Facies-Controlled Sedimentary Distribution and Hydrocarbon Control of Lower Cretaceous Source Rocks in the Northern Persian Gulf

Abstract

The two-phase source rocks deposited during the Lower Cretaceous in the Persian Gulf Basin play a pivotal role in the regional hydrocarbon system. However, previous studies have lacked a macroscopic perspective constrained by the Tethyan Ocean context, which has limited a deeper understanding of their developmental patterns and hydrocarbon control mechanisms. To address this issue, this study aims to clarify the spatiotemporal evolution of the two-phase source rocks and their hydrocarbon control effects, with a particular emphasis on the critical impact of terrestrial input on the quality improvement of source rocks. Unlike previous studies that relied on a single research method, this study employed a comprehensive approach, including time series analysis, sequence stratigraphy, lithofacies, well logging, well correlation, seismic data, and geochemical analysis, to systematically compare and analyze the depositional periods, distribution, and characteristics of the two-phase source rocks under different sedimentary facies in the region. The goal was to reveal the intrinsic relationship between the Neo-Tethyan Ocean context and regional sedimentary responses. The results indicate the following: (1) the late Tithonian–Berriasian and Aptian–Albian source rocks in the Northern Persian Gulf were deposited during periods of extensive marine transgression, closely aligning with the global Weissert and OAE1d anoxic events, reflecting the profound impact of global environmental changes on regional sedimentary processes; (2) in the early stages of the Neo-Tethyan Ocean, controlled by residual topography, the Late Tithonian–Berriasian source rocks exhibited a shelf–intrashelf basin facies association, with the intrashelf basin showing higher TOC, lower HI, and higher Ro values compared to the deep shelf facies, indicating more favorable conditions for organic matter enrichment; (3) with the opening and deepening of the Neo-Tethyan Ocean, the Aptian–Albian source rocks at the end of the Lower Cretaceous transitioned to a shelf–basin facies association, with the basin facies showing superior organic matter characteristics compared to the shelf facies; (4) the organic matter content, type, and thermal maturity of the two-phase source rocks are primarily controlled by sedimentary facies and terrestrial input, with the Aptian–Albian source rocks in areas with terrestrial input showing significantly better quality than those without, confirming the decisive role of terrestrial input in improving source rock quality. In summary, this study not only reveals the differences in the depositional environments and hydrocarbon control mechanisms of the two-phase source rocks, but also highlights the core role of terrestrial input in enhancing source rock quality. The findings provide a basis for facies selection in deep natural gas exploration in the Zagros Belt and shale oil exploration in the western Rub’ al-Khali Basin, offering systematic theoretical guidance and practical insights for hydrocarbon exploration in the Persian Gulf and broader tectonic domains.

1. Introduction

The Persian Gulf, located in the western Asian segment of the Tethyan domain, is part of the Arabian Plate. It comprehensively records the entire process of the opening and evolution of the Proto-Tethyan Ocean, Paleo-Tethyan Ocean, and Neo-Tethyan Ocean [1]. This region not only formed the Paleo-Tethyan hot shales, but also developed Lower Cretaceous source rocks under the evolution of the Neo-Tethys Ocean [2]. During this evolutionary process, not only were the hot shales of the Paleo-Tethyan Ocean period formed, but high-quality Lower Cretaceous source rocks were also developed under the evolution of the Neo-Tethyan Ocean [3]. The multiple marine transgression events of the Tethyan Ocean (e.g., the Silurian hot shales, influenced by global large-scale transgressions, exhibit macroscopically uniform distribution characteristics) have resulted in significant spatiotemporal evolutionary features of the source rocks within the region [2,4]. During the opening stage of the Neo-Tethyan Ocean, the Arabian region experienced multiple marine transgressions in the Cretaceous, leading to the formation of two phases of source rocks: the Late Tithonian–Berriasian and Aptian–Albian of the Lower Cretaceous.

These two phases of source rocks are widely distributed in the Northern Persian Gulf, characterized by high organic matter abundance, and serve as the core source rocks of the regional hydrocarbon system. Previous studies have conducted extensive research on them, indicating that the sedimentary facies and tectonic background of the Early Cretaceous source rocks in the Northern Persian Gulf are crucial factors controlling the preservation of organic matter, the evolution of maturity, and subsequent hydrocarbon generation [4,5,6,7,8,9,10]. Meanwhile, numerous scholars have demonstrated the influence of sedimentary environments on organic matter types and maturity through organic geochemical parameters, providing a quantitative basis for the evaluation of source rocks [11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29].

