Direct Dating of Natural Fracturing System in the Jurassic Source Rocks, NE-Iraq: Age Constraint on Multi Fracture-Filling Cements and Fractures Associated with Hydrocarbon Phases/Migration Utilizing LA ICP MS

Abstract

This study provides a detailed geochronological paragenesis of fracture systems from the Upper Jurassic petroleum source formation in NE Iraq, utilizing U-Pb dating, integrated with microprobe analyses and petrographic studies. Five fracturing stages are recognized (FI–FV), indicating significant tectonic and temperature changes from the Late Jurassic to Pliocene times (approximately 5.2–5.5 Ma). The burial history curve shows continuous subsidence events, starting with initial burial of the Barsarin Formation reaching depths of 1000–1200 m by 110 Ma, this depth interval coincides with the first fracturing stage (FI). The buffered system of FI by pristine facies and geometrical cross-cutting of FI with early stylolite formation show a prior formation of stylolite. Subsequent fracturing stages FII (28.6 ± 2 Ma, Oligocene) and FIII (19.83 ± 0.43 Ma, Early Miocene) were contemporaneous with tectonic deformation phases and hydrocarbon generation times. Microprobe and optical analyses demonstrate variations in mineralogical composition, particularly in FIV/FV-filled calcite and dolomite cements (12.2 ± 1.5 Ma and 5.5 Ma), highlighting the periods of conduit formation for the hydrocarbon migration. Backscattered electron (BSE) imaging reveals a textural alteration of these cements, especially those associated with fluorite precipitation, which further support the hydrothermal entrapment associated with the hydrocarbon migration. The hydrocarbon entrapment appeared in at least two episodes under subsurface setting under temperatures exceeding 100 °C. In summary, the significant meaningful ages and compositional analyses obtained from this study reveal crucial insights into the dynamics of fracture-filling cements and hydrocarbon entrapment mechanisms within the petroleum source rock formation. The novelty of these data would enhance our understanding of the complex relationship between structural geology and migration conduits, highlighting the influence of fracture-filling cements on hydrocarbon accumulation and reservoir quality as a main target for hydrocarbon field development.

1. Introduction

The Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) for U-Pb dating of carbonate rocks is a high-resolution technique to obtain absolute geochronological data in complex diagenetic settings. The U-Pb dating attracted several scholars, who highlighted the development and advantage of U-Pb geochronology in carbonates, with low uranium and lead concentrations. Despite their heterogeneous distribution in carbonate minerals, an absolute dating can be obtained with LA-ICP-MS [1,2,3,4].

Recently, in situ U-Pb dating was utilized to constrain the timing of fractures by integrating regional tectonic sequences and fracture diagenetic sequences with the integration of relative timing, geochemical analyses, and rock properties to constrain the origin of fracture formation [5]. The absolute dating method is applied to characterize the fault activity period, in relation with the hydrocarbon migration (Hb) and accumulation, which is essential to basin structural studies and oil and gas exploration [6]. Fractures are crucial for hydrocarbon storage and assist as channels for fluid movement [7,8]. Fractures are also considered as significant conduits for hydrothermal fluids, allowing the formation of saddle dolomites [9], and as a consequence, hydrothermal dolostone reservoirs [10,11]. The fractures can be assessed by evaluating the paleo-fluid filling times, the fracture development periods, and dissolution times [12,13].

Natural fractures can significantly affect the movement of fluids, especially in low-permeability rock formations [14]. They create pathways between organic-rich source layers and matrix pores during hydrocarbon charging, as well as between matrix pores [15]. The presence of cement in fractures, whether cemented or cement-lined, significantly influences fracture porosity/permeability, hence affecting well productivity and reservoir performance [16,17]. Furthermore, the types and amounts of fracture cements can affect fracture spacing [18], and, among other factors, cement in pre-existing fractures can influence subsequent fracture propagation [19,20]. Understanding the processes of cement deposition in cracks is essential for predicting the flow and storage properties of these rocks [14].

Therefore, the significance of our study focuses on multi-stages of calcite and dolomite fracture-filling utilizing a powerful application (LA U-Pb dating). To reconstruct precisely the fracture stages within petroleum source rocks of our studied Upper Jurassic formation, backscattered electron (BSE) imaging integrated with micro-probe analyses was conducted on each fracture-filling cement. Investigating the interactions between fractures, matrix pores, and cement deposits within these geological settings will provide new insights into hydrocarbon charging processes and reservoir behavior. This study contributes new knowledge to the field by examining how fracture characteristics, such as porosity, permeability, and the nature of cement, directly influence well productivity and overall reservoir performance within the reservoir blocks in NE Iraq. The advanced aspect of this project is the application of U-Pb dating, a method that offers meaningful insights into the timing of fracturing events and their relationship to hydrocarbon migration. By integrating micro-scale imaging with precise U-Pb dating, our study will provide a novel evaluation of the complex interaction between fracture systems and hydrocarbon migration. In addition, this project will bridge the gap between continuous debates on the absolute timing of fracturing systems and hydrocarbon migration.

2. Geological Setting

The predominant oil in Iraq originates from Jurassic formations and is confined within Cretaceous and Tertiary reservoirs, located in the Mesopotamian Basin, the Zagros Basin, and the Zagros Fold Belt. Other Jurassic rocks, particularly those in central and northern Iraq, are also significant reservoirs [21]. The Upper Jurassic Barsarin Formation is located in the High Folded, Imbricated, and northern Thrust Zones of Iraq (Figure 1), and likely extends into the corresponding structural zones in Turkey and Iran [22]. This formation developed from the complex tectonic activity of the Arabian plate and the evolution of the southern Neo-Tethys Ocean [23]. The Neo-Tethys Ocean underwent considerable sea level fluctuations that affected evaporitic sabkhas in the basin during the Jurassic-Cretaceous period [24]. During this time, northern Iraq formed a euxinic basin, separated from the main Neo-Tethys by a rifted basin with shallow water carbonates [23]. The Late Jurassic tectonic event of the region was marked by extension associated with the opening of the Neo-Tethys Ocean, resulting in a succession of horsts and grabens that influenced sediment deposition [25].

