Detailed sedimentological log of the Quwarah Member of the Qasim Formation was constructed to capture bed thickness, lithology, color, grain size, sorting, sedimentary structures, types of bed contacts, and fossil content. The log was constructed for two, shallow stratigraphic sections, which were generally 400 m apart and about 10.5 m in total thickness (Figures 1B; Figure 2). The sections were selected based on the presence of basaltic sills, which directly overly them. A total of 17 samples were collected in the measured sections (Figure 2).

Seventeen (17) standard petrographic thin sections were prepared and impregnated with blue-dye resin for identification of porosity. The thin sections were studied using an Olympus BX53F petrographic microscope. Point-count analysis was performed on the thin sections to determine the percentage of detrital framework grains, authigenic minerals, matrix content, primary and secondary porosities. The analysis was carried out using the Petrog software package, based on 300 counts per thin section. Grain size was measured by measuring the long axes of 100 fresh detrital quartz grains using the Petrog, at the end of which the average grain size and sorting were determined for each sample by the software.

Bulk X-ray diffraction (XRD) analysis was conducted to complement the point-count data, scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS) analyses; to establish and quantify the sandstone constituents; and to guide the thin section analysis. The analysis was conducted at the Inorganic Geochemistry lab, College of Petroleum Engineering and Geoscience, King Fahd University of Petroleum and Minerals, Saudi Arabia. Prior to running the analysis, about 4 g of each sample was crushed, powdered using a Retsch RM 200 mill. The mill was cleaned with ethanol before and after each run. Powdered samples were prepared and loaded into sample holders with circular geometry and grooved shallow wells. XRD analysis was then carried out on all samples using Malvern PANalytical Empyrean Cu LFF HR diffractometer with Cu Kα1 radiation at a wavelength of 1.5406 Å and X-ray tube voltage and current set at 40 mA and 45 kV, respectively (Amao et al., 2022). Each bulk sample was scanned between 4^ο and 63^ο 2θ with a step size of 0.013^ο and scan step time of 8.67 s. However, to confirm the presence of dickite in some samples, the samples were scanned between 4^ο and 70^ο 2θ range. Qualitative analysis of all XRD data acquired was performed using the search-match module Highscore Plus software package (v. 4.9), with the reference database of ICDD PDF-4 2022. After the mineral phases were identified with Highscore Plus, they were further analyzed using the Rietveld quantitative XRD fitting tool.

An automated QEMSCAN^® system was used for identifying and mapping of mineralogical composition of seven representative sandstone thin sections. In particular, the QEMSCAN technique was employed to establish the presence of chlorite and its relationship with kaolinite within intergranular pores. The instrument consists of Quanta 650 FEG scanning electron microscope (SEM) coupled with XFlash, Bruker Inc. energy-dispersive X-ray spectrometers (EDS). Before running the analysis, thin sections were carbon-coated using Q150T Quorum EMS 150R ES and then loaded into the QEMSCAN. The QEMSCAN analysis was conducted at the Center for Integrative Petroleum Research, King Fahd University of Petroleum and Minerals, Saudi Arabia. For this study, the QEMSCAN data were acquired using an X-Ray beam produced by accelerating voltage of 15 kV and a sample current of 10 nA (±0.05) (Amao et al., 2016). Field-scan mode was selected with a covered area of 1 cm² and 5 µm point spacing. The data was acquired and processed using iMeasure and iDiscover software packages, respectively.

Stub samples were studied by scanning electron microscopy using Zeiss Germini 500 scanning electron microscope equipped with an Aztec energy-dispersive spectrometer (EDS) and a backscattered electron detector (BSE), to identify minerals based on crystal morphological features, establish textural relationships between detrital grains and pore-filling materials, and determine diagenetic features. The stub samples were coated with 10.0 nm platinum using a Leica EM 900, prior to running the SEM analysis. The SEM analysis was carried out at 5–15 kV voltage and current of 1–3 nA working conditions. The EDS analysis was performed using Aztec EDS (manufactured by Oxford Instruments, United Kingdom) and permitted mineral identification using spot chemical analysis to acquire semi-quantitative mineral composition.