Although previous studies have conducted detailed localized research on the two phases of source rocks (Late Tithonian–Berriasian and Aptian–Albian) in the Persian Gulf region, such as in the East Baghdad, Darquain, Nasiriya, Dukhan, and Halfaya oil fields, there remains a lack of comprehensive understanding regarding the regional differences in the depositional distribution and hydrocarbon generation characteristics of these two phases of source rocks during the evolution of the Neo-Tethyan Ocean [11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29]. Existing literature primarily focuses on localized sedimentary facies and hydrocarbon exploration case studies, lacking a systematic explanation of the synergistic effects of transgression events, terrestrial input, and sedimentary facies belts on the formation and spatial distribution of source rocks within the overall evolutionary framework of the Neo-Tethyan Ocean.

To address the aforementioned issues, this study built on previous measured data and employed comprehensive methods such as high-resolution sequence stratigraphy, lithofacies analysis, and biofacies assemblage modeling. Unlike previous research that focused solely on localized oil fields, this study constructed a sequence model encompassing the main sedimentary facies belts within the region, enabling a multi-scale, multi-method-integrated analysis from local to regional scales. The aim was to systematically explore the sedimentary characteristics and hydrocarbon generation patterns of the two phases of source rocks during the evolution of the Neo-Tethyan Ocean in the Persian Gulf region and to reveal the main controlling factors influencing the quality and distribution of source rocks. The findings can provide a basis for facies selection in deep natural gas exploration in the eastern margin of the Arabian Plate (Zagros Belt) and shale oil exploration in the western margin (Rub’ al-Khali Basin), thereby offering systematic theoretical guidance and practical insights for hydrocarbon exploration in the Persian Gulf and broader tectonic domains. Based on this, the following hypotheses are proposed:

(1)

The two phases of source rocks in the Persian Gulf region exhibit significant differences in depositional environments, lithofacies types, and associations;

(2)

Multiple transgression events during the evolution of the Neo-Tethyan Ocean played a decisive role in the organic matter enrichment and spatial distribution of the source rocks;

(3)

The input of terrestrial materials and the synergistic effects of sedimentary facies belts jointly control the hydrocarbon generation potential of the source rocks in the region.

2. Geological Background

The Persian Gulf Basin is located at the convergence of the Arabian Plate and the Eurasian Plate, situated at the edge of the ancient Neo-Tethys Ocean. From the source area to the depositional area, it can be divided into three major geological units: the stable Arabian Shield in the west, the stable Arabian Platform in the center, and the Zagros Foreland Deformation Zone in the east [29] (Figure 1a). In addition, the northern passive marginal basin consists of the Western Arabian Basin, the Widyan-Mesopotamian Basin, the Central Arabian Basin, and the Rub’ al-Khali Basin [29]. The northward unidirectional subduction of the Neo-Tethyan Ocean and multiple phases of continental block collisions led to the formation of basement rift zones, passive continental margins, and foreland basins. These tectonic processes controlled the evolution of sedimentary environments and laid the foundation for the subsequent development of source rocks [2,29]. The basin is filled with sediments ranging from the Paleozoic to the Cenozoic, with the Paleozoic dominated by clastic rocks and the Mesozoic to Cenozoic primarily composed of marine carbonate rocks. During this evolutionary process, two critical phases of source rock deposition were formed:

(1)

Late Jurassic–Early Cretaceous (Late Tithonian–Berriasian, approximately 129–140 Ma).

During this period, the Sarmord and Chia Gara formations correspond to the Diyabakir and Kirkuk regions, the Garau and Yamama formations to the Lorestan region, and the lower Fahliyan Formation to the Khuzestan and Fars regions. The lithology is dominated by micritic limestone and calcareous shale, with GR curves showing high-amplitude serrated patterns, reflecting anoxic-high productivity environments in intrashelf basins. The back-arc extension during the initial expansion of the Neo-Tethyan Ocean (~145 Ma) caused differential subsidence along the northern margin of the Arabian Plate, forming multiple isolated intrashelf basins that provided closed conditions for organic matter preservation (average TOC of 4–5%). The early stage featured Pterocera from the Late Jurassic, while the late stage included Watznaueria barnesae and Haqius circumradius from the Berriasian [12], which contributed abundant organic matter for the formation of source rocks.