The Barsarin Formation is formed within the foreland context, representing the evolution of sediment deposition influenced by both marine incursions and continental uplift processes during the formation of the Main Foreland Basin. The Upper Jurassic Barsarin Formation has a thickness of approximately 18 m and comprises stromatolitic limestones, dolomitic limestone, interbedded shales with deformed and conglomerate layers, as well as partially secondary gypsum and anhydrite [26]. The upper section of the Barsarin Formation, characterized by a lagoon setting, was formed in a tidal setting, whilst the bottom section was deposited in a shallow subtidal environment [27,28]. During the Late Jurassic, increased depletion resulted in the deposition of the evaporitic Gotnia and Barsarin formations in the middle Kimmeridgian to early Tithonian periods. The formation contains only age correlatives in Iraq. The Barsarin Formation of the High Folded, Imbricated, and Thrust Zones is typically equivalent to the upper parts of the Gotnia anhydrites [22].

3. Materials and Methods

Intensive fieldwork was performed in the studied section. A total of 27 samples from the Barsarin Formation were obtained. Thirty-three specimens were prepared for thin-section analysis utilizing a LEICA DM2700P polarized optical microscope (Soran University, Soran-Erbil, Iraq), focusing on the identification of diverse carbonate phases, the geometric relationships of fractures and veins, and the examination of cementation types and their origins (see Section 4.1, Section 4.2 and Section 4.3). The thin sections were used for the electron microprobe analysis (EMPA), which allows a precise point analysis leading to a correct characterization of the minerals [29]. The BSE complements the polarized light optical microscopy by identifying minerals by their mean atomic number, thus giving qualitative information on the mineral phases in terms of their chemical composition and element distribution, and provides the basis of chemical mapping. The samples were cleaned with distilled water, and thin sections were coated with carbon under vacuum. The analyses were performed on a JEOL JXA-8900R at 15 kV accelerating voltage (Alberta University, Edmonton, Canada). Operating conditions were 40 degrees takeoff angle and a beam energy of 15 keV. The beam current was 20 nA, and the beam diameter was 10 microns. Up to hundreds of micro-spot analyses (SA) were obtained for each sample. The analyses were performed at the Department of Earth & Atmospheric Sciences, University of Alberta.

U-Pb ages were obtained from polished thin sections of dolomite and calcite cements in multiple fractures with LA-ICP-MS at Goethe University Frankfurt. The ablated spot size used in this study was 235 µm, with a crater depth of around 15 µm; some scholars have utilized a 100 µm ablated spot size. Data acquisition during laboratory operations was entirely automated. Each spot analysis comprised a 20 s background capture, followed by 20 s of sample ablation and a 25 s washout period. During a 42 s data acquisition, the signals of ²⁰⁶Pb, ²⁰⁷Pb, ²⁰⁸Pb, ²³²Th, and ²³⁸U were identified using peak jumping in pulse counting mode, with a total integration time of 0.1 s, yielding 420 mass scans. Before analysis, each area was pre-ablated for 3 s to eliminate surface contamination. Soda-lime glass SRM-NIST 614 served as a reference material alongside two carbonate standards to encompass sample analysis.

A ThermoScientific Element 2 sector field ICP-MS was integrated with a Resolution S-155 (Resonetics) 193 nm ArF Excimer laser (CompexPro 102, Coherent) that features a dual-volume ablation cell (Laurin Technic, Australia). Samples were ablated in a helium environment (0.6 L/min) and combined in the ablation funnel with 0.7 L/min argon and 0.04 L/min nitrogen. The signal strength at the ICP-MS was optimized for peak sensitivity while maintaining oxide formation below 1% (UO/U). Static ablation used a spot size of 213 µm and a fluence of less than 1 J/cm² at a frequency of 6 Hz. This resulted in a depth penetration of approximately 0.5 µm s⁻¹ and an average sensitivity of 420,000 cps/µg g⁻¹ for 238 U in SRM-NIST 614. The detection limits for 206 Pb and 238 U were around 0.1 ppb and 0.03 ppb, respectively. However, at a U signal of less than 1000 cps (~2 ppb), the data were typically disregarded due to increased scatter in the isotope ratios. This tool has been utilized at the Goethe University Frankfurt (Germany).

The correction of data was obtained using the MS Excel^© spreadsheet application [30,31]. After background correction, outliers (±2 s) were excluded based on the time-resolved ²⁰⁷Pb/²⁰⁶Pb and ²⁰⁶Pb/²³⁸U ratios. The ²⁰⁷Pb/²⁰⁶Pb ratio was adjusted for mass bias (0.3%), and the ²⁰⁶Pb/²³⁸U ratio for inter-element fraction (about 5%), accounting for drift during the 12 h sequence duration, utilizing SRM-NIST 614. Data were represented in a Tera–Wasserburg diagram, and ages were determined as lower intercepts utilizing Isoplot 3.71 [32]. All uncertainties are presented at the 2σ level.

Phase contrast microscopy was used in the examination of both organic and inorganic materials, with a focus on hydrocarbon fluid inclusions that are typically difficult to detect using traditional microscopy techniques. By utilizing ultraviolet fluorescence in conjunction with the principles of reflection and absorption, this microscopy method enhances the resolution and contrast of images, thereby allowing for significant advancements in the study of fluid inclusions.

4. Results

4.1. Optical Observation

The pristine facies of the Barsarin Formation consist of stromatolitic boundstone with fenestrae and microspar. The stromatolites range in size from mm to cm. This facies is predominantly associated with sub- to euhedral dolomite crystals following the horizontal distribution pattern of stromatolite textural facies (Figure 2). Silicification is also observed in the work by Salih et al. [24]. The formation experienced intensive diagenetic alteration with multi-phased dolomite crystallization (Figure 2). The earliest dolomites are brecciated dolomite (D_B) and anhedral-subhedral dolomite (D_A), characterized by sub-planar crystals with persistent micrite relicts [31]. Petrographically, rhombohedral dolomite and radiaxial dolomites postdate these earlier dolomites [31]. Saddle dolomites with larger crystals, curved faces, and an undulatory extinction followed the preceding ones, suggesting they may have precipitated from high temperature in magnesium-rich fluids. In addition to these stromatolitic facies, the Barsarin Formation contains various kinds of fracturing stages.

