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Article

The Facies Analysis, Evolution, and Coal-Bearing Source Rock Features of the Middle–Late Triassic Shallow-Water Delta in the North Carnarvon Basin, Northwest Shelf of Australia

by
Zhiwei Zeng
1,2,3,*,
Wei Wang
2,3,*,
Hongtao Zhu
2,3,
Xianghua Yang
2,3 and
Dan Li
4
1
School of Geophysics and Geomatics, China University of Geosciences, Wuhan 430074, China
2
School of Earth Resources, China University of Geosciences, Wuhan 430074, China
3
Key Laboratory of Tectonics and Petroleum Resources, China University of Geosciences, Wuhan 430074, China
4
China National Offshore Oil Corporation Research Institute, Beijing 100028, China
*
Authors to whom correspondence should be addressed.
Energies 2023, 16(5), 2265; https://doi.org/10.3390/en16052265
Submission received: 21 January 2023 / Revised: 17 February 2023 / Accepted: 25 February 2023 / Published: 27 February 2023
(This article belongs to the Special Issue Natural Gas Hydrate and Deep-Water Hydrocarbon Exploration)
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Versions Notes

Abstract

:
The sedimentary facies, architecture, and depositional mechanism of deltaic systems have been one of the global research hotspots in recent decades; however, the detailed distribution, sedimentary evolution, source rock potential, and major control factors of the coal-bearing shallow-water delta are still unclear. A typical shallow-water braided delta with coal-bearing source rocks developed in the Middle–Late Triassic Mungaroo Formation of the North Carnarvon Basin, which can be a good study area for an analysis of coal-bearing source rocks. In this study, the sedimentary facies, distribution and evolution, and coal-bearing source rock features of the Triassic strata were analyzed based on the integrated study of wireline logs, drilled cores, thin sections, seismic facies and attributes, and geochemical data. A range of shallow-water delta sedimentary facies was identified, including the proximal delta plain channel/interdistributary bay, distal delta plain channel/interdistributary bay, and the delta front. The coal-bearing shallow-water delta system of the Middle–Late Triassic Mungaroo Formation was characterized by the largest scale delta system with relatively broad proximal and distal delta plains and relatively narrow delta front subfacies. The scale of the delta system showed a trend of increasing from the Early Triassic Locker Shale to the Middle–Late Triassic Mungaroo Formation and then decreasing to the Late Triassic Brigadier Formation. The distal delta plain subfacies of the Mungaroo Formation should have the highest potential coal-bearing source rock, and the proximal delta plain also can be a favorable target for source rock evaluation. The major control factors of the coal-bearing source rocks of the Mungaroo shallow-water delta mainly included the Triassic megamonsoon climate, the topographic features, eustatic changes, and provenance supply. The proximal and distal delta plains of the shallow-water delta system with thin coal seams, carbonaceous mudstone, and dark mudstone lithologies’ association could be a favorable source rock exploration facies for the next stage of natural gas field exploration.

