INTRODUCTION

The Carbonate rock is class of sedimentary rocks which majorly comprises of carbonate minerals and are subdivided into two main categories i.e. limestone and dolostone [1]. The hydrocarbon perspective of these carbonate are generally measured for a good hydrocarbon reservoirs, but in some places the carbonate rocks of low energy conditions can also be considered as a good hydrocarbon source rocks [2] and thus, the earlier research work mostly focused on carbonate reservoir characterization rather than carbonate hydrocarbon source rock potential in general. However, the increased research in carbonate rocks suggests that the hydrocarbon bearing carbonate rocks are extensively distributed throughout and hold a remarkable Proterozoic to the Cenozoic. Many researchers claimed that the API gravity of the hydrocarbons generated from the carbonate source rocks are relatively lower with reference to the oil generated from argillaceous source rocks [3].

The carbonate rocks are highly unpredictable to its reservoir behavior due to diagenesis, but the fossils content could be highly effective to act as source rock due to complete preservation of organic content unlike those of clastic source rocks. Similarly, the spatial and temporal distribution reservoir and the reservoir heterogeneities are the key factor to manage the production competency from carbonate reservoirs. The reservoir heterogeneities due to alterations because of rock and pore fluid interaction which influence the reservoirs in both adverse and favorable by destruction and creation of porosity respectively. The prime events that enhance the porosity and permeability includes dissolution, clay minerals precipitation that inhabit quartz overgrowth, mineral replacement by dolomitization, while cementation and dolomite cementation and dedolomitization eliminate the depositional as well as diagenetic porosity. The sum of all later events influences the reservoir properties in carbonate sediments [4]. However, the diagenetic events itself are not enough to bring alteration, the chemical and physical characteristics of host limestone and the physio mechanical conditions are also playing very vital role parallel to diagenesis [5]. Therefore, the entire system is important to understand to have a grasp over the reservoir characterization and interpret the impact of diagenesis on the parent rock and determine the interrelationship between the host limestone and pore fluid chemistry [6]. During the Late Jurassic, the high rates of organic carbon burial and the production of petroleum source rock were globally widespread [7].

The worldwide Jurassic deposits mostly act as a good source rock like the Upper Jurassic sequence of the Hanifa-Arab Formation of Arabian-Iranian Basin, the Bazhenov Formation deposits of Siberia, Taman Formations in the Gulf of Mexico, and the Kimmeridgian Shale in the Northwestern European Shelf records increased organic carbon accumulation and hence provide productive source rocks potential [7]. Most of these sequences are comprises of shale and carbonate lithology which marks the existence of oil proven organic matter due to a substantial rise in global eustatic sea level [7].

The present study focuses on the first ever detailed study on the source potential of Isha formation and diagenetic alteration with parallel focus on the influence of depositional fabric and depositional sequences. Earlier, Stuart [8] carried out some geological observations on the North Waziristan Agency and reported the occurrence and distribution of Jurrasic and Cretaceous sequence of the region. The present discovery of gas in North Waziristan attracted to gain more importance to be explored with different aspects such as source rock potential, reservoir characterization, diagenetic history, dolomitization and heterogeneities distribution. This is the first paper providing a detail insight to source potential and established facies association for paleoenvironmental studies along with the sequence stratigraphic setup in axial belt, North waziristan, Pakistan.

REGIONAL GEOLOGY AND STRATIGRAPHY

The Indo-Afghan collision zone lying in the Kurram-Waziristan region of northwestern Pakistan consists of stacks of thrust sheets originating from different parts of the continental shelf-seafloor transition that were formerly located on the northwestern edge of the Indian plate. Tracing from SW to NE, these thrust sheets comprise of Waziristan Ophiolite, Khaisora Nappe, Shahur Tangi-Kahi Nappe and Isha Nappe. These thrust stacks cause multiple deformation and unconformabaly overlain by the Late Paleocene and younger shallow-fluvial marine sediments [9]. Four tectonic stages including the Middle Cretaceous, Early Paleocene, Post Middle Eocene and quaternary are recognized in the Waziristan collision zone [9]. The middle cretaceous orogeny represents the initial intraoceanic ophiolite obduction of about 90 Ma and produce Shahur Tangi-Kahi Group of sediments in a fore-deep setting [9]. During the Early Paleocene tectonic event (65 Ma), a major thrust occur which causes the displacement of the extensive Nappe of Shahur Tangi- Kahi Group over the continental shelf of the Indian Plate [9]. Post Middle Eocene (45 Ma) tectonic evolution is marked by a shallow marine transgression which is followed by a major hiatus and finally the onset of fluvial sedimentation take place. During the Quaternary evolutionary event (<2 Ma) the molasses sediments of Waziristan-Kurram foreland are uplifted, tilted and folded. The quaternary tectonic event causes the development of Drasmand and Khandimak antiforms along with associated thrust sheets and deformations [9].

