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Mumtaz M. Shah, Saifullah Afridi, Emad U. Khan, Hamad Ur Rahim, Muhammad R. Mustafa, "Diagenetic Modifications and Reservoir Heterogeneity Associated with Magmatic Intrusions in the Devonian Khyber Limestone, Peshawar Basin, NW Pakistan", Geofluids, vol. 2021, Article ID 8816465, 18 pages, 2021. https://doi.org/10.1155/2021/8816465
Diagenetic Modifications and Reservoir Heterogeneity Associated with Magmatic Intrusions in the Devonian Khyber Limestone, Peshawar Basin, NW Pakistan
In the present study, an attempt has been made to establish the relationship between diagenetic alterations resulting from magmatic intrusions and their impact on the reservoir properties of the Devonian Khyber Limestone (NW Pakistan). Field observations, petrographic studies, mineralogical analyses, porosity-permeability data, and computed tomography were used to better understand the diagenetic history and petrophysical property evolution. Numerous dolerite intrusions are present in the studied carbonate successions, where the host limestone was altered to dolomite and marble, and fractures and faults developed due to the upwelling of the magmatic/hydrothermal fluids along pathways. Petrographic studies show an early phase of coarse crystalline saddle dolomite (Dol. I), which resulted from Mg-rich hydrothermal fluids originated from the dolerite dykes. Coarse crystalline marble formed due to contact metamorphism at the time of dolerite emplacement. The second phase of dolomitisation (Dol. II) postdates the igneous intrusions and was followed by dedolomitisation, dissolution, and cementation by meteoric calcite. Stable isotope studies likewise confirm two distinct dolomite phases. Dol. I exhibits more depleted δ18O (-15.8 to -9.1‰ V-PDB) and nondepleted δ13C (-2.05 to +1.85‰ V-PDB), whereas Dol. II shows a relatively narrow range of depleted δ18O (-13.9 to -13.8‰) signatures and nondepleted δ13C (+1.58 to +1.89‰ V-PDB). Dolomitic marble shows a marked depletion in δ18O and δ13C (-13.7 to -8.5‰ and -2.3 to 1.95‰, respectively). The initial phase of dolomitisation (Dol. I) did not alter porosity (5.4-6.6%) and permeability (0.0-0.1 mD) with respect to the unaltered limestone (5.6-6.9%; 0.1-0.2 mD). Contact metamorphism resulted in a decrease in porosity and permeability (3.3-4.7%; 0.1 mD). In contrast, an increase in porosity and permeability in Dol. II (7.7-10.5%; 0.8-2.5 mD) and dolomitic marble (6.6-14.7%; 8.2-13.3 mD) is linked to intercrystalline porosity and retainment of fracture porosity in dolomitic marble. Late-stage dissolution and dedolomitization also positively affected the reservoir properties of the studied successions. In conclusion, the aforementioned results reveal the impact of various diagenetic processes resulting from magmatic emplacement and their consequent reservoir heterogeneity.
Igneous bodies are widely distributed in frontier basins worldwide, but less attention has been envisaged to better understand the impact of these igneous bodies on hydrocarbon systems, source rock maturity modification, and alteration of mineralogy and reservoir properties of the associated sedimentary successions (e.g., [1–8]). Recent studies have shown that magmatic intrusions may result in the development of hydrocarbon traps, seals, and migration pathways within sedimentary successions, as well as contribute to the thermal maturation of source rocks [1–3, 9–23].
In general, the intrusion of high-temperature magmatic bodies results in contact metamorphism of host carbonate successions, and the associated Mg-rich hydrothermal fluids cause dolomitisation [24–25]. Such dolomitized platform carbonates resulting from hydrothermal fluids have been extensively studied focusing on the source of magnesium, the driving forces for fluids, and the geometry of these geobodies (e.g., [9–18]). In recent studies, it is demonstrated that mafic intrusions may be the source of magnesium, which can result in the formation of high-temperature dolomite [25–26]. These high-temperature dolomites (i.e., hydrothermal) are potential hydrocarbon reservoir rocks in many parts of the world, including major reservoirs of Saudi Arabia and Canada (e.g., [13, 25–33]).
In addition, detailed studies have been carried out to investigate the influence of igneous intrusions on the reservoir properties of carbonate rocks [34–38]. However, key parameters controlling the diagenetic alteration of limestone due to their interaction with intrusive bodies are still poorly constrained in the studied formation.