1. Introduction

The sedimentary facies, stratal architecture, depositional evolution, and mechanism of modern and ancient deltaic deposits have been one of the global research hotspots in recent decades, including the continental shelf marginal delta system [1,2,3,4,5,6,7,8,9,10] and lake basin delta system [11,12,13,14,15,16]. The shallow-water delta is a special type of deltaic system, which was first proposed by Fisk et al. (1954) [17] with a relatively gentle slope, a broad and shallow water environment or low accommodation, a stable tectonic background, and an abundant sediment supply [17,18,19,20], based on the outcrop analysis and seismic-based interpretation [8,11,12,21,22,23,24]. Shallow-water deltas obviously differ from the Gilbert deltas [25] and are generally characterized by a different scale of braided distributary channels with a large-scale delta plain and a small range delta front [8,12,26]. The delta area in the dry period differs greatly from that in the flood period [11,12]. Previous studies have shown that the topographic features [11], climatic conditions (humid, semiarid, or arid) [27,28,29,30], river inflow, and tectonic subsidence [21,31,32] would be the significant controlling factors for the sedimentary architecture and depositional process of the shallow-water delta.
Coal-bearing source rocks are widely developed in the coastal delta environment, with many natural gas fields in the global Tethys region [33], such as the Northwest Shelf Basins of Australia [8,34], the Central Sumatra Basin of Indonesia [35], the Brunei–Sabah Basin of Malaysia [36], the East China Sea Shelf Basin [37], and the Yinggehai Basin [38] and Pearl River Mouth Basin [15,39] of the northern South China Sea. Generally, the coal-bearing source rocks are closely related with a coal seam and carbonaceous and dark mudstone with a high content of organic matter (rich with terrigenous higher plants) [37]. This special type of source rock shows a significant relationship with the natural gas exploration, and more than 70% of the gas fields in China were located in coal-related source rock [40]. However, the maceral composition in the coal-bearing source rocks of different delta subfacies or microfacies are obviously different, which could have a significant impact on the evaluation and prediction of the hydrocarbon generation potential. Meanwhile, the development characteristics, detailed distribution, sedimentary evolution, and major control factor of the favorable coal-bearing source rock of a shallow-water delta are still unclear. This might seriously restrict further exploration in the related oil and gas fields. Therefore, it is urgent to select a favorable research area to carry out these studies.
The Northwest Shelf (NWS) of Australia has become an important oil and gas exploration region in recent years [41,42,43]. The Middle–Late Triassic Mungaroo Formation of the large-scale and coal-bearing shallow-water delta in the North Carnarvon Basin (NCB) also has been a hot research target in this area, which was affected by the Pangaea megamonsoon [8,44], especially during the Carnian Pluvial Episode (CPE) [45]. The Carnian stage witnessed major changes in both the marine and terrestrial ecosystems and underwent a climate change event associated with global warming, the eruption of the Wrangellia large igneous province (LIP) [44], and the strong enhancement of the hydrological cycle [46]. Based on the previous studies [8,47], a typical shallow-water braided delta with coal-bearing source rocks was developed in the NCB during the Mungaroo Formation. The NCB has experienced many years of oil and gas exploration, with a large amount of drilling and 3D seismic data, which makes it a good study area to carry out the comprehensive analysis of the sedimentary facies, depositional evolution, and hydrocarbon generation potential of the coal-bearing source rocks of the shallow-water delta system.
Therefore, based on the integrated analysis of wireline logs, drilled cores, thin sections, seismic facies and attributes, and a series of geological and geochemical data in the NCB, the motivations of this research mainly include: (1) to characterize the stratal architectural features of various sedimentary subfacies and microfacies in the shallow-water delta system, especially during the Middle to early Late Triassic, (2) to analyze the source rock types, maceral compositions’ characteristics, and hydrocarbon generation potential of different delta subfacies, (3) to investigate the sedimentary evolution of the deltaic system from the Early Triassic to the Late Triassic and to establish a representative depositional model of the Mungaroo Formation, (4) to further discuss the major control factor of the development of coal-bearing source rock, and to predict the favorable source rock distribution in different Triassic sequences.

2. Geological Setting

The North Carnarvon Basin (NCB) is located at the southern part of the Northwest Shelf of Australia (Figure 1a); the basin is a giant oil and gas bearing basin formed by continuous rifting and subsidence from the Late Paleozoic to the Cenozoic [41,43]. Since the first oil field discovery in the NCB in 1954, more than fifty oil and gas fields have been put into hydrocarbon production, including the Jansz, Gorgon, North Rankin, Perseus, Goodwyn, Sarborough, Pluto, Wheatstone, Geryon, and Clio gas fields (Figure 1b). The land and sea regions of the basin cover approximately 115,000 km2 and 535,000 km2, respectively, and the maximum water depth is approximately 3500 m [8,47].
Six significant evolution stages developed in the NCB, including: (1) the pre-rift stage (Silurian to Toarcian), (2) the early syn-rift stage (Toarcian to earliest Callovian), (3) the main syn-rift stage (earliest Callovian to Berriasian), (4) the late syn-rift stage (Berriasian to Valanginian), (5) the post-breakup subsidence stage (Valanginian to mid-Santonian) and (6) the passive margin stage (mid-Santonian to present) [48]. In this study, the thick Triassic strata was deposited in the pre-rifting stage (Late Permian to Triassic) with typical siliciclastic sediment deposits (Figure 2).
The study intervals are the Triassic Locker Shale (SQ1), the Mungaroo Formation (SQ2 and SQ3), and the Brigadier Formation (SQ4) (Figure 2). The Early Triassic Locker Shale was characterized by marine claystone and siltstone with shelfal limestone. The Middle–Late Triassic Mungaroo Formation was characterized by typical fluvial–deltaic to marine environments, and the large-scale coal-bearing shallow water delta covered much of the offshore NCB [47]. The palynologic associations published in previous studies [49,50] indicated typical fluvial–deltaic facies deposition. During the Latest Triassic to Early Jurassic, rapid subsidence caused the deposition of the transgressive Brigadier Formation, which was characterized by thin and interbedded claystone, marl, and sandstone. After the drowning of the Mungaroo delta, the depositional environment of the Brigadier Formation was deltaic to marine deposits.