The stratigraphy of study area comprises of six distinctive units of Mesozoic rocks sequence ranging from Late Triassic to Late Cretaceous, previously which were named as Kurram Group rocks [9]. From older to younger, these units comprise of Triassic Spalga Formation, the Jurassic Sarobi and Isha Formations, Chashmai Kharasai, Marsi Khel and Zerghar Formations of Cretaceous age. During Palaeocene the Ophiolite emplacement take place and thrust over the Zerghar Formation which followed by the deposition of Eocene successions. The study area lies in the axial belt (Fig. 1a) along the Miran Shah-Mirali Road (Fig. 1b). The Sarobi and Isha Formations represents the Jurassic Carbonates of the area. The Sarobi Formation consist of thick and thin bedded dark grey to brownish grey micritic limestone. Toward the top, the limestone is dominated by shale. The Isha Formation is predominantly comprising of massive and thick bedded limestone with some minor intercalation of shale at places. The limestone varies in color from dark grey to light grey and is crosscut by various calcite veins. It has a lower gradational contact with Sarobi Formation, while the upper contact is disconformable with the Chashmai Kharsai Formation (Fig. 1c) [9].

Fig. 1.
figure 1

(a) Location map of the study area; (b) geological map where the orange circle shows the location of study area [69] (c) stratigraphy of the study area [69].

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MATERIAL AND METHODS

The outcrop of Jurassic Isha Formation is well exposed near the Isha Check post along the Miran Shah-Mirali Road which lies about 8.5 km toward east from Miran shah. The Formation occur within Latitude 32°58′14″ N, and Longitude 70°06′18″ E. A  detail field investigation was conducted to study area, where the outcrop was measured and sampled bed to bed. Samples were also collected from the area where some diagenetic features were observed. The samples were cut and polished for petrographic studies which were carried out through Olympus microscope (CX31) along with DP-21 camera attachment. The thin section examinations were carried out in the Department of Earth Sciences, Quaid-I-Azam University, Islamabad, Pakistan. The thin sections were studied under plane polarize light for understanding the facies assemblages and detail diagenesis. To differentiate between dolomite and calcite phases, the thin sections were selectively stained with alizarin-red S and potassium ferricyanide solution. The total organic content (TOC) and Rock-Eval pyrolysis were conducted in the Hydrocarbon Development Institute of Pakistan (HDIP), to evaluate the hydrocarbon potential in term of organic matter type and their maturity.

RESULTS

Field Observations

86-meter-thick outcrop of Jurassic Isha Formation of is well exposed along the Miran Shah- Mirali road in North Waziristan (Fig. 2a). The section majorly comprises of limestone which was measured and sampled. The limestone is greyish in color having ooids along with intraclasts (Fig. 2b). The limestone is cross laminated and burrowed at places and dominantly consists of bivalve, brachiopods, and gastropods (Fig. 2c). Thickness of interval is 2.5 m. At places this bioclastic limestone has a contact with dolomitic limestone crosscut by calcite veins (Fig. 2d). Medium bedded limestone, with calcite filled fractures and transgressive erosional rounded clasts are seen within this rock unit (Fig. 2e). The bioclastic limestone in turn has been altered by the brown color dolomite as several intervals (Fig. 2f). The middle part of the Isha Formation is marked by the oolitic bioclastic grainstone interval having low to high amplitude stylolite (Fig. 2g). At various intervals calcite cementation has been observed crosscutting each other. The calcite vein occurs in the form of veins as well as in the form of patches (Fig. 2h). Dolomites were distinctly differentiated from the limestone based upon the color contrast (Fig. 2i). In the studied section, the lower part has cyclic intervals of gainstone and mudstone facies having intraclasts (Fig. 2j). The central part of the succession mostly consisted of bioclastic grainstone, and wackestone facies, with the cyclic interval of peloidal packstone, and bioclastic wackestone diagenetically altered by dolomite (Fig. 2k).

