Recognizing and classifying the naturally fractured reservoirs (NFRs) system for the cretaceous carbonate formation exposed along the Bagnotar-Baragali Section, north Pakistan: Implications from surface fractures, petrography and geochemistry

doi:10.21203/rs.3.rs-2192959/v1

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Recognizing and classifying the naturally fractured reservoirs (NFRs) system for the cretaceous carbonate formation exposed along the Bagnotar-Baragali Section, north Pakistan: Implications from surface fractures, petrography and geochemistry

https://doi.org/10.21203/rs.3.rs-2192959/v1

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The current work deals with the classification of NRFs system for reservoirs characterization of Cretaceous Kawagarh Formation along Bagnotar-Baragali Section, Lesser Himalaya Northern Pakistan. Surface fracture data were collected including orientation, attitude (Dip/Strike), fracture density (F.D), frequency and their distribution. The methodology implemented comprises of surface fracture data and subsequent statistical calculations by Monte Carlo equation followed by scanline circle inventory method. The data was collected from 198 different stations with 75 fractures inventories. The statistical data is then compared with the petrography, SEM and EDX data already available. Additional, the NFRs system was classified and recognised for the studied carbonate units based on the comparative analysis. Likewise, stress analysis was integrated with the results which displays that major stresses in NW directions whereas minor stresses were acting in NEE direction. The Stereonet diagram shows that the maximum fractures are NW-SE oriented, average amount of fracture are NE-SW oriented while Minimum amount of fracture are oriented in NEE-SWW direction. The fractures direction show three sets of joints making X-shape, which indicate that these fractures are interconnected and due to which the reservoir potential can be heightened. The average Fracture Density (FD), Porosity and permeability were calculated as 0.043136, 3.212509 and 0.282299 values respectively. The X-plots of Fracture Density (FD) vs. Porosity and Permeability is weak which decrease reservoir potential while the interrelationship of Fracture Porosity and permeability is very strong which yields supposed to enhancing reservoir potential of Kawagarh formation. The analysed formations show intermediate to low porosity and permeability and classified as Type 2 or Types 3 in NFRs system based on the Visual porosity (V.P) calculated from petrography and the fracture data from the outcrop. This further shows that, the fractures provide essential permeability only in case of types 2 while in case of type 3 the fractures provide a permeability assistance. Classification of Natural Fractured Reservoir (NFR) (Nelson, 2001) Shows that Kawagarh formation is type three (Poor) reservoir. The comparative analysis also shows that Kawagarh formation has a poor reservoir potential in Qamar Mela (Nizampur Basin) and Kala Chitta Ranges.

NFRs system

Carbonate units

Petrography

Geochemistry

Fractures data

Stress implication

Pakhtunkhwa

Pakistan

Bara Gali Section near Nathia Gali (Galyat), Northern Pakistan was selected for the current study having coordinates 34° 21ʹ 0ʺ North, 72° 21ʹ 0ʺ East. The area is about 30 km away from Abbottabad city which lies to its west as shown in Fig. 1. Kawagarh Formation is well exposed at different section along Nathia Gali. Reservoir quality determination is a difficult and complex process for which different methods were to be carried out and different factors are to be examined in detail for achieving such a difficult goal. For that purpose, we do analysis and interpret outcrops fractures of Kawagarh formation with different methods and technique, and made a comparative analysis with other thin sectional and SEM based on Kawagarh formation. The fractures data of Kawagarh formation were recorded along Kala Bagh Track at University of Peshawar, Bara Gali campus and along the main rout of Nathia Gali where scanned line method was used. The area is well exposed and fractured, consist of dense and non-systematic fracture patterns in which most fracture are in NW direction, Normal to the bedding plans with different opening and length (Ahsan and Naveed, 2020).The detail analysis of Stresses, Fracture Density, Porosity, Permeability and their mutual relationship was undertaken in context of Regional Stresses. Comparative analysis with other work was also carried out to evaluate reservoir potential of Kawagarh Formation at study area, for which SEM and Petrographic study was correlated with present work. The aims and objectives of this study are as follow, which are done on Kawagarh Formation at Bara Gali near Nathia Gali (Galyat), Lesser Himalaya, and Northern Pakistan.