3. Dataset and Methods

This study focuses mainly on the comprehensive research of wireline logs, drilled cores, thin sections, seismic facies and attributes, and a series of geological and geochemical data to promote a comprehensive understanding of the sedimentary facies, evolution, and the related coal-bearing source rock potential of the Triassic delta, especially for the Mungaroo Formation.
There were over 50 wells with detailed drilled wireline logs that penetrated the Triassic stratum, and approximately 1100 m of drilled cores were available from six of these wells. The cored samples were mainly derived from the Mungaroo Formation. A 3D seismic volume covered an area of over 8200 km2, and the NCB covered 2D seismic data over 24,000 km (Figure 1a). The 3D seismic data were represented by inline and xline spacings of 12.5 m and 18.75 m, respectively. The vertical sample interval was 4 ms, and the signal-to-noise ratio was relatively high at ~8.3 with the dominant frequency of 40–50 Hz (measured velocity of ~2520 m/s). All geological data applied in this comprehensive study were provided by the CNOOC. All the drilled wells used in this research revealed the Mungaroo and Brigadier formations, and only few of them (such as Bruce-1, Hampton-1) penetrated the Locker Shale strata. The detailed sedimentary facies interpretations were based on the analysis of the well wireline logs, cores features, and microscopic thin sections. The thin sections and scanning electron microscope (SEM) micro-photos provided favorable records for the depositional environment. The seismic facies and seismic attributes (RMS attribute used in this study) were also useful to support the analysis of the facies’ dispersal pattern and depositional evolution.
In addition, a series of geochemical analyses of mudstone, carbonaceous mudstone, and coal seam source rock samples (>200) from different sedimentary facies or subfacies of the shallow-water delta system were also carried out in this study. The test items of hydrocarbon source rock mainly included the total organic carbon (TOC), hydrocarbon generation potential (S1 + S2), hydrogen index (HI), maximum pyrolysis peak temperature (Tmax), and organic maceral compositions in various sedimentary facies. These geochemical elemental analyses were carried out at the State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences (Wuhan). Based on replicate analyses of the Chinese national standard GB/T 19145-2003 [51], the analytical precision of the TOC measurements was better than 0.5 wt%.

4. Results

4.1. Microscopic Sedimentological Characteristics

The thin sections of the Middle–Late Triassic Mungaroo Formation showed different petrographic features with characteristics of various mineral compositions and assemblages (Figure 3). The sandstone samples of the Mungaroo Formation were characterized as quartz sandstone and subarkose with poor and medium sorting and angular round quartz grains (Figure 3a,b). Some primary pores, secondary pores, siderite, organic matter filled, and quartz overgrowth were also identified in the thin section graphs. In addition, kaolinite cements, banded organic matter, rootlet debits (Figure 3c,d), and coal seams (Figure 3e,f) also developed within the delta plains of the Mungaroo delta system.

