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Unconventional energy resources such as shale gas are becoming an increasingly important exploration and production targets. Organic-rich shales are widely distributed in the sedimentary basins of Pakistan. Among these shales, organic-rich Sembar shale of the lower Cretaceous age is a strong potential shale gas play in the lower Indus basin of Pakistan. However, seismic and petrophysical characterization of this organic-rich shale is not well understood because of the complex dependency of the seismic and petrophysical properties of these rocks on mineralogical, heterogeneities, organic matter, clay content and thermal maturity. Therefore, to understand the seismic and elastic character of Sembar shales, rock physics modeling and seismic attribute analysis have been done. The results reveal that in-situ mineral composition, total organic content (TOC) and thermal maturity can influence significantly in inverse order to the seismic and elastic parameters of organic-rich shale. This work will facilitate the workers to interpret shale gas play and develop the exploration and production strategies for gas exploration and production.

Spreadsheet-based programming has become a worldwide computational tool and a powerful platform for executing scientific calculations in different engineering disciplines. The inherent advantage of spreadsheets exists in integrated storage and the ability to manipulate matrices along with wide-ranging mathematical calculations, plotting, and diagrammatic presentation. This paper presents a user-friendly and interactive spreadsheet program “AVA-anisotropy” to characterize the primary P-wave amplitude versus incident angle (AVA) responses in vertically transverse isotropic (VTI), horizontally transverse isotropic (HTI), and orthorhombic media. The set of Ruger’s equations for isotropic, VTI, HTI, and orthorhombic media are solved to describe the seismic energy partitioning at an anisotropic interface. The spreadsheet program has three main sections: basic concepts, data input, and AVA modeling for anisotropic media. The interactive data input and on-screen mathematical and graphic presentation of the spreadsheet enables the students to have an in-depth examination of the effects of changing input data and the resulting model. This greatly helps them in understanding the energy partitioning in anisotropic medium.

The proper understanding of earthquake geometry and location is very important to understand the seismicity of an area. In the present study, 185 earthquakes that occurred in the northwest Himalaya of magnitude Mw > 0.6 between 2010 and 2013 were relocated using the Modified Joint Hypocenter Determination method. After examining seven velocity models, average VP/VS ~ 1.73 computed by Wadati diagram is used to improve the hypocenter locations of the earthquakes. Based on hypocentral distributions, these earthquakes were classified into four groups. Group 1 is along the Punjal Thrust in Hazara Kashmir Syntaxes. Group 2 belongs to the Indus-Kohistan Seismic Zone whereas group 3 is along the Besham dome and the fourth group is along the Main Mantle Thrust. Shallow crustal earthquakes were relocated to 0–50 km, deep earthquakes were relocated to 50–100 km, and much deep earthquakes were relocated to 250–350 km depth. This analysis will be helpful to understand the seismicity of the northwest Himalaya of Pakistan.

In the above published article in October 2018 issue of the Journal of the Geological Society of India (v.92, pp.465–470).

Petrophysical evaluation and rock physics analysis are the important tools to relate the reservoir properties like porosity, permeability, pore fluids with seismic parameters. Nevertheless, the uncertainties always exist in the quantification of elastic and seismic parameters estimated through wireline logs and rock physics analysis. A workflow based on statistical relationships of rock physics and logs derived elastic and seismic parameters with porosity and the percentage error exist between them is given. The statistical linear regressions are developed for early Eocene Chorgali Formation between various petrophysically factors determined from borehole logging of well Ratana–03 drilled in tectonically disturbed zone and the seismic and elastic parameters estimated through rock physics modeling. The rock physics constraints such as seismic velocities, effective density and elastic moduli calculated from Gassmann fluid substation analysis are in harmony and close agreement to those estimated from borehole logs. The percentage errors between well logs and rock physics computed saturated bulk modulus (K sat ), effective density (ρ eff ), compressional and shear wave velocities (V P and V S) are 1.31%, 4.23 %, 5.25% and 4.01% respectively. The permeability of reservoir intervals show fairly strong linear relationship with the porosity, indicating that the reservoir interval of the Chorgali Formation is permeable and porous thus having large potential of hydrocarbon accumulation and production.