Analysis of recorded Fractures data to calculate and find out Stresses acting on the study area.
Analyse these factors for reservoir characterization of Kawagarh Formation.
To evaluate Reservoir Potential of Kawagarh Formation from Fractures data based on Natural Fractured Reservoir Classification (NFR) by Nelson (2001) at study area.
Doing a detail comparative assessment for Reservoir potential of Kawagarh formation based on current study and previous works at different section of KP.

The fieldwork was conducted to Bara Gali, Galyat area located in Lesser Himalayan in Northern Area of Pakistan. For collection of data several techniques and methods was owned. At field, scan line method was used for strike, dip data of Fractures using Brenton Compass and the Outcrop images were taken. Photo-geological interpretation were carried out for structural determination of data. For stresses analysis and Orientation of Fractures, rose diagram and stereo net diagrams were used with the help of recorded data using GeoRose Software. The illustration and Calculation were done using Adobe Illustrator and Microsoft Excel Sheet respectively. Evaluation and Characterisation of the Formation was carried out using Natural Fractured Reservoir Classification (NFR) of Nelson (2001) as recently done by Anjum et al., 2022 in Nizampur Basin, KP Pakistan.

Scanline Method

Scanline method is a systematic way to record fractures and joints data of an outcrop for different analysis through Fractures data of Rocks and Formation. In this method, simply a line with known length was drawn on the surface of an outcrop. Then recorded all the data of fractures which intersects that line, the data include strike, dip, type of fracture, Orientation and a Photograph of the outcrop.

Data Collection

The Strike and Dip data was measured through Brenton Compass while the coordinates of the study area were determines by GPS device. The length of the fracture was measured through a handheld measuring tape. The images/photographs were clicked through mobile phones. These data were recorded in notebooks in a proper sequence having station name on the top of the page to reduce confusion in further analysis.

Illustration Work

Illustrations are used to convey more detailed information about something and make a mind map graphically to others. Therefore, for that purpose we use Adobe Illustrator, which is Graphic Designing Software package, to Illustrate Figures, Tables, Maps and Field Photographs for better understanding of data Interpretation.

Mapping Software

GeoRose Software were used for plotting Strike, Dip data on Rose diagrams and Stereo net diagrams. These diagrams were used to calculate different stresses and its direction, Dipping Directions of Fractures, Fractures Orientations and mainly used for structural determination and evaluation of an area.

Regional Geology And Tectonic Setting

In early Triassic to Middle Jurassic all, the continents were present as a super-continents called Pangea (Kent et al., 2003). During the breaking of Pangaea, the subcontinents become isolated from the southern part of the Pangaea and that isolated part is called Gondwanaland more than 140 million years ago. Around 120 million years ago, Indian plate is breaking off and migrating slowly towards north about 5 cm per year and after that, suddenly 80 million years ago its speed increases at the rate of 15 cm per year. Indian plate kept this speed another 30 million years before hitting the brakes, after that Indian plate collided with Eurasian plate 50 million years ago which gives rise to the Himalayas. The collision of Indian and Eurasian plate formed the Himalayan mountain belt that stretches from Salt range to Main mantle thrust (MMT). The collision of Indian and Eurasian plate produced the Himalayan Arc which has a complex geometry having northwest-southeast trends in India, the after bending around Hazara Kashmir syntaxes its orientation becomes nearly east-west in Pakistan and its orientation become almost north-south near the western boundary of Pakistan. The Indo-Pakistan plate, towards Eurasian plate is still in motion at the rate of 2 mm per year. (Patriat and Achache, 1984). Pakistani Himalayas thrust system Main Karakoram thrust (MKT), Main mantle thrust (MMT), Main boundary thrust (MBT) and Salt range thrust (SRT) (Treloar, 1989), (Kazmi and Rana, 1982). The main stratigraphic regimes of northern Pakistan are separated by major fault system is given below. (Ahmad et al, 2004).

Main Karakoram Thrust (Mkt)

Main Karakoram Thrust formed in Cretaceous age (Coward et al. 1986). Main Karakorum thrust is formed due to the collision of Kohistan Ladakh arc and Karakoram block about 100 million years ago, Eurasian plate is thrusting over Kohistan island arc (Boreman, 2015). Main Karakoram thrust (MKT) shows the boundary between Eurasian plate and KIA (Kayal, 2008), (Boreman et al. 2015). Seismically MKT is an active thrust with a large number of earthquakes of low to medium intensity.