Fig. 2.
figure 2

Field photographs. (a) Paranomic view of the outcrop in the study area; (b) oolitic grainstone bed along with intraclast; the yellow arrow shows stylolites; (c) bioclasts and skeletal grains of Gastropod indicated by arrow; (d) different limestone bed cross cut by calcite veins; (e) intraclasts represented by arrow in bioclastic limestone; (f) dolomite present in bioclastic limestone; (g) low to high amplitude stylolites; (h, i) multiple calcite veins cross cutting each other; (j) intraclasts; (k) dolomite bed present in limestone.

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Microfacies Analysis

Based on collected outcrop data and petrographic studies, which reveals the lithological characteristics, texture, sedimentary features and fossil content a total of eight microfacies were recognized which were associated with three sedimentary environments including the peritidal, lagoon and carbonate shoal settings. These microfacies analyses were based on the original facies model for ramps by Flügel [10]. Table 1 shows the detailed features of distinct microfacies in accordance with their facies associations, as documented within the studied intervals.

Table 1. Overall summary of microfacies of the Middle Jurassic Isha Formation
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Dolomudstone (MF1)

The dolomudstone microfacies dominantly comprise of anhedral to euhedral dolomite crystals (70–80%) floating in the mudstone matrix (Fig. 3a). The dolomite rhombs are medium to coarse grained, non-homogeneous, and probably non-ferroan. The facie is barren from any kind of biota and fossils and display fenestral fabric in a dolomicrite matrix. The very fine quartz grains are rarely present and the dolosparite recrystallization to dolomicrite is locally documented. Diagenetic processes have highly influenced this microfacies.

Fig. 3.
figure 3

Photomicrographs representing different microfacies. (a) Dolomudstone (MF1) where the blue color shows the porosity; (b) mudstone (MF2); (c) bioclastic mudstone (MF3); (d) bioclastic wackestone (MF4); (e) bioclastic packstone (MF5), the blue color indicates the porosity; (f) ooidal bioclastic grainstone (MF6), where the arrow shows porosity; (g) ooidal grainstone (MF7), where the arrow shows porosity; (h) peloidal grainstone (MF8).

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Interpretation

The lack of well preserved fossils indicates peritidal environment [11]. The scarce fossils and/or completely absence of it recommend an adverse environmental releams of greater salinity for organism survival [11]. The dolomudstone microfacies is attributed to peritidal environment and very comparable to those defined by Aghaei et al. [11]. The MF1 can be compare with the modern Persian Gulf [12], which is associated with the warm and arid region depositional environment in the upper tidal flat. It can be correlated to RMF 22 of Flügel, [10].

Mudstone (MF2)

This microfacies comprises of muddy matrix with absence of any type of biota. The bioclast are replaced by cement precipitation and the muddy matrix is highly altered by the processes of bioturbation and micritization, it can be observed that multiple calcite veins crosscut this microfacie (Fig. 3b).

Interpretation

The deposition of the muddy matrix is characteristically related to the lagoonal environment [12]. The non-laminated muddy behaviors indicate calm energy condition devoted to lagoonal deposition. It can be correlated to RMF19 of Flügel [10]. It has caused the obliteration of ooids inner laminations, whereas at some places it has micritized the whole ooid. The micritization is common and widespread in Isha Formation indicating slow sedimentation rate.

Bioclastic Mudstone (MF3)

It is comprised of pure mudstone about (80–90%), and bioclast (10–20%) (Fig. 3c). The minor content includes oval shape fine quartz in places rarely distributed [13]. This microfacies is devoid of skeletal grains and the bioclasts are preserved to some extent. The intraclasts in very minor amount is also present appear as black pebbles, silt to sand-sized quartz, and oxide-coated grains. Multiple calcite veins crosscut these microfacies.

Interpretation

The association of rare scattered very fine quartz grains together with fragments of bivalve, ostracods, and echinoid in the muddy matrix is typical of lagoons [13]. The interpreted MF3 microfacies can be related with RMF-19 of Flügel [10].

Bioclastic Wackestone (MF4)

The bioclastic wackestone microfacie comprise of bioclast about (30–40%), disarticulated and scrappy shells, dispersed forams, algae, ostracods, pelecypods, bivalves and echinoid fragments in a mud supported matrix (Fig. 3d). Other minor components are pellets, rare detrital quartz grains, uneven patches dolomite grains, and peloids. It is non-laminated and bioturbated, but the bioturbation is non-apparent due to micritization.

