Microfacies analysis of the Palaeocene Lockhart limestone on the eastern margin of the Upper Indus Basin (Pakistan): Implications for the depositional environment and reservoir characteristics

A detailed sedimentological analysis of the Palaeocene Lockhart Limestone has been conducted to evaluate the depositional environment, diagenetic processes and hydrocarbon potential of the eastern margin of the Upper Indus Basin. From bottom to top, there are three microfacies recorded. The lower microfacies, composed of fine‐grained micrite and some diagenetic dolomite, reflect the low energy and calm palaeo‐current in the shallower section (1–2 m) of the inner shelf close to shore. The middle microfacies contain algae that suggest 5–15 m of water depth, especially along the inner‐middle shelf, but fractured and mixed bioclasts in micrite material indicate calm to moderately active water close to the wave base. Progressing from the lower microfacies to the middle microfacies, a gradual shift from orthochem to allochem components is observed. The top microfacies is dominated by massive benthic microfossils, indicating moderate energy‐water conditions with normal salinity. However, the presence of limestone intraclasts surrounded by microspar, miliolids and nummulites at the top indicates a high‐energy environment with increasing salinity and water depths from 20 to 130 m. These findings show that the Lockhart Limestone was deposited in a shallow shelf environment, spanning the inner‐mid shelf. Diagenetic processes observed include micritisation, cementation, dissolution, replacement, physical and chemical compaction, and fracture filling by calcite cement. The Lockhart Limestone represents a deepening upward sequence deposited below the shelf margin system tract and highstand systems tract in a regressive environment that could reflect good reservoir characteristics, has the potential to serve as an excellent hydrocarbon reservoir rock, and could be a primary target for future hydrocarbon exploration.


| INTRODUCTION
The Indus Basin (IB), a major sedimentary basin of Pakistan, is approximately 873,000 km 2 in size ( Figure 1A) (Wandrey et al., 2004). Tectonically, the basin is bounded from the north-eastern and western sides by the Main Boundary Thrust (MBT) and the Chaman Transform Fault (CTF) respectively Awais et al., 2019). The IB has been further subdivided into the Upper Indus Basin (UIB), Central Indus Basin (CIB) and Southern Indus Basin (SIB), and it is a major source of hydrocarbons in the area ( Figure 1B) (Ehsan et al., 2021;Shah, 2009). Highs separate the UIB and CIB from the south and the Kurram Thrust towards the west, while MBT delineates the northern and eastern boundaries of the UIB (Badshah et al., 2000). The UIB consists of sedimentary rocks ranging in age from the Precambrian to the recent. However, the research region is located on the UIB's eastern boundary and is composed of sedimentary, metamorphic and igneous rock types ( Figure 2; Table 1).
In recent years, emphasis has been placed on the stratigraphy, structure, geophysics and palaeontology of the UIB's nearby eastern margin (Afzal et al., 2009;Akhtar et al., 2019;Khan et al., 2018;Munir et al., 2006). However, a few researchers have conducted reconnaissance studies focussing on particular geological aspects such as tectonic framework and mineralogy (Figure 2A,B) (Ashraf & Chaudhary, 1984;Nawaz & Mn, 1980) and the geological characteristics of the area are difficult to understand as this region is part of a fold and thrust belt.
The meta-sediments of Dogra Slates (Neo-Proterozoic Era) are unconformably overlain by the Cambrian Kailar Formation (Table 1). The Kailar Formation consists mostly of dolomites and dolomitic limestone (Ashraf & Chaudhary, 1984). The upper contact of the Kailar Formation with the Permian Panjal Formation is also unconformable (Ashraf & Chaudhary, 1984). A large unconformity separates the Panjal Formation from the Palaeocene-Eocene rocks (Bilal & Khan, 2017). The upper contact between the Middle Eocene to Early Oligocene Kuldana Formation and the Early Miocene Murree Formation is also unconformable.
At the top of the succession, the Murree Formation and the Recent Alluvium are separated by an angular unconformity (Table 1) .
Due to its type location near Fort Lockhart in the Samana Range, the nodular limestone of Palaeocene age is referred to as the Lockhart Limestone (Davies, 1930;Shah, 2009). The Lockhart Limestone is a hydrocarbon reservoir in the UIB (Awais et al., 2019;Shakir et al., 2019). Most of the nodular limestone deposits on the shallow shelf, particularly in the inner-mid-shelf, are highly significant because they have historically served as suitable reservoirs across Europe, the Middle East and Asia (Awais et al., 2019;Reza, 2014;Yang et al., 2017). In addition to being exposed across the UIB, it is also exposed at the eastern margin of the UIB although detailed investigation of the Lockhart Limestone in this area was beyond the scope of this study.
The lack of information regarding the Lockhart Limestone, particularly with regards to age, diagenesis and depositional environment, is a significant issue in the research region that has to be addressed. This research will be used to cross-correlate the Lockhart Limestone regionally and throughout the UIB and to comprehend the geological and sedimentological characteristics of the Lockhart Limestone in this region ( Figure 2B). Additionally, the depositional and diagenetic features of the Lockhart Limestone were addressed to determine its reservoir characteristics. These were derived through in-depth analyses of lithobiofacies used to reconstruct depositional settings and identify relevant diagenetic processes for the petroleum industry.