In this study, an attempt has been made to understand the diagenetic alterations (i.e., cementation, dissolution, dolomitization, dedolomitisation, etc.) resulting from the emplacement of high-temperature magmatic intrusions into the Devonian Khyber limestone in the Peshawar basin of NW Pakistan. In the present study, the relationship between the intrusive bodies and the host limestone is investigated with the help of field observations, petrographic studies, and geochemical analyses, which help to better understand the role of diagenetic alterations in the modification of petrophysical properties.
2. Geological Settings
In the northern Pakistan part of the Indian Plate, upper Devonian reefal carbonates were deposited in a northward deepening epicontinental sea . Subsequent rifting during the Early Carboniferous  was accompanied by the development of NE-striking normal faults and by alkaline magmatism, which continued to the middle Carboniferous. A second phase of magmatism resulted in the emplacement of porphyritic alkaline rhyolites, diabase dikes/sills, and basalts during the Late Carboniferous-Permian, followed by postrift thermal subsidence resulting in marine sedimentation during the Late Triassic .
Northward drift of the Indian plate occurred during the Mesozoic and ended up in its convergence with the Eurasian plate during the early Paleocene. This resulted in the formation of regional-scale faults including the Main Karakoram Thrust (MKT), Main Mantle Thrust (MMT), the Panjal Thrust (PT), and the Main Boundary Thrust (MBT), respectively (e.g., [37–48]). The Kohistan-Ladakh batholith (an island arc complex) is sandwiched between the sutured Indian and Eurasian plates (Figure 1). The Main Mantle Thrust (the western extension of the Indus Tsangpo Suture Zone) marks the southern boundary of the batholith and separates it from the Indian plate; the northern boundary with the Eurasian plate is marked by the Main Karakoram Thrust (MKT) (Figure 1; ).
The Khyber Range covers the western margin of a metasedimentary Fold-and-Thrust Belt  and is located to the north of the Punjal-Khairabad Fault System extending westwards from Gari Habibullah to the Khyber Pass region . The Khyber Range forms a transition zone between the igneous-metamorphic complex in the north and the sedimentary cover in the south . The Khyber Limestone (the focus of this study) is located at the western extremity of the Peshawar Basin (part of the Himalayan Crystalline Nappe-and-Thrust belt) in the Khyber Range, in the foothills of the Main Boundary Thrust (MBT; Figure 1).
According to Khan et al. , Paleozoic sedimentary successions exposed in the Khyber Ranges include the Shagai Formation (Cambrian), the Landi Kotal Formation (Ordovician-Silurian), the Ghundai Sar Formation (Silurian-Devonian), and the Khyber Limestone (Devonian). The Khyber Limestone is a platform carbonate which crops out to the NW of the city of Peshawar in an area extending westwards into Afghanistan (Figure 2). Dolerite intrusions of the middle Carboniferous age, ranging in thickness from 10 to 25 m, occur within the Khyber Limestone over an area of about 100 km2 .
In the study area, the Khyber Limestone is ~1000 m thick with very few argillaceous and arenaceous intervals, whereas a maximum thickness of 1300 m is attained in the type locality near the village of Ali Masjid (Figure 2). This limestone is medium to thick bedded, grey, and medium to fine grained and locally passes into medium to coarse crystalline marble and medium-crystalline, massive dolomite . The northern part of the Khyber Limestone is dissected by basic dykes and sills (~5-6 m thick and traceable for hundreds of metres), which occur in the upper part of the formation . The Khyber Limestone has upper and lower faulted contacts with the Landi Kotal and Shagai Formations, respectively [52, 55–56].