Interpretation

The green algae in this facie represent the accessibility of sunlight which indicates the deposition in shallow waters. The wackestone fabric and lack of significant faunal diversity both are indicators of a restricted lagoonal environment [13]. The microfacies are formed in shallow brackish lagoonal with restricted circulation and can be correlated RMF-17 of Flügel [10].

Bioclastic Packstone (MF5)

The MF10 microfacies include abundant bioclasts (30–40%) (Fig. 3e). The bioclasts includes fragments of brachiopods, ostracods echinoderms, gastropods, and rare well-preserved forams and sponge spicules. The other minor components include intraclasts shell fragments, rare ooids. The rims of bioclasts are micritized and microstylolites structures filled with black micrites.

Interpretation

The diverse fauna and micrite indicate the deposition of this facie in low energy lagoonal setting [11]. This microfacie can be correlated RMF-17 of Flügel [10].

Ooidal Bioclastic Grainstone (MF6)

It consists of deformed ooid, bioclasts and intraclasts in grainstone with intergranular space filled cements (Fig. 3f). The intergranular and/or shelter porosity is documented in this microfacies.

Interpretation

The facies contain ooids and skeletal fragments produced under moderate to high energy condition in shallow waters results in ooids formation by reworking of bioclasts. This grainstone reflects the exceedingly high energy condition probably carbonate banks and shoal [14].

Ooidal Grainstone (MF7)

This microfacies includes ooids (80–90%) in grainstone. The ooids are not well rounded, non-concentric, and display internal laminations of minor light and major deeply dark bands (Fig. 3g). The cores of ooidal grains could be possibly of previously existing peloids, small size ooids, and bioclastic fragments. While the minor components found in association with are fragments of bioclasts, peloids, and small benthic forams. The rounded and sorted ooids are cemented by micro spar and/or sparry calcite. In some places a portion is covered by coarse grained blocky calcite between ooidal grains.

Interpretation

The occurrence of well-sorted and concentric coarse ooids in the grainstone facie indicates high energy environment [14]. The deposition of the high energy conditions are usually linked with shoals and bars environments either on or near the seaward side of carbonate factories [10]. The rare intraclasts indicate intraformational reworking. The plenty of ooids, paucity of micrite, and the occurrence of well-sorted ooids represent high energy shoal environment above fair-weather wave base [14]. The facie is comparable to those described by [14]. This facie can be linked with RMF-29 of Flügel [10].

Peloidal Grainstone (MF8)

The MF8 microfacie include abundant peloids (80–95%) (Fig. 3h). The other minor component includes ooidal grains, and rare microbial crusts. The peloids are well-sorted cemented by granular and blocky calcite cement. The micrites are present in this facie occurs as void and fractures fillings.

Interpretation

The absence of muddy matrix and well sorted fecal peloids signify high energy carbonate banks and shoal depositional deposition. The peloidal grains deposits in a shoal environment are characterized by minimum circulation, warm water at low depth [11]. Since ever, the peloids cannot stand with a high energy condition therefore, the preserved well sorted fecal pellets signify quick cementation in shoal environment. The richness of fecal pellets and in association non/minor ooidal grains implies high energy environment [14]. This facie is compatible with RMF-29 of Flügel [10].

Diagenesis

In sedimentary rocks, the carbonates are the more vulnerable to diagenetic alterations. Due to the heterogeneity of carbonates, their post-deposition alterations are more complex than other types of relatively homogeneous rocks [4]. The carbonate unit of Jurassic Isha Formation has also undergone through different diagenetic stages.

Micritization and Bioturbation

The micritization in mostly developed along the ooids and bioclasts rims. The micritization resulted in destruction of ooids internal laminations, while in some facies it fleets the entire texture of ooidal grain (Fig. 4a). The micritization is common and widespread in different facies, which indicates the slow rate of sedimentation, hence microbial alterations occur before burial under very favorable environment [4]. Overall diagenetic environment of micritization is marine phreatic conditions [10]. The bioturbation is mostly influence by drilling effects of digger organism in lower energy environment lagoonal setting (Fig. 4b). Such effect generates heterogeneities in texture and color contrast of sediments due to penetration of oxygen.