| METHODS AND MATERIALS
The texture and composition of carbonate rocks can be used to understand their depositional setting (Janjuhah et al., 2017a. For the purpose of petrographic microfacies identification, 40 limestone samples from the Haveli Section (LC1 to LC40) were collected at equal intervals of 3.7 m (Figure 3). The Haveli Section is located on the eastern margin of UIB and is bordered by latitudes of 33 55′0″ to 33 58′0″ North and longitudes of 74 8′0″ to by calcite cement. The Lockhart Limestone represents a deepening upward sequence deposited below the shelf margin system tract and highstand systems tract in a regressive environment that could reflect good reservoir characteristics, has the potential to serve as an excellent hydrocarbon reservoir rock, and could be a primary target for future hydrocarbon exploration.

K E Y W O R D S
carbonate microfacies, depositional environment, diagenetic processes, Lockhart limestone, palaeoenvironmental reconstruction, sequence stratigraphy 74 11′0″ East ( Figure 2B). The thickness of the formation was determined using the Jacob's Staff method, which led to the preparation of the section's litholog. Field images were taken using a digital camera ( Figure 3A through E). Thin section preparation was carried out at the Shandong University of Science and Technology (China). A polarising microscope, the LEICA-DM 750P with a connected LEICA-EC3 camera (Leica Microsystems Ltd.), was then used in the laboratory to identify and categorise the fossil content and to determine the mineralogical composition. F I G U R E 2 (A) Geological map of the Hazara Kashmir Syntaxis (HKS) and UIB's eastern margin, based on Nawaz and Mn (1980). (B) A geological map of the study area.
T A B L E 1 Stratigraphy of different areas in the IB (Shah, 2009  The petrographic and field data were interpreted according to Dunham's (1962) classification scheme for establishing microfacies by comparing it with those of Flügel and Munnecke (2010) and Wilson (1974). Microfossils in the same thin sections were identified to evaluate their specific depositional realm and for stratigraphic purposes in a relative geochronological study. The bibliography of Flügel and Munnecke (2010) was used to determine foraminiferal microfossils. The Imagej software was then used to compute the effective porosity using the image threshold approach ( Figure 3F,G,H).

| RESULTS
During the Palaeocene, a broad carbonate shelf covered the northern parts of the IB. The resultant carbonate rocks of the Lockhart Limestone are variably classified as the source, reservoir and cap rocks within the basin, giving them a crucial role not just in basin analysis, but also in hydrocarbon exploration within the basin. The following litho-biofacies are identified by field and petrographic investigations of the Lockhart Limestone along the eastern margin of the UIB.

| Integrated carbonate lithofacies and biofacies
In general, the limestone is hard and exhibits a light to dark grey colour within the study area ( Figure 3A through E). A diversity of fossils, including algae and foraminifers ( Figure 3F,G,H), were detected throughout the section. Fossils are only present in the middle (above 40-84 m, towards the middle of the section) and at the top (above 121-151 m, towards the middle of the section) of the sequence. After an erosional contact, the shale of the Patala Formation covers the limestone rocks ( Figure 4).