4.2. Well-Based Sedimentary Facies’ Characteristics

The facies and vertical associations in this study were identified based on the analysis of the well-based wireline-log signatures, lithologies, grain-size, and sedimentary structures/fabrics (Figure 4, Figure 5 and Figure 6). In this study, a range of shallow-water delta sedimentary environments was recognized, including the proximal delta plain channel/interdistributary bay, distal delta plain channel/interdistributary bay, delta front, beach bar, carbonate platform, and prodelta or littoral neritic sea. In the study area, the distributary channel sandstone of the delta plain was characterized by box or bell shapes in the GR logging curve and was mainly composed of thick gravel coarse sandstone with a typical scouring surface that was shown in the cores (Figure 4b). The interdistributary bay in the delta plain was mainly characterized by the frequent interbedding of sand and mudstone with typical rhythmic bedding (Figure 4a), and the GR logging curve was characterized by toothed features and interbedded with typical coal seams. The delta front was characterized by the interbedding of fine sandstone, siltstone, and mudstone with a certain reverse cycle feature (upward coarsening) and a visible crossbedding and bioturbated structure (Figure 4c).
In addition, the cross-wells stratigraphic profile A–A′ showed that the Early Triassic Locker Shale (SQ1) consisted of thick (40–200 m) mud-rich lithofacies integrated with siltstone (0.3 m–2.0 m) (Figure 5). The thin sheet siltstones were characterized by low gamma (GR) peaks.
The Middle–Late Triassic Mungaroo Formation (SQ2 and SQ3) was dominated by coal-bearing and braided delta with broad proximal and distal delta plains and a relatively narrow range delta front (Figure 5). The proximal and distal delta plains were characterized by deltaic distributary channels with thick (12–30 m) channels of conglomeratic sandstones (with box or bell shapes in the GR curve) interbedded with coal seams and carbonaceous mudstone (Figure 4 and Figure 5). The delta plains also displayed multiple fining upward fluvial aggradational cycles (FACs; 0.1–1.2 m) with deposition of a braided distributary channel and an interdistributary bay (Figure 4). The deposition of the thin coal beds (Figure 1a) may be the consequence of the redeposit of coals that developed at the delta system. In addition, there was a certain reverse grain sequence in the delta front of the Mungaroo Formation (Figure 4 and Figure 6). It is worth noting that the scale, thickness, and grain-size of the sediment deposits of the SQ2 were obviously larger than that of the SQ3, especially in the sedimentary facies of the proximal and distal delta plain distributary channels (Figure 4).
The Late Triassic Brigadier Formation (SQ4) was characterized by thin (2.2 m–4.0 m) fine stone, mudstone, and limestone with low gamma characteristics (Figure 4, Figure 5 and Figure 6). Compared with the SQ2 and SQ3, the delta-scale and grain-size of the deltaic deposits obviously decreased, which was well indicated by North Gorgon-1 (Figure 4). During the deposition of the Brigadier Formation, the features of the FACs also decreased from bottom to top.

4.3. Seismic Facies and Attribute Features of the Coal-Bearing Shallow-Water Delta

The thickness map of the Triassic strata in the North Carnarvon Basin (Figure 1b) shows that the topography of the study area had a relatively gentle slope with a gradient of approximately 1.2° to 2.3°. The cross-well seismic profile C–C′ (Figure 7) presented a typical low-angle shingled progradational seismic reflection configuration of the SQ2 and SQ3, while the underlying Locker Shale (SQ1) and overlying Brigadier Formation (SQ4) were characterized by a parallel or subparallel seismic configuration (Figure 7). Integrated with the drilled-well based sedimentary facies analysis (Figure 4, Figure 5 and Figure 6), the shingled progradational reflection in this research corresponded to the large-scale proximal and distal delta plain, with typical distributary channel gravel-rich or sand-rich deposits. In contrast, the parallel or subparallel seismic reflection could be linked to the interdistributary bays with mud-rich facies, carbonaceous mudstone, or coal seam deposits (Figure 7).
The root mean square (RMS) seismic amplitude attribute map (Figure 8) also can be used to depict the planar distribution features of the various-scale channel sandstone geobodies of the shallow-water delta system. The distributary channel belts of the Mungaroo Formation were obviously imaged, showing the dominant paleoflow direction (SE–NW orientation), characteristics, and dispersal pattern. The features of the distributary channels also further supported the analysis of the delta system of the Mungaroo Formation in the study area.