Fig. 4.
figure 4

Photomicrographs representing various diagenetic features. (a) Micritization (Mc); (b) bioturbation; (c) isopachous fibrous cementation (fb); (d) isopachous fibrous (fb) and dog tooth (dt) cements; (e) granular (gr) and blocky (bl) cements; (f) coarse grained vein filling blocky cements in grainstone; (g) compaction in the form of grain deformation along with fracture indicated by arrow; (h) chemical compaction in the form of stylolite; (i) pyritization; (j, k, l) dolomitization.

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Cementation

Isopachous Rim Cement

Isopachous fibrous cement comprises of fibrous, bladed, or microcrystalline calcite crystals developed around grains as single or more equant rims as first-generation cement (Figs. 4c, 4d). It mostly forms by constituting marine first-generation cements in meteoric phreatic environment [15]. In the studied section it occurs around the rims of ooids in ooidal grainstone facie and does not show any dissolution effect which indicates the original calcite mineralogy [4].

Dog Tooth Cement

The cement consists of sharply outward projected elongated calcite cement crystals of either on grain surface or on the top of another cement (Fig. 4d). This cement display different origin includes meteoric, shallow burial conditions and sometimes even in marine environments [10]. The rarely seen result from petrographic investigation of the present work shows marine phreatic environments of the dog-tooth cement [4].

Granular Cement

Granular mosaic cement comprises of fine-grained pore filling calcite crystals of non-uniform size as well as unpreferred orientation. The type of cement implies burial meteoric environment [4]. The granular cements have patchy dispersion, same size crystals and promote the formation of the intergranular and pore-filling cements (Fig. 4e).

Blocky Cement

The blocky cement comprises of medium to coarse euhedral to subhedral crystalline margins crystals of calcite with non-ideal alignment. It resulted to the reduction of porosity by filling and cementation of calcite spar directly into pre-existing grains, or cements (Fig. 4f). Blocky cement is present in-between the grains, fractures, and voids. Such pore filling blocky calcite cement represents deep burial diagenetic environment [10].

Mechanical and Chemical Compaction

The studied section is highly deformed and has gone through various stages of deformation which is evident by observing the physical and mechanical compaction features. The mechanical compaction is also obvious from re-orientation fracturing and deformation of grains (Fig. 4g). The chemical compaction can be observed from the formation of stylolite and pressure dissolutions seams (Fig. 4h). The Isha Formation has experienced inconsistent shallow to deep burial compaction due to overlying deposition. The fractures in carbonate sediments are either caused by tectonics deformation or burial over burden pressure in final stage of burial diagenesis [15].

Pyritization

Pyritization in carbonate rocks generally occurred in the form of framboides which seems to be disseminated as well (Fig. 4i). It is formed in shallow to deep burial environments that suggests reducing conditions and associated with nearby sulfur rich diagenetic fluids [16].

Dolomitization

The variation in dolomite texture depends on the framework composition of host limestone, their formation process as well as the timing and origin of dolomitizing fluids. The dolomites are recognised through their crystal size and crystal shape. It can be observed that the zoned dolomites present between the carbonates grains and have filled the spaces between them (Fig. 4j). It is suggested that the fractures acted as conduit for the dolomitizing fluids to cause the dolomitization. Moreover, the dolomites are well developed euhedral crystals (Figs. 4j, 4k, 4l), such dolomites seem to be crystalized from shallow burial depth and suggest a late-stage diagenetic event [10].

Sequence Stratigraphy

In sequence stratigraphy, the type of carbonate facies can be used to describe the paleoenvironments and subsequent relative sea-level change [10]. The sea-level curve for the Isha Formation is also constructed (Fig. 6). The rock unit is divided into 10 Ma time with an equal time interval by assuming the constant rate of deposition [17]. The interpreted 2nd and 3rd order cycles are fitted into different time intervals using Embry and Johannessen [17] model for carbonates. Based on interpreted depositional environments and chronostratigraphic framework, it is recommended that Isha Formation is deposited in a single second-order cycle while it is in turn comprised of multiple Transgressive Systems Tracts (TSTs) and Regressive Systems Tracts (RSTs) (Fig. 6). Each TST is bounded by the maximum flooding surface (Fig. 6). The details of systems tracts are given below.