Main Mantle Thrust (Mmt)

Main mantle thrust MMT represents the boundary between metamorphic shield and platform rock of the Indian plate hinterland and dominantly mafic and ultramafic rock of the Kohistan-Ladakh arc complex. Faults define the MMT vary in age from Quaternary to Late cretaceous. The diffing direction of the MMT is north direction generally dipping with 25 to 45 degree. Main composition of MMT is more than 15 km thick Protozoic gneisses and schist (Madin, 1986).

Main Boundary Thrust (Mbt)

Main boundary thrust MBT has different tectonic singularity along the Himalayan belt. Hazara syntaxial zone are surrounded by main boundary thrust. Main boundary thrust is an active fault for earthquakes and the hanging wall of MBT carries the pre-collisional Paleozoic and Mesozoic sedimentary and metasedimentary rocks of the northern deformed fold and thrust belt.

Salt Range Thrust (Srt)

Salt range thrust is terminates in the west against Kalabagh fault. Along the Salt range thrust SRT different age of rock exposed at different area i.e. Precambrian are exposed in salt range, Cambrian rock in Kohisor ranges (Alam et, al) and Permian rocks in Surghar ranges (Ali, 2005).

Local Geology

Tectonic Framework of the Study Area

Our study area is in between MBT main boundary thrust to MCT main central thrust or Panjal thrust i.e. called Lesser Himalaya. The lesser Himalayan are separated from the sub-Himalayan by the main boundary thrust MBT. Our study area i.e. the Galyat area is located in the Lesser Himalaya that have been uplifted along the Nathia Gali Fault which is also known as Hirrastang Fault at the west which is the splay of main boundary thrust (MBT) or Murree fault. The study Area is surrounded by the following Geological structures in Northern Pakistan as Shown in Fig. 2.

Nathia Gali Fault
Panjal Fault
Hazara Kashmir Syntaxes
Hazara Basin

At Nathia Gali area there is splay of Murree fault/Main boundary thrust MBT from east to west side, which is known as Nathia Gali fault and Hirrastang Fault at the west. Seismically Nathia Gali fault is an active fault which extends from north of Hazara Kashmir syntaxes to west of Afghanistan. At north side of the Nathia Gali fault there is another splay of Murree fault i.e. Panjal thrust which is also known as (khairabad fault at the west) or MCT main central thrust at west side of study area. (Gansar, 1981). The MBT is making the Hazara Kashmir Syntaxes to the east of Galyat area as shown in Fig. 2.1 due to which the overall geology of lesser Himalaya is very complex. Hazara basin is located to the west of Galyat area the rocks of which is uplifted in the Galyat area by Nathia Gali fault. The rocks and formation of the basin were faulted and folded because these rocks were subjected to high stresses at the time of convergent of Indian and Eurasian plate.

Generalized Stratigraphy

The rocks and formations of Galyat area is part of the Hazara basin, which is uplifted due to Nathia Gali fault. Nathia Gali fault is the splay of main boundary thrust, which is in between the MBT, and Panjal thrust fault. The rocks exposed in this area is from early Jurassic to middle Eocene age. The rock present in study area is highly weathered, fractured and deform due to different faulting and folding activities, these activities are strongly effected the rocks. The stratigraphy of the area is very complex. However, geologist have done their research and given a proper sequence. The sequence of rocks/formations from older to younger is as Datta Formation, Samanasuk Formation, Chichali Formation, Lumshiwal Formation, Kawagarh Formation, Hungu Formation, Lockhart Formation, Patala Formation, Margalla Hill Formation, Chor Gali Formation and Kuldana Formation as Shown in Fig. 3. The Kawagarh formation was deposited in the basin where subsidence was caused by northward drift of the Indian plate (Ahsan and Chaudhry, 2008). Formation was deposited inner to outer ramp based on the identified micro-facies and bio-clastic wackstone micro-facies (Asif et al. 2012). The Kawagarh formation is one of the significant formation shows two different facies (Ahsan and Chaudhry 1998) north and south of Nathia Gali fault. Kawagarh formation is in the sections north of the Nathia Gali fault mostly comprised of fine grain light grey limestone, with inter-beds of marls and clay. Kawagarh formation are well exposed near Giah (M Nawaz Ch. 2008). Light medium grey to light yellowish grey on fresh surface while the limestone is light yellowish grey to whitish grey on weather surface are exposed. According to (Fatmi, 1972) fossils, foraminifera species can be recorded. Kawagarh formation is unconformable overlying Hungu formation and conformable underlying Lumshiwal formation.