5. Discussion

5.1. Depositional Evolution and Dispersal Pattern of the Triassic Coal-Bearing Delta System

Based on the integrated analysis of the well-based lithologies, seismic facies and attributes, and microscopic sedimentary characteristics, in this study, the sedimentary facies distribution and evolution from the Early Triassic Locker Shale (SQ1) to the Middle–Late Triassic Mungaroo Formation (SQ2 and SQ3) and Late Triassic Brigadier Formation (SQ4) were well characterized (Figure 9a).
The Early Triassic Locker Shale (SQ1) was characterized by a relatively small-scale delta system that was mainly distributed along the coast of the Northwest Shelf of Australia. The feature of the Locker Shale sedimentary facies distribution was consistent with the characteristics of a relatively mud-rich lithologic association revealed by a large number of existing wells (Figure 5 and Figure 6). The lower Mungaroo Formation (SQ2) corresponded to the depositional stage of the largest coal-bearing shallow-water delta systems that developed in the NCB (Figure 9a). It was characterized by a large-scale delta system with relatively broad proximal and distal delta plains and relatively narrow delta front subfacies. The feature of the lower Mungaroo Formation (SQ2) was also well validated by the drilled wells with thick distributary channel pebbly sandstone and widely developed thin coal seams (Figure 4, Figure 5 and Figure 6). Based on the previous studies [8,44,46], it is believed that the large-scale delta system of the Mungaroo Formation might have a good inner relationship with the impact of the Triassic megamonsoon and the influence of Carnian Pluvial Episode (CPE). During the upper Mungaroo Formation (SQ3), the delta system was characterized by the interbedding of medium–fine channel sandstone and siltstone, mudstone, and thin coal beds. During the Brigadier Formation (SQ4), the delta system’s scale decreased rapidly (Figure 9a) and was characterized by thick limestone and dolomite and interbedded with thin siltstone and mudstone.
Therefore, based on the sedimentary facies’ distribution and evolution of the Triassic sequences, a three-dimensional (3D) conceptual model from bottom to top is also provided (Figure 9b). The 3D model obviously displays the dispersal pattern of the sedimentary facies or subfacies association, lithologies distribution patterns, and the spatiotemporal distribution characteristics. Overall, the scale of the delta system showed a trend of increasing from SQ1 to SQ2 and then decreasing from SQ2 to SQ4. At the same time, the lithologic association and distribution also showed a matching evolution trend.

5.2. Coal-Bearing Source Rock Characteristics and the Potential of Different Sedimentary Subfacies

In this study, the Mungaroo Formation was the major study interval for detailed coal-bearing source rock analysis with different sedimentary facies or delta subfacies (Figure 10 and Figure 11; Table 1 and Table 2). The organic matter of the Mungaroo Formation was mainly represented by thin coal seams, carbonaceous mudstones, and mudstone-rich terrigenous organic matter. Among them, the thin coal seams and carbonaceous mudstone were mainly concentrated in the distal delta plain. Due to the distributary channels being well-developed in the proximal delta plain, the effect of river scouring was strong, and the fine sediment such as peat swamp deposited in the early stage was difficult to preserve. Most of the organic matter in the sediment was oxidized and decomposed in the Mungaroo Formation. The oxidized vitrinite was also seen in some sample analysis. In addition, the delta front was mainly enriched in silty mudstone and siltstone with typical dispersed organic matter.
Generally, the Mungaroo Formation was characterized by coal-bearing source rock with obvious organic matter that derived from terrestrial plants (ferns and seed ferns), showing the organic matter type dominated with type-III kerogen in different facies (Figure 10). The organic matter types of the source rocks in different sedimentary facies zones also had certain differences. Among them, the proximal delta plain and the distal delta plain were characterized by type-II2 and -III kerogen (Figure 10a,b), while the organic matter types of the source rock in the delta front, prodelta, and carbonate platform were mainly represented by type-III kerogen, and a few samples fell into the type-II2 area (Figure 10c,d).
The contents of the organic macerals in different sedimentary facies zones also had obvious differences. The organic matter of the coal-bearing delta system was mainly represented by two different types, including the coal and dispersed organic matter. The macerals of coal mainly consisted of vitrinite, and the content of inertinite was relatively low, which mainly developed within the distal delta plain (Table 1). The content of vitrinite in the dispersed organic matter was lower than that in the coal, while the content of the inertinite and exinite was relatively higher, which mainly developed within the proximal delta plain and delta front subfacies (Table 1). From the proximal delta plain to the distal delta plain and then to the delta front and prodelta (littoral neritic sea), the content of vitrinite increased (from 23% to 54%) and then decreased (from 54% to 21%), of which the content of relative vitrinite in the distal delta plain was the highest (54%), while the content of exinite had a significant increasing trend (from 12% to 16% and 41%). Overall, the macerals of organic matter in the proximal and distal delta plains of the Mungaroo Formation in the NCB were mainly vitrinite and inertinite, and the content of exinite was relatively low. The content of vitrinite and inertinite in the macerals of the proximal delta plain and the distal delta plain showed an obvious mutual growth and decline relationship (Table 1). However, the content of exinite in the delta front was relatively high, and some samples even contained exinite alone.
In addition, the terrigenous organic matter content and hydrocarbon generation potential of the dark mudstone and coal seam in different sedimentary facies zones of the Mungaroo Formation were quite different (Figure 11; Table 2). The thin coal seams in the proximal and distal delta plain were well developed with a high average TOC content (33.3% to 37.4%; Table 2). Overall, the distal delta plain was characterized by coal seams and thick carbonaceous mudstones rich in terrigenous organic matter, which can serve as good or very good source rocks. The average total organic carbon (TOC) content of the dark mudstones in the distal delta plain reached 4.11% (Table 2). In the proximal delta plain, the carbonaceous mudstone was not developed, and the TOC content in the dark mudstone was also relatively high, with an average of 1.16%, which could be medium to good hydrocarbon source rock (Figure 11; Table 2). The delta front sedimentary facies belt was relatively narrow with a relatively low TOC content in the dark mudstones (average 1.05%), which might not be a favorable source rock facies belt in this area. The samples of the prodelta/littoral neritic sea and the carbonate platform were also represented by low TOC content features (average 0.96%), which should not be the source rock targets in NCB exploration.