(2)

Late–Early Cretaceous (Aptian–Albian, approximately 90–110 Ma)

During this stage, sedimentation was concentrated in the Mesopotamian Foredeep (Nahr Umr Formation) and the Fars Platform margin. The lithofacies association is characterized by deep-water slope facies, including argillaceous sandstone and siliceous nodular limestone. Bioturbation structures and pyrite enrichment indicate a bathyal reducing environment. Key biomarkers include Biticinella berggensis [8] and diverse ammonoid fauna [14], which provided the material basis for the formation of high-quality source rocks and reservoirs. The organic matter is predominantly Type II kerogen, with TOC reaching up to 12%.

The unidirectional subduction of the Neo-Tethyan Ocean and multiple phases of continental block collisions regulated the formation of basement rift zones and basin types. By controlling sediment supply and depositional environments, these tectonic processes influenced the lithology and sequence evolution of the two depositional stages. The transgression events triggered by tectonic movements altered sedimentary facies and material input, leading to significant differences in lithology and sequences between the Late Tithonian–Berriasian and Aptian–Albian stages. These differences in depositional environments and lithology are directly reflected in the organic matter enrichment and spatial distribution of the source rocks, providing critical insights for regional hydrocarbon generation and exploration.

3. Materials and Methods

This study adopted the official units of the International Commission on Stratigraphy (ICS), dividing the Cretaceous into two series and twelve stages. The target intervals were located in the Early Cretaceous Late Tithonian–Berriasian and Aptian–Albian stages. Based on the paleogeographic reconstructions by Christopher R. Scotese using the Mollweide projection of GIS software (ArcMap 10.8.2, ESRI, Redlands, CA, USA) [22] (Figure 1b) and the Arabian Plate evolution reconstructed by Niu Binbin et al. [31] (Figure 1c), the internal tectonic and depositional settings of the Arabian Plate during the unidirectional subduction of the Neo-Tethyan Ocean were clarified.

The data collection and organization for this study primarily include data from multiple research wells within the region and publicly available database resources. Sampling points involve wells such as D-4 and A-3, (Resform, Version 5.1) with well locations shown in Figure 1. Seismic data were independently acquired, and the profile locations are illustrated in Figure 1. Geochemical data were mainly sourced from English-language publications and the HIS database [1,8,11,12,14,15,17,18,19,22,23,24,25,26,27,28,32,33,34,35,36,37,38,39]. The data cover literature and datasets were published between 1990 and 2024, with selection criteria including complete sampling information, well-documented instrument calibration records, and peer-reviewed recognition (specific data ranges are provided in Supplementary Table S1). To ensure the scientific identification of depositional sequences and hydrocarbon-bearing boundaries, the following methods were employed: high-resolution stratigraphic analysis: Acycle (Version 2.7) was used to perform high-precision stratigraphic division of the Lower Cretaceous time series from Well D-4, with a detailed recording of the sampling and temporal parameters. Seismic data analysis: for seismic data from Well A-3 (Petrel, Version 2018), reflection amplitude, frequency variation, and continuity analysis were applied to identify seismic interfaces, flooding surfaces, and maximum flooding surfaces. Sequence stratigraphy and corresponding hydrocarbon-bearing boundaries were determined through comparative analysis. Lithofacies classification: based on well log descriptions, reflection internal structures, and well correlation profiles, lithofacies were classified for different depositional environments.

For the geochemical analysis of the two-phase source rocks in the region, the following steps were taken. (1). Data collection and parameter determination: data from well samples, including TOC, HI, OI, Tmax, Ro, and S1+S2, were collected. The measurement ranges for these parameters were as follows: TOC 0–4%, HI 50–600 mg/g, Tmax 420–500 °C, and Ro 0.6–3% (2). Data organization and chart generation: the collected source rock characteristic indicators were systematically organized based on different facies and lithologies. Charts reflecting depositional environments and source rock distribution patterns were generated, including C27–C29 ternary diagrams, HI-OI cross-plots, pristane-phytane cross-plots, as well as HI–Tmax, NA-TOC, and S2-TOC cross-plots to identify organic matter types, maturity, and quality. Through chart comparison, the differences in depositional environments and hydrocarbon control mechanisms between the Late Tithonian–Berriasian and Aptian–Albian stages were clarified.