| Pore types
Most of the pores in the studied samples are either mouldic or intraparticle ( Figure 3F,G,H). Choquette and Pray (1970) classify pores into intraparticle, mouldic, vuggy, fracture and interparticle pores, features observable in Figure 5A through G.

| Quantitative analysis
An analysis of all Lockhart Limestone thin sections revealed that the rock is composed of fine to medium-sized grains that are free of mud. The majority of the rock is limestone (90%) and dolomite (10%). ( Figure 5A). Packstone was identified in 35% of thin sections from all rock samples (lower, middle and upper sections), whereas the remaining 65% were classified as wackestone, mudstone or grainstone ( Figure 5B). A total of 40% of the Lockhart Limestone consists of grains, while the other 54% is composed of matrix and cement. The remaining 6% can be attributed to porosity ( Figure 5C). In a qualitative study of the visible porosity, five distinct pore types were identified ( Figure 5D), vuggy (35%), intraparticle (25%), interparticle (20%), fracture (10%) and mouldic (10%) ( Figure 5D).
F I G U R E 5 Graphics depicting: (A) the quantitative distribution of microfacies, (B) the rock components and textures, (C) the quantitative distribution of grains, matrix and cement and (D) the various pore types in the studied samples. Description: This microfacies is characterised by 1-5 cm of thickly bedded limestone, which is dark grey to brownish in colour and moderately fractured ( Figures 3A and 6A Table 2). Thin section analysis characterised the rock as mudstone and wackestone. Allochems (5%) and micrite matrix (90%) are the major constituents, with 5% of the grains consisting of chert, with sizes ranging from 0.02 to 0.3 mm (Table 2). Spar, calcite veins and stylolites are prominent features of this microfacies and define the sub-microfacies shown in Figure 6C,D,E.
Thin sections show internal deformation, tectonic veins and pressure dissolution features ( Figure 6E).

| Thin laminated lateral subfacies
A thinly laminated subfacies ( Figure 6G) with several calcite veins that cross-cut one another and lamina that appear as lateral expansions along the undulating laminated chert sub-microfacies ( Figure 6C,F).

| Micrite-dominated dolomite subfacies
Micrite ( Figure 6H) has often been transformed into dolomite in this subfacies ( Figure 6I). It is also characterised by a lack of organic material.
3.4.5 | Spar-dominated micrite to dolomite subfacies There is a variation in which a complete sequence of alteration from micrite to dolomite ( Figure 6J) is observed, which is thereafter converted into sparite ( Figure 6K). The transformation of dolomite into sparry dolomite ( Figure 6L) illustrates the de-dolomitisation process. 3.4.6 | Algal-foraminiferal mixed bioclastic mudstone-packstone microfacies (LMF2) Description: LMF2 is composed of medium-bedded light grey limestone (0.5-3 m) ( Figure 7A,B). The upper and lower limits of the adjacent microfacies are defined gradationally. Thin section examination indicates a micrite concentration of up to 25%. The grain-clast content is 75% and 25% respectively. The clasts, which are mostly bioclasts, vary in size from 0.2 to 1 mm ( Figure 7B). Dascycladacean algae, Miscellanea spp., Gymnocodiacea, Coskinolina, echinoid spines, and a variety of broken bioclasts are the most notable fossils within this facies (Figure 4).

| Fossiliferous and micrite matrix dominated subfacies
Micrite predominates after fossils in this subfacies, along with some stylolites ( Figure 7C). Micrite usually contains mixed and broken fossil fragments ( Figure 7D,E,F). This is a less abundant subfacies.