Systems Tracts

The lower part of Isha Formation comprises of high energy shoal facies which indicates sea-level high stand and is marked by transgrassive system tract (TST) during Bajocian time (Fig. 6). Afterwards the drop down of sea-level has been perceived in the form of appearance of peritidal facies on top of high energy shoal facies and is marked Regrassive (RST). The Late Bathonian time is distinguished by the occurrence of sand shoal facies in the studied Formation. Similarly, multiple TST’s covers a short span and is soon overlain by a series of alternating facies of lagoon and peritidal environments from Early Callovian to Middle Oxfordian. This sequence overall shows regressive episode over the TST which is marked as RST. The topmost part of Isha Formation is deposited in the Late Oxfordian and is marked by TST by the deposition of high energy shoal facies over the regressive lagoonal and tidal facies. Overall, the Formation shows deposition majorly is high energy shoal with thicker sequences followed by lagoonal and peritidal settings. The constructed sea-level curve based on microfacies interpretation is correlated with the local sea-level curve. The correlation of the current study with the global sea-level curve shows that the curve generated for the lower part of Isha Formation can be correlated with the global sea-level. However, a mismatch is observed in the sea-level curve of middle and upper part of the rock unit with a global sea-level curve. In the current study, the middle part of Isha Formation shows a perfect match with the sea-level curve. However, the lowermost and uppermost part of the rock strata cannot be correlated. Such a difference in the local sea-level curves indicates a variation in the deposition of carbonates even in the same basin due to ambient tectonics in the region [17]. Furthermore, the correlation indicates that the deposition of Isha Formation is partially controlled by global events. In addition to this same phenomenon is also observed in the Samana Suk Formation and it can be correlated with it based on microfacies analysis and sea level curves. In other words, it can be suggested that the Isha Formation is same as Samana Suk Formation in upper Indus basin and Chiltan Formation in the lower Indus basin.

Source Rock Evalution

The organic richness of the carbonate samples is analysed in term of TOC and Rock-Eval pyrolysis. Various plots i.e., Hydrogen (HI) versus oxygen index (OI), HI versus Tmax, PI versus Tmax and GP versus TOC were generated. Hydrogen (HI) versus oxygen index (OI) plot is carried out order to determine the potential and organic richness present in the source rock. The Van Krevelen diagram (HI versus OI) (Fig. 8a) indicates that all the analysed samples comprise primarily of Types III kerogens having capability to generate gas at suitable temperature and depth.

The HI and Tmax compared plot are very useful in order to avoid the influence of the OI for defining kerogen type [18]. The HI versus Tmax plot was applied to demonstrate the relationship between kerogen type and their maturity level of the analysed carbonate samples. The relationships between HI and Tmax (Fig. 8b) indicate that the samples have HI values of 30–200, which shows the presence of gas-prone type III kerogen. Based on Rock-Eval Tmax and PI values kerogen maturity was evaluated (Fig. 8c). Tmax values vary between 420 and 440°C which specify a thermal maturity level at the beginning of the oil window. A PI value varies from 0.25 to 0.4 showing impregnation by migrated petroleum (Fig. 8d). The genetic potential i.e. (S1 + S2) is the ability of a rock to generate kerogen when subjected to appropriate time and temperature [18]. Genetic potential depends upon kerogen nature and its richness which is generally controlled by the original organic contribution during sedimentation. The source rock with GP values in between 2 and 5 are fair, 5 to 10 are good and GP > 10 is very good in term of source rock potential [18]. The potentiality of the studied samples was determined in term of hydrocarbon generation by plotting GP values against the TOC values (Fig. 8c). The results of analyzed samples indicate fair hydrocarbon generating potential [19]. For the assessment of type kerogen and thermal maturity different rock eval pyrolosis parameters and cross plots have been used. Based on these plots the analysed sample from studied Formation contains gas proven type III kerogen which is mature for oil but not for gaseous hydrocarbon generation at present condition.