This study ensured the completeness and reliability of data sources, providing a structured and reproducible experimental workflow for depositional sequence analysis, lithofacies classification, and source rock evaluation. Detailed information (e.g., sampling data, specific parameter ranges, depositional sequence identification, and lithofacies classification criteria) offers a solid foundation for subsequent research, enhancing the rigor of the methodology and the reproducibility of the experiments.

4. Results and Discussion

4.1. Climate Evolution and Determination of Hydrocarbon Generation Periods Following the Formation of the Neo-Tethys Ocean

During the Lower Cretaceous, the Neo-Tethys Ocean was in a continuous subduction phase beneath the Eurasian Plate [40], leading to the rapid growth of oceanic crust. An intra-oceanic subduction zone developed within the Arabian Plate, accompanied by intensified magmatic activity and extensive volcanic eruptions. The CO₂ released from these eruptions contributed to a greenhouse effect [41], resulting in higher sea surface temperatures. The warm and humid conditions during this greenhouse period promoted biodiversity [42]. By identifying and tracing sequences and flooding surfaces on seismic interfaces from Well A-3, the hydrocarbon generation environments of the two source rock intervals were further clarified. Both the Aptian–Albian and Late Tithonian–Berriasian source rocks were deposited during periods of sea-level rise, corresponding to the regressive cycles of third-order sequences (Figure 2). Onlap and the most distal onlap points were observed within the source rock intervals, while downlap was evident in the overlying strata. Additionally, anomalous shifts in δ¹³C [31,33,40] (Figure 2) indicate significant carbon burial events associated with rapid sea-level rise [33], suggesting that both source rock intervals were deposited during extensive marine transgressions, providing a favorable foundation for the formation of source rocks (Figure 2).

The deposition of source rocks in the study area was influenced by various astronomical orbital parameters. This study utilized Acycle to analyze the time series of the Cretaceous in Well D-4, identifying two favorable source rock intervals in the north of the Persian Gulf during the Lower Cretaceous: the Late Tithonian–Berriasian stage (129–140 Ma) and the Aptian–Albian stage (approximately 97 Ma). These intervals exhibited good synchronicity with the Weissert anoxic event and the OAE1d anoxic event of the Cretaceous, consistent with previous studies [26,43,44] (Figure 3). During anoxic events, the bottom water became oxygen-depleted, while the surface water exhibited high productivity, reflecting the climatic evolution of the Lower Cretaceous and establishing a strong correlation with sea-level changes. This provides robust evidence for the evolutionary background of the source rocks and highlights the contribution of anoxic events to their formation.

4.2. Evolution of the Neo-Tethys Ocean and Paleogeographic Filling Patterns

During the Late Tithonian–Berriasian stage, the Neo-Tethys oceanic crust subducted in a northeastward direction beneath the Eurasian Plate, resulting in significant differential subsidence in the Mesopotamian Basin of Iraq and southwestern Iran [47]. This subsidence led to the separation of the Arabian Plate from the Taurides and Batlis microcontinental blocks. These microblocks, located in the northeastern part of the Arabian Plate, were tectonically connected to the western block of the Arabian Plate, which inherited the Triassic Rutba Uplift and Mosul High [47]. In eastern Oman, a narrow deep-sea zone filled with clastic sediments developed, transitioning westward into a narrower intermediate zone, and further west into the Mesopotamian Basin, which was characterized by a broad shallow-water carbonate depositional area. The Arabian Plate exhibited relatively confined tectonic features, forming a differential subsidence pattern with a restricted western region and a broad, gentle eastern region. This differential subsidence pattern dominated the regional depositional framework, particularly influencing the depositional centers of the Late Tithonian–Berriasian stage. During this period, three main depositional centers were distributed, showing distinct east–west isolation in space, with depositional environments including shallow shelf, deep shelf, and intrashelf basin facies (Figure 4a), indicating significant spatial variations in depositional environments. In the restricted western environment, the development of intrashelf basins was prominent, primarily filled with argillaceous limestone. The well correlation profile from Well B-1 to Well B-4 (Figure 4g) mainly includes deep shelf and intrashelf basin facies, where the deep shelf lithology is dominated by packstone (Figure 4b,c), and the intrashelf basin is characterized by mudstone (Figure 4d,e). The well correlation profile from Well D-1 to Well D-6 (Figure 4f) includes shallow shelf, deep shelf, and intrashelf basin facies. The shelf lithology is rich in dolomite, indicating a warm and humid shallow marine environment. Based on the E–F seismic profile (Figure 4h), the Late Tithonian–Berriasian shelf environment is characterized by low-frequency, weak-amplitude, and poorly continuous high-energy conditions, with downlap phenomena observed in the overlying strata. In the G–H seismic profile (Figure 4i), the Late Tithonian–Berriasian intrashelf basin is characterized by medium-frequency, medium-amplitude, and well-continuous low-energy conditions.