3. Materials and Methods
In the study area, detailed field investigations of igneous intrusions in the Khyber Limestone (Devonian) were carried out. To investigate the effect of igneous intrusion within the host limestone, sampling was performed with particular emphasis on collecting the diagenetic phases identified in the field. Sampling was performed from dyke-limestone contacts to unaltered limestone to collect the various diagenetic phases. In total, 87 samples were collected representing unaltered limestone, its metamorphosed equivalent (marble), and dolomite phases. Petrographic studies of 68 thin sections representing various lithological types were carried out to unravel the diagenetic history of the carbonate rock and to establish a detailed paragenetic sequence. Besides the host limestone, different dolomite facies were distinguished on the basis of crystal boundaries (planar and nonplanar) and crystal size distribution following the classification scheme of Sibley and Gregg . This study was performed using a conventional microscope (Olympus CX31 with a DP-21 camera attachment at the Department of Earth Sciences, Quaid-i-Azam University, Islamabad). Cathodoluminescence microscopy was carried out at the Institute of Geology, Jagiellonian University, Krakow (Poland), using an optical microscope with a cathodoluminescence stage (CL8100 MK5) with operating conditions of 12 to 17 kV gun potential, 350 to 600 μA beam current, and 0.05 Torr vacuum.
Based on petrographic studies, 38 samples representing various phases were selected for X-ray diffraction analyses (XRD) to determine the bulk mineralogical composition. XRD analyses were performed at the Geosciences Advanced Research Laboratory, Geological Survey of Pakistan in Islamabad, where powdered samples were prepared from the whole rock of the selected samples, followed by analyses using PANalytical X’Pert PRO X-ray diffractometer (Cu-Kα radiation ~45 kV, 40 mA). The scan speed was set at 0.2°θ min-1 and the sampling interval at 0.001°θ per step. Furthermore, seventeen samples showing characteristic features of various phases (i.e., host limestone, calcite, and dolomite cements) were collected using a dentist drill for stable isotope studies (δ18O and δ13C), whereas analyses were carried out in the Isotope Application Division, Pakistan Institute of Nuclear Science and Technology, Islamabad (Table 1). All stable isotope values are reported in per mil (‰) relative to the Vienna Pee Dee Belemnite (V-PDB) by assigning a δ13C value of +1.95‰ and a δ18O value of – 2.20‰ to NBS19. The carbonate powders were reacted with 100% phosphoric acid (; ) at 75°C in an online carbonate preparation line (Carbo-Kiel-single sample acid bath) connected to a Finnigan Mat 252 mass spectrometer. Dolomite isotopic composition values are corrected by the fractionation factors given by [58, 59].
In order to understand the reservoir heterogeneity in the studied successions, plug scale porosity and permeability analyses and pore network distribution in various diagenetic phases were carried out. The porosity and permeability of 21 unaltered fresh plugs (~2.8 cm long and ~1.5 cm in diameter) were analyzed at the Hydrocarbon Development Institute of Pakistan, Islamabad, using standard laboratory techniques (Core measuring Sampler-100) under a net helium gas pressure of 1000 psi (6.89 kPa). In addition, four samples representing each type, i.e., limestone (LS), dolomarble (DM), saddle dolomite (Dol. I), and dolomite (Dol. II) were selected for computed tomography to better understand the 3D pore network distribution using a medical CT scanner .
4.1. Field Observations
In the study area in the western Peshawar Basin, the Khyber Limestone in general consists of limestone, dolomite, and marble. Limestone is light to medium grey and medium to thick bedded in the lower part, and the beds decrease in thickness in the upper part (Figure 3). These carbonates are intruded by 10 to 15 m thick, irregular-shaped dolerite dykes (Figures 3(a) and 3(b)). The intrusions occur mostly in the lower and middle parts of the Khyber Limestone, and no intrusions are reported in the upper part (Figures 3(a) and 3(b)). Transformation of the limestone into marble and progressive dolomitisation occurs near the intrusive bodies (Figure 3(d)). Contact metamorphism resulted in the formation of marble at the margins between the limestone and the intrusions, and the intensity of metamorphism decreases away from the intrusive body (Figure 3(b)). The contact between the intrusive bodies and the surrounding steeply dipping metamorphosed limestone (marble) is irregular (Figures 3(a) and 3(b)). In addition, a high density of fractures is observed in the intrusive body as well as in the surrounding marble units (Figure 3(b)). Isolated, unaltered blocks of limestone contain pore-filled dark brown dolomite, which has a friable texture (Figures 3(d) and 4(a)). Detailed investigations showed two distinct dolomite types, which include the following: (i) light brown massive dolomite (Dol. I), exhibiting replacive dolomite texture and observed as isolated clasts in contact with host limestone and postdated by magmatic rocks (Figures 4(b) and 4(c) and (ii) medium to dark brown, fracture-filled dolomite (Dol. II), which mostly occurs within dissolution cavities and fractures in the metamorphosed limestone (Figure 4(d)).
