiary, the breaking of the slab, which caused the dragging of the and Oligocene aged unconformities. In the Joya Mair mantle below the Indian Plate (Yoshida and Santosh, 2018). area, Pre-Cambrian salt is overlain by the Jhelum Group Due to the prevailed thrusting between these two plates, of Cambrian age that, in turn, is unconformably overlain a series of thrusts developed from north to south (Main by Nilawahan Group (Moghal et al., 2007). Due to the Karakoram Thrust to Salt Range Thrust) and divides the prevailed tectonic conditions, Zaluch Group of Late NW-Himalayas into different tectonostratigraphic zones Permian age was not deposited and Nilawahan Group (Chatterjee et al., 2013; Goswami et al., 2020). is directly overlain by clastic as well as carbonates of Upper Indus Basin is a consequence of the collision of Paleocene and Eocene age (Shami and Baig, 2002). Rocks two continental landmasses, i.e. Eurasian Plate and Indian of Paleocene-Eocene age contains hydrocarbon prospects, Plate, that resulted in the deformation of Pliocene and and significant hydrocarbons have been taken from these Pleistocene rocks of the Potwar basin (Moghal et al., 2007). zones. Himalayan orogeny did not allow the deposition The hydrocarbon potential of the Upper Indus Basin is during Oligocene time, and Eocene strata followed by comparable with the world’s total hydrocarbon resources. molasses of Miocene to Pleistocene (Moghal et al., 2007). Joya Mair area lies in the S-SE direction of the Potwar The stratigraphic column of the study area is shown in basin and is bounded by Jhelum River and Surghar Ranges Figure 2. on the eastern and western sides, respectively, while Soan Syncline (SS) and Salt Range Thrust (SRT) marked the 3. Hydrocarbon potential northern and southern extremity of the Joya Mair area. Potwar Basin contains all the components of the petroleum The stratigraphic sequence of the northern part of the basin system and favorable conditions that allow the hydrocarbon 35
RATHORE et al. / Turkish J Earth Sci Figure 2. Stratigraphic succession in Joya Mair area (modified after Shami and Baig, 2002). 36
RATHORE et al. / Turkish J Earth Sci entrapment (Kozary et al., 1968; Khan et al., 1986, Shami In the current research, two types of techniques, and Baig, 2002). Marine facies of the Cambrian, Paleocene- including seismic interpretation and petrophysical Eocene are potential reservoirs (Kadri, 1995). Jurassic and analysis, were used to develop the understanding of the Paleocene shales are generating the hydrocarbons, while subsurface structure and fracture identification of the Joya the shales and evaporites of different ages are providing Mair area. sealing for hydrocarbons (Khan et al., 1986). 4.1. Seismic interpretation 3.1. Source rock Seismic interpretation is a key step to delineate the Shales of Triassic (Mianwali Formation), Jurassic subsurface structure and to explore the hydrocarbon (Datta Formation), and Paleocene (Patala Formation) potential of the targeted area. After seismic tie, a synthetic provide source facilities for hydrocarbon generation. seismogram was generated with the help of density Pre-Cambrian oil shales with an average of 32% TOC (RHOB) and sonic (DT) curve. Acoustic impedance was (Total Organic Carbon) are also a potential source of calculated and, in turn, used to determine the reflection hydrocarbons (Shami and Baig, 2002). coefficient (Figure 4). Horizons of different formations 3.2. Reservoir rock were marked after the well to seismic tie to delineate the Cambrian to Eocene strata acts as good reservoirs. subsurface structure (Mo’men et al., 2017). To check the Secondary porosity increases the productivity of the time and depth of the targeted zone, a time contour map and reservoir, while the contribution of primary porosity is depth contour map were generated. GVERSE Geophysics minor. However, in the case of clastic reservoirs, primary software was used for seismic data interpretation of the porosity has a significant role in producing hydrocarbons. Joya Mair area. Eocene limestones are the main producing reservoirs in Joya Mair area, where primary porosity is only 1%. 