Turbidite-embedded lowstand prograding wedges are hot topics in the sequence stratigraphic consortium. The lowstand prograding wedges within the lowstand system tract are mud-prone in deep-water settings, and hence, sandstone reservoirs become ultra-thin to be detected using bandlimited seismic. The prediction of thin-bedded (hydrocarbon-charged) lowstand prograding wedge reservoirs along the structural closure is therefore a challenge. We delineate thick and porous source and reservoir segments by interpreting the seismic stratigraphy and performing continuous wavelet transforms of spectral decomposition tools on the Indus Offshore, Pakistan. The 48 Hz resolves the shallow-marine coarse-grained hydrocarbon-bearing oblique-tangential clinoforms, faults, source rock, and shale out zone, which captured the lowstand prograding wedges play. The 48 Hz images the lowstand prograding wedge geomorphology and high-energy coarse-grained turbidity channelized basin floor fans source beds. The bandlimited seismic facies simulation predicted a 15 to 25 % porosity and a thickness of 5.7 m for porous oblique-tangential prograding clinoform facies. The CWT-based spectral seismic facies and porosity simulations predicted a 15 to 35 % porosity and enhanced reservoir thickness of 11 m for oblique-tangential prograding clinoform facies, which suggested robust indicators for explorations of lowstand prograding wedge plays in the Indus Offshore and other similar basins.

This study describes a method of combining geostatistical analysis with geophysical inversion of 1D electrical resistivity data acquired in the Bannu Basin, northwestern Himalayas of Pakistan. The data has been integrated and broadened from single dimension resistivity data into a 2D model that can be fully visualized and interpreted in a spatial sense. By interpreting and calibrating the electrical resistivity curves with the lithologies and geophysical logs of boreholes, it has been possible to identify unique sedimentary accumulations that occur throughout the basin. Through the use of variogram modelling the spatial extent of these accumulations can be identified and associated with measured hydraulic properties. It has become possible to map the extents and facies of the alluvial systems of the Bannu Basin formed during periods of heightened tectonic activity. The coarser sediments are associated with higher levels of resistivity as measured in the electrical surveys, whereas the finer sediments exhibit characteristically lower resistivities. Thus, the zones of high and low resistivity values become indicative of sediments associated with alluvial fans and lacustrine environments, respectively. The sediments of alluvial fans show relatively low gamma ray levels and higher transmissivities, whereas those of the piedmont deposits as well as lacustrine and sandy plains tend to show medium to high gamma ray values and lower transmissivities. Gross transmissivities were estimated by studying regression relationships between transmissivities measured in boreholes and the apparent formation factors of sediments. These relationships indicate an exponential increase in transmissivity with increasing apparent formation factor. These trends suggest a contrast in transmissivities between alluvial fan and other environments where lacustrine and sandy plains are characterized by low transmissivities. These estimated transmissivities do not represent absolute values but are merely indicative of any high or low trends in transmissivity of sediments, throughout the basin.

The use of seismic direct hydrocarbon indicators is very common in exploration and reservoir development to minimise exploration risk and to optimise the location of production wells. DHIs can be enhanced using AVO methods to calculate seismic attributes that approximate relative elastic properties. In this study, we analyse the sensitivity to pore fluid changes of a range of elastic properties by combining rock physics studies and statistical techniques and determine which provide the best basis for DHIs. Gassmann fluid substitution is applied to the well log data and various elastic properties are evaluated by measuring the degree of separation that they achieve between gas sands and wet sands. The method has been applied successfully to well log data from proven reservoirs in three different siliciclastic environments of Cambrian, Jurassic, and Cretaceous ages. We have quantified the sensitivity of various elastic properties such as acoustic and extended elastic (EEI) impedances, elastic moduli (K sat and K sat–μ), lambda–mu–rho method (λρ and μρ), P-to-S-wave velocity ratio (V P/V S), and Poisson’s ratio (σ) at fully gas/water saturation scenarios. The results are strongly dependent on the local geological settings and our modeling demonstrates that for Cambrian and Cretaceous reservoirs, K sat–μ, EEI, V P/V S, and σ are more sensitive to pore fluids (gas/water). For the Jurassic reservoir, the sensitivity of all elastic and seismic properties to pore fluid reduces due to high overburden pressure and the resultant low porosity. Fluid indicators are evaluated using two metrics: a fluid indicator coefficient based on a Gaussian model and an overlap coefficient which makes no assumptions about a distribution model. This study will provide a potential way to identify gas sand zones in future exploration.