The studied Qasim sandstones are very fine-to medium-grained, with sorting ranging from moderately well-sorted to well sorted. Detrital grains are well-to very well-rounded. Quartz occurs as monocrystalline grains and ranges from 20% to 67% in abundance (Table 1). Detrital plagioclase prevails (trace to 1.3%) over K-feldspar (trace to 0.7%) (Table 1). Muscovite is the predominant mica (trace to 1.5%), whereas biotite occurs in trace amounts (<1%). Lithic fragments are mainly sedimentary in origin (trace to 1%). Glauconite and zircon are accessory minerals, with abundances generally ≤1% each. Compositionally, the sandstones have been classified as mainly quartz arenites and rarely subarkoses (Figure 3), with an average, present-day composition Q₉₈F₂L_0, based on Folk. (1980) classification scheme.

Results of the bulk XRD analysis show that detrital quartz is the main framework mineral in all samples (Figure 4). Kaolinite, dickite, calcite, goethite, biotite, and muscovite are the remaining predominant minerals (Figure 4), whereas hematite, ilmenite, and chlorite occur in very few to one samples (Table 2).The bulk XRD results show that the amount of quartz ranges from 47.5% to 99.2%, biotite from 0% to 2.8%, muscovite from 0% to 1.9%, kaolinite from 0% to 43.5%, dickite from 0% to 11.4%, calcite from 0% to 16.2%, goethite from 0% to 22.6%, hematite from 0% to 0.9%, ilmenite from 0% to 0.6%, garnet (andradite) from 0% to 1.4%, and chlorite from 0% to 0.8% (Table 2).

Three main depositional facies have been established, including offshore, lower shoreface, and upper shoreface (Figures 5A–D). These depositional facies are overlain by basaltic sills (Figures 5E–G), which commonly range in thickness from 0.4 to 4 m and are relatively laterally extensive >300 m (Figure 5F). Sedimentary facies and interpretations.

Thin-section optical microscopy and SEM analysis indicate that some minerals have undergone dissolution and replacements. For instance, detrital quartz has been partly and pervasively dissolved, creating secondary intragranular porosity (Figures 14A–C). In addition, the secondary intragranular porosities within detrital quartz are often filled by Fe-oxide cement (e.g., goethite; Figure 14A), dickite, kaolinite, and chlorite (Figures 14B, C). The dissolution of detrital quartz occurred in two forms: 1) dissolution restricted to the center of grains (Figures 14A, B); and 2) dissolution around the edges of detrital quartz (Figure 14D).

Furthermore, quartz overgrowths, notably the rounded ones, show major evidence of dissolution (Figure 14D). The overgrowths exhibit partial dissolution, breaking their continuity around detrital quartz grains and often creating embayments (Figure 14D). However, quartz overgrowths with angular, pointed edges show no evidence of dissolution in the analyzed sandstones (Figures 11C, D). In addition, intergranular, pore-filling calcite and goethite have undergone dissolution and create secondary intergranular porosity (Figures 14E, F).

Two types of porosities were observed in the studied Qasim Formation, including primary, intergranular and secondary, intraganular porosities. The intergranular porosity is both opaque-mineral lined (Figures 12A, B) and non-opaque mineral lined, with percents ranging from 0% to 15.6% (average 7.7%) and 1.3%–15.7% (average 6.2%) (Table 1), respectively. Total intergranular porosity varies from 3% to 25% (average 13.9%; Table 1).Results of petrographic point count indicate that secondary porosity related to detrital quartz dissolution ranges from 0 to up to 1.7%, averaging 0.5%. However, because feldpars generally occur in trace amounts in the sandstones, SEM analysis has revealed that they often exhibit evidence of dissolution, with kaolinite replacing the altered grains (Figure 9C). Additionally, dissolved or altered mica grains (e.g., muscovite) show evidence of dissolution and replacement by kaolinite (Figures 9A, B). Thin section petrography shows that blocky kaolinite and dickite occur around altered muscovite (Figure 9A). Nevertheless, the secondary porosity created due to calcite and goethite dissolutions (Figures 14E, F) are volumetrically insignificant, as they occur in trace amounts (<1% each).