Illustration Of Fractures And Fracture Analysis

The interpretation and data analysis of fractures were done in this chapter. We calculated various parameter of fractures by mathematical equations and calculation in excel sheet. The fractures data were recorded in field area for each station as shown in Table 1, and were illustrated separately in outcrop photographs as Shown in Fig. 4. The stresses were calculated and shown by Rose Diagram. The Calculation of Fracture Density FD, Fracture Porosity FP (%) and Fracture Permeability (K) were find out using the following Formulas:

Table 1

Showing Collected Fractures data At 18 Stations.
Recorded Data At Field
		Strike	Dip direction	Dip angle	Length (cm)	Width (cm)
Station No	Joint No	Strike	Dip direction	Dip angle	Length (cm)	Width (cm)
1	1	343	73	80	46.3305	0.78
1	2	40	310	74	28.7044	0.37
1	3	333	63	84	52.5209	0.68
1	4	27	297	86	51.1341	0.95
1	5	48	318	87	51.2436	2.5
1	6	286	16	88	52.9634	0.53
2	7	59	329	71	30.5305	0.25
2	8	45	315	87	33.3298	0.22
2	9	326	56	87	34.5897	0.99
2	10	61	331	61	53.2271	0.53
2	11	51	321	67	56.4003	0.24
2	12	350	80	82	54.7041	0.54
3	13	37	307	75	56.6617	0.29
3	14	52	322	64	49.6034	0.21
3	15	302	32	84	33.1975	0.22
3	16	71	341	62	38.3813	0.32
3	17	335	65	67	72.9747	0.19
3	18	7	277	72	47.3345	0.23
4	19	320	50	32	47.9276	0.3
4	20	17	287	40	106.052	1.56
5	21	75	165	62	63.4215	0.18
5	22	44	314	51	82.4941	0.91
6	23	323	53	54	51.1754	0.33
6	24	322	52	51	51.8966	0.32
6	25	319	49	44	53.7041	0.24
6	26	329	59	48	51.7118	0.29
7	27	321	51	45	51.1575	0.82
7	28	329	59	43	51.5548	1.8
8	29	310	40	53	53.6151	2.51
8	30	298	28	62	43.3613	3.36
9	31	33	303	70	50.1961	1.23
10	32	313	43	67	45.1517	0.28
10	33	297	27	53	39.1553	0.24
10	34	300	30	49	56.6064	0.2
10	35	80	350	87	92.7517	1.03
10	36	306	36	45	29.1499	0.26
11	37	324	54	43	53.8124	2.5232
11	38	356	86	22	72.1431	1.16
11	39	317	47	41	52.1291	1.22
11	40	296	26	41	37.1989	0.89
12	41	337	67	70	36.2085	0.53
12	42	293	23	49	61.2329	1.27
12	43	319	49	51	74.5821	0.42
12	44	76	346	75	61.1353	0.83
12	45	4	274	73	66.4646	0.53
12	46	300	30	73	23.4935	0.21
12	47	299	29	78	55.4056	0.48
13	48	315	45	44	51.5506	2.31
13	49	86	356	86	27.5092	0.65
13	50	315	45	43	50.5259	1.48
13	51	302	32	52	43.5126	0.72
14	52	330	60	41	48.0322	0.38
14	53	337	67	77	51.1321	0.36
14	54	334	64	57	56.1456	0.32
14	55	346	76	50	54.2264	0.1
14	56	300	30	85	55.6533	0.77
14	57	335	65	54	59.0132	1.2
15	58	300	30	62	51.1539	0.14
15	59	349	79	54	33.1172	0.19
15	60	323	53	57	53.6732	0.31
15	61	316	46	53	46.5898	0.34
16	62	8	278	85	58.9311	0.45
16	63	350	80	87	46.2381	0.19
16	64	286	16	76	30.2271	0.23
16	65	70	340	63	58.1491	0.48
16	66	58	328	69	55.5483	1.99
17	67	46	316	61	51.0951	0.48
17	68	46	316	72	40.8322	0.5
17	69	33	303	46	55.0381	0.34
17	70	42	312	58	54.8841	41
17	71	35	305	45	50.1116	1.11
18	72	41	311	77	52.8974	0.42
18	73	280	190	70	104.46	0.64
18	74	43	313	67	54.7271	0.44
18	75	55	325	64	28.5143	0.39

Fracture Density: FD = ∑L/A (Davis et al. 1996).