4.2. Petrophysical interpretation Therefore, all the contribution for significant production is Well logging is used to check the petrophysical properties secondary fracture porosity (Shami and Baig, 2002). Open of the encountered formation that in turn used for the fractures are the youngest out of the three fracture sets that formation evaluation or for the reserve estimation of the existed in the Potwar Basin. All the fractures are oriented field (Moore et al., 2011). It has a vital role in exploration in different directions as E-W, NE-SW, NW-SE. Parallel, and reservoir modeling (Szabo, 2011). Petrophysical perpendicular, and oblique fractures formed due to the analysis of Minwal-X-1 (Joya Mair area) was done by using high-rate of compressional forces in the folding. Open density, resistivity, caliper, neutron porosity, photoelectric fractures often contribute to the secondary porosity that absorption (PEFZ), Gamma Ray, and spontaneous significantly helps in the migration of hydrocarbons in the potential logs with the help of GVERSE Petrophysics. study area. A zone of interest was marked after analyzing the 3.3. Cap rock petrophysical properties of the targeted well. In Joya Mair area, Kuldana shales (Upper Eocene) and Murree clays (Miocene) act as roof rock for Eocene 5. Results reservoirs (Shami and Baig, 2002). Murree clays are 5.1. Seismic interpretation excellent seal rocks due to their exceptional sealing Analysis of the seismic section and synthetic seismogram properties. gives information about the depth at which different 3.4. Geothermal gradient formations were encountered. Minwal-X-1 drilled on An average of 3150 m is regarded as a depth of the oil seismic line of Joya Mair area (S93-MN-08), and the window in the study area with 2 °C per 100 m geothermal interpreted reflectors of different seismic horizons along gradient. A minor variation was noticed in the geothermal with the faults (Fault and Fault A) can be seen in Figure 5. gradient of some areas of the Potwar Basin (i.e. 2.05 °C per Triangular zone was a delineated structure developed due 100 m for Balkassar area, Shami and Baig, 2002). to compressional forces (thrust fault and back thrust) and 4. Data and methodology good for trapping the hydrocarbons. To delineate the subsurface structure of the Joya Mair area, The time contour map across time shows the triangular well log data along with different geophysical techniques zone that demarcates the Top Eocene at the base of the were opted. The Directorate General of Petroleum map (Figure 6). In time contour map, the area between Concessions (DGPC), Pakistan provided the data for this 2030–2310 ms transient time indicates the deepest, while purpose. Only Minwal-X-1 well log data were provided 1247–1510 ms shows the shallowest area. In other words, along with nine seismic lines. Joya Mair area lies in the the area lies between the faults was deeper as compared 42N UTM zone. The orientation of seismic lines and the to the other one that indicates the dropped down area location of a well can be seen in Figure 3. (triangular zone or pop-down structure) (Figure 6). 37
RATHORE et al. / Turkish J Earth Sci Figure 3. Seismic line orientation on the base map of study area. Similarly, the depth contour map was generated by Potential, Bit Size and Caliper) run in the first track using the velocity and time values. The map indicates the ,while resistivity logs (Micro Spherically Focused Log, same results as the time contour map. Near the faulted Laterlog Shallow, and Laterlog Deep) run in the second zone, the drilled Minwal-X-1 can be visualized from the track. Porosity logs (Neutron, Sonic, Density) along with map (Figure 7). PEF (lithology) and DRHO (density correction) run in Because of the best visualization, in the next phase, the Track-03. Similarly, the volume of shale, measured 3D grid map was generated (Figure 8). The generated grid porosities, net reservoir, and saturation run in the next map showed the locational information along with the four tracks. Saturation values were not beneficial because displacement of the faulted zone (Figure 8). of the compacted limestone. Some cut off were marked, 5.2. Petrophysical interpretation i.e. volume of shale should be 1%. lithology of the targeted formation. With the help of color 5.2.2. Delineation of the fracture zone bar, gamma ray values were shown on the z-axis (Figure Only FMI (Image logging) can directly demarcate the 9). Pure limestone was the interpreted lithology