4.2. Petrographic Studies
Various facies exhibiting depositional and diagenetic imprints are petrographically identified. These include (i) host limestone, (ii) nonplanar dolomite (Dol. I), (iii) dolomarble, and (iv) planar dolomite (Dol. II) (Figures 5–6). (i)Host Limestone. The Khyber Limestone mostly consists of coralline, wacke- to packstone facies, which contain abundant fractures and vugs. The fractures are filled with fracture-filling dolomite (Figures 4(c) and 5(a)).(ii)Nonplanar Dolomite (Dol. I). Such matrix dolomites are coarse crystalline, nonplanar, and anhedral in appearance with crystal sizes ranging from 1000 to 5000 μm, showing undulose extinction and curved crystal boundaries (like horse’s saddle) and therefore termed “saddle dolomite” (Figure 5(b)). Dol. I shows dull luminescence with occasional bright red luminescence bands (Figures 5(c) and 5(d)). Intercrystalline pore spaces are limited and are occluded by clayey material (Figures 5(b) and 5(c)). In addition, twinned calcite represented by grey and dark blue colored bands is also observed in limestone (Figure 5(e)). Partial modification of nonplanar saddle dolomite (Dol. I) into twinned calcite is evident which shows its transformation into marble (Figure 5(f)).(iii)Dolomarble. It mostly consists of white to off-white-colored metamorphosed calcite with occasional brown-colored dolomite cement mostly associated with fracture and vugs fillings (Figure 5(e)). Petrographically, twinned calcite is typically associated with the metamorphism and is replacing nonplanar dolomite (Dol. I) (Figure 5(f)).(iv)Planar Dolomite (Dol. II). Medium to coarse crystalline, planar dolomite mostly occurs as thick (tens of cm) fracture and vug fillings in the preexisting metamorphosed limestone (i.e., twinned calcite) and Dol. I (Figures 6(a) and 6(b)). Petrographic observations reveal that Dol. II presents a dark brown color with distinguishable dark and bright zones present in it (Figures 6(a) and 6(b)). It mostly postdates twinned calcite, which is in contact with nonplanar saddle dolomite (Dol. I; Figure 6(a)). Cathodoluminescence analyses show that dolomite presents a bright red luminescence color, which makes it easily distinguishable from Dol. II (Figures 6(b) and 6(c)). A progressive change in luminescence from bright red to dull orange color is associated with dolomite calcitization (Figures 6(c) and 6(d)). Dol. II mostly occurs as fracture/cavity fillings in marble, while some intercrystalline pore spaces are present (Figure 6(e)). Large dolomite crystals may however extend into pore spaces (Figure 6(e)). Overgrowths of white-colored dolomite cement around Dol. II indicate late-stage dolomite cementation (Figure 6(e)). Finally, dissolution is restricted to dark-colored bands in dolomite crystals, as shown in Figure 6(f).(v)Dedolomite. These are rusty brown-colored calcite, mostly present on the weathered surfaces. It appears to be rhombohedral shaped under polarized light, whereas to nonluminescent mottled appearance under CL (Figures 6(d) and 6(f)).
4.3. Mineralogical Analysis and Isotopic Signatures
Quantitative mineralogical composition of matrix dolomite and distinct phases of pore-filling cements help to better understand the probable source of the identified minerals, as well as the porosity/permeability distribution in the analyzed samples. Nonplanar dolomite (Dol. I) forms a major part of the whole rock geochemistry in the thirty-eight analyzed samples (Table 1; Figures 7(a) and 7(b)), whereas palygorskite mostly occurs in the intercrystalline pore spaces of dolomite (Figure 5(c)). Besides this, other accessary minerals, which include ankerite and diopside, occur in association with dolomite (Figure 7(c)).
Seventeen samples representing the characteristic features of various diagenetic phases (i.e., host limestone, dolomite (Dol. I, Dol. II), and dolomitic marble) were selected for stable isotope studies (δ18O and δ13C). The correlation of these phases is done with known seawater signatures of Devonian derived from Veizer et al. . The approximate values of δ18O ranges from −7.32 to −3.98‰ V-PDB, whereas δ13C signatures varies from -2.01 to +3.14‰ V-PDB, respectively (Figure 8).
