REVISED LITHOFACIES AND FRACTURE ANALYSIS OF MIOCENE MURREE FORMATION, ISLAMABAD

ABSTRACT


INTRODUCTION
Islamabad is facing Water issues for a quiet period of time.The main aim of this research work is to investigate the geological and hydrological characteristics of the Murree Formation through Lithofacies and fracture analysis in Islamabad region.Through lithofacies analysis the environment of deposition of the Murree Formation will be investigated while Fracture analysis purpose is to study hydrological characteristics.Previously all studies were carried out at regional level (Yar et al., 2021).This study was concluded on local scale so that minor details can be observed.Also the fracture analysis results were utilized for hydrological purposes previously fracture analysis results were used for hydrocarbon exploration (Mughal et al., 2018).
The lithofacies analysis was conducted using the traditional visit observational study and later identifying those critical indicators to identify its history.The term "facies" refers to the properties of a rock unit that represent its origin and set it apart from others around it (Richard and Stephen 1995).Lithofacies analysis is to investigate each sedimentary facies of the rock Formation with the naked eye.Each facies is formed in a specific type of environment (Moore, 1992).After collecting all the field data, it is plotted on a lithological column, and all the indicators are crosschecked.Figure 6 is showing the lithological column.
Fracture analysis aims to calculate fractured porosity and permeability using the Monte Carlo method.For this a circle of known radius was drawn on the exposed outcrop and then the details of the fractures were measured (Davis, 1996).A fracture is defined as the surface along which rocks break due to loss of cohesion (Twiss and Moores, 1992).Fracture is usually defined for a thing which is broken "any separation in the rock" (Gary, 1872).The density or abundance of discontinuities is evaluated or described through fracture density (Davis, 1996).Fracture density can be evaluated in different ways i.e.Cumulative addition of all the fractures divided by the area of the circle.Fractures enhance the porosity and permeability of rocks.Porosity and permeability are crucial parameters in reservoir characterization and are essential in present-day research.Fracture analysis is essential for evaluating flow parameters.Porosity relies on width, lengths and densities of discontinuities (Jadoon et al., 2003).
Various types of fractures exist, and brittle structures in the upper crust result from brittle deformation.These structures, such as joints, fractures, and veins, contain valuable information about multiple deformation events and stress orientations (Hodgson 1961;Heman, 2005).Fractures exhibiting parallel and perpendicular displacements to the fracture surface are referred to as mixed mode or oblique extension fractures.Extensional fractures like veins typically have small post-shear displacements and occur when dilation stresses exceed the tensile strength of rock.As a result of minor stretching adjacent to the fold axis, extension fractures develop (Yeats et al., 1987).A high angle to the opening direction characterizes extensional veins or tensional gashes.Extensional fractures can be classified into two types: systematic extensional fractures, which occur in sets parallel to sub-parallel orientations, and non-systematic extensional fractures, which display different orientations and irregular surfaces extending to a systematic fracture set (Herman, 2005).
For study ten different locations were selected from which parameters were noted, and then fractured porosity and permeability were calculated using the Monte Carlo method.The description includes the length, Width, and type of the fracture.Both analyses aim to investigate the characteristics of the Murree Formation in the Shah Alla Ditta region of Islamabad for development Projects on a local scale so that precision can be maintained.

Study Area
Figure 1: Showing the study area, i.e., Islamabad, located in the Potohar region in upper Pakistan.
The study area encompasses Shah Alla Ditta and the D-12 sector of Islamabad, located near the Margalla Hills.The primary study location is approximately 6 km from Islamabad (Shah Alla Ditta).The Murree Formation is exposed in various parts of Islamabad, with the most prominent outcrops found in Shah Alla Ditta region.The Shah Alla Ditta area is in the foothills of the Margalla Hills ranges, part of the N.W. Himalayan Foreland Fold and Thrust Belts of Pakistan are associated with the main zone of Himalayan convergence (Jadoon et al., 2002).