4.2. The Petrographic Study of Multi-Stages of the Fracture System in the Upper Jurassic Source Rock

In addition to the depositional and early diagenetic facies, the following multi-fracture stages were recognized under optical microscope:

Fracture I (FI): The earliest fracture formation was identified in the Barsarin Formation, a fracture filled by calcite cement with variable-sized crystals in a mosaic shape (Figure 3 and Figure 4). The FI-filled calcite cement cross-cuts the early stylolite. The early stylolite was characterized by a low-amplitude pattern (LAS) and usually followed the stromatolitic facies pattern (Figure 3a–f and Figure 4). However, the late stylolite was characterized by a high-amplitude pattern developed also within the various matrix facies (Figure 3g).

Fracture II (FII): This fracture type is also filled by calcite crystals and post-dates the FI and the initial bedding-parallel stylolite (LAS). The crystal shape was characterized by an anhedral shape within a compacted texture; in places, subhedral shapes of crystal were also observed (Figure 5).

Fracture III (FIII): This type of fracture is characterized by euhedral to subhedral grains with variable sizes of calcite crystals, usually with the medium-sized crystals growing close to the fracture wall and the size growing toward the fracture core (Figure 6 and Figure 7).

Fracture IV (FIV): FIV is the largest fracture filled by elongated shapes of calcite crystals in a complex set of fractures. It is filled by radiaxial calcite crystals, likely formed from re-opening an earlier fracture (Figure 8). The re-opened fracture extends through host limestone/dolomite facies. The re-opened fracture was likely formed under high pressure and deep burial conditions due to the corrosive and dissolution surface of the radiaxial cement and the sweepy/curved shapes of the crystals filling this fracture. The development of suture contact compacted zigzag shapes between dolomite crystals, and dissolution features support deep burial diagenesis.

Fracture V (FV): This fracture is associated with saddle dolomite formation and contains disturbed solid bitumen inclusions, likely hydrocarbon inclusions (Figure 7c). Suture contact between previous dolomitization and fracture-filled saddle dolomite, corrosion, and compacting of SD crystals were likely linked to an increasing subsidence rate and overlying sedimentary rocks. The diagenetic setting of the last phase of dolomitizing is similar to that of the deep diagenetic “Mesogenesis Phase”. Saddle dolomite formation and its mechanism can be recognized by their appearance and features under deep conditions.

4.3. Electron Microprobe and Backscattering Image (BSE)

Electron microprobe analysis (EMPA) tests were conducted on various diagenetic phases with closer focusing on targeted fracture-filling cements from the Barsarin Formation samples (

Table 1. Fracture paragneiss, mineralogical/elemental composition with obtained meaningful ages for identified fractures.

S. NO.	Phase/ Fracture Paragenesis	Mineralogical Composition	CaO (%)	MgO (%)	SiO2 (%)	FeO (%)	Additional Elemental Oxides	U-Pb Dating
B17	FI	Calcite cement	55.55 n = 25	0.21 n = 25	0.03 n = 1		MnO = 0.05 n = 1 SrO = 0.16 n = 1
B10	FII	Calcite cement	56.19 n = 23	0.45 n = 23	0.12 n = 6		SrO = 0.15 n = 1	28.6 ± 2 Ma
FB9A, F4S9, F2S9	FIII	Calcite cement	56.09 n = 26	0.38 n = 26	0.03 n = 3			19.83 ± 0.43 Ma
B14 (FA, FB, FC)	FIV	Calcite cement	56.10 n = 22	0.32 n = 22	0.03 n = 3		SrO = 0.07 n = 5	12.2 ± 1.5 Ma
B14 (FC), B9 (F3), SD (B19)	FV	SD	32.01 n = 45	20.39 n = 45	0.03 n = 2	0.08 n = 1	SrO = 0.10 n = 2	5.20 ± 0.47 Ma–5.53 ± 0.27 Ma B17

). The measurements include major and minor elemental analyses of mineralogical composition. In addition, several back scattering images (BSE) were obtained.

The fracture-filling cements are considered according to their fracture chronologies and also as a function of their chemical composition. The first stage of fracturing, FI (early fracturing filled by calcite cement), has a CaO concentration ranging from 55.04 to 55.87 wt.% (av. 55.55 wt.%; n = 25), MgO from 0.04 to 0.4 wt.% (av. 0.21 wt.%; n = 25), SiO₂ content (av. 0.03 wt.%; n = 1), MnO (0.05 wt.%; n = 1), and SrO (0.16 wt.%; n = 1) (Table 1). The second stage of fracturing, FII, filled by calcite cement, is identified by microprobe and shows a CaO concentration ranging from 55.57 to 57.84 wt.% (av. 56.19 wt.%; n = 23), MgO concentration ranging from 0.16 to 0.61 wt.% (av. 0.45 wt.%; n = 23), SiO₂ concentration ranging from 0.03 to 0.26 wt.% (av. 0.12 wt.%; n = 6), and SrO content (0.15 wt.%; n = 1). The third stage of the fracture system, FIII, filled by calcite cement, has a CaO concentration ranging from 55.12 to 75.51 wt.% (av. 56.09 wt.%; n = 26), MgO concentration ranging from 0.14 to 0.62 wt.% (av. 0.38 wt.%; n = 26), and SiO₂ content (av. 0.03 wt.%; n = 3). The fourth stage of fracturing, FIV, filled by calcite, has a CaO concentration ranging from 59.21 to 43.97 wt.% (av. 56.10 wt.%; n = 22), MgO concentration ranging from 0.18 to 0.5 wt.% (av. 0.32 wt.%; n = 22), SiO₂ content (0.03 wt.%; n = 3), and SrO concentration ranging from 0.06 to 0.09 wt.% (av. 0.07 wt.%; n = 5). The fifth stage of fracturing, FV, filled by dolomite cement, had a measured CaO concentration ranging from 30.83 to 33.87 wt.% (av. 32.01 wt.%; n = 45), MgO concentration ranging from 19.41 to 21.22 wt.% (av. 20.39 wt.%; n = 45), SiO₂ content (0.03 wt.%; n = 2), SrO content (0.10 wt.%; n = 2), and FeO (0.08 wt.%; n = 1), and the mineralogical component was dolomite.