| Benthic foraminiferal wackepackstone microfacies (LMF3)
Description: This microfacies is characterised by 2-3 m of thick-bedded, nodular, light grey limestone with a significant fossil content ( Figure 8A,B). This microfacies has a thickness interval of 32 m. Upper and lower boundaries are gradational with the adjacent microfacies. The matrix and (bio-) clast content of the samples may reach up to 20% and 75% respectively. A considerable number of fossils ( Figure 8C through G) originate from algae and benthic foraminifera: Lockhartia spp. (Figure 8C), Operculina spp. (Figure 8D), Rotallid spp. (Figure 8H), Miscellanea spp. (Figure 8I,J), Assilina spinose, Quinqueloculina and additional gastropods ( Figure 8B,E) and broken bioclasts ( Figure 8F,G), This microfacies also contains fragments of limestone surrounded by spar ( Figure 8J). This subfacies shows a relatively large grain size. The spar surrounds the limestone clasts, which are scattered in the micritic material. This subfacies is characterised by the presence of large bioclasts and spar ( Figure 8K,L). This is a lateral extension of the micrite-dominated fossiliferous wackestone subfacies.

| Interpretation of the Lockhart limestone microfacies
The first microfacies (LMF 1) from the base of the Lockhart Limestone is similar to FZ8 and FZ9 of Flügel and Munnecke (2010). The abundance of lime mud in this microfacies suggests deposition in a low-energy environment with relatively calm, shallow water, most likely on a shallow shelf in an inner ramp setting (Flügel & Munnecke, 2010;Vaziri-Moghaddam et al., 2006). Dolomitic recrystallisation of micrite during diagenesis results in the development of micritic dolomite (Janjuhah et al., 2019;Xi et al., 2015). A wavy laminated subfacies may indicate the influence of fair-weather wave base. The second microfacies (LMF 2) from the middle section of the formation is the same as the SMF-18 of Flügel and Munnecke (2010) and Wilson (1974). It is interpreted as forming below wave base in calm, shallow water. The presence of Dascycladacean algae, in particular, indicates shallow water depths of 5-15 m in an inner to upper middle ramp . These algae are typical of the depositional environment found between the inner and middle shelf areas. Awais et al. (2019) referred to similar facies as 'electrofacies 3' in their studies, describing them as deposits formed within the inner-middle shelf region. The appearance of (green algae) Gymnocodiacea and the dominance of benthic foraminifers directed the depositional environment in the photic and oxygenated middle shelf region just below the storm wave base (Abasaghi et al., 2020;Flügel & Munnecke, 2010). This mudstone-packstone microfacies is dominated by a visible pattern composed of algae, mixed and broken fossils,

Miscellanea miscella and Coskinolina bioclasts.
The third microfacies (LMF 3) from the top section of the Lockhart Limestone has the same fossil composition as Flügel and Munnecke (2010) facies assemblages SMF8 and 9, and Wilson (1974) FZ4, FZ5. The presence of a high micrite concentration indicates deposition in a low-energy environment. Micritisation, cementation and spar are common in a middle shelf environment (Wadood et al., 2021). Deposition on the middle shelf is suggested in conjunction with the fossil content of these facies. The majority of the fossils found indicate normal salinities during deposition of this microfacies. Gastropods suggest a shallow-marine shelf environment with normal salinity, while the presence of small-sized Milliolid and Nummulites spp. supports relatively high salinities and subtidal depths of 20-130 m with lime mud precipitation (Reiss & Hottinger, 2012). Limestone clasts surrounded by spar represent a highenergy environment. Clasts with similar textures indicate intraclast origins within the formation. The original micrite has recrystallised to form microspar, enclosing the hardened