In the studied samples, limestone and various diagenetic phases affected by igneous intrusions show a broad range of depleted δ18O values (–15.83 to –6.11‰ V-PDB), while δ13C values (-2.7 to +2.5‰ V-PDB) are close to those reported for carbonates precipitated from Devonian seawater . The unaffected limestone showed δ18O signatures ranging from -5.87 to -4.57‰ V-PDB (Figure 8 and Table 1). The δ13C values range from +1.49 to +1.85‰ V-PDB. Both δ18O and δ13C values are within the range of Devonian seawater signatures (Figure 8; Table 1). Coarse crystalline, replacive nonplanar dolomite (Dol. I) shows δ18O signatures ranging from -15.83 to -9.1‰ V-PDB, exhibiting depleted values compared to the marine signatures (Figure 8 and Table 1). The δ13C values ranging from -2.05 to +1.85‰ V-PDB are within the range of seawater signatures (Figure 8; Table 1). The marbles also show considerable depletion in δ18O values (i.e., –9.15 to –8.78‰ V-PDB), while the δ13C values ranging from (-2.25 to -1.98‰) are slightly depleted with respect to the Devonian seawater (Figure 8; Table 1). The cavity-filling dolomite cements (Dol. II) exhibit depleted δ18O isotope signatures (i.e., –13.93 to –13.76‰ V-PDB; Figure 8). The δ13C values (i.e., +1.58 to +1.89‰ V-PDB) are within the Devonian seawater signature area (Figure 8(a)). The dolomitic marbles (DM) have δ18O values ranging from -13.69 to -8.47‰ V-PDB (Figure 8 and Table 1), while δ13C values range from -2.69 to +1.85‰ V-PDB. The δ18O values are significantly depleted with respect to the marine signatures, and δ13C values are slightly depleted compared to seawater signatures (Figure 8; Table 1). Furthermore, the calcite (CC) and dedolomite (Dedol.) represent depleted O/C signatures, which are -6.11‰ V-PDB (δ18O) and -7.12‰ V-PDB (δ13C) for calcite (CC) and -6.41‰ V-PDB (δ18O) and -5.14‰ V-PDB (δ13C) for dedolomite (Dedol.), respectively.
4.4. Reservoir Properties
(i) Petrographic and Porosity-Permeability Analyses. Limestone samples analyzed for porosity and permeability studies exhibit negligible porosity as the pore space (i.e., fractures) is filled with calcite cement (Figure 9). The measured porosity and permeability values of the precursor limestone range from 5.6 to 6.9% and 0.1 to 0.3 mD, respectively (Figure 9). Coarse crystalline, nonplanar dolomite (Dol. I) occurs as a replacive phase with a narrow range of porosity and permeability (i.e., 5.3-6.6%; 0.0-0.1 mD) (Figures 9 and 10(a)). Intercrystalline pore spaces in Dol. I are mostly filled with detrital clays, which also reduce porosity (Figures 9 and 6(b)). Marble exhibits low porosity (3.25-4%) and permeability values (0.5-0.9 mD), which may be due to recrystallization of the original limestone and occlusion of the pore space (Figure 10(a)). Dolomitic marble shows considerably higher porosity-permeability values (6.7-14.7% and 0.8-13.3 mD), contrasting with preexisting marble with lower porosity-permeability values (Figures 9 and 10(a)). Medium-crystalline pore-filling dolomite (Dol. II) shows relatively high dissolution porosity and low permeability due to its corrosive nature, which makes it highly porous but not permeable (Figures 6 and 10(a)). Pore-filling dolomite (Dol. II) contains porosity values ranging from 7.6 to 10.5% and low permeability values (0.1-0.2 mD) (Table 1).
In some samples where dolomite is abundant, the porosity ranges from 6.2 to 13.2% (Figure 10(b)). The presence of twinned calcite occludes the pore space and hence decreases porosity (Figure 5). In the studied samples, containing low permeability values with correspondingly high porosity values is due to late-stage calcite and clay minerals, which were precipitated in the pore spaces and in fractures to form veins (Figures 5, 6(a), and 6(b)).