The lower Indus basin is one of the largest hydrocarbon producing sedimentary basins in Pakistan. It is characterized by the presence of many hydrocarbon-bearing fields including clastic and carbonates proven reservoirs from the Cretaceous to the Eocene age. This study has been carried out in the Sanghar oil field to evaluate the hydrocarbon prospects of basal sand zone of lower Goru Formation of Cretaceous by using complete suite of geophysical logs of different wells. The analytical formation evaluation by using petrophysical studies and neutron-density crossplots unveils that litho-facies mainly comprising of sandstone. The hydrocarbons potentialities of the formation zone have been characterized through various isoparameteric maps such as gross reservoir and net pay thickness, net-to-gross ratio, total and effective porosity, shaliness, and water and hydrocarbons saturation. The evaluated petrophysical studies show that the reservoir has net pay zone of thickness range 5 to 10 m, net-togross ratio range of 0.17 to 0.75, effective porosity range of 07 to 12 %, shaliness range of 27 to 40 % and hydrocarbon saturation range of 12 to 31 %. However, in the net pay zone hydrocarbon saturation reaches up to 95%. The isoparametric charts of petrophysically derived parameters reveal the aerial distribution of hydrocarbons accumulation in basal sand unit of the lower Goru Formation which may be helpful for further exploration.

The quantification and assessment of groundwater are of great significance for the urbanization and town planning in an area. The current study has been carried out for quantification and assessment of groundwater quality zones by using geo-electrical technique in Faisalabad area (73° 06′ 08″–73° 11′ 59″ E; 31° 35′ 00″–31° 40′ 39″ N) of Pakistan. Vertical electrical soundings (VES) at six locations were conducted by Schlumberger configuration up to 300-m depth of investigation. Modeling technique is used for the interpretation of apparent resistivity, and the true resistivities of various subsurface lithologies were recorded from 2.10 to 55 Ω m for the depth of investigation. Three sand-dominant aquifers were identified at depth intervals of 0–50 m, 50–100 m, and 100–300 m. The VES results are compared and calibrated with the available borehole data taken from the tube wells drilled in the vicinity of the study area. The GIS technique is applied for the preparation of water quality zonation maps. Some of the groundwater quantification parameters such as total volume of the alluvium, total volumes of the water content, and active storage are estimated through GIS applications. Top geo-electrical layer (0–50 m) is dominated by low resistivity zone (10–25 Ω m), having an active storage of 0.71 km3. Intermediate geo-electrical layer (50–100 m) comprised coarse sand with low-to-medium resistivity zone of 10–35 Ω m, showing a low active storage (1.01 km3). Deep geo-electrical layer (100–300 m) is dominant by very low resistivity zone (

This research deals with the sensitivity of the hypsometric integral (HI) and hypsometric analysis to map active sedimentation and tectonics in the southern Suleiman fold and thrust belt (SSFTB)—a part of extensive folds and thrusts system vastly distributed along the western margin of the Indo-Pakistan sub-plate. We used SRTM DEM with 90 m spatial resolution to determine HI values in the SSFTB in Pakistan. In order to estimate maximum, minimum, and mean elevations of the study area, grid of regular squares of different sizes was analyzed. The active erosional stages and topographic development using digital elevation model (DEM)-derived HI were analyzed. The Shuttle Radar Topography Mission (SRTM)-based DEM is employed to carry hypsometric investigation and isobase codings. For the tectonic geomorphological analysis, the HI and hypsometric curves (HCs) were used to decode the recent and active surface deformation and its influence on the local topography of SSFTB. Spatial autocorrelation is applied to compute the distribution of HIs to see the possible clustering, dispersion, or randomization. Spatial autocorrelation demonstrates that the HIs are auto-correlated because of higher positive values of Z-scores. The spatial statistical analyses indicate that the HI clusters and hot spot are spatially correlated with each other having high positive Z-scores. The study concluded that the central parts and north-north-eastern and north-north-western parts of the SSFTB show more hot spots as compared to south-eastern and south-western parts. The analysis reveals that the resolution of the hot spots depends largely on the grid size and its quality decreases with increase in grid size and vice versa. Hot spots corresponding to higher values of HI are steady with uplifting and active topography and reveal a strong connection with the different structures in the study area. Cold spots correspond to the shallow depth earthquakes and active sedimentation close to fault lines.