| Diagenesis
Diagenesis is a process of classical transformation in the rocks from deposition to burial and is most commonly found in carbonate reservoir rocks. However, in the Himalayas, post-depositional diagenesis occurred due to the intense compressional tectonic forces in the region (Bilal et al., 2022). Micritisation, cementation, dissolution, replacement, compaction (both physical and chemical), fracturing and calcite filling are all examples of diagenetic processes that have occurred in the Lockhart Limestone ( Figures 5, 7 and 9). These modifications have had significant effects on the porethroat structure of the rocks. Janjuhah and Alansari (2020) suggest that as these systems adapt to changes in their environment, they change in a unique way. As shown by the petrographic features of the examined samples, diagenetic processes have overprinted the pore size distribution.
The micrite envelope, as seen in Figures 6C,D and 9, is the first diagenetic process to occur in low-energy and slow-sedimentation conditions ( Figure 7C,D) Morad et al., 2018;Saleem et al., 2020;Tucker & Wright, 2009). Syn-sedimentary micritisation is defined by Brett and Brookfield (1984) as the development of micritic bio-erosion fringes that penetrate deep into the grain. Modification and deposition of micrite envelopes occur simultaneously (Ge et al., 2020;Tomašových et al., 2022).
Moderate to complete grain micritisation occurred in the skeletal components ( Figure 7H,I,K). The presence of boring organisms and significant levels of microbial activity have also been reported. The micrite matrix indicates a later stage of micritisation ( Figure 7I).
Indications of calcite and dolomite cement have been observed ( Figure 6C,D,F through I,K). It has been reported that calcite cement precipitates under marine diagenetic conditions (De Boever et al., 2017;Mueller et al., 2020;Shuster et al., 2018;van Smeerdijk Hood & Wallace, 2012). Foraminiferal skeletons exhibit syntaxial overgrowth cement, which is characterised by a micrite envelope with an incompletely formed outer layer ( Figure 8D,E). Mechanical compaction and certain pore-lining cements come after the syntaxial cements (Al Areeq et al., 2016). Syntaxial overgrowths are associated with bioclasts and open-grained sedimentary textures that are replaced by pore fluids precipitations (Alsuwaidi et al., 2021). Calcite cement fills the intraparticle, mouldic and vuggy pores to a far greater extent than any other material.
Carbonate minerals dissolve mostly because of their solubility (Toner & Catling, 2020). The 'dissolution' of metastable bioclasts in meteoric water is referred to in the literature (Budd, 2002;Janjuhah et al., 2017b;Tucker & Wright, 2009). Secondary porosity develops during the early-late phases of porosity development, and it shows a high degree of dissolution in the samples studied ( Figures 6E and 9). Seawater or a shallow burial depth might have facilitated the early dissolution of aragonite (Ge et al., 2022). Mouldic and vuggy porosities were formed during the late stages due to the dominance of fabric-to-non-fabric-selective dissolution ( Figure 6K,I). Overburdened stresses are the primary source of compaction, but temperature changes (geothermal gradient) may also play a role by increasing pore pressure and leading to changes in pore structure as a result of chemical interactions between water and rock grains (Swarbrick, 2012). Overburden pressure is responsible for the observed grain-to-grain contacts, the observed F I G U R E 9 Based on petrographic observations, a proposed diagenetic process for the studied area. distribution pattern of various grains, and the reported fractures ( Figure 7B,D). Grain fracture, breaking and close packing are all signs of mechanical compaction.
Fine-grained calcite matrices have subhedral and euhedral calcite crystals that are between 5 and >20 μm in diameter (Ionov & Harmer, 2002). In the Lockhart Limestone, the alteration of high-magnesium calcite (micrite) into low-magnesium calcite (microspar) is a common occurrence, characterised by the formation of isolated patches of microspar ( Figure 7F). Many stylolites have been identified in the Lockhart Limestone ( Figure 5D). Stylolites are a late stage diagenetic process characterised by irregular seams of insoluble residues of the parent rock and formed when compacted grains continue to dissolve at planar and sutured contacts. Carbonate rock fractures also occur as significant secondary structures as a result of either compaction or as a result of local tectonic activity (McMahon et al., 2017). Additionally, micro-fractures may be caused by extensional movements and natural hydraulic fracturing Jin et al., 2021). The filling of the fractures by calcite demonstrates that diagenesis occurred long after the rocks were buried.