4.4. Geochronological Results

The uranium-lead isotopic analyses confirmed the significant age for the selected samples across several diagenetic environments. The obtained lower intercept age of the second fracture-filling cement (FII) was 28.6 ± 2.0 Ma (MSWD = 0.46; n = 14), with the initial ²⁰⁶Pb/²⁰⁷Pb ratio being approximately 0.84. This age belongs to the Early Oligocene (Rupelian). The absolute age for the second cement was 19.83 ± 0.43 Ma (MSWD = 1.5; n = 15), with the initial ²⁰⁶Pb/²⁰⁷Pb ratio being approximately 0.82. This numerical age belongs to the Early Miocene (Burdigalian). The lower intercept age of the third cement was 12.2 ± 1.5 Ma (MSWD = 1.04; n = 11), with the initial ²⁰⁶Pb/²⁰⁷Pb ratio being 0.73. This age belongs to the Middle Miocene (Serravailian). The lower intercept absolute age of the fourth cement was 5.53 ± 0.27 Ma (MSWD = 1.8; n = 22), with the initial ²⁰⁶Pb/²⁰⁷Pb ratio being 0.83. The last lower intercept age of fracture-filled cement was 5.20 ± 0.47 Ma (MSWD = 3.3; n = 20), with the initial ²⁰⁶Pb/²⁰⁷Pb ratio being 0.82. The ages obtained from the last event of the fracturing fourth and fifth cements therefore belong to the Early Pliocene (Zanclean). The obtained meaningful ages were plotted later on Burial History Curve (BHC). The BHC was prepared previously by English [33].

5. Interpretation and Discussion

5.1. Integrating U-Pb Direct Dating and Micropore Analyses (EMPA) for Tracking the Geochemical Evolution and Geochronology of Fracture-Filling Carbonates

Detailed direct dating (U-Pb) and electron microprobe analysis (EMPA) revealed a paragenetic sequence consisting of at least five distinct fracture generations (FI–FV), with transitions from early, meteoric-influenced low-Mg calcite to late-stage dolomite formation. Such transitions highlight the complex history of fluid chemistry and thermal conditions, which impacted the host carbonate rocks over millions of years [34]. The geochemical and geochronological data (Table 1 and

Table 2. The highest uranium concentrations are observed in dolomite cement B17, whereas the lowest values are noted in calcite cement-filled fracture B14, predominantly below 0.15 ppm. The mean ²³⁸U/²⁰⁶Pb ratios fluctuate between 1.29 and 731.3, while the initial ²⁰⁷Pb/²⁰⁶Pb ratios span from 0.73 to 0.84.

Fracture Stage	Sample	Initial207Pb/206Pb	238U/206Pb	U–Pb Age (Ma)	MSWD	n Spot No.	U Content		Pb Content		Geochronological Event
							Avg (ppm)	2sd	Avg (ppm)	2sd
Fracture-filling calcite (FII)	B10	0.84	1.29–45.45	28.6 ± 2	0.46	14	1.22	4.02	0.391	0.966	Early Oligocene Rupelian
Fracture-filling calcite (FIII)	B9	0.82	6.29–296	19.83 ± 0.43	1.5	15	0.33	6.18	0.006	14.87	Early Miocene Burdgalian
Fracture-filling calcite (FIV)	B14	0.73	4.94–247.7	12.2 ± 1.5	1.04	11	0.13	9.7	0.023	7.27	Middle Miocene Serreavalian
Fracture-filling calcite (FV)	B17	0.83	65.94–731.3	5.53 ± 0.27	1.8	22	1.26	5.50	0.01	6.27	Early Pliocene Zanclean
Fracture-filling dolomite (FV)	B17	0.82	102.7–595.9	5.20 ± 0.47	3.3	20	2.42	4.10	0.022	4.50	Early Pliocene Zanclean

) indicate a complex diagenetic history of fluid evolution within carbonate fracture systems. The transition from low-Mg calcite to dolomite formation records a significant change in the diagenesis, probably associated with extensive tectonic and burial history.

The establishment of our geochronological model, related to the burial history, derived from the detailed information of the five generations of fracture-filling cements (Figure 3, Figure 4, Figure 5, Figure 6 and Figure 7). The concentrations of U and Pb exhibit significant variability, with average values spanning from 130 ppb to 2.42 ppm and 6 ppb to 491 ppm, respectively (Table 2).

5.1.1. Early Fracturing Stage (FI)

The microprobe data obtained from the early fracture stage (FI) are distinguished by a narrow range of elemental analyses, with low MgO (0.003%–0.40%) and elevated CaO values (55.0%–56.0%; Figure 4 and Figure 9). These values could be linked to meteoric-dominated fluids in a shallow burial setting [35]. The ratio of Mg/Ca in FI-filled calcite (0.003) suggests a restricted incorporation of Mg into the calcite lattice under low temperatures and/or low Mg/Ca ratios in the involved fluids. This phase of fracture probably confirms an early diagenetic occurrence, either linked to the first fractures during early burial or to late fractures in a telogenetic setting with penetration of meteoric waters [36]. In any case, low Mn concentration in FI-filling calcite (av. 0.05 wt.%) supports a closed system [37]. Low SiO₂ (0.03%) values point to very weak siliciclastic input (Table 1). Furthermore, in places, distinctive concentrations of MgO, CaO, SiO₂, and SrO overlap the precursor carbonates (See Table 1; B10), confirming that FI is the earliest fracturing stage that formed in the shallow environment. Additionally, the geometrical cross-cutting shows that FI post-dated the early stylolite formation and FI-filled cement buffered the precursor carbonate rocks (Figure 4). The radiogenic isotope measurement to obtain the meaningful age is based on three requirements: (1) a significant U/Pb isotope variation, (2) an adequate U and Pb concentration, and (3) a homogeneity of initial Pb isotope composition. Therefore, U/Pb variation within the fracture-filling calcite (FI) was not sufficient to obtain a qualified isochron for an absolute numerical age.