of the secondary or fractured porosity. Due to some data formation by cross-plot. limitations, conventional logs were opted in this research, 5.2.1. Well logging of Minwal-X-1 to indirectly identify the fractured porosity in Minwal-X-1. The CPI (Computer processed interpretation) of Chorgali For Chorgali Formation, Sonic values, with Gamma Ray Formation and Sakesar Limestone of Eocene age are and average porosity values, were plotted against each shown in Figure 10. All the logs that run during the well other to cross-plot them for fracture identification (Figure logging operation can be seen in total seven tracks, and 11). Limestone was the interpreted lithology by Gamma the petrophysical analysis was done on all the logs of first Ray log, while fracture porosity was determined by SPI three tracks. Four log curves (Gamma Ray, Spontaneous (secondary porosity index) points that lie below the blue 38
RATHORE et al. / Turkish J Earth Sci Figure 4. Synthetic seismogram built using DT and density logs run in Miwal-X-1 well. Figure 5. Interpretation of Seismic line S93-MN-08 showing different reflectors and two faults (Fault 1 and Fault 2) forming pop- down structure. 39
RATHORE et al. / Turkish J Earth Sci Figure 6. Time Contour Map of Joya Mair area generated on the top of Eocene. line. Based on SPI, fracture zones were marked between the 6. Discussion depth of 2046 m and 2048 m. Similar results were found by Joya Mair area is a part of the Potwar Basin, which is log responses (Figure 12). Low GR responses indicated the structurally deformed due to the compressional forces. clean formation, but due to the fractured zone or bad hole The triangular zone is the major structure, which conditions, the caliper values were not accurate. Therefore, controls the structural geometry of the area (Zahid et al., other log responses were also considered for the true 2014). The time contour map and depth contour map also delineation of the responses. The separation between the confirm the triangular zone structure. 3D grid maps give a resistivity curves due to the mud invasion also indicated three-dimensional view of the subsurface structure of the the fracture porosity. All these responses indicated a study area. fractured zone in Chorgali Formation. High Gamma-Ray values indicate the presence of Similar methodology was opted for Sakesar Limestone. shale, while low values indicate clean formation (Fertl, Two fractured zones were identified based on SPI (Figure 1979, Katahara, 1995). In both Chorgali and Sakesar 13). Then, these zones were compared with log responses limestones, the Gamma-Ray value is low, showing that that support the results of cross-plots (Figure 14). lithology is pure limestone. In short, three fractured zones were delineated in To confirm the lithology as defined by the Gamma the Eocene limestones of Minwal-X-1. The fact that Ray log, a cross-plot of density-neutron is made. All the the Minwal-X-1 is producing hydrocarbons from the points lie on the limestone line (density value ~ 2.7), fractured zones that were the same as the ones marked in whereas porosity is 0–0.04. Cross-plot confirms that the present study indicates the authenticity of the research lithology is pure limestone with no or minimum porosity and convention well log response technique. (Figure 9). Minimum porosity in the limestone is due to 40
RATHORE et al. / Turkish J Earth Sci Figure 7. Depth contour map of Joya Mair area generated on Eocene Top. the compaction and cementation (Mukharjee and Kumar, porosity. The property of sonic wave to travel through the 2018). Thus, for getting the production from carbonates, shortest possible path makes them an excellent fracture the secondary porosity is crucial. indicator. It can be seen in Figures 12 and 14 that the sonic The caliper tool measures the diameter of the borehole. values in the marked zones are approximately 55–60 µs/ft. The caliper values in both formations are showing In tight formations, such a small sonic value indicates the significant variations that occurred due to the bad hole fracture porosity. conditions or fractured zones in the formations. The Secondary porosity index (SPI) of Chorgali Formation resistivity track has three curves that show separation and Sakesar Limestone were identified in this paper. In only when the invasion