DISCUSSION

The Jurrasic Isha Formation is well exposed along the Mirali–Miran Shah road. Detail field work was arranged to the exposed outcrop where the formation was measured, logged and sampled comprehensively. Different depositional and diagenetic features were recorded during the field. The association of field observation with microscopic examination leads to establish a complete paragenetic sequence for Jurrasic Isha Formation expressing various diagenetic episodes. From the microscopic observation, the relative abundance of various constituent and matrix type were noted. The constructed microfacies includes dolomudstone (MF1), mudstone (MF2), bioclastic mudstone (MF3), bioclastic wackestone (MF4), bioclastic packstone (MF5), ooidal bioclastic grainstone (MF6), ooidal grainstone (MF7) and peloidal grainstone (MF8) microfacies. The dolomudstone microfacies lack any skeletal remnants and has been inferred to be deposited in peritidal carbonate setting [10–12]. The mudstone facies comprise of carbonate mud with scare biota. The dominant micritic matrix and the absence marine dominated biota suggest a restricted and low energy lagoonal environment [14]. Such kind of homogenous and non-fossiliferous microfacies is also observed in Jurassic Samana Suk Formation at Nizampur and Kohat Basin which is interpreted to be deposited in lagoonal environment [14]. The bioclastic mudstone is dominated by bioclasts and intraclasts infilled with thin silty quartz describes a low energy lagoon condition (Fig. 3c) [13, 14]. In the bioclastic wackestone facies, the bioclastes of bivalves and gastropods embedded in micrite. Such scare biodiversity and limited fauna show the deposition in medium energy, shallow subtidal lagoonal environment [10, 14]. Similar bioclastic packstone microfacies identified in the Jurassic Samana Suk Formation from Nizampur Basin also indicate low energy shallow lagoonal environment [14]. The ooidal bioclastic grainstone facies is characterized by abundant ooids of platform margin. Such assign environment is also supported by the grains dominated texture. The concentric ooids along with peloids and occurrences of some shell fragments supports high energy shoal environments [11, 14]. The well-sorted grainstone facies comprises of concentric coarser ooids which suggests high energy environment above fair-weather wave base [11]. Such type of deposition characterizes shoals and bars environment [10–12]. The peloidal grainstone facies comprises of predominantly peloids and peloids and occasional ooids. Such condition is developed in high-energy conditions [10, 13, 14].

Based on detail petrographic examinations, eight types of microfacies (MF1–MF8) were constructed which is characterized by skeletal and non-skeletal components. From the above mentioned microfacies analyses, the Jurassic Isha Formation clarify its deposition in the ramp environment and are further allocated into three facies association including the peritidal flat which refers to the supratidal and intertidal zone, lagoons, carbonate sands and shoals (Fig. 7, [20]). The studied microfacies are grouped into three facies assemblage on carbonate ramp based on standard Paleozoic and Mesozoic carbonates ramp facies by [10]. The mudstone and wackestone microfacies denote proximal lagoon to distal peritidal carbonates successions. Such microfacies are characterize by the hypersaline fauna and subdued open marine biota of restricted circulations while generous carbonate mud, bioturbation, and conservation of laminated fabric suggests low energy settings. The packstone and grainstone facies generally represents deposition in carbonate shoals. These facies are dominated by both skeletal rich heterozoan and non-skeletal photozon carbonates. The presence of ooids, intraclast, intense cementation and lack of any organism activity represents high energy environments of the carbonate shoals while bioclasts indicates open marine conditions [12]. The Isha Formation comprises of dominant carbonate shoals and lagoon peritidal facies. Same kind of microfacies is also reported from the Jurassic Samana Suk Formation studied in the Upper Indus Basin, and globally these Jurassic carbonates can be correlated to Jaisalmir Formation, India, Sahtan Group in central Oman, and Lusitanian Basin, Portugal. From both field investigation and petrographic analysis, it is obvious that the facies contain pure carbonate accumulation with rare terrigenous influx. Such conditions is controlled by sea-level changes or shoreline variation and not by the shift in the clastic sedimentation [12].

Fig. 5.
figure 5

Detail paragenetic sequence and the different diagenetic realms of Isha formation.

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Fig. 6.
figure 6

Stratigraphic log of Isha Formation representing microfacies, depositional environment, and sequence stratigraphy.

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Fig. 7.
figure 7

Depositional model of Isha Formation representing the deposition of individual microfacies.

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Fig. 8.
figure 8

Organic geochemistry. (a) Kerogen type; (b) maturity level; (c) source rock quality; (d) maturity; (e) organic environment.