During the late Lower Cretaceous Aptian–Albian stage, the continental margin of the Arabian Plate experienced collapse, manifested by the southwestward retreat of the Rutba Uplift and Mosul High. From the perspective of tectonic evolution, the changes in the depositional framework during the late Lower Cretaceous were closely related to the retreat and subsidence of the Arabian Plate’s continental margin. The increase in coarse clastics from the Arabian Shield caused the shallow-water carbonate depositional area to shift eastward, while the migration of shelf carbonate depositional areas further offshore indicated an increased supply of terrigenous clastics. The shallow-water carbonate depositional area expanded, while the extent of coastal plain deposits decreased. This tectonic event led to a transformation in the overall depositional framework of the Arabian Plate, resulting in a broader and more uniform depositional environment, with more orderly distribution of depositional centers. The depositional centers of the Aptian–Albian source rocks exhibited a uniform distribution, with diverse sedimentary facies, including the western delta-influenced shelf–basin facies and the eastern non-influenced shelf–basin facies, primarily filled with mudstone and shale (Figure 5b–e). Based on lithofacies, the shelf area was dominated by argillaceous limestone and mudstone (Figure 5b,c), while the basin area was mainly composed of mudstone (Figure 5d,e). The well correlation profile from Well A-1 to Well A-8 (Figure 5f) includes shallow marine shelf and deep marine shelf facies, with the shallow marine shelf primarily consisting of thick sandstone sequences and the deep marine shelf dominated by argillaceous limestone. The well correlation profile from Well D-1 to Well D-5 (Figure 5g) is mainly composed of basin facies, with lithofacies dominated by mudstone and shale (Figure 5). These changes indicate that the depositional environment of the Arabian Plate during the late Lower Cretaceous transitioned from a restricted depositional pattern to a more uniform and widespread depositional pattern.

In summary, the formation of the depositional centers during the Late Tithonian–Berriasian and Aptian–Albian stages is closely related to the differential subsidence, marine transgressions, and tectonic evolution of the Arabian Plate. The depositional environment in the region gradually transitioned from a restricted western depositional setting to a more uniform depositional pattern.

4.3. Distribution and Characteristics of Source Rocks

In a sedimentary basin, only effective source rocks can provide commercial hydrocarbon accumulations. To be considered effective, source rocks must contain sufficient organic matter, have favorable organic matter types, and undergo the process of organic matter transformation into hydrocarbons [49]. This study used total organic carbon (TOC) to describe the quantity of organic matter. For poor, fair, good, very good, and excellent source rocks, the TOC values are 0–0.5, 0.5–1, 1–2, 2–4, and greater than 4, respectively [50]. The hydrogen index (HI) was used to describe the type of kerogen. For Type III kerogen, HI ranges between 50 and 200, indicating a gas-prone nature and a marine depositional environment. For Type II kerogen, HI ranges between 200 and 600, indicating good oil and gas generation potential and a transitional marine–terrestrial depositional environment. For Type I kerogen, HI is greater than 600, indicating an oil-prone nature and a terrestrial depositional environment [50]. The vitrinite reflectance (Ro) is used to describe the thermal maturity of organic matter. Ro values between 0.6 and 1 indicate the oil generation window, values between 1 and 1.35 indicate the wet gas window, and values between 1.35 and 3 indicate the dry gas window.