Fracture Porosity: FP (%) = (1/A) ∑i = 1(Li × Wi) ×100 (Baitu et al. 2008).

Fracture Permeability (K) = (3.5 × 10⁸)(1/A) ∑i = 1 (Li × Wi³) (Muskat; 1949).

The overall data of Fracture Density FD, Fracture Porosity FP (%) and Fracture Permeability (K) were shown in Table 2. First, scanned line method is used for recording the fracture data for those fractures which is cross cut by a given line on the outcrop as Shown in Fig. 4. Furthermore, the related data of strike and dip were collected. After that, we used Adobe Illustrator for the illustration and finding of length and width of fractures in every individual stations. Rose diagram is consist of one or more concentric circles, which are superimposed on radial line (Anjum et al., 2022; Marshak and Mitra 1988). The rose diagram has radial lines arranged after some specific degree to the geographical quadrant. We have arranged the radial line after each 10 degree to the geographical quadrant. The concentric circles in the rose diagram shows the number of fractures in the same direction. In the rose diagram, the data were plotted according to the scale of fracture data we have recorded. It gives the visual and graphical estimation of fracture data. We used GeoRose software for plotting the rose diagram for the analysis of fractures data we have collected at field. The stereo net diagrams are used for plotting the geological data of planes and lines for determination. The stereo net consist of two type of circles the greater one and the smaller ones, which are perpendicular to each other. The circles, which passes through the centre of the sphere, are called the greater circles and those, which are not passes through the centre of sphere, are called smaller circles. The great circles shows the longitudinal lines while small circles shows latitudinal lines on the globe. These great and small circles determines the location of stereographic projection.

Table 2

showing details of all fractures recorded on each station.
Station no	Formation name	Total no of fractures	Calculated maximum stress direction	Fracture density in cm^− 1	Fracture porosity in %	Fracture permeability in Darcy
1	Kawagarh	6	NE	0.056579	5.744593	0.62465328
2	Kawagarh	6	NE	0.052556	2.409913	0.036198527
3	Kawagarh	6	NW,N,NE, NEE	0.059631	1.423724	0.003170242
4	Kawagarh	2	NW,NNE	0.030796	3.596382	0.282737557
5	Kawagarh	2	NW,NNW	0.029183	1.72971	0.043774525
6	Kawagarh	4	NW	0.041698	1.227604	0.00388027
7	Kawagarh	2	NW	0.020542	2.694956	0.230211941
8	Kawagarh	2	NW	0.019395	5.605357	1.744858958
9	Kawagarh	1	NE	0.010039	1.234824	0.065385786
10	Kawagarh	5	NW	0.052563	2.729485	0.072694951
11	Kawagarh	4	NW,N	0.043057	6.323399	0.768554861
12	Kawagarh	7	NW	0.075705	4.915553	0.131278765
13	Kawagarh	4	NW	0.03462	4.861405	0.576116111
14	Kawagarh	6	NW	0.064841	3.474358	0.094008338
15	Kawagarh	4	NW,NNW	0.036907	0.918661	0.002658363
16	Kawagarh	5	N,NWW,NE,NEE	0.049819	3.614183	0.315167804
17	Kawagarh	5	NE	0.050392	2.835621	0.059664358
18	Kawagarh	4	NE	0.04812	2.48543	0.026359065

Interpretations

In this study, we calculated and plot diagrammatically the stress analysis, and Reservoir characterization that includes relationship between Fracture density, Fracture Porosity and Fracture Permeability of Kawagarh Formation exposed in the Lesser Himalaya at Galyat Area near Bara Gali, which lies on Nathia Gali thrust Fault the splay of Main Boundary thrust MBT. As we know that the area is located in Northern area of Pakistan, which is highly exposed to regional stresses, produce by the convergence of Indian and Eurasian Plate several million years ago, which cause the uplifting, Folding and Faulting of the area. Several techniques and Mathematical formulas are used to calculate stresses and fracture orientation, density, porosity and permeability from the fractures data recorded at each station in the area. The overall data of maximum stresses direction, no of stresses per station, Fracture Density, Fracture porosity, fracture permeability for each station was comprehensively shown in table 19. In addition, the stresses calculated by Rose diagram are plotted on geological map of study area as shown in Fig. 5.