The analyzed Qasim sandstones are overlain by a basaltic sill (Figures 5E–G), which ranges in thickness from 0.4 to 4 m. To investigate the role of the contact between the sill and sandstones on diagenesis, cross plots of diagenetic minerals in relation to the proximity to the sill contact have been constructed (Figures 15A–D). The results show that the amounts of authigenic kaolinite (Figure 15A) and dickite (Figure 4) increase away from the sill contact. However, the plot of quartz overgrowths against the distance below the sill contact shows that the cement relatively decreases away from the sill contact (Figure 15B). Additionally, the results show that while the volume of pore-lining Fe-oxide (Figure 15C) minerals increases towards the sill contact, pore-filling Fe-oxide minerals increase away from the sill contact (Figure 15D). Furthermore, values for compactional porosity loss have remained relatively similar at and away from the sill contact (Figure 15E). However, cementational porosity loss increases away from the sill contact (Figure 15F).

The types of grain-grain contacts and orientation of ductile grains (e.g., mica) can record the impact of mechanical compaction in sandstones (Figures 7A–F) (Chuhan et al., 2002; Al-Ramadan et al., 2013; Haile et al., 2019; Bello et al., 2021). While sandstones with abundant sutured grain contacts indicate intense mechanical and chemical compaction, sandstones with floating, point and long grain contacts (Figures 7A–C) indicate a low degree of mechanical compaction (Heald, 1955; Haile et al., 2019). In the studied Qasim sandstones, the predominance of floating, point, and long grain contacts (Figures 7A–C) suggests that the sandstones have been subjected to little mechanical compaction, probably buried to a depth below 2 km before being uplifted. This is consistent with mica grains being flat to slightly deformed around detrital grains (Figure 7E, F). Concavo-convex contacts are very uncommon in the sandstones (Figure 7D). In addition, the Qasim sandstones in the Tabuk area are interpreted to have been uplifted following the deposition of the studied, sandy Quawarah Member of the Qasim Formation (Laboun, 2010), suggesting that the sandstones have undergone only shallow burial eodiagenesis (<2 km; <70 ^⸰C) (sensu Morad et al., 2010) before being exhumed and subjected to telodiagenesis (related to uplift). Furthermore, petrographic observations indicate that all diagenetic cements such as early calcite cementation (Figures 13A–C), kaolinite, feldspar, and mica dissolution show that the sandstones did not reach the quartz cementation window (>65°C) before being exhumed. The observed quartz overgrowths with rounded edges (Figures Figure11A, B) suggest that they were not formed in-situ during burial and, hence, were most likely recycled or inherited (Sanderson, 1984; Hellevang et al., 2021), which resulted in the rounding of the edges of the overgrowths during transport. Consequently, all the above-mentioned suggest that the studied Qasim sandstones have been subjected to shallow burial, probably ≤2 km.

Most of the authigenic kaolinite occurs as a pore-filling cement, formed from alteration of feldspar and mica grains at shallow burial depth related to meteoric flushing either during shallow burial, eodiagenesis, or after uplift. Kaolinite is engulfed by goethite and hematite (Figure 14F), and therefore, they are interpreted to have formed during telodiagenesis related to uplift.