5.3. Coal-Bearing Source Rock Prediction, Controlling Factors, and Implications for Exploration

The Triassic coal-bearing source rocks in the NCB mainly developed during the Mungaroo Formation (SQ2 and SQ3) with typical proximal and distal delta plain deposits (Figure 9 and Figure 12a). Based on the comprehensive analysis of the organic matter types, organic maceral compositions, TOC content, and hydrocarbon generation potential (Figure 10 and Figure 11; Table 1 and Table 2) above, the distal delta plain subfacies of the SQ2 and SQ3 should have the most potential coal-bearing source rock that developed in the NCB (Figure 12b). The proximal delta plain could also be a favorable target for source rock evaluation. The narrow delta front, the prodelta, littoral neritic sea, and carbonate platform facies do not have favorable hydrocarbon generation potential.
The depositional evolution and dispersal pattern of the source rocks of the Mungaroo delta were significantly affected by the Triassic megamonsoon, the basin geomorphology, eustatic changes, and provenance from the orogenic belt. Changes in the Triassic climate affected the rainfall, which then influenced the fluvial erosion, sediment supply, and discharge from the source to sink of the drainage system. During the Late Triassic, especially the Carnian stage, the NCB was located in a temperate to warm humid and monsoonal climate, which also benefitted the megamonsoon that developed related to the Pangaea supercontinent. In addition, the gentle topography during the Mungaroo Formation of the NCB was advantageous for the formation of the coal-bearing delta with a broad delta plain. During the Middle–Late Triassic, the major source in the study area was derived from the provenance regions of Antarctica and Southeastern Australia [8], which provided sufficient sediments to form the large-scale delta.
In summary, the broad proximal and distal delta plains of the shallow-water delta system of the Mungaroo Formation (SQ2 and SQ3) with thin coal seams, carbonaceous mudstone, and dark mudstone lithologies’ association could be advantageous coal-bearing source rock exploration layers in the NCB of the Northwest Shelf of Australia, which also could be the major targets of reservoir and source rock for the next stage of natural gas field exploration.