Tectonics
The Formation of the Himalayas occurred when the Indian plate in the south collided with the Eurasian plate in the north approximately 50-65 million years ago (Gansser, 1964).The Indian plate separated from the giant Gondwanaland landmass and gradually moved northward around 130 million years ago (Johnson et al., 1985).This movement caused the Neo-Tethys, a sea located between the Indian and Eurasian plates, to start shrinking (McKenzie and Slater, 1971).The magmatism of this arc continued for about 40 million years (Petterson et al., 1985).The collision between the Kohistan-Ladakh arc and the Eurasian plate in the north resulted in the Formation of an Andean-type continental margin due to the closure of the back-arc basin.This collision gave rise to a boundary known as the Main Karakorum Thrust, separating the Kohistan Island Arc (KIA) from the Karakoram Block.The Neo-Tethys continued to subduct beneath the Kohistan-Ladakh arc until the leading edge of the Indian Plate was wholly consumed and finally collided with the Kohistan-Ladakh arc (Powell, 1979).This collision occurred 50-65 million years ago and resulted in the Formation of the Main Mantle Thrust.The Main Boundary Thrust forms the boundary between the Himalayas and the southern deformation, the MMT forms the boundary between the Himalayas and the Kohistan Island Arc, while the MKT forms the boundary between the Karakoram and Kohistan (Tahirkheli, 1979(Tahirkheli, , 1982;;LeFort, 1975;Patriate and Achache, 1985;Khan, 2011).

Geology
The Murree Formation derives its name from the Murree hills in Rawalpindi district.The Formation comprises siltstone and dark red to maroon clay, interbedded with red and reddish-gray sandstone.The Formation belongs to the Miocene epoch periods and is part of the Rawalpindi group (Kazmi and Jan, 1997).The convergence and collision of the Pakistan-India and Eurasian tectonic plates primarily influence the geology of the Islamabad area.This collision has developed complex structures and stratigraphy in Islamabad (Sheikh et al., 2007).The exposed sedimentary rocks in the Islamabad area span from the Jurassic to the Quaternary period.From approximately 150 to 24 million years ago, there was relatively slow marine deposition and minor tectonic activity.From 24 to 1.9 million years ago, rapid and extensive continental deposition was accompanied by gradual subsidence.However, during the last 1.9 million years, intense tectonic activity, extensive erosion, and coarse clastic continental sediment deposition in localized areas.The lithologies exposed in the study area range from the Jurassic to the Miocene.

METHODOLOGY
The first phase of the research work includes lithofacies analysis.Fieldwork was conducted to perform lithofacies analysis, primarily focusing on the physical characteristics of the exposed Formation in the Shah Alla Ditta region.The variations in lithologies were observed and recorded for each bed, and these observations were then plotted in a lithological column.A facies is defined by its sediment nature, lithology, texture, fossils, geometry, and contact relationships.When studying sedimentary rocks, two essential approaches are lithostratigraphy and lithofacies analysis.The term "facies" is subject to debate in geology.Facies analysis is a significant step in sedimentology and has applications in analyzing igneous and metamorphic rocks (Moore, 1992).Facies analysis is commonly used as the initial step in studying the depositional environments of sedimentary rocks.Each rock unit is studied, considering grain size, color, weathering, and sedimentary features.Finally, all the results are correlated, leading to a conclusion.The second phase consists of fracture analysis.There are two methods commonly used for fracture analysis.The Selection method and the Circle Inventory method.The Selection method involves choosing specific joints for measurement and study.This method focuses on analyzing continuous joints and shear fractures that are visibly associated with similar fractures in appearance and orientation.Determining which fractures should be measured and which ones can be ignored at each location can be challenging.
On the other hand, the Circle Inventory method involves measuring all fractures within a designated circular area.This is more effective as the density of a fixed area is calculated, and then the average value is taken.If random fractures were taken, we might measure more readings in some areas compared to others.So the density per square area will not be justified.To apply this method, a circle with a known or predetermined diameter of 100 inches was drawn on an exposed outcrop of fractured rock using spray paint (Davis, 1996).Ten such circles were drawn at different Shah Alla Ditta region locations.Taking readings from different locations was to get a mean average value.Each fracture's aperture and trace length within the circle is then measured.To avoid duplicating measurements, the entire length of each joint can be traced with chalk after measuring it.For this fracture analysis, the Circle Inventory method was used.The filling nature of the fracture was also noted because filled fractures are closed and unsuitable.After collecting all the readings, Monte Carlo equations were used to calculate fractured porosity and permeability.The first step in this process is to calculate the fracture density of the area, and then the fracture density readings are used to calculate the fractured porosity.Finally, fractured permeability was calculated using the values of fractured porosity in the Monte Carlo equation.

RESULT AND DISCUSSION
This study aims to analyze the depositional environment of the Murree Formation by studying lithofacies and investigating fractured porosity and permeability through fracture analysis.Figure 5 shows the circles from which the details of the discontinuity were observed and noted.The details include Fracture length, Width, and infilling material.The details are shown in tables from 1 to 10.