5.1.2. Transitional Early-Intermediate Fracturing Stage (FII)

The meaningful FII age from U-Pb dating reveals that FII postdated FI and formed not later than the Late Oligocene interval. The compositional analyses exhibit significant differences in the calcite cementation compared to those of FI. FII is characterized by significant increases in MgO (av. 0.45%) and CaO (av. 56.19%) contents relative to FI (Figure 5 and Figure 9). Consequently, a greater variability in MgO (0.15%–0.6%) and a broader range of CaO values (55.8–57.8) characterize FII (Table 1). This significant bimodal compositional distribution suggests a substantial alteration in the fluid chemistry, indicating an open burial setting system around 28.6 ± 2.0 Ma, as suggested by the common lead (²⁰⁶Pb/²⁰⁷Pb) intersection on the Y-axis, giving 0.84 (Figure 7; Table 2). The Mg/Ca ratios of the FII calcite cements gave a 0.006 ratio, pointing to the increased Mg incorporation with higher Mg/Ca ratios that flooded into the Barsarin Formation under deep burial settings. Our results obtained from FII-filled cement are consistent with those reported by Morse JW, Mackenzie [38]. The elevation in SiO₂ concentration (0.12%) further supports the influx of fluids that interacted with silicate minerals. This transitional period probably shows a mixing zone between the preceding meteoric system and the basinal or marine-derived fluids, and could be at the base of pore water chemistry changes during burial diagenesis [39,40].

5.1.3. Intermediate Burial Fracturing Stage (FIII)

The FIII-filling calcite cement generation reveals a significant range of MgO values, between 0.13 and 0.63%, accompanied by clustered CaO concentrations (55.0%–56.5%; Figure 6 and Figure 9). The variability in MgO content with the non-linear relationship between MgO and CaO suggests precipitation under highly fluctuating fluid conditions, which is consistent with intensive fluid–rock interactions [41]. This compositional heterogeneity may indicate a transitional phase during intermediate burial, characterized by variable temperatures and the mixing of multiple fluid sources within the fracture system [42,43].

FIII-filling calcite cement shows a minor reduction in MgO content (av. 0.38%, n = 26) compared to FII, while preserving comparable CaO levels (av. 56.09%) and bimodal elemental distribution (Figure 9; Table 1). The increased incorporation of Mg in the calcite lattice likely reflects rising temperatures, since the partition coefficient of Mg in the calcite lattice increases with temperature [44,45]. These findings suggest a progressive burial phase during fracture formation (FIII stage), where elevated temperatures facilitated greater uptake of Mg in the calcite lattice. Based on U-Pb dating, this fracture dated back to 19.83 ± 0.43 Ma and shows a slight decrease in MgO content (0.38%, n = 26) compared to the FII generation, while maintaining similar CaO levels (56.09%, n = 26). This variation linked to the partial stabilization in fluid chemistry occurred approximately 9 million years after the FII formation.

FIII postdated the stylolitization formation (early and late when the burial depth was higher than earlier times during stylolitization process). However, [46] documented a significant uplift of the entire Zagros margin during the Burdigalian epoch. Our results show that the meaningful age of FIII is 19.83 ± 0.43 Ma, with a common ²⁰⁶Pb/²⁰⁷Pb ratio of 0.844 (Figure 7). Since the MSDW is lower than 2 and uncertainties less than 1 Ma, the age of fracture-filling cement is Early Miocene (Burdigalian) (Table 2). Integrating the optical observation, geochemical, and geochronological analyses of the FII and FIII, we found an elevated temperature continuing from FII to FIII within increasing burial depth during the period of subsidence, while the source of diagenetic fluids pumped to the Barsarin Formation was linked to the Early Miocene intervals, where the Jurassic rock was subjected to various intrusive fluids with burial depth, forming carbonate rocks when exposed to Mg-poor fluids [39,47].

5.1.4. Burial Fracturing Stage (FIV)

Microprobe analyses from FIV-filling calcite cement show a deviation trend from a wide range of major elemental oxides (MgO, CaO) to a narrower range compared to FII and FIII, with uniform MgO concentrations (0.23%–0.5%) alongside elevated CaO values (56.0%–57.8%) reflecting further stable geochemical composition/conditions during the calcite precipitation (Figure 9; Table 1). The Mg/Ca ratios of 0.005 further support the consistent fluid chemistry in this system, suggesting continuous burial conditions [41,48]. The fluids involved in this calcite precipitation were consistent with continuous Sr concentration (592 ppm), which is absent in the previous fracture fillings. Sr distribution in carbonates typically follows a regular pattern, and strontium content tends to be more enriched in brine fluids [45], thus being more abundant in deep layers due to the prolonged duration of fluid–rock interaction [49]. Therefore, the fluid origin and diagenetic settings control the strontium mobility and incorporation into the carbonate minerals lattice [50]. The stable strontium levels recorded in the FIV cements may thereby present significant constraints on the fluid supply and diagenetic conditions during this stage of fracture mineralization [51].

This phase of FIV reveals a burial condition roughly 7.6 Ma after FIII precipitation, with fluid chemistry characterized by a semi-equilibrium condition. The close clustering of geochemical compositions suggests uniform pressure–temperature conditions during precipitation, perhaps associated with a specific tectonic or burial event consistent with the subsidence process [39,52]. Furthermore, the lower intercept age of FIV-filling calcite cement with the Y-axis, giving a ²⁰⁶Pb/²⁰⁷Pb ratio of 0.73 (within the Tera–Wasserburg diagram), is significantly different from the previous fracture-filling cements (FII, FIII; Figure 8; Table 2). The differences in lead isotopic composition could indicate a re-opening of the fracture system during a period of enhanced tectonic activity in the context of the Zagros fold-thrust belt [53]. This isotopic ratio potentially favored the uptake or loss of lead from surrounding depositional environments. Furthermore, the elemental oxides obtained from FIV partly overlap with those from previously dated fracture stages (such as FII and FIII). This overlap suggests a buffering mechanism and emphasizes a reactivation in the fracture systems [54] which modified the fluid pathways [55] and the isotopic and chemical signatures. This highlights the role of fluid–rock interactions as a conduit for the migration of fluids during tectonic deformation [56].