(ii) Computed Tomography (CT). In order to better constrain the distribution and interconnectivity of the pore network, the precursor limestone and selected diagenetic phases, i.e., dolomite Dol. I (coarse crystalline, saddle nature), dolomarble (dolomite marble), and dolomite Dol. II (pore-filling), were examined (Figure 11). The results show that the precursor limestone samples exhibit lower porosity (lack of dark spots) compared to dolomite (Dol. II) and dolomarble (relatively high concentrations of dark spots) in transversal views of selected samples (Figure 11). The amount of pore space in dolomite (Dol. II) and dolomarble is higher compared to the above-mentioned phases, whereas saddle dolomite (Dol. I) shows very few pore spaces (negligible dark spots; Figure 11).
In the cross-sectional and 3D views, different phases exhibit diagnostic pore network connectivity (Figure 11). Limestone exhibits less fracture-related linear open spaces and low microfracture development, whereas dolomarble and Dol. II show intercrystalline pore spaces having a high rate of pore interconnectivity linked to associated dolomite Dol. II, hence exhibiting relatively high permeability (Figure 11).
The studied Devonian Khyber Limestone has been extensively altered by hydrothermal processes. Field and petrographic studies reveal that the magmatic intrusions are emplaced within the carbonate succession into the middle and bottom parts of the sedimentary unit. As discussed earlier, the area was subjected to rifting and associated normal faulting during the middle Carboniferous, and this resulted in alkaline magmatism [35, 36]. These normal faults were the most favorable conduits for channelizing hydrothermal fluids [13, 25] (Figures 3–6). The occurrence of these hydrothermal dolomites in association with intrusive igneous bodies along the normal faults suggests its origin from the same hydrothermal fluids. Field observations show sharp contacts between the limestone and the intrusive bodies. The presence of broken, isolated blocks (>1 m in size) of metamorphosed limestone within the intrusive dolerites indicates the intensity of upwelling of magmatic fluids through the studied carbonate successions (Figure 4(a)). These detached, broken blocks of metamorphosed limestone exhibit no reaction with the surrounding igneous material due to the fact that they are marbleized, which made them impervious to the flow of hydrothermal fluids (Figure 4(a) and Table 1). During the emplacement of igneous intrusions, the dolomite phases were formed from the hydrothermal fluids originated from the magma source (Figure 4(b)). The high-temperature origin of these dolomite phases is confirmed by the sharp color contrast and nonplanar crystal habit  (Figures 3(c), 4, 5(a), and 5(b)). In addition, the presence of high-temperature minerals (i.e., ankerite and diopside) may have resulted from the interaction between hydrothermal fluids with the host limestone, which is also confirmed by various studies where diopside mineral is usually found in skarn deposits  (Figures 8(a) and 8(b)).
5.1. Diagenetic Alterations and Interpreted Paragenetic Sequence
The detailed paragenetic sequence has been established on the basis of the above-mentioned observations (Figure 12). This includes two stages of diagenetic modifications, which are as follows: (i) Mg-rich, hydrothermal fluids affected the host limestone soon after the magmatic emplacement, which resulted in hydrothermal dolomitisation (Dol. I) as evidenced by field observations and petrographic studies and supported by isotopic signatures, (ii) low-grade contact metamorphism and the formation of marble, and (iii) late-stage dolomitisation (Dol. II) before dissolution and dedolomitisation associated with uplift and meteoric recharge.
Dol. I mostly consists of dark grey, massive matrix-replacive dolomite, which exhibits a nonplanar, very coarse crystalline texture (saddle type), and was formed before the intrusion of magmatic material and contact metamorphism of the host limestone, which was later converted into coarse crystalline, twinned marble (Figures 6(e) and 6(f)). Dol. II consists of light brown, sugary textured, fracture-filled dolomite cement, which is medium to coarsely crystalline, whereas intercrystalline pore spaces are filled with detrital clays (Figures 6 and 7). Dol. II developed after the intrusions of the basic dyke as such dolomite postdate twinned marble (Figure 7(a)). In addition, such dolomite (i.e., Dol. II) mostly occurs as fracture and pore fillings in the metamorphosed limestone and the intrusive body, respectively (Figures 6(b) and 6(e)). Subsequently, the growth of Dol. II continued resulting in the formation of growth zones in the form of white dolomite cement along crystal boundaries (Figure 7(e)). Dissolution-related porosity in general develops along partially dedolomitised dolomites [63, 64]. Such a phenomenon was also observed in the present study, and Dol. II underwent dedolomitisation (Figure 7(f)).