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Diagenesis accounts for all those processes that occur after the initial deposition and lead to the stabilisation and lithification of sediment. Prior to diagenesis, these carbonate rocks set a geochemical equilibrium with marine environment in which it forms [5]. Later, its contacts with shallow marine, meteoric and deep burial fluids cause alteration in its mineralogical composition, fabric transformation, stabilisation and lithification. The carbonate diagenesis is mainly controlled by the original mineralogical arrangement, diagenetic fluids including marine, meteoric, burial diagenetic fluids and the burial history comprises of burial brines, temperature and pressure [19]. The Jurassic of Isha Formation comprises of thick carbonate sequence that has undergone through several diagenetic phases which reflects a widespread water distribution in the depositional setting. These diagenetic events comprise of micritization, bioturbation, cementation, chemical and mechanical compaction, pyritization and dolomitization. All these phases indicate three major diagenetic releams including early marine, meteoric and later burial settings which are compiled in a single detailed paragenetic sequence (Fig. 5). During eogenetic stage the primary depositional structures are altered due to extensive micritization and bioturbation. Such large scale micritization signifies a sufficient time for microbial activities (i.e., micritization) in marine phreatic environment [10]. Similarly, the reworking of sediments by organism (i.e., burrowing) cause the destruction of texture in low energy marine phreatic environment [10]. The isopachous rim cement displays bladed fabric with aragonite composition and postdates micritization. The dog-tooth cements are diagenetically form high magnesium calcite [10]. This cement demonstrates both origins of meteoric and shallow burial conditions and sometimes even in marine environments [10]. The observed granular equant calcite cementation filled the fractures and free spaces and ultimately decreases reservoir quality of Isha Formation. Such kind of cementation is frequently occurring in near-surface meteoric regime. The microscopic observations show medium to coarse grained blocky calcite cement crystals which are present in between the grains, fractures, and voids. Such pore filling blocky calcite cement shows deep burial diagenetic environment [10]. Pyrite represents reducing conditions of shallow to deep burial [16]. Dolomite is a common diagenetic product of deeply buried carbonates. Dolomitization is also observed in Isha Formation. The polymodal dolomite crystals partially filled the free spaces (Fig. 4j) as well as also completely replace the original limestone fabric (Figs. 4k, 4l). This indicates late-stage diagenetic processes and seems to be originated from shallow burial conditions [14].

For exploration purposes, categorization and identification of reservoir as well as source rock is very important. The source rock identification and its ultimate categorization are carried out by studying the organic richness, kerogen type, and thermal maturity of the studied rock [19]. The TOC generally quantifies the organic matter irrespective of the quality of organic matter [19]. The marine sedimentary rocks hold higher organic content and possess higher hydrocarbon generation potential. For carbonates the minimum required TOC values is 0.3%. According to Peters and Casa [19] the carbonates containing 0.0–0.2 wt % TOC are poor source rock while those fall with the range of 0.2–0.5 wt% TOC are considered as fair source rocks. The values of TOC for good, very good and excellent source rock ranges from 0.5–1.0, 1.0–2.0 and more than 2 wt %, respectively. The TOC results of the analysed outcrop carbonates samples of Jurassic Isha Formation falls within the range fair potential source rock (Fig. 8). Furthermore, the carbonate successions of Jurassic Isha Formation were evaluated through Rock Eval Pyrolysis. From the obtain results of Rock Eval Pyrolysis, various cross plots have been generated (Fig. 8). Based on these plots the evaluated carbonate samples from studied section of Isha Formation contains gas proven type III kerogen which is mature for oil but not for gaseous hydrocarbon generation at present condition (Fig. 8).

CONCLUSIONS

• Excellent exposures of Jurassic carbonate succession is present along the Miransha-Mirali road.

• The Isha Formation is thick to thin bedded limestone unit having ooids along with peloids and intraclasts that is diagenetically altered.

• Petrographic analysis revealed that the Jurassic carbonate unit consists of eight microfacies (MF1–MF8) that are deposited in the inner part of homoclinal ramp environment. The mircofacies MF1 is deposited in peritidal environment. MF2–MF5 microfacies are deposited in lagoonal settings, whereas MF6–MF8 microfacies are deposited in carbonate sands and shoal settings.

• The Isha Formation went through different diagenetic processes which include micritization, bioturbation, compaction, cementation, pyritization and dolomitization.

• The sequence stratigraphy of shows that the formation is deposited in a single 2nd order cycle and then multiple 3rd order transgressive and regressive cycles where the thickness of TST’s is greater then RST’s. The TSTs are mostly grainstone and packstone microfacies whereas the RSTs are wackestone and mudstone microfacies.

• The evaluation of type kerogen and thermal maturity various rock eval pyrolysis parameters and cross plots have been used. Based on these plots the analysed samples from studied formation contains type III kerogen which is mature for oil but not for gaseous hydrocarbon generation at present condition and has poor to TOC.

• Based on field investigations microfacies analysis and sequence stratigraphy it can be suggested that the Isha Formation can be correlated with Jurassic Samana Suk Formation or probably it can be Samana Suk Formation.