Breakthroughs in shale gas exploration and production technology in China point to a possible solution to Pakistan's current energy crises. In this study, we evaluate the shale gas prospects in the Kohat-Potwar Plateau of Pakistan by establishing an integrated approach involving the analysis of fundamental elastic and petrophysical properties, Rock-Eval pyrolysis, and the sealing mechanism of shale. Detailed geochemical and petrophysical evaluation of the Patala Formation in the Kohat-Potwar Plateau indicates the good potential for shale gas with the following characteristics similar to the Longmaxi shale of Sichuan Basin China, i.e., (i) complex structural types sandwiched by tight limestone strata with low porosity (less than 3%), ultra-low permeability, high density, and large thickness which provides strong sealing capacity for gas preservation and enrichment; (ii) multiphase tectonic evolution of Patala Formation allowed various slippage processes to develop fractures and enhance the porosity and permeability; (iii) high organic matter content and thermal maturation stage (TOC > 2%, 416–445 °C); (iv) high brittle minerals content (e.g., calcite and dolomite > 40%); and (v) large formation thickness (>30 m) at shallow burial depth

Porosity is one of the key factors of a reservoir system that is typically distributed in a spatially non-uniform and non-linear manner. Nevertheless, spatial distribution of porosity in stratigraphic traps (channel sand bodies) is challenging due to the variations in depositional environments, shale intercalation, and repetition of textural changes in the pore sizes. This paper presents a strategy of joint inversion that combines support vector machine (SVM) and particle swarm optimization (PSO) algorithms to predict the spatial distribution of porosity using well logs and seismic data. In addition, Gaussian simulation algorithms (sequential Gaussian and Gaussian indicator simulations) were used to predict lithology and porosity distribution using wireline logs and core data. Rock physics modeling was performed to bridge the gap between the elastic and petrophysical properties for seismic-inversion based reservoir characterization. The results show that the joint inversion strategy leads to the most stable prediction of AI and porosity distribution in the lower Goru heterogeneous reservoir of Sawan Gas Field, Pakistan. The calibration of the individual spatial distribution of lithology and porosity from wireline logs data using Gaussian simulation algorithms and post-stack seismic inversion using SVM and PSO; revealed favorable matching. In fact, the spatial pattern of low AI corresponds with the high porosity and sandstone lithofacies. The study results provide valuable insights for identifying the sweet spots in the heterogeneous lower Goru clastic packages of Sawan Gas Field through quantitative analysis of rock physics, petrophysics, seismic inversion, and fuzzy optimization. The proposed joint inversion strategy may be considered useful for the prediction of porosity in similar context of extremely heterogeneous stratigraphic traps in other basins of the world.

Petrophysical parameters of a petroleum reservoir rock, used in formation evaluation, are generally estimated from wireline logs and core plugs. However, uncertainty or percentage error in quantitative interpretation of wireline logs and petrophysical parameters is generally not well quantified. Formation evaluation analysis and rock physics modeling are very useful techniques to link physical properties of reservoir interval extracted from wireline logs at well location with petrophysical, elastic and seismic properties of the reservoir interval far away from the well location. However, several types of uncertainty in the estimation of these parameters can affect this link seriously. In this work, a complete workflow is proposed to find out various petrophysical and rock physics parameters and to estimate percentage error in the extraction of these parameters. This work flow is based on traditional wireline log interpretation and is successfully applied on Basal sands of four wells drilled in the lower Indus Basin of Pakistan. An uncertainty analysis using statistical techniques is performed on different petrophysical parameters such as volume of shale, porosity, permeability estimated from wireline logs of these wells. Based on our statistical approach, linear and nonlinear regression relationships are developed between different petrophysical parameters. Furthermore, these parameters are used as input in rock physics model to predict elastic moduli and seismic velocities of the reservoir interval. The rock physics parameters such as velocities, densities and elastic moduli derived from well data are in close agreement to those predicted from rock physics model.