Fracture Density
Fracture density determines the abundance of fractures at each station and point (Davis, 1996).In the circle inventory method, fracture density is calculated by dividing the sum of all fractures within a circle by the area of that circle.To calculate the area of a circle, the radius of that specific circle needs to be known (Jadoon et al., 2002).Fracture density provides insights into the hydrological conditions of the area under study.In areas with low porosity and permeability, it takes years for surface water to infiltrate the groundwater.However, surface runoff is reduced in highly fractured areas, and a significant amount of water is stored in the underlying aquifer (Fetter, 2003).The mathematical equation used to calculate fracture density was derived from the work (Jadoon et al., 2002).

Fractured porosity
Fractured porosity depends on the fractures' widths, lengths, and densities in an inventory circle.This data is combined using Monte Carlo techniques to estimate fracture porosity and permeability (Jadoon et al., 1992).The average value of Width was taken for each fracture.The fractured porosity was calculated using the following mathematical equation.

Fractured Permeability
The permeability within inventory circles (assuming no matrix) is estimated from the following equation (Muskat, 1949).Fracture provides both permeability and storage in rocks.An estimate of porosity and permeability is vital for any reservoir or aquifer.For any aquifer to exist in fractured lithologies under suitable hydrograph and climatic conditions, fractured porosity (Sandstone) must range from 1.8 % to 6.5 % (Jadoon et al., 2002).The fractured porosity of the Murree Formation in the study area ranges from 0.6 % to 3.4 %.With reference to those published values, Murree Formation in this region is not considered suitable for an aquifer system.Only three circles (circle inventory method) show prominent signs of aquifer conditions, while the remaining seven are below ideal values.

Lithofacies Analysis
The leading indicators, i.e., reddish sediments, asymmetrical ripple marks, cross-laminated sandstone, and conglomerate, indicate that the Murree Formation was deposited in a fluvial environment.The thick clay deposits indicate that the river system was meandering and in its mature stage.The clay, shale, and very fine sand represent flood plan deposits.
The conglomerate (well sorted) was deposited as a channel deposit during floods.The medium to fine-grained sandstone with fining upward sequence, ripple marks, and small cross lamination represents the point bar of the river.The coarse-grained sandstone with cross-lamination represents channel deposits.Below are field photographs of different facies.

CONCLUSIONS
Fracture provides both permeability and storage in rocks.An estimate of porosity and permeability is vital for any reservoir or aquifer.The fractured porosity of the Murree Formation in the study area ranges from 0.6 % to 3.4 %.With reference to those published values, Murree Formation in this region is not considered suitable for an aquifer system.Only three circles (circle inventory method) show prominent signs of aquifer conditions, while the remaining seven are below ideal values.
The leading indicators, i.e., reddish sediments, asymmetrical ripple marks, cross-laminated sandstone, and conglomerate, indicate that the Murree Formation was deposited in a fluvial environment.The thick clay deposits indicate that the river system was meandering and in its mature stage.The clay, shale, and very fine sand represent flood plan deposits.The conglomerate (well sorted) was deposited as a channel deposit during floods.The medium to fine-grained sandstone with fining upward sequence, ripple marks, and small cross lamination represents the point bar of the river.The coarse-grained sandstone with cross-lamination represents channel deposits, and the Parallel laminated sandstone represents different energy conditions of the channel.

Figure 2 :
Figure 2: Tectonic map of northern Pakistan.Yellow colored area showing the location of study (Modified after GSP, 1993)

Figure 4 :
Figure 4: Flow chart of Circle Inventory Method used in fracture analysis

Figure 5 :
Figure 5: Showing field photographs of outcrops (circle inventory) from which fracture data was collected for further analysis.

Figure 10 :Figure 11 :
Figure 10: Showing Contact between Sandstone and Conglomerate

Table 1 :
Description of Circle 1 exposed in an outcrop.

Table 2 :
Description of Circle 2 exposed in an outcrop, Coordinates

Table 3 :
Description of Circle 3 exposed in an outcrop.

Table 4 :
Description of Circle 4 exposed in an outcrop, Coordinates

Table 5 :
Description of Circle 5 exposed in an outcrop.

Table 6 :
Description of Circle 6 exposed in an outcrop.

Table 7 :
Description of Circle 7 exposed in an outcrop.

Table 8 :
Description of Circle 8 exposed in an outcrop.

Table 9 :
Description of Circle 9 exposed in an outcrop.

Table 10 :
Description of Circle 10 exposed in an outcrop.

Table 11 :
Fracture Densities of each circle in the inventory method

Table 12 :
Fractured porosity of each circle in the inventory method

Table 13 :
Fractured permeability of each circle in the inventory method Fractures in the study area were open and closed (filled with calcite).