In summary, the diagenetic evolution of the Ordovician Khyber limestone has close relationship with the intrusive magmatic body of mafic origin, which provided heat and necessary fluids for the diagenesis of the host limestone units. The pervasive, coarse crystalline dolomite (Dol. I) formed during or soon after the emplacement of the magmatic intrusions. The tectonic activation/reactivation and the resulting normal faulting in the area may have provided pathways for Mg-rich fluids of hydrothermal origin. The second episode of dolomitisation resulted from the fracture-filled dolomite (Dol. II) in marble (i.e., dolomite marble) and vein- and fracture-filling dolomite, respectively.
5.2. O/C Isotopic Signatures
Analysis of the cross-plot between δ13C and δ18O of diagenetic phases of dolomite and calcite following an inverted J pattern, which represents fluid-rock interaction at various stages of diagenesis [65, 66]. In addition, the depleted δ18O values of the diagenetic phases indirectly reflect the temperature of the diagenetic fluids [65, 66]. Oxygen isotope values of the diagenetic phases (Dol. I, marble, Dol. II, and dolomitic marble) reveal depletion of δ18O (Figures 8(a) and 8(b)), which may either be due to an increase in temperature and/or δ18O-depleted fluids as suggested by various studies (e.g., [9, 12, 13, 24, 65–70]). According to Bindeman , fluid-rock interaction between host carbonate rock and dyke/magmatic rock results in the depletion of ẟ18O signatures, where dykes are more depleted (i.e., at least 10‰) in ẟ18O than marine carbonates. It appears more probable that the investigated dolomites associated with magmatic driven fluids were formed at elevated temperatures. In order to determine the δ18O (‰ SMOW) fluid composition of Dol. I, Dol. II, M, and DM, the values of δ18O (V-PDB) were plotted against the precipitation temperature by using the fractionation equation of Land . The plot shows that the inferred δ18O (‰ SMOW) fluid values of Dol. I and Dol. II are +10.5 and +10.0‰ (Av. V-SMOW), respectively, showing their origin from magmatic waters (Figure 13). Similarly, the δ18O (Av. ‰ SMOW) fluid signatures of marble (M) and dolomitic marble (DM) are +15.6 and +13.5‰ (Av. V-SMOW), respectively, showing their magmatic origin (Figure 13). Based on the above stated discussion, it is confirmed that various dolomite and marbleized phases are derived from magmatic fluid interaction with the host limestone (Figure 8(b)). Nevertheless, during the late stages, the area was subjected to uplift due to the Himalayan orogeny, which caused exposure and involvement of surface-derived fluids contaminated by soil-CO2, which affected limestone and dolomite units and caused dedolomitisation. This is represented by less depleted δ18O and depleted δ13C values probably formed near meteoric recharge (Figure 8; e.g., [72–77, 15]).
5.3. Influence of Diagenetic Alterations on Reservoir Properties
Diagenetic changes had both positive and negative effects on the reservoir properties of the host rock (; Figure 12). Early dolomitisation (Dol. I) associated with Mg-rich hydrothermal fluids resulted in a replacive phase and had less effect on porosity modification in the host limestone. This was followed by the emplacement of dolerite intrusions resulting in recrystallization due to contact metamorphism, further reducing the porosity (marble) (Figure 10(a)). However, in fracture development and late-stage dolomitisation (Dol. II), postdating magmatic emplacement resulted in considerable enhancement of porosity and permeability (Figure 10(a)).
The second dolomite phase (i.e., fracture and pore-filled; Dol. II) contributed to the reservoir properties of the studied carbonates by retaining secondary porosity and intercrystalline pore spaces within Dol. II, which significantly enhanced the pore network (up to 13% porosity; Figures 6(e), 10(a), and 11). Dol. II comprises most of the dolomite in the study area, and the reservoir properties of the studied rocks are therefore enhanced through Dol. II. In addition, fractures as well as dissolution seams further enhanced the pore networks (Figure 6(f)). Uplift and exposure resulted in a meteoric influx that caused dedolomitisation of the dolomite phases observed (Figures 10(a) and 12).