Mansehra Granitic Complex (MGC) lies in the NW Himalaya of Pakistan. The MGC magmatic rocks are peraluminous, calc-alkaline S-type granitoids. Prior to this study the Mansehra Granite had produced ages of 83 Ma by K/Ar, 215 Ma using Ar/Ar on biotite, and 516 ± 16 Ma, using the whole rock Rb/Sr method. The Susalgali Granite Gneiss, a sheared facies of the Mansehra Granite previously regarded as older than the Mansehra Granite, was dated at 79 Ma using K/Ar on biotite. Hakale Granite, which is intrusive into the Mansehra Granite, had yielded K/Ar muscovite age of 165 Ma. The age of the leucogranites was not reported before this contribution. We have presented the revised geochronology of the MGC magmatic bodies, employing SHRIMP and LA-ICP-MS U-Pb zircon chronometry, to constraint precise crystallization ages and tectonic setting of the NW Himalaya, Pakistan. Dates of emplacement of the Mansehra Granite, leucogranites and Hakale Granite are ca. 478, 475 and 466 Ma, respectively. These new ages are comparable to U-Pb zircon and Rb/Sr dates of other granites and granite gneisses in the Lesser Himalaya to the east, in India, Nepal, south Tibet and SW China. The age components of ca. 1900–1300, 985–920, 880–800 and 690–500 Ma are interpreted as inherited grains. Geochronological and field evidence suggest that the MGC of the NW Himalaya are the product of an Andean-type Cambro-Ordovician accretional orogenesis with continental-continental settings along the northen margin of east Gondwana. On the basis of new age data of the MGC plutonic rocks it is inferred that Cambro-Ordovician accretional event commenced from SW China and extends at least up to NW Pakistan along the northern margin of east Gondwana. However, granitic rocks of Pan African affiliation prevail in central Iran and Turkey along northern and western margins of Gondwana.

Delineation of saline water and freshwater interfaces is a primary objective in groundwater studies and groundwater management. Karak valley, NW Himalaya of Pakistan is situated in a semiarid region where the continuous supply of freshwater is challenging because of high salinity values in groundwater. In this study, vertical electrical resistivity and borehole data are used to mark the freshwater and saline water interfaces. Borehole data collected from different locations in the study area is used to interpret various subsurface lithologies and to calibrate the modeled resistivity curves. The electrical resistivity data indicates that a thick cover of Quaternary sediments is present in the valley. The re-inversion of electrical resistivity data with modern analysis techniques is highly effective to map the boundary between fresh and saline water aquifers in the valley. Inversion technique is applied to generate 2D and 3D subsurface resistivity maps for the delineation of fresh and saltwater surfaces. The presence of saline water aquifer with clay rich sediments is identified by very low resistivity values on the northern part of the valley whereas relatively high resistivity values with sand and gravel sediments on the southeastern part of the valley indicate the presence of freshwater aquifer. The generated 2D resistivity surface maps at different depth levels above and below the water table and formation resistivity distribution map are effectively used to mark the boundary between fresh and saline water zones. The zone of brackish water is clearly seen in the resistivity inverse model at a depth of 5–30 m.

Gas hydrates gained a remarkable attention as an unconventional energy resource recently. In order to interpret gas hydrates (part of fluid) and free gas saturated zone accurately, it is essential to implement new technique related to seismic attenuation and velocity dispersion. P wave attenuation and velocity dispersion in porous media made promising imprints for exploration of gas hydrates. The most prominent phenomenon for attenuation and velocity dispersion in porous media is wave induced fluid flow in which wave inhomogeneities are larger than pore size but smaller than wavelength. Numerical simulation technique is applied to analyze frequency dependent velocity dispersion and attenuation in gas hydrates and free gas layer in Makran offshore of Pakistan. Homogeneous and patchy distribution patterns of gas hydrates and free gas within pore spaces of host sediments at lower and higher frequency regime are considered. It is noted that the attenuation and velocity dispersion increase with the increase in gas hydrates saturation. The maximum attenuation is observed at 66% saturation of gas hydrates in the area under investigation. However, in case of water and gas mixture the maximum attenuation and velocity dispersion occur at low gas saturation (~15%). Therefore, based on our numerical simulation, velocity dispersion and attenuation can be used as seismic attributes to differentiate various gas saturations and gas hydrates saturation for Makran offshore area of Pakistan.

Sandstone intervals of the Lower Goru Formation of the Cretaceous age are proven reservoir intervals in the middle Indus Basin, Pakistan. However, based on seismic data interpretation, it is difficult to differentiate payable gas sand from non-payable gas sand. Various types of post-stacks seismic attributes are extracted and analyzed to differentiate payable gas sand from non-payable. The analysis reveals that average instantaneous frequency, peak spectral frequency, average instantaneous phase and sweetness seem to be more effective attributes along with energy half time. Sweetness attribute map shows two sweet spot below the targeted window which indicate to the presence of gas play. Statistical analysis is also carried out to find the associations between seismic attributes and thickness of gas layer values. Cross plots of these attributes were quantified through regression analysis. A good correlation with regression coefficient (R2) > 0.78 is found between seismic attributes and gas layer thickness except average instantaneous phase attribute, which lays the foundation for predicting the thickness of gas reservoir.