The FIV filling corresponds to a period of continuous deformation as the Zagros fold-thrust belt continued to evolve, and U-Pb dating gives a meaningful age of 12.2 ± 1.5 Ma with a MSWD of 1.04 (Figure 8). The tectonic evolution can create pathways that enhanced fluid flow and re-equilibrated the system within the succession of fracture phases, which linked to the observed initial lead values in this study. The isotopic constraints revealed that the fracture-filling carbonates likely precipitated from fluids [54] contemporaneous with seawater composition [53].

5.1.5. Late Burial Fracturing Stage (FV)

The final phase in the fracture-filling sequence (FV) represents a fundamental shift in carbonate mineralogy of the studied series, with MgO content ranging from 19.4% to 21.2% and CaO from 30.5% to 34.0%, typical of dolomite composition. The inverse relationship between MgO and CaO suggests stoichiometric conditions during dolomite precipitation approaching a Ca:Mg ratio of 1:1, while those with lower MgO and higher CaO represent a less stoichiometric dolomitizing process (Figure 10; Table 1). This stoichiometric trend favors burial dolomitization processes that formed in more stoichiometric compositions under conditions with sufficient time for ordering the cations within the crystal lattice of dolomite [57]. This fracture-filling dolomite indicates a significant change in fluid composition with higher Mg/Ca ratios, likely related to deep burial conditions and the influx of magnesium-rich basinal brines or hydrothermal fluids [42,58]. The integration of petrography with microprobe analyses and BSE shows the characteristics of dolomite under hot reducing conditions, likely saddle dolomite. The high concentrations of Fe (622 ppm) and Sr (846 ppm) are related to the influx of Mg-rich basinal brines or hydrothermal fluids [59]. The FV- and FIV-filling cements also share high Sr concentration values; this overlap is another indicator that both fracture stages developed within deep burial settings.

To evaluate this event associated with FV-filling dolomite, two samples were analyzed utilizing laser ablation U-Pb absolute dating. Both samples gave two meaningful ages (5.20 ± 0.47 Ma and 5.53 ± 0.27 Ma); these two ages perfectly coincide with Early Pliocene times (Figure 11). Furthermore, the resolution of radiogenic dating is very precise, as the uncertainties are too small, and MSWD in one sample was less than 2; moreover, the data scattering within the isochron was very small. These data support the dolomitization fluids under a deep burial setting and reflect the dynamic nature of these fractures under the influence of reactivation of tectonics/and continuous subsidence processes during this event.

Late Miocene tectonic reactivation and dolomitization are constrained by U-Pb geochronology to the period of 5.20 ± 0.47 Ma to 5.53 ± 0.27 Ma, indicating a notable temporal gap of roughly 7 Ma after the precipitation of FIV calcite (Table 2). During the Late Miocene event, a tectonic reactivation of pre-existing structural features was produced (fractures III–IV), enabling the influx of dolomitizing fluids that induced the substitution of calcite with saddle dolomite cement. The identical tectonic pulse concurrently triggered the formation of new fractures (FV), producing a complex network of Mg-rich fluid conduits. This accurate U-Pb dating with dated cross-cutting relationship offers essential insights into the episodic fluid migration history of the basin and an estimate of the significance of integrated structural, diagenetic, non-conventional geochronological methodologies to explain the evolution of the complex carbonate basin.

5.2. Geochronological Evolution of Multi-Fracture System in Upper Jurassic Formation

The absolute ages and geochemical analyses of fracture-filling cements within the Upper Jurassic Barsarin Formation reveal significant insights into the diagenetic fluid sources and geochronological events, which were concomitant with the tectonic evolution of northeastern Iraq. The initial calcite-dominated fracture phases (fractures FI–FIV) exhibit a consistent CaO composition (ranging from 55.55% to 56.19%) with low MgO and low trace element contents. These values suggest a precipitation of carbonate cements from marine-derived formation fluids with minimum external fluid influence [60]. This is further confirmed by minor SrO concentrations, typical of carbonate-dominated systems, as suggested by [61,62].

Similar patterns of geochemistry within multi fracture-filling calcite cements (FII and FIII) and overlapping values reflect the homogeneity in fluid origin following multiple deformation stages across the Zagros fold-thrust region. The estimated temperatures of these fractures were likely around 60 to 90 °C within the oil generation window for the Barsarin Formation, similar to the thermal models reported by [46]. The minimum contributions of siliciclastic rocks, as inferred from a distinctive increasing of Si concentrations in FII (561 ppm), suggest an interaction with adjacent formations during fluid migration.

The geochemical transition, radiogenic and non-radiogenic ones, from fracture-filling calcites to FV-filling saddle dolomites indicates a significant alteration in both digenetic fluid composition and thermal conditions. These observations, with the support of optical observations in FV, such as the nucleation of saddle dolomites and the crystal properties of dolomite, reflect reducing conditions associated with hydrothermal fluid influx [63]. Furthermore, MgO enrichment (20.39%) favors external sources of Mg²⁺, probably from deep basinal brines, which were mobilized during maximum burial or by interactions with Mg-rich basement rocks [64]. The presence of a high concentration of Fe (622 ppm) further indicates a reducing environment, commonly associated with deeper burial settings or hydrocarbon-charged fluids [39]. Such mineralogical shifts illustrate the complex relationship between diagenetic fluid composition, diagenetic processes, and the structural evolution during fluid/hydrocarbon migration.

The integration of the direct U-Pb dating with microprobe analyses, BSE, and optical microscopy of multi-fracture filling cements shows an extensive deformation history spanning at least approximately 28.6 million years. The meaningful age using micro-spot laser ablation shows an oldest fracture generation (28.6 ± 2 Ma), which coincides with the onset of the Arabia–Eurasia collision, correlating with the formation of the Zagros foreland basin and early compressional forces [65]. Our geochronological data add new insight in understanding the timing of key tectonic events that affected the hydrocarbon migration. Consequently, the direct dating highlights a model with integrated subsidence and structural evolution and finally provides a constraint on the structural development of the northeastern part of the Zagros fold-thrust region in Iraq.