The kerogen types in the Late Tithonian–Berriasian stage are mainly Type II and Type III. Comparing the organic matter abundance, kerogen type, and maturity indicators of the source rocks, mudstones showed relatively higher values compared to marlstones (Table S1). For Late Tithonian–Berriasian mudstones, the lowest TOC value was found in the Azadegan oilfield at 0.9%, with an HI of 157.18 mg/g. The highest TOC value was found in the Gashun oilfield at 19.16%, with an HI of 537.05 mg/g. For Late Tithonian–Berriasian marlstones, the lowest TOC value was found in the Ab-e Zimkan oilfield in the Lorestan segment at 0.19%, with an HI of 159.10 mg/g and Tmax of 418 °C. The highest TOC value was found in the Kushk oilfield in the Khuzestan segment at 4.52%, with an HI of 89.29 mg/g and Tmax of 441.57 °C. Mudstones were mainly developed in intrashelf basins, while marlstones were mainly developed on shelves.

For the Aptian–Albian source rocks, marlstones and mudstones were predominant. Mudstones were mainly developed in basins, while marlstones were mainly developed on shelves. Mudstones showed relatively higher values compared to marlstones. For Aptian–Albian mudstones, the lowest TOC value was found in the Mand oilfield at 0.09, with an HI of 377. The highest TOC value was found in the Marun oilfield at 4.99, with an HI of 323.24. For Aptian–Albian marlstones, the lowest TOC value was found in the Kuh-e Khaki oilfield at 0.19, with an HI of 355. The highest TOC value was found in the Farur oilfield at 2.03, with an HI of 158. Evaluation indicators of source rocks during the Late Tithonian–Berriasian and Aptian–Albian are hydrocarbon generation periods (see Supplementary Material Table S1).

During the Late Tithonian–Berriasian stage, with the opening of the Neo-Tethys Ocean, the study area lacked terrigenous input, and the depositional differentiation was influenced by residual paleo-highs, resulting in a suite of shelf–intrashelf basin source rocks. By the late Lower Cretaceous Aptian–Albian stage, water depth increased, and paleo-continents retreated, leading to a broad and uniform depositional environment in the study area. The input from the western delta caused depositional differentiation, resulting in a suite of shelf–basin source rocks, with terrigenous-influenced source rocks in the west and non-terrigenous-influenced source rocks in the east.

The distribution maps of Late Tithonian–Berriasian source rock indicators show that high TOC values were mainly distributed in intrashelf basins, with maximum values exceeding 4, and three high-value evolution centers. Shallow and deep shelves were dominated by TOC values of 0–1. High HI values were mainly distributed in intrashelf basins and deep shelves, with two thermal evolution centers. Intrashelf basins had lower HI values than deep shelves, indicating a marine environment. Ro values ranged from 0.4 to 1.0, indicating that most of the source rocks were within the oil generation window, with higher maturity in intrashelf basins, reflecting the contribution of burial depth to their thermal evolution (Figure 6a).

The distribution maps of Aptian–Albian source rock indicators show that high TOC values are mainly distributed in basin facies, with maximum values exceeding 4. Shelf facies had TOC values of 2.0–3.0, indicating lower organic matter content compared to basin facies. High HI values were mainly distributed in shelf facies, with a maximum value of 400, indicating a transitional marine-terrestrial environment. Ro values ranged from 0.4 to 1.0, indicating that most of the source rocks were within the oil generation window, with higher maturity in basin facies, reflecting the contribution of burial depth to their thermal evolution (Figure 6b). The distribution of TOC, HI, and Ro in Late Tithonian–Berriasian and Aptian–Albian source rocks indicates facies-controlled constraints.

The C²⁷, C²⁸, and C²⁹ sterane ternary diagram, pristane–phytane cross-plot, and HI-OI cross-plot indicate a deepening of the water environment from the Late Tithonian–Berriasian shelf–intrashelf basin to the Aptian–Albian shelf–basin (Figure 7).

4.4. Hydrocarbon Control Mechanisms

The hydrocarbon source rocks of the Late Tithonian–Berriasian stage were primarily distributed in intrashelf basins and on the shelf (Figure 8). The HI-Tmax cross-plot indicates that samples from the intrashelf basin facies exhibited a rich variety of organic matter types (Figure 8a), including Type I, II, and III, with the overall organic matter entering the oil window, predominantly in the mature to highly mature stage, and a small portion in the overmature stage. In contrast, samples from the shelf facies showed poorer organic matter types (Figure 8b), mainly Type II and III, with lower thermal evolution. Although they have entered the oil window, they were primarily in the mature stage.