occurs in the formation. As the Figures 11 and 13, the porosity cross plot can be seen cross-plot in Figure 9 shows that the formations are very where X-axis shows total porosity, and Y-axis shows that tight; thus, the separation between the resistivity curves in the values of sonic porosity in Minwal-X-1 well. It can be marked zones is due to fractures in the formation. In case observed that the plotted points are distributed equally in of bad borehole conditions, the best porosity measuring both groups. The points in the secondary porosity zone tool is a sonic tool. The sonic tool measures the transient (zone below the line) indicate good fractured Chorgali time of the elastic waves passing through the formation, and Sakesar Limestone zones in the Minwal-X-1 well. and less transient time indicated the more significant Based on the SPI log plots, one fractured zone is marked in 41
RATHORE et al. / Turkish J Earth Sci Figure 8. 3D grid maps of study area showing wells’ locations. the Chorgali Formation while two in Sakesar Limestone. tectonics (thrust fault and back thrust) that was good for Chorgali Formation is subjected to fracturing at a depth of trapping hydrocarbons. Shales of Patala Formation are 2046 to 2048 m, whereas the Sakesar Limestone is subjected hydrocarbon sources, whereas Chorgali Formation and to fracturing at two different depths, i.e. 2078–2082 m Sakesar Limestone of Eocene age act as reservoirs in the and 2086.9–2090 m. Fracturing can be considered as the Upper Indus Basin. The key aim of this research was to main reason for this secondary porosity. By analyzing the use conventional well logs (Gamma Ray, Caliper, Sonic, conventional log responses, it can be assumed that Chorgali Density, Resistivity, and Neutron logs) for the prediction Formation and Sakesar Limestone in the Minwal-X-1 well of fractured zones in Eocene limestones of Chorgali have some fractured zones, as all the log responses support Formation and Sakesar Limestone. Good secondary the presence of fractures in the formation. porosity has been observed in one zone of Chorgali The current study depends entirely on post-stack Formation and in two zones of Sakesar Limestone. Both seismic and conventional petrophysical logs. Conventional limestones are tight, as shown by cross plots and log laboratory testing results cannot be obtained due to the responses. The reduced porosity in carbonates is due to unavailability of core data. By acquiring the core data, the compaction and cementation. However, few possible validity of the results will be more significant. fractured zones in both the formations were marked by analyzing conventional log responses and secondary 7. Conclusion porosity index (SPI). All the conventional log responses Joya Mair area lies in the south-southeast of the Eastern and cross plots are supporting the presence of fracture Potwar, Upper Indus Basin. The triangular zone was zones in Eocene limestones. There are many brownfields a delineated structure developed by compressional worldwide in which image logging had not been acquired. 42
RATHORE et al. / Turkish J Earth Sci Therefore, the identification of fractured zones can be Acknowledgment made possible in these areas with the help of the proposed This research is a part of M. Phil research work of Palwasha technique. The availability of more datasets can enhance Shahzad Rathore and submitted to the Higher Education the quality of the current research. Commission (HEC) for the partial fulfillment of the degree. Figure 9. Cross plot made between neutron and density values showing the limestone lithology in the zone of interest. 43
RATHORE et al. / Turkish J Earth Sci Figure 10. CPI (Computer Processed Information) of Eocene Limestone (Chorgali Formation and Sakesar Limestone). 44
RATHORE et al. / Turkish J Earth Sci Figure 11. Cross Plot between Average porosity and sonic porosity for finding SPI (Standard porosity index) of Chorgali Formation. Figure 12. Log plot showing the log signatures in the fractured zone of Chorgali Formation. 45
RATHORE et al. / Turkish J Earth Sci Figure 13. Cross Plot between Average porosity and sonic porosity for finding SPI (Standard porosity index) of Sakesar Limestone. Figure 14. Fractured zones in Sakesar Limestone. 46
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