Lower Goru sand intervals of the Cretaceous age in Kadanwari field of the middle Indus Basin, Pakistan, are interpreted as reservoir intervals. Based on seismic interpretation and geology, several wells were drilled in these sands. However, most of the wells drilled were dry and declared abundant because highly saturated brine was encountered with minute amount of hydrocarbon gas. The main reason for drilling dry wells was limitation of seismic reflection data to differentiate seismic response of hydrocarbon-saturated reservoir interval from non-hydrocarbon-saturated interval. Amplitude versus offset (AVO) analysis is widely used to differentiate hydrocarbon intervals from background lithology. In this work, AVO-derived attributes are used for fluid discrimination in the sand intervals. Amplitude differences are associated with density and compressibility of fluid under in situ temperature and pressure conditions of the reservoir interval. The results reveal that seismic amplitude anomalies associated with AVO analysis can differentiate hydrocarbon fluids from non-hydrocarbon fluids and thus can optimize the exploration of gas. The gas sand anomaly in the study area is of weak impedance as compared to brine sand. The intercept and fluid factor are very promising attribute pairs to discriminate gas sands from background sands. The results may be helpful to discriminate low gas saturation or fizz water from high gas saturation.

Partially saturated reservoirs are one of the major sources of seismic wave attenuation, modulus defect and velocity dispersion in real seismic data. The main attenuation and dispersion phenomenon is wave induced fluid flow due to the heterogeneity in pore fluids or porous rock. The identification of pore fluid type, saturation and distribution pattern within the pore space is of great significance as several seismic and petrophysical properties of porous rocks are largely affected by fluid type, saturation and fluid distribution pattern. Based on Gassmann-Wood and Gassmann- Hill rock physics models modulus defect, velocity dispersion and attenuation in Jurassic siliclastic partially-saturated rocks are studied. For this purpose two saturation patterns - uniform and patchy - are considered within the pore spaces in two frequency regimes i.e., lower frequency and higher frequency. The results reveal that at low enough frequency where saturation of liquid and gas is uniform, the seismic velocity and bulk modulus are lower than at higher frequency where saturation of fluid mixture is in the form of patches. The velocity dispersion and attenuation is also modeled at different levels of gas saturation. It is found that the maximum attenuation and velocity dispersion is at low gas saturation. Therefore, the dispersion and attenuation can provide a potential way to predict gas saturation and can be used as a property to differentiate low from high gas saturation.

The Khewra Sandstone is widely distributed throughout the Potwar Basin of Pakistan. Its reservoir aspects are evaluated by the interpretation of well log data recorded from Kal-01 and Kal-02 wells in the Kal Oil Field, eastern Potwar Basin. The encountered thickness of the Khewra Sandstone in Kal-01and Kal-02 varies from 60 m to 120 m. On the basis of lithological interpretation from wireline logs the formation is divided into four facies: 1) shale interbedded with minor siltstone and sandstone, 2) sandstone with alternating siltstone and claystone/shale, 3) claystone/shale, siltstone interbedded with fine to medium grained sandstone and 4) fine to coarse grained sandstone with channels. The log derived average porosity in Khewra Sandstone is about 14–22% with permeability ranges from 20–58 mD. Thus, it possesses a large storage capacity for moveable hydrocarbons in Kal Oil Field. Facies 4, in the upper part of the Khewra Sandstone, is interpreted as potential reservoir interval with hydrocarbon saturation ranges from 77–79%. Based on isopach map, the formation is interpreted to be well developed in NE-SW which gradually thins basinward to the west and northwest direction. The play fairway analysis suggested very high chances of hydrocarbon entrapment is in the Kal Oil Field.