The TOC-NA cross-plot reveals that samples from the intrashelf basin facies were generally of moderate-to-good quality (Figure 8c), with some being excellent. Samples from the shelf sedimentary facies ranged from poor to good (Figure 8d). In terms of organic matter type, maturity, and source rock quality, the intrashelf basin facies were superior to the shelf sedimentary facies, indicating that sedimentary environments significantly control the organic matter type, maturity, and quality of hydrocarbon source rocks.

The hydrocarbon source rocks of the Aptian–Albian stage were mainly distributed in areas with and without terrigenous input (Figure 8). The HI-Tmax cross-plot shows that samples from areas with terrigenous input had a diverse range of organic matter types (Figure 8f), including Type I, II, and III, with the organic matter generally entering the oil window, ranging from immature to mature stages. In contrast, samples from areas without terrigenous input were dominated by Type II or III organic matter, with thermal evolution primarily in the immature stage and a small portion reaching the mature stage (Figure 8e).

The S2-TOC cross-plot indicates that source rocks in terrigenous input areas were generally of moderate-to-very good quality, with a uniform distribution (Figure 8g). In contrast, source rocks in areas without terrigenous input ranged from poor to good, with a small portion being good (Figure 8h).

The HI-Tmax cross-plot further indicates that samples from the Aptian–Albian stage terrigenous input areas had richer organic matter types and higher thermal evolution compared to those from the Late Tithonian–Berriasian intrashelf basin facies. Additionally, the organic matter content in the Aptian–Albian stage source rocks was generally higher than that in the Late Tithonian–Berriasian stage, highlighting the significant contribution of terrigenous input to the quality and characteristics of hydrocarbon source rocks.

The hydrocarbon source rocks of the Late Tithonian–Berriasian and Aptian–Albian stages represent significant periods of source rock development in the Neo-Tethys Ocean. The formation of these source rocks primarily relied on favorable conditions in the shelf environment during extensive marine transgressions. Shelf areas are typically shallow marine environments with moderate water depths, providing excellent conditions for organic matter accumulation. During the formation of the Late Tithonian–Berriasian and Aptian–Albian source rocks, widespread marine transgressions created relatively stable marine conditions, facilitating the enrichment of organic matter. Additionally, the supply of nutrients from surface ocean waters promoted biological productivity, further enhancing the production of organic matter.

In the process of source rock formation, terrigenous input significantly improved the quality and distribution of the source rocks. In particular, under the conditions of upwelling seawater and continuous replenishment of sedimentary organic matter, both the quality and thickness of the source rocks were enhanced (Figure 9).

5. Conclusions

Through time series analysis and high-resolution sequence stratigraphy, this study established the synchronicity of the two-phase source rocks (Late Tithonian–Berriasian and Aptian–Albian) in the northern Persian Gulf during the Early Cretaceous with global anoxic events (Weissert Event and OAE1d), demonstrating the significant contribution of widespread marine transgressions to the formation and organic matter enrichment of source rocks.

The research indicates that during the early stages of the Neo-Tethyan Ocean, the influence of residual topography led to the development of source rocks within the shelf–intrashelf basin facies association during the Middle–Late Jurassic to Early Cretaceous, showing a higher TOC, lower hydrogen index (HI), and higher maturity (Ro). As the Neo-Tethyan Ocean opened and water depths increased, the Aptian–Albian source rocks at the end of the Early Cretaceous primarily formed in the shelf–basin facies association, with the basin facies exhibiting higher organic matter abundance and maturity compared to the shelf facies.

Furthermore, this study reveals the following:

(1)

The organic matter content, type, and thermal evolution of the Late Tithonian–Berriasian source rocks are mainly controlled by sedimentary facies, with the intrashelf basin facies outperforming the shelf facies;

(2)

The characteristics of the Aptian–Albian source rocks are primarily regulated by terrestrial input, with the quality of basin source rocks in terrestrial input areas being significantly better than those in non-terrestrial input areas and contemporaneous shelf source rocks, highlighting the critical role of terrestrial input in improving source rock quality.

In summary, this study not only supplements the understanding of the depositional control mechanisms and organic matter evolution of the two-phase source rocks in the northern Persian Gulf, but also provides systematic theoretical and practical guidance for regional hydrocarbon exploration. Future research could further explore the dynamic evolution of terrestrial and marine influences within different tectonic units through multi-scale geochemical and sedimentary modeling methods, aiming to provide more refined predictions for source rock accumulation in broader tectonic domains.