Various diagenetic phases played a significant role in controlling the reservoir characteristics of the studied rocks. An increase in the content of Dol. II increased the pore space and contributed to porosity enhancement (Figure 10(b)). However, Dol. I shows low porosity due to the nonplanar nature of crystals and presents occluded pore spaces by clays (Figure 10(b)). A progressive increase in porosity was observed in the dolomite marble, in which fracture-filled Dol. II forms a major part of the analyzed samples. In summary, marble with less fracture-filled dolomite (i.e., Dol. II) showed low porosity values, compared to marbles consisting of more than 80% Dol. II which had elevated porosity values (Figure 10(a)).
Porosity and permeability studies are further validated by microtomography, where the high intensity of microfracture development and high rate of interconnectivity confirm high porosity and permeability values in the dolomarble phase (i.e., up to ) (Figures 10(a) and 11). Dol. II exhibits relatively high porosity (up to 10%) and low permeability (up to 2 mD) due to the moderate nature of fracture distribution but negligible pore connectivity (Figures 10(a) and 11). In contrast, the low microfracture development and interconnectivity of pore spaces justify less porosity and permeability values in the precursor limestone, saddle dolomite Dol. I, and marble (i.e., up to ; ) (Figures 10(a) and 11). This leads to the conclusion that the host limestone, nonplanar dolomite (Dol. I), and marbles acted as barriers to fluid migration, whereas dolomite marble and fracture-filling dolomite (Dol. II) contributed to the enhancement of reservoir properties in the studied rocks (Figures 5 and 12). This leads to the suggestion that postmarble tectonic activity resulted in brittle deformation and the corresponding fractures were filled by Dol. II, hence retaining considerable intercrystalline porosity as evidenced in Figure 6(e).
This excellent outcrop analogue shows the effect of igneous intrusions on the reservoir behavior of a carbonate succession, whereby modifying the mineralogical composition of the host limestone due to the associated fluid-rock interaction. The key conclusions from the present study are as follows: (i)Field observations reveal that hydrothermal fluid emplacement followed by igneous intrusions of basic origin in the host limestone resulted in dolomitisation and transformation of limestone to marble(ii)Sharp contacts between the intrusive bodies and host limestones and the presence of isolated limestone clasts of variable sizes suggest a rapid interaction of the magmatic fluids with the host limestone(iii)Two distinct dolomite phases, which postdate magmatic intrusions, have been reported. The first phase of dolomitisation resulted in matrix-replacive saddle dolomite, whereas the second-phase dolomites are mostly found as fracture and/or pore fillings. Dedolomitisation, dissolution, and infilling of meteoric calcite mark the end of the diagenetic history(iv)The first-phase saddle dolomite cement (Dol. I), related to the hydrothermal fluids, mostly occluded the pore space and negatively impacted the reservoir properties. The second-phase pore-filling dolomite (Dol. II) forms the major part of the diagenetic stage and exhibits good porosity (~15%) and permeability values, hence making them potential reservoirs with considerable fracture density and interconnected pore network. Fracture development after the magmatic intrusion resulted in porosity enhancement and infilling of Dol. II. Dedolomitisation followed by dissolution-related phenomenon affected the second phase of dolomite and hence generated late dissolution-related porosity(v)The δ18O and δ13C isotope fractionation trajectories suggest that the dolomites were formed at a temperature between 150°C and 340°C from the hotter fluids that interacted with the regional limestone. The source of dolomitizing fluids is probably from the magmatic intrusion. Later on, the surface-weathering agents caused enrichment in δ18O and δ13C values of the early formed phases(vi)Correlation of petrographic and porosity-permeability studies revealed that reservoir properties are associated with specific diagenetic phases. Computed tomography (CT) further confirmed the porosity-permeability results and helped to understand the porosity-permeability evolution in various diagenetic phases.
Field data acquired from outcrop has been analyzed and interpreted using techniques mentioned in “Materials and Methods.”
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Funding for this research was received from the University Research Fund (2016). The authors wish to thank Frontier Constabulary, Peshawar, for providing security during field visits. We are indebted to Geology & Reservoir (G&R) Laboratory of Oil & Gas Development Company Limited (OGDCL) for poropermeability analyses. We are also grateful to Geosciences Advance Research Laboratory (GARL), Geological Survey of Pakistan (GSP), for performing XRD analyses.
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