Pakistan is situated very close to the subduction zone of Indian-Arabian plates and Indian-Eurasian plates. Thus, it is a country with moderate to high seismicity with a large number of major earthquakes. Therefore, the severity of seismic hazards of major earthquakes is not ignorable. The seismicity of northern areas of Pakistan (33–37°N and 70–76°E), which are the part of North West Himalaya, is studied. Three earthquake catalogues are selected and then completed for this purpose. The spatial distribution of their epicenters, as a function of depth, shows that most of the earthquakes (about 84 %) occurred in shallow to intermediate depths. Relationship between major faults and their source mechanism is also investigated. The Focal mechanism solutions of 39 significant earthquakes in this area, occurred in the period of 1976–2014 with magnitude M w > 5.0, are also analyzed. The focal depth of these events is in the range of 10–244 km. Among 39 focal mechanisms, the majority of them located in lower crust. Global centroid moment tensor solutions indicate that the majority of the seismic events show strike-slip faulting with a left-lateral sense of motion, followed by thrust and normal faulting. Most of these focal mechanisms located in the Hindu-Kush, a most seismically active, region. The major source of these events and thrusting is the subduction of the Indian plate under the Eurasian Plate. At shallow to intermediate depth, low-to-intermediate angle thrust faulting is dominant. It is seen that in a large number of seismic events the compressive stress is acting nearly in NNW–SSE to N–S directions.

The accurate estimation of b-value of Gutenberg–Richter (GR) frequency-magnitude relationship is very crucial in seismic hazard analysis of a region. In this study, we focused on the b-value estimated by using Gumbel’s extreme distribution and GR methods for Karachi region of Pakistan. The seismotectonic setting of Karachi region is of great significant because it is surrounded by many active faults systems such as Makran subduction, Run of Kutch, Kirthar, Pab fault etc. In the past, low to moderate seismicity has been occurred in Karachi region. However, because of its seismotectonic settings there is the possibility of occurrence of major seismic event. The probability of recurrence of potential earthquake is also investigated in Karachi region. An earthquake catalogue is developed by taking moment magnitude (M w ) as a homogenous magnitude for all events. The completeness of this catalogue is done through taking instrumental data of earthquakes from 1973 onwards. A total of 530 events, occurred in 300 km surrounding of Karachi city, are selected for analysis of b-value and probability of recurrence. The regression analysis is carried out by GR-relationship to analyze the pattern of earthquake distribution and b-value. The statistical analysis of past earthquakes has computed b-value 0.49 showing inadequate earthquake events. Therefore, to find out b-value for this region with low number of earthquake, lack of data and deficiency of relevant earthquake magnitude, GR methodology is not much appropriate. In this situation, Gumbel’s extreme distribution methodology is used, which have shown better results. The regression analysis through Gumbel’s method indicates b-value of 0.8. The probability of recurrence and return periods for different magnitudes is determined by using Gumbel’s extreme distribution value approach.

Accurate prediction of subsurface structures, lithologies and pore fluids, is of great interest for petroleum prospecting and reservoir characterization. Seismic reflection data are widely used to mark subsurface structures and lithologies. However, only seismic data are not sufficient to mark fluid heterogeneities present into the pores. Therefore, the use of integrated approach is vital to map subsurface heterogeneities with more accuracy. Based on seismic interpretation, the limestone of Chorgali Formation present in Ratana area of Northern Potwar, Pakistan is interpreted as reservoir rock. Structural interpretation revealed that the study area lies in compressional regime and structures formed are thrust and popups. The reservoir properties such as lithology, porosity, permeability, depositional environments, shale volume, fluid saturation, net pay thickness are determined from petrophysical analysis which confirms that reservoir characteristics of Chorgali limestone are enough to permit hydrocarbon production. Fluid substitution modeling is used to estimate different rock physics attributes such as compressibility, Lame’s parameters and their product with density, P to S-wave velocity ratio, impedances and Poisson’s ratio are computed as a function of pore fluid type (oil, gas, brine etc.). Sensitivity analysis is performed to derive fluid indicator coefficient which indicates the most appropriate and sensitive rock physics attribute that can be crossplotted to discriminate the rock saturated with different pore fluids (gas/brine/oil).