5.3. Hydrocarbon Migration Within Deep Burial Fracturing System: Insights from Regional Tectonic Framework, Burial History Curve, U-Pb Dating, and Hb Fluid Entrapments

The geochronology of the fracture system, determined via direct U-Pb dating integrated with spot analyses from EMPA, offers new insights into the tectonic development of NE Iraq and modification of the burial history curve (BHC; Figure 12) for future exploration throughout the Zagros fold-thrust belt. The geochronological meaningful ages/data show five distinct fracturing stages, initiating probably from the Late Jurassic to the beginning of the Pliocene. The burial history curve shows a relatively rapid subsidence in the Barsarin Formation during the Late Jurassic to Early Cretaceous interval, reaching depths of approximately 1000–1200 m by 150 Ma. The interval of the initial burial phase coincides with the earliest fracturing stage (FI) (Figure 12). The BHC shows continued subsidence through the Cretaceous period, with depths of approximately 1300 m during the Late Cretaceous (~90 Ma).

The second generation of fracture filling (FII) via U-Pb direct dating (28.6 ± 2 Ma; Oligocene), produced after a significant period of continued subsidence, corresponds with the initial collision phase of the Zagros orogeny, consistent with the regional initiation of continent–continent collision between 35–25 Ma [66]. This collision drove the structural formation of early foreland basin systems and the phase of compressional deformation in the northern Zagros region [67]. The FII-cement filling shows evidence of elevated temperatures based on petrographic studies [31].

During the Oligocene to Early Miocene, the Upper Jurassic Barsarin Formation likely entered the oil generation window [68]. This interval coincides with the oldest two fracture generations (28.6 ± 2 Ma and 19.83 ± 0.43 Ma; See Figure 12), suggesting the initial fracture formation developed synchronously with hydrocarbon generation; consequently, the primary migration conduit was developed [69]. Similar fracture-filled calcite has been reported previously as a petroleum migration conduit in the Zagros fold-thrust belt [70]. To draw and understand the reservoir charging history, the timing between fracture development and structural trap formation has to be provided. Therefore, second-dated fractures (28.6 ± 2 Ma) predated the main phase of structural trap formation, potentially allowing a later timing for the earliest period of hydrocarbon migration, probably before trap development. This phase occurred during the Oligocene after a significant period of subsidence, while later developed fractures FIII and FIV (19.83 ± 0.43 Ma to 12.2 ± 1.5 Ma) coincided with the main period of fold development and trap formation [46]. FIII aligns with the increase in convergence rates between the Arabian and Eurasian plates throughout the Early Miocene [71]. This age is consistent with deformation ages determined by [72] in the Zagros-Folded Belt and coincides with the early formation of significant fold structures that subsequently created hydrocarbon traps in the area [73]. The authors determined the Early Miocene deformation stages in the Northern Zagros region of Iraqi Kurdistan via integrated structural and thermochronological investigations.

Furthermore, the ages associated with FII- and FIII-filled calcite cements are consistent with the constant burial depth. This consistency is revealed from sharing the bimodal elemental distribution patterns and the wide range of CaO concentration (FII and FIII; Figure 9). However, both fracture-filled cements formed with a relatively constant burial depth, but the increase in Mg incorporation into the calcite lattice suggests increasing temperatures from the FII to FIII stage, as confirmed by the enhancement of magnesium uptake (Figure 6).

With continuation of burial depth, exceeding 2500 m, the absolute U-Pb dating reveals a younger age of fracture that is consistent with the Middle Miocene ages (12.2 ± 1.5 Ma; FIV). FIV coincides with the principal phase of folding and thrusting in the NE Zagros, based on magnetostratigraphic and structural analyses [74]. This interval represented the maximum orogenic activity in the central Zagros, marked by peak shortening rates and the formation of significant thrust faults [75]. The simultaneous occurrence of fractures throughout the region indicates a broad tectonic influence, probably associated with variations in plate convergence rates or mechanical stratigraphy [76].

The most recent fracture-filling cements (5.20–5.53 Ma; FV) show a clear shift as deduced from optical, mineralogical, and elemental composition (Figure 9); FV is associated with late-stage deformation and a continuous subsidence process, exceeding the depth of 3500 m (Figure 10). During this age, the Barsarin Formation remained at the maximum depth before the region experienced the main uplifting phase. This phase of subsidence was accompanied also by textural alteration on the dolomite formation (Figure 11). The late-stage deformation aligns with a recorded shift from mostly compressional to transgressional tectonics in certain regions of the belt, especially at oblique convergence zones [77]. The alteration in fracture-filling cement and shifting in stress regime may explain the transition from calcite to dolomite mineralization measured in FV.

Interestingly, the subsequent formation of fracture-filling calcite and dolomite (FIV, FV) was synchronous with the occurrence of euhedral fluorite formation. The microprobe analyses measured an extra increase in CaO (up to 74.88%), which indicates a fluorite formation within the fracturing system. In addition to microprobe analyses, the BSE shows floating large crystals of fluorite within dolomite cements filled in the late fracture stage (Figure 13 and Figure 14). Fluorite mineral deposits were identified along the fault zone using fluid inclusions suggesting two distinct categories: those formed at high temperatures, ranging from 240 to 280 °C, and those formed at lower temperatures, between 100 and 150 °C [78]. Therefore, at a minimum, fluorite precipitation required a temperature higher than 100 °C, and mostly originated from deep basinal-derived brines [79,80]. If we consider a standard geothermal gradient of 25 °C/km [81], the temperatures of FIV and FV would have reached up to 63 °C and 88 °C (Figure 12). However, the formation of fluorite requires at least 100 °C, and the saddle dolomite also requires a higher temperature to precipitate. Consequently, the fluids responsible for calcite and dolomite precipitation within FIV and FV were likely hydrothermal-rich brines. Similar cases of association of hydrothermal/hot fluids with fluorite precipitation, related to the circulation of deep-seated basinal brines, have been reported [79,82]. Consequently, the systematic geothermal gradient is not applied to the FIV and FV stage, since the temperatures responsible for fracture-filled calcite/dolomite (FIV and FV) are higher than the surrounding temperature. Other scholars have explained that the fluorite deposition occurred at lower temperature (130–175 °C) conditions compared to the normal thermal condition for fluorite formation (higher than 200 °C) due to the acidic environment and the upward migration of deep-seated fluids as a result of fluid–rock interaction, originating from the HT-dolomitized rocks.