| Depositional environment
The Lockhart Limestone was deposited during the Thanetian transgression of the Ceno-Tethys Ocean , which resulted in flooding of the Indian Plate margins (Shah, 2009). The continuous collision of the Indian and Eurasian plates resulted in cyclic movement of the coastline and the formation of sedimentary sequences characterised by limestone and chert deposition (Jade et al., 2017). Microfacies analysis, incorporating information from the fossil content ( Figure 4B), indicates that the Lockhart Limestone was deposited on a carbonate shelf that formed in a shallower, inner-middle shelf environment ( Figure 10). In the inner-middle shelf of the Ceno-Tethys Ocean, a large group of benthic foraminifera was found to include Rotaliids, Miscellanea, Lockhartia, Operculina and Nummulites species (Figure 10A through I).
This environment is often observed in shallow shelf to open-marine settings. Access to the open ocean is commonly through a narrow channel or flooding waves. The chert content of the lower Lockhart Limestone varies (LMF 1 and LMF 2). (Table 2). This is produced by the precipitation of silica shelled organisms, which is especially common during periods of falling sea level.
A decrease in mudstone facies and Dascycladacean green algae, along with an increase in benthic foraminifera, indicates a change from an inner shelf to middle shelf environment ( Figure 10). Also, both Yates and Bradshaw (2018) and Ameen (2008) stress the importance of the Thanetian succession in the Tethyan realm. However, the Thanetian transgression should not be misinterpreted as a regional sea-level rise. In general, the UIB's hinterland experienced regionally unequal changes in elevation from the late Cretaceous to the early Palaeocene, i.e., during India's collision with Eurasia. The northern part of the basin, for example, the Kohistan Island Arc, was uplifted earlier than the eastern part of the basin's marginal areas (Ullah et al., 2020). This is why the Thanetian transgression has different geographical and temporal effects throughout the UIB. Furthermore, the eastern part of the basin witnessed a down-folding event during the Thanetian owing to an angular motion of the Indian Plate (Jade et al., 2017). This finally resulted in the formation of the HKS on the eastern margin of UIB. Finally, towards the end of the sedimentary sequence, a drop in sea level is visible, corresponding to an uplift in the later Palaeocene emergence. This regression finally resulted in sedimentary conditions common to the overlying strata, which favoured the development of tropical coal and bauxite deposits (Qasim et al., 2020).

| Sequence stratigraphic analysis
In the case of the Lockhart Limestone, depositional interpretation (or facies analysis) may aid in sequence identification and correlation while also providing knowledge of the sedimentary conditions in the examined location. The Lockhart Limestone originated on a typical carbonate homoclinal shelf, which develops at low slope angles during times of minimal tectonic activity, resulting in relatively uniform deposition over large regions. Transitional deposition occurs below the wave base from the inner (near shoreline) to the middle shelf (deeper) zones (Adams & Diamond, 2019). Following a basin-wide unconformity, facies alternations (together with changes in fossil material) between LMF1, LMF2 and LMF3 (Figures 3 and 4) indicate two very shortterm cycles of deepening and shallowing upwards, coinciding with transgression and regression cycles in the area. These short-term cycles might be explained by the continuing Himalayan Orogeny and the associated down-throw subsidence of single tectonic blocks in the region. The sequence boundaries are clearly evident. As they moved from the first (LMF1) to upper microfacies, the targeted strata formed a shelf margin systems tract (SMST) (a sequential basal subdivision) (LMF2, 3). A reverse succession from LMF2 and 3 to LMF1 overlays this system tract, suggesting a highstand systems tract (HST) and pointing to the maximum regression. As a result, the Lockhart Limestone deposition may be classified as a type 3 transgressive-regressive systems tract. These cycles are characterised by generally shallow but below-wavebase transgression shelf deposits with a deepening upward sequence, shelf conditions shifting landward, and favourable reservoir features (Kontakiotis et al., 2020;Posamentier, 2002). These reservoirs are generally a few tens of metres thick (Steela et al., 2000). In this case, however, the formation thickness averages 151 m. as the upper member 3 of the Zongpu Formation in the Tibet region of China (Jiang et al., 2016). Furthermore, the limestone of the Zongpu Formation is nodular, thickly bedded and classified as wackestone or packstone ( Figure 11A). Miscellanea and Operculina species were identified as the dominant fossils (Wan et al., 2010). At the same time, the Lockhart Limestone was deposited in the IB's Salt Range ( Figure 11B) and Murree areas, with similar nodular habits and medium to large bedded limestone, composed of a wide range of Miscellanea spp, Discocyclina spp, Lockhartia spp, gastropods, Milliolid spp, pelecypods, Operculina spp and ostracods Sameeni et al., 2013;Yaseen et al., 2011).