Magmatic intrusions can have impacts on the petrophysical properties of sandstone reservoirs and on the thermal maturation of hydrocarbon source rocks (Karlsen et al., 1998; Ochoa et al., 2007; Grove et al., 2017; Haile et al., 2019; Duffy et al., 2021; Rong et al., 2021). Geochemical reactions are often employed to understand the influence of magmatic systems in sedimentary basins characterized by anomalously high paleotemperatures compared to conventional burial history models (Ochoa et al., 2007). In this study, however, basaltic sill-induced diagenetic processes can be differentiated from normal diagenetic processes. This is because the Qasim sandstones in Tabuk region were only at shallow burial before their uplift (Laboun, 2010), and the diagenetic signatures in the studied sandstones indicate only shallow burial processes, except for those affected by quartz dissolution and conversion of kaolinite into dickite and chlorite.

The establishment of paragenetic sequence is crucial for understanding the sequence of diagenetic events that affect sandstone reservoirs, which can significantly improve their reservoir quality prediction. In the studied sandstones, the formation of calcite, pyrite, and kaolinite cements and mechanical compaction are considered as the earliest diagenetic processes that affected the sandstones (Figure 16; Figure 17), which occurred during eodiagenesis and presumably at a temperature of <70 and burial depth of <2 km. Early calcite occurs in sediments with loose packing and floating grain contacts (Figure 13A) and often occurs as poikilotopic cement (Figure 13B). The origin of the early, poikilotopic calcite in marine sandstones has been linked to recrystallization from skeletal debris, which serve as substrates and nucleation sites for calcite cement to precipitate (Hakimi et al., 2012). Framboidal pyrite forms from microbial sulfate reduction near sediment-water interface. However, the framboidal pyrite in the Qasim sandstones has probably been completely oxidized and replaced by rounded hematite during the uplift-related diagenesis (Figure 12C &17) (Grove et al., 2017). Eogenetic kaolinite occurs around partly or pervasively dissolved muscovite grains (Figures 9A, B; Figure 17), and was formed probably due to meteoric flushing during sea-level low-stands and progradation of shallow marine sands (Morad et al., 2000; Ketzer et al., 2003). Eogenetic kaolinite engulfs and, thus, postdates the early diagenetic calcite (Figure 13C).

Telogenetic alterations related to uplift in the studied sandstones include the dissolution of early calcite formation of goethite, kaolinite, goethite, and hematite. Early calcite in the studied Qasim sandstones has undergone dissolution (Figure 14E), presumably due to the influx of acidic, meteoric fluid during the uplift, creating secondary intergranular porosity within the calcite (Figure 14E). However, it is unclear whether the dissolution of the early calcite was related to percolation of CO₂-rich fluid from the magmatic source. Additionally, authigenic kaolinite continued to form during the uplift probably due to meteoric flushing or the influx of CO₂-rich fluid from magmatic intrusions. This is corroborated by the presence of kaolinite, which has transformed into dickite and chlorite, within the dissolved detrital quartz grains (Figure 14C). Goethite and hematite are important Fe-rich minerals that form under oxidizing conditions (Berner, 1969; Chan et al., 2000; Busigny and Dauphas, 2007). The mixing of Fe-rich fluid, probably during the uplift, with oxidizing groundwater has resulted in the precipitation of goethite and hematite (Chan et al., 2000; Busigny and Dauphas, 2007). However, in the analyzed sandstones, rounded hematite was observed to engulf goethite, indicating that it postdates the goethite (Figure 14F). This is probably because the hematite was formed from the dehydration of goethite, as the latter is chemically unstable relative to the former (Berner, 1969).

The formation and distribution of magmatic-related diagenetic minerals within pore networks is strongly influenced by the type of diagenetic minerals prior to the magmatic intrusion (Mckinley et al., 2001). Therefore, understanding the “normal” diagenetic mineralogy is crucial to understanding the heating effects on the sandstones. The earliest diagenetic mineral related to the heating effect on the sandstone was dickite (Figure 14C; Figure 17). The dickite engulfs crystals of kaolinite, indicating that it was formed after the formation of kaolinite. As stated above, petrographic data and SEM analysis suggest that the sandstones have experienced shallow burial diagenesis, with depths and temperatures too shallow and low, respectively, to form dickite through normal diagenesis. While pseudomorphic chlorite, which replaces kaolinite, grows between dickite crystals (Figures 10B,C), rosette chlorite engulfs dickite (Figure 10E), suggesting that the conversions of kaolinite into chlorite and dickite occurred at the same or similar time.