Stress Analysis

Stresses have very importance in Structure Geology, Hydro Geology, Engineering Geology, Basin modelling and Reservoir Characterization. Here we have calculated stresses through Rose diagram from the fractures data recorded during field. The study show that Kawagarh formation is exposed mainly to three types of stresses that are calculated through Rose diagram using Geo Rose Software which is shown in Fig. 6. (A), where δ1 show the maximum stresses direction, δ2 shows intermediate stresses direction and δ3 shows minimum stresses direction. After analysis of 75 fractures recorded at 18 stations, the formation has experienced three types of stresses i.e. δ1, δ2 and δ3.

These stresses are discussed briefly as follow:

NW- SE Directed Stresses (δ1)
NE-SW directed stresses (δ2)
NEE-SWW Directed Stresses (δ3)

Nw- Se Directed Stresses

According to Table 2 stresses acting on the formation having NW-SE direction are recorded in 12/18 station. Therefore, it is assumed that maximum stresses are acting on the formation in NW-SE direction, which are in stations 3,4,5,6,7,8,10,11,12,13,14 and 15.These stresses are plotted on Rose diagram and illustrated on Google Earth image of the study area, which are denoted by δ1 in both as shown in Fig. 6. (A) and Fig. 7. respectively.

Ne-sw Directed Stresses

There are 7/18 stations having stresses in NE-SW direction as shown in Table 2. These stresses are categorized as Intermediate stresses. These stresses are recoded in stations 1, 2,3,9,16,17 and 18 which acting on the formation in a NE-SW direction. These stresses are plotted on Rose diagram and illustrated on Google Earth image of the study area, which are denoted by δ2 in both as shown in Fig. 6. (A) and Fig. 7 respectively.

Nee-sww Directed Stresses

The minimum stresses acting on the study area are in NEE-SWW direction. The maximum stresses in this direction are only recorded in 2/18 station which are station 3 and 16.These stresses are shown on Rose Diagram and Google Earth image, which are denoted by δ3 in both as shown in Fig. 6. (A) and Fig. 7 respectively.

Orientation Of Fractures

The orientation of fractures are find out from the strike and dip data recorded at each station during field. After the analysis of all the fractures we concluded that the fractures are mainly oriented in three direction which are NW, NE and NEE. Maximum orientation of fractures are in NW direction, in the NE, the fractures are oriented intermediately while the minimum orientation of fractures are in NEE direction as shown in Figs. 8 and 9. These orientations of fractures are illustrated on google earth image in Fig. 8.

Relationship Between Fracture Density And Fracture Porosity

The fracture density and porosity data were plotted on scattered chart against each other to analyse the mutual relationship between these two factors of Kawagarh formation of the study area. The Graph shows that there is no proper linear relation between these two factors and the data is variably scattered as shown in Fig. 10. The highest value of Fracture porosity is 6.3233% at station 11 and highest value of fracture porosity is 0.0757 at station 12.

Relationship Between Fracture Density And Fracture Permeability

The data of fracture density and fracture permeability were plotted against each other to find out the mutual relationship between these two factors of Kawagarh Formation of study area. According to the graph, there is no proper relationship between these two factors because the data is very variable as shown in Fig. 11. It is because of difference in width and length of fractures in the formations. The highest fracture permeability is 1.7448 encountered at station 8.

Relationship Between Fracture Porosity And Fracture Permeability

The fracture porosity and fracture permeability of Kawagarh formation were plotted against each other to calculate their mutual relationship at study area. According to the graph shown in Fig. 12, the data shows the linear trend between these two factors, which means that as the fracture porosity increase, fracture permeability also increases, which is a very important characteristic of a good reservoir.