| Regional correlation
The basin's nodular carbonates serve as excellent reservoirs (Awais et al., 2019;Shakir et al., 2019). Similar rocks have been identified in Iran's Zagros Basin as part of the Jahrum Formation. The fossil assemblage in this formation is comprised of similar biota (Reza, 2014; Figure 11C). Shallow shelf conditions in the Ceno-Tethys Ocean during the Thanetian Palaeocene resulted in the deposition of the Lockhart Limestone on the eastern margin of UIB ( Figure 11D). The Lockhart Limestone under study is comparable to the Zongpu Formation upper member 3 and the Jahrum Formation middle part (Table 3). The limestone in the study area is nodular in habit and contains a diverse range of benthic foraminifers. The examined formation seems to be capable of sustaining a hydrocarbon reservoir in the IB and surrounding parts of the world. Khattak et al. (2017) studied the Lockhart Limestone near Islamabad (Potwar Basin). That contribution recognised the first microfacies as LH-MF 1 from the end of early transgression (corresponding to LMF3; Figure 3A). In the centre of the section, where a reverse succession is redeposited, the research revealed an identical microfacies to LH-MF 2 with reference to LMF1 above the centre of our study area (LMF1) ( Figure 4A). This contribution found T A B L E 3 Regional correlation of Upper Palaeocene rocks. Marine units are shown in blue while, white colour indicate non-marine sequences (modified from Jiang et al. 2016). Colum's are modified from (1) Berggren and Pearson (2005), (2) Jiang et al. (2016), (3) Bilal et al. (2022), (4) Bhatia and Bhargava (2006), (5) Tiwari and Tripathi (1987), (6) Pivnik and Wells (1996), (7) Najman et al. (2008). the uppermost ( Figure 3A) microfacies (LMF2) equal to the third microfacies (LH-MF 3) of Khattak et al. (2017). This comparison shows that the lower 52 m of the succession is absent in their study area. This may also explain the reduced thickness of the Lockhart Formation on the UIB's eastern margin in Khattak et al. (2017) compared to this study.
Furthermore, not only are equivalent microfacies detected in both places, but so is the same depositional environment, which is referred to in both studies as the inner-middle shelf. This is an important discovery because the similar palaeo-depositional environment demonstrates the extensive reach of a shallow water carbonate shelf in the Thanetian Palaeocene from the Potwar Basin to the eastern margin of the UIB (>100 km), suggesting deposition in a single basin.

| CONCLUSIONS
Microfacies analysis and diagenesis studies were conducted (Lockhart limestone) in order to shed light on the composition of the carbonate shelf and reservoirs in Palaeocene strata. On the eastern margin of UIB, the Thanetian age Lockhart Limestone was deposited in a shallow carbonate shelf environment. From base to top, a gradual change from orthochems to allochems has been recorded in these three microfacies (LMF1, LMF2 and LMF3). The Lockhart Limestone and other formations found along the eastern margin of UIB share similar depositional environments, pointing to a single basin of deposition. Deposition of the Lockhart Limestone was accompanied by a type 3 transgressive-regressive systems tract, or fourth-to sixth-order cycles, as well as a SMST and a HST.
Micritisation, dissolution, cementation, fracturing and physical and chemical compaction all played a role in determining the final reservoir characteristics in the Lockhart Limestone. Therefore, in the Palaeocene limestone, stylolite features and fractures have enhanced the reservoir quality. Nodular rocks of Lockhart Limestone are found synchronous with the Zongpu Formation of China and the Jahrum Formation of Iran, which act as excellent reservoirs in their respective areas. A careful examination of microfacies and diagenesis may provide a better basis for selecting reservoir targets in the study region. As a result of the Lockhart Limestone's displaying favourable reservoir characteristics the formation's rocks have the potential to become a significant hydrocarbon reservoir.