6. Conclusions

The sedimentary facies’ analysis, distribution and evolution, and coal-bearing source rock features of the Middle–Late Triassic shallow-water delta in the North Carnarvon Basin, Northwest Shelf of Australia, were comprehensively analyzed based on the integrated study of wireline logs, drilled cores, thin sections, seismic facies and attributes, and a series of geological and geochemical data. The major conclusions are presented as follows:
(i)
A range of shallow-water delta sedimentary environments was recognized in the Triassic strata of the North Carnarvon Basin, especially during the deposition of the Middle–Late Triassic Mungaroo Formation, including the proximal delta plain channel/interdistributary bay, distal delta plain channel/interdistributary bay, delta front, beach bar, carbonate platform, and prodelta or littoral neritic sea. The coal-bearing shallow-water delta system of the Mungaroo Formation was characterized by a large-scale delta system with relatively broad proximal and distal delta plains and relatively narrow delta front subfacies. The scale of the delta system showed a trend of increasing from SQ1 to SQ2 and then decreasing from SQ2 to SQ4.
(ii)
The Triassic coal-bearing source rocks in the NCB mainly developed during the Mungaroo Formation (SQ2 and SQ3) with a relatively high TOC content and hydrocarbon generation potential. The distal delta plain subfacies of the Mungaroo Formation should be the highest potential coal-bearing source rock. The proximal delta plain also could be a favorable target for source rock evaluation. The delta front, prodelta, littoral neritic sea, and carbonate platform facies do not have favorable hydrocarbon generation potential.
(iii)
The depositional evolution and dispersal pattern of the source rocks of the Mungaroo delta were affected by the Triassic megamonsoon climate, the gentle basin topographic feature, eustatic changes, and enough sediment supply from the orogenic belt. The proximal and distal delta plains of the shallow-water delta system with thin coal seams, carbonaceous mudstone, and dark mudstone lithologies’ association could be advantageous coal-bearing source rock exploration layers, which could serve as a reference for other regions with a similar background.

Author Contributions

Conceptualization, investigation, writing—original draft preparation, review, validation, and editing, Z.Z.; methodology, validation, formal analysis, and editing, W.W.; supervision, methodology, project administration, and data curation, H.Z.; methodology, supervision, project administration, and funding acquisition, X.Y.; software, data curation, and project administration, D.L. All authors have read and agreed to the published version of the manuscript.

Funding

This project was supported by the “CUG Scholar” Scientific Research Funds at China University of Geosciences (Wuhan) (Project No.2022148). This research was also funded by the National Science and Technology Major Projects (No. 2011ZX05030-002-002), the China National Postdoctoral Program for Innovative Talents (Grant No. BX20200310), the China Postdoctoral Science Foundation (Grant No. 2020M682522), and Research funding for postdoctoral innovation in Hubei Province (Grant No. 257236). This research was also funded by the National Natural Science Foundation of China (Grant No. 42172127, Grant No. 41872149).

Data Availability Statement

Not applicable.