Quantitative And Qualitative Reservoir Potential

The lithology of Kawagarh formation is limestone and was highly exposed to several stresses that cause three types of joint sets in the area. The formation was evaluated for reservoir characterization based on collected data of fractures, which is then used to find out length, width, Fracture density, fracture porosity and Fracture Permeability using different statistical and mathematical approaches for each station as shown in Table 2. We have also analyse the Relationship between fracture density, fracture porosity and fracture permeability, which are very important factors for reservoir characterization. Generally the calculations and comparison of Tables 3, 4,5 and 6, based on Fracture data of 18 stations, shows that Kawagarh formation at study area cannot act as good reservoir although it is exposed to high compressional stresses. Using this statement it should be in mind that it was generalized statement based on fracture data of Kawagarh Formation at lesser Himalayan where no thin section study was evaluated.

Table 3

showing average fracture Density, Porosity and Permeability of 18 stations.
Factors effecting reservoir potential of Kawagarh formation
Average Fracture Density in cm^− 1	Average Fracture Porosity (%)	Average Fracture Permeability (10⁸⁾
0.043136	3.212509	0.282299

Table 4

Shows Relationship between Fracture density, Fracture Porosity and Fracture Permeability that are the main factors for reservoir evaluation.
Relationship between factors effecting reservoir potential of Kawagarh Formation
Fracture Density vs Fracture Porosity	Fracture Density vs Fracture Permeability	Fracture Porosity vs Fracture Permeability
Poor	Poor	Good

Table 5

Showing Qualitative Classification of Natural Fractured Reservoir (NFR) Modified from Nelson (2001).
Classification of Natural Fractured Reservoir NFR
Types of Fractured Reservoir
S. No	NFR Type	Definition	Porosity in %	Permeability in Darcy
1	Type 1	Fractures provide essential porosity and permeability	High	High
2	Type 2	Fracture provide essential permeability only	Intermediate	Intermediate
3	Type 3	Fractures provide permeability assistance	Low	Low
4	Type 4	Fractures do not provide significant additional porosity and permeability	Extremely low or Zero	Extremely low or Zero

Table 6

Showing the Qualitative Classification of Kawagarh formation according to Nelson (2001) category type based on Quantitative data.
Classification of Natural Fractured Reservoir Nelson (2001)
Average Fracture Density in cm^− 1	Average Fracture Porosity (%)	Average Fracture Permeability (10⁸⁾	NFR Type
0.043136	3.212509	0.282299	Type 3 & 4

Table 7

Showing Results Comparison of Current study, Paper I and Paper II.
Conclusion from the comparison of Current study, Paper I, Paper II and Paper III to evaluate Reservoir Potential of Kawagarh Formation at different section of KPK using different techniques.
Current study results	(Bilal Wadood et al. 2019)	(Samiullah et al. 2017)
Limestone Facie	Limestone Facie	Limestone and Marl Facie
Well fractured	Well fractured	Well fractured
Avg porosity is 3.212509%	Avg porosity is 2.80%	Avg porosity is 1.69%
Avg permeability is 0.2823 Darcy	------	Avg permeability is 0.26 Darcy
Poor reservoir Potential	Poor Reservoir Potential	Poor reservoir Potential

Comparative Analysis

The results of this studies is compared with the nearby available work on the exposed formation to check mutual relationship between these results for better understanding of characterization of reservoir potential of Kawagarh Limestone. Bilal Wadood et al. 2019 conducted studies on thin section visual porosity analysis and diagenetic features. The porosity of Kawagarh carbonate noticed in this paper was mostly vuggy, Stylolitic and Fracture Porosities in different thin sections. The vuggy and Stylolitic porosity are recorded as about 3.88%, while the average porosity is 2.80%. They stated that “Based on the present study, Kawagarh Formation is interpreted as poor quality reservoir rock.” Furthermore, Samiullah et al. 2017, studied the same carbonate unit based on micro-facies analysis, depositional environment and Reservoir characterization. He concluded that, the factors enhancing reservoir quality at section are dissolution, fractures and dolomitization while factors reducing reservoir quality are cementation, compaction and pyritization. Plug porosity and permeability of the formation was recorded as 1.69% and 0.26 ka/md. Moreover, the SEM study of Kawagarh formation at Nizampur Basin show vuggy porosity, shelter porosity, quartz, acicular cementation and micro-pores as shown in Figs. 13, 14 and 15. From the analysis of these SEM Plates, it is concluded that Vuggy porosity, Shelter porosity and pores enhanced the porosity of Kawagarh formation. While the porosity is reduced by cementation and quartz precipitation.