The injection of magmatic fluids into cooler sandstone reservoirs can have significant impacts on porosity (Einsele et al., 1980; Senger et al., 2014; Sydnes et al., 2019b; Haile et al., 2019). The extent of magmatic sill-induced diagenetic alterations in sandstones largely depends on their proximity to the sill, the thickness of the sill, the depth at which the sill intrusions occur, and the clusters of the sill (Aarnes et al., 2011; Sydnes et al., 2019b; 2019a; Chen et al., 2021). Previous studies on the role of magmatic intrusions on sandstones porosity evolution and diagenesis have come to contrasting conclusions of both positive and negative impacts on porosity (Feng and Tang, 1997; Mckinley et al., 2001; Grove et al., 2017; Chen et al., 2021). In the present study, intergranular porosity decreases away from the magmatic sill contact (Figure 18A). This can be attributed to three factors: (1) primary depositional controls (e.g., grain size and sorting); (2) diagenetic controls; and (3) impact of magmatic intrusions. Sandstones with coarser grain size tend to have better sorting, and porosity increases with increasing grain size and better sorting (Figures 18B,C). Intergranular porosity increases with increasing volume of grain-coating Fe-oxide (Figure 18B). However, it could not be established whether or not the grain-coating Fe-oxide cements have inhibited quartz cementation. This is mainly because the quartz overgrowths might have been predominantly inherited, as they are well rounded (Figures 11A, B).

Additionally, pore-filling kaolinite is one of the major cements that has negatively affected the intergranular porosity of the sandstones (Figure 16; Figure 17; Figure 18E), and kaolinite increases in volume away from the sill contact (Figure 15A). This is because most of the kaolinite is associated with muscovite grains. Due to their grain shape, low density, and hydrodynamic processes, mica grains (e.g., muscovite) are deposited in low-energy environments (e.g., offshore) (Mansurbeg et al., 2008; Marchand et al., 2015; Bello et al., 2021). Thus, kaolinite could form in sandstones in close proximity to offshore environment, resulting in the occlusion of intergranular porosity if meteoric flushing occurred. Additionally, XRD data show that dickite increases away from the sill contact (Figure 4). This is probably because: 1) the dickite is strongly associated with muscovite grains and kaolinite; and 2) there could be another heat source below the studied outcrops (i.e., sill or dike) (Figure 17), which might have supplied sufficient heat for the conversion of kaolinite into dickite. Based on XRD, SEM, and QEMSCAN analyses, chlorite was only observed in few sandstone samples and, therefore, its occurrence is localized. As a result, it might have less impact on the intergranular porosity of the sandstones. Pore-filling opaque minerals, mainly consisting of goethite and hematite, have negatively impacted the intergranular porosity of the sandstones (Figure 18F), and their volume percent increases away from the sill contact (Figure 15D). This is presumably because of the close proximity with Fe-rich oxidizing groundwater that facilitate their formation.

The values for compactional porosity loss have relatively remained similar both at and away from the sill contact (Figure 15E). This is presumably because the sill intrusions are relatively thin (0.4–4 m) and, thus, could not exert enough vertical loading on the sandstones to cause significant compaction. However, cementational porosity loss decreases outwards from the sill contact, probably because the acidic fluids supplied by the intrusions have caused the dissolution of early calcite, creating secondary intergranular porosity (Figure 14E). In addition, the increase in cementational porosity loss away from the contact has been contributed by the increasing kaolinite and dickite from muscovite, due to its density and hydrodynamic processes. Overall, porosity is reduced mainly due to compaction rather than cementation.