A large number of valleys and basin systems are present in the northwestern part of the Himalayas in Pakistan which form significant aquifers in the region. Hydrogeophysical investigations in the western part of Nowshera District, a part of the intermontane Peshawar basin, were undertaken to help to determine the availability of groundwater resources in the region. Thirty vertical electrical resistivity soundings (VES) were acquired using a Schlumberger expanding array configuration with a maximum current electrode spacing (AB/2) of 150 m in delineating the groundwater potential in the study area. The results of the interpreted VES data using a combination of curve matching technique and computer iterative modeling methods suggest that the area is underlain by 3 to 5 geo-electric layers. The interpretation results showed that the geo-electrical succession consists of alluvium comprising of alternating layers of clay, silty clay, fine to coarse sands, sand with gravels and gravels of variable thickness. High subsurface resistivity values are correlated with gravel–sand units and low resistivity values with the presence of clays and silts. The modeled VES results were correlated with the pumping tests results and lithological logs of the existing wells. The pumping test suggests the transmissivity of the aquifer sediments is variable corresponding to different sediments within the area. The gravel–sand intervals having high resistivity value show high transmissivity values, whereas clay–silt sediments show low transmissivities. It is concluded that majority of the high resistive gravel–sand sediments belong to an alluvial fan environment. These gravel–sand zones are promising zones for groundwater abstraction which are concentrated in the central part of the study area.

International audience; In seismic applications, the bulk modulus of porous media saturated with liquid and gas phases is often estimated using Gassmann's fluid substitution formula, in which the effective bulk modulus of the two-phase fluid is the Reuss average of the gas and liquid bulk moduli. This averaging procedure, referred to as Wood's approximation, holds if the liquid and gas phases are homogeneously distributed within the pore space down to sizes well below the seismic wavelength and if the phase transfer processes between liquid and gas domains induced by the pressure variations of the seismic wave are negligible over the timescale of the wave period. Using existing theoretical results and low-frequency acoustic measurements in bubbly liquids, we argue that the latter assumption of "frozen" phases, valid for large enough frequencies, is likely to fail in the seismic frequency range where lower effective bulk modulus and velocity, together with dispersion and attenuation effects, are expected. We provide a simple method, which extends to reservoir fluids a classical result by Landau and Lifshitz valid for pure fluids, to compute the effective bulk modulus of thermodynamically equilibrated liquid and gas phases. For low gas saturation, this modulus is significantly lower than its Wood's counterpart, especially at the crossing of bubble point conditions. A seismic reflector associated to a phase transition between a monophasic and a two-phase fluid thus will appear. We discuss the consequences of these results for various seismic applications including fizz water discrimination and hydrocarbon reservoir depletion and CO2 geological storage monitoring. © 2013 Saudi Society for Geosciences.

Naturally occurring gas hydrates are potential future energy source. A significant amount of gas hydrates is interpreted through seismic reflection data in the form of bottom simulating reflector (BSR) present in the sediments of the convergent continental margin of Pakistan. However, the seismic character of these hydratebearing unconsolidated sediments is not properly investigated. Since no direct measurements are available for quantitative estimation of gas hydrate and free gas in these sediments, therefore detailed knowledge of seismic velocities is essential. Seismic velocities of the gas hydrate-bearing sediments in the study area are estimated by using the effective medium theory and the fluid substitution modeling. The results show that the presence of gas hydrates increases the stiffness of the unconsolidated sediments; whereas the presence of free gas decreases the stiffness of these sediments. It is noted that seismic velocities and density of hydrate-bearing sediments are highly affected by saturation and distribution pattern of gas hydrates. The hydrate-bearing sediments seem to be characterized not only by high P-wave velocity (about 2800 m/s) but also by anomalously low S-wave velocity (about 850 m/s). As pure gas-hydrates have much higher seismic velocities than those of host sediments, presence of gas-hydrate increases the seismic velocities, whereas free-gas below the hydrate-bearing sediments decreases the velocities. Seismic reflection from the BSR exhibits a wide range of amplitude variation with offset characteristics, which depend upon the saturation and distribution of hydrates above and free gas below the BSR. We have also demonstrated that some attributes like acoustic and shear impedances, and AVO can be used as important proxies to detect gas hydrate saturation.

The presence of gas hydrates in Makran area of Pakistan is confirmed by seismic evidences in the form of a strong reflector known as bottom simulating reflector (BSR). Amplitude versus offset (AVO) analysis is performed to analyze seismic behavior of free gas zone beneath BSR. This analysis is carried by considering a free gas zone beneath a gas hydrate-bearing zone and overlying a fully water saturated interval. Different pairs of AVO-derived attributes are applied to differentiate free gas and gas hydrates saturated sediments. Based on our analysis, it is concluded that low and high saturation of gas hydrates can be deduced from seismic amplitude anomalies. Low and high saturation of gas hydrates can be distinguish able from background trend. The fluid factor, pore space modulus and Poisson reflectivity are found more sensitive attributes for discrimination of gas hydrates saturation.