The BSE and microscopic analyses obtained from FIV and FV-filling calcite/dolomite cements (Figure 13 and Figure 14) reveal significant tracks for hydrocarbon migration pathways within deep burial carbonate settings. Both primary and secondary fluid inclusions containing hydrocarbons (Hb) were observed within the saddle dolomite and calcite crystals, indicating a multi-phase migration history. The Hb inclusions trapped the fracture and vuggy fabric; these fabrics could be created during tectonic events and traced as fluid migration channels, particularly in tight carbonate systems where matrix permeability is limited [39]. These porosity and permeability fabrics are responsible for entrapment of Hb inclusions, as observed in BSE (Figure 13 and Figure 14). The primary fluid inclusions represent hydrocarbon entrapment contemporaneous with dolomite precipitation (SD; Figure 13 and Figure 15), while secondary hydrocarbon entrapment was found along micro-fractures, demonstrating post-crystallization hydrocarbon migration events (Figure 15). The spatial distribution of both Hb inclusions (primary and secondary Hb inclusions) were observed either as clustering Hb inclusions or crosscutting the boundaries of the crystal faces of FIV and FV filling calcite/saddle dolomite (Figure 11). These analyses indicate the consistency of hydrocarbon migration within the same period of hydrothermal influx that was responsible for the deformation stage (FV) during further subsidence and dolomitization (Figure 15). The secondary migration of Hb was initiated after the influx of hydrothermal fluids, most likely during further subsidence of the Barsarin Formation (Figure 11). These data and observations are consistent with those in [42], that hydrocarbons commonly migrate along with metal-rich brines during saddle dolomite formation, and during the Late Miocene, possibly representing a second phase of migration from deeper, more thermally mature source rock intervals [68]. The relationship between late HT dolomitization and hydrocarbon migration has been archived in carbonate reservoirs, where it significantly impacts reservoir quality through porosity and permeability alteration [83].

The presence of suture contacts (SCs) alongside the hydrocarbon-bearing saddle dolomite provides critical evidence regarding the timing and pressure conditions during migration (Figure 12). These pressure solution features indicate significant overburden pressure during the deep burial phase [59]. The chronological relationship between the suture contacts and hydrocarbon inclusions suggests that hydrocarbon migration occurred during or before maximum burial conditions within the Upper Miocene interval (not later than 12 Ma). This timing is significant because it indicates that hydrocarbon generation, migration, and entrapment occurred within a relatively narrow temporal window during late-stage diagenesis. Scholars have reported previously that the late diagenetic saddle dolomite formation coincides with peak hydrocarbon generation windows [43]. The presence of saddle dolomite and fluorite formation provide temperature constraints on the hydrocarbon migration system (Figure 12). Scholars established formation conditions for saddle dolomite typically require temperatures up to 160 °C [60]. These elevated temperatures are also conductive to hydrocarbon maturation, supporting the interpretation that hydrocarbon generation and migration were thermally coupled with the hydrothermal system responsible for saddle dolomitization. The Hb phase is represented by the bright/dark blue luminescent inclusions, and these inclusions were trapped also inside the euhedral shape of fluorite crystals, with numerous fractures and microcracks visible as dark linear features throughout the sample. The contrasting luminescence colors between primary and secondary fluid inclusions indicate different hydrocarbon phases, providing insights into the timing and evolution of fluid migration during fracture formation and cementation processes (Figure 14).

The fracture-controlled nature of the calcite/saddle dolomite and associated hydrocarbons points to significant tectonic influence during the Miocene and Pliocene intervals. The stage FIV–FV fractures likely formed during a pulse of tectonic activity that created the conduits necessary for both hydrothermal fluid circulation and hydrocarbon migration (Figure 15). This tectonic-hydrocarbon system is consistent with compressional tectonic pulses in foreland basin settings that often trigger hydrocarbon remigration events through the formation of overpressure migration pathways [4]. Consequently, the parallel occurrence of hydrocarbon migration pathways with fracture systems suggests that structural deformation provided the primary control on fluid distribution rather than stratigraphic factors. The structurally controlled hydrocarbon distribution patterns are characteristic of hydrothermal dolomite/calcite, where faults and fracture networks create a complex pore-space network for enhancing the reservoir quality.

6. Conclusions

The study of fracturing stage via Laser Ablation direct U-Pb dating, integrated with electron micro-probe analyses and BSE within the Upper Jurassic Formation in the NE Iraq region, provides new geological insights into the temporal and spatial distribution of fracturing stages in petroleum reservoirs from the Late Jurassic to Pliocene times. Five distinctive fracture types were recognized (FI–FV) and characterized by a considerable mineralogical and elemental composition and enough variation in uranium and lead isotopic compositions to obtain a reasonable isochron within a Tera–Wasserburg diagram. These two approaches reveal a progressive fracture evolution consistent with significant diagenetic/tectonic activities within the Zagros orogeny.

The obtained data integrated with the burial history curve show significant subsidence, reaching depths critical for hydrocarbon generation and migration. Specifically, the initial fractures (FII), which dated back to approximately 28.6 ± 2 Ma, predated the major structural trap formations, indicating an early hydrocarbon migration phase. The consequent formation of various fracture stages (FIII and FIV) shares bimodal distribution patterns of the CaO and MgO compositional fluids; these data are synchronous with the development of tectonic compressional phases, which explains the main key periods of structural evolution and petroleum accumulation. These fracture stages, particularly FII (28.6 ± 2 Ma, Oligocene) and FIII (19.83 ± 0.43 Ma, Early Miocene), are linked to tectonic deformation processes and significant hydrocarbon generation periods. The temporal correlation between these fracture stages and known tectonic deformations strongly suggests that fluctuations in subsurface pressures and temperatures were influenced by the involvement of hot fluid and Hb migration. With continuing subsidence (2500–3500 m), the last two phases of fracturing were produced, the last one within approximately the maximum depth formation of FIV/FV-filled calcite and dolomite (12.2 ± 1.5 Ma and 5.5 Ma). These two stages of fracturing highlight the conduits for hydrocarbon migration. In brief, utilizing high-resolution data integrated with other tools contributed significantly to the understanding of subsurface fluid evolution within a multi-fracture system in the studied reservoir rocks, focusing on the influence of hydrofracturing/tectonics on hydrocarbon migration pathways and reservoir characteristics.