Implications Of Stress Analysis

Stresses have an important role in Geology, because it deform rocks structurally and makes different Geological structure including Fold, faults and Fractures etc. It is also considered as an important factor in evaluating reservoir potential of a formation. Here, we will discuss the stresses analysis we do on Kawagarh formation at Nathia Gali section, Northern Pakistan. After the fracture analysis of Kawagarh formation at study area, it was observed that the Kawagarh formation was exposed to three types of stresses δ1, δ2 and δ3 shown in Fig. 4.2 and 4.3. Where the major stresses δ1 is acting in NW-SE direction on the formation, Intermediate Stresses δ2 acting in NE-SW direction on the formation while Minor stresses acting in NEE-SWW direction on Kawagarh formation at study area. These stresses generate three types of fractures (Three Joint Sets) in the Kawagarh formation shown in Figs. 4.4, 4.5, which enhance the porosity and permeability of the formation at the study area. So it is concluded from the stress analysis that the reservoir potential of Kawagarh formation was enhanced by these stresses because it produce three types of fractures in the formation which are opposite in direction and crosscut each other making X-shapes fracturing shown in Figs. 3.5.b, 3.9.b, 3.13.b, 3.15.b, 3.19.b, 3.21.b, 3.23.b, 3.27.b, 3.31.b and 3.35.b, which means that the fractures are interconnected due to which permeability increased which is suppose as an important factor for reservoir potential.

This study was based on Fracture Analysis of Kawagarh Formation at Galyat Area, Lesser Himalayan, Northern Pakistan. The fractures data of 18 stations including 75 Fractures were evaluated to find out the Stress analysis and Reservoir Potential of Kawagarh Formation at Study Area. The results calculated using several approaches are as follow:

Major Stresses acting on Kawagarh Formation are NW-SE directed.
Intermediate Stresses Direction acting on Kawagarh Formation are NE-SW.
NEE-SWW directed stresses are minor stresses acting on Kawagarh formation.
Maximum Orientation of Fractures are in NW-SE direction.
Intermediate Orientation of Fractures are in NE-SW direction.
Minimum Fractures are Oriented in NEE-SWW Direction.
Maximum density cm^-1, Porosity % and Permeability (Darcy) of Fractures were encountered at station 11, 12 and 8 having values 0.0757, 6.3233 and 1.7448 respectively.
Fracture Density and fracture Porosity has a poor Relation.
Fracture Density and fracture Permeability has also a poor relation.
Fracture Porosity and Fracture Permeability has good relation.
Average density cm^-1, Porosity % and Permeability (Darcy) of total Fractures are 0.043136, 3.212509 and 0.282299 respectively.
Kawagarh formation was kept as Type 3 to Type 4 Reservoir according to Classification of Natural Fracture Reservoir (NFR) Nelson (20021) which shows that it is a poor Reservoir.

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No competing interests reported.

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Recognizing and classifying the naturally fractured reservoirs (NFRs) system for the cretaceous carbonate formation exposed along the Bagnotar-Baragali Section, north Pakistan: Implications from surface fractures, petrography and geochemistry

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Abstract

Figures

Introduction

Materials And Methods

Scanline Method

Data Collection

Illustration Work

Mapping Software

Regional Geology And Tectonic Setting

Main Karakoram Thrust (Mkt)

Main Mantle Thrust (Mmt)

Main Boundary Thrust (Mbt)

Salt Range Thrust (Srt)

Local Geology

Tectonic Framework of the Study Area

Generalized Stratigraphy

Illustration Of Fractures And Fracture Analysis

Interpretations

Stress Analysis

Nw- Se Directed Stresses

Ne-sw Directed Stresses

Nee-sww Directed Stresses

Orientation Of Fractures

Relationship Between Fracture Density And Fracture Porosity

Relationship Between Fracture Density And Fracture Permeability

Relationship Between Fracture Porosity And Fracture Permeability

Quantitative And Qualitative Reservoir Potential

Comparative Analysis

Implications Of Stress Analysis

Results And Discussion

References

Additional Declarations

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