Acknowledgments

The Beijing Research Institute of the China National Offshore Oil Corporation is thanked for providing data used in this study and for the permission to publish the results.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) Regional geological maps of the North Carnarvon Basin in the Northwest Shelf of Australia, including the drilled wells which penetrated the Triassic conglomerate or coal seam and the location of the connecting-well sections and seismic profile used in this study. (b) Thickness map of the Triassic strata in the North Carnarvon Basin and a series of significant gas field in this region.
Figure 1. (a) Regional geological maps of the North Carnarvon Basin in the Northwest Shelf of Australia, including the drilled wells which penetrated the Triassic conglomerate or coal seam and the location of the connecting-well sections and seismic profile used in this study. (b) Thickness map of the Triassic strata in the North Carnarvon Basin and a series of significant gas field in this region.
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Figure 2. Stratigraphic map of the NCB, showing the Triassic chronology, sea-level curve, lithology, formation, sequences, depositional facies, and evolution stage. The major interval is the Triassic strata (especially the Mungaroo Formation) with a typical fluvial–deltaic to marine environment.
Figure 2. Stratigraphic map of the NCB, showing the Triassic chronology, sea-level curve, lithology, formation, sequences, depositional facies, and evolution stage. The major interval is the Triassic strata (especially the Mungaroo Formation) with a typical fluvial–deltaic to marine environment.
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Figure 3. Photomicrographs of different petrographic features of the Mungaroo Formation in the NCB. (a,b) show the shapes and distribution patterns of the quartz (Q), feldspar (F), primary pores (P), secondary pores (SP), and a small amount of siderite (S) and quartz overgrowth (QO), 4050.6–4051.0 m, well North orgon-1. (c,d) show the microscopic characteristics of quartz (Q), kaolinite (K), organic matter (O), and rootlets, 4035.4–4035.7 m, well Bluebell-1. (e,f) display the features of the coal seam, SEM, 4159.0 m, well Bluebell-1. See the location of the wells in Figure 1a.
Figure 3. Photomicrographs of different petrographic features of the Mungaroo Formation in the NCB. (a,b) show the shapes and distribution patterns of the quartz (Q), feldspar (F), primary pores (P), secondary pores (SP), and a small amount of siderite (S) and quartz overgrowth (QO), 4050.6–4051.0 m, well North orgon-1. (c,d) show the microscopic characteristics of quartz (Q), kaolinite (K), organic matter (O), and rootlets, 4035.4–4035.7 m, well Bluebell-1. (e,f) display the features of the coal seam, SEM, 4159.0 m, well Bluebell-1. See the location of the wells in Figure 1a.
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Figure 4. The gamma ray (GR) wireline log, lithologies and vertical evolution, delta–marine sedimentary facies, and the related cores and crossbedding sedimentary structures from North Gorgon-1. The depositional strata are related to the Mungaroo and Brigadier Formation (BF) from SQ2 to SQ4. The drilled cores’ photos indicate the depositional features of the (a) coal seam and carbonaceous mudstone, (b) coarse grained sandstone and conglomerate of the distributary channel sandstone, and (c) interbedding of the distributary channel sandstone and mudstone of the interdistributary channel bay.
Figure 4. The gamma ray (GR) wireline log, lithologies and vertical evolution, delta–marine sedimentary facies, and the related cores and crossbedding sedimentary structures from North Gorgon-1. The depositional strata are related to the Mungaroo and Brigadier Formation (BF) from SQ2 to SQ4. The drilled cores’ photos indicate the depositional features of the (a) coal seam and carbonaceous mudstone, (b) coarse grained sandstone and conglomerate of the distributary channel sandstone, and (c) interbedding of the distributary channel sandstone and mudstone of the interdistributary channel bay.
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Figure 5. The cross-wells’ stratigraphic profile A–A′ of the Triassic Locker Shale (SQ1), Mungaroo Formation (SQ2 and SQ3), and Brigadier Formation (SQ4), showing the characteristics of the gamma ray (GR) wireline log, lithologies, lithological association, and vertical evolution from bottom to top. See Figure 1a for the location of the cross-wells profile A–A′.
Figure 5. The cross-wells’ stratigraphic profile A–A′ of the Triassic Locker Shale (SQ1), Mungaroo Formation (SQ2 and SQ3), and Brigadier Formation (SQ4), showing the characteristics of the gamma ray (GR) wireline log, lithologies, lithological association, and vertical evolution from bottom to top. See Figure 1a for the location of the cross-wells profile A–A′.
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Figure 6. Cross-well profile B–B′ showing the sedimentary facies (proximal delta plain channel/interdistributary bay, distal delta plain channel/interdistributary bay, delta front, beach bar, carbonate platform, and littoral neritic sea) of the shallow-water delta system of the Middle–Late Triassic Mungaroo and Brigadier Formation (BF) from SQ2 to SQ4. The scale of the delta is gradually decreasing accompanied by vertical changes in the sedimentary facies and lithologic association. See Figure 1a for the location of the cross-wells profile B–B′.
Figure 6. Cross-well profile B–B′ showing the sedimentary facies (proximal delta plain channel/interdistributary bay, distal delta plain channel/interdistributary bay, delta front, beach bar, carbonate platform, and littoral neritic sea) of the shallow-water delta system of the Middle–Late Triassic Mungaroo and Brigadier Formation (BF) from SQ2 to SQ4. The scale of the delta is gradually decreasing accompanied by vertical changes in the sedimentary facies and lithologic association. See Figure 1a for the location of the cross-wells profile B–B′.
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Figure 7. Cross-well seismic profile C–C′ presents a typical progradational seismic reflection of the SQ2 and SQ3. See Figure 1a for the position of the seismic profile C–C′. Abbreviation: TWT = two-way travel time.
Figure 7. Cross-well seismic profile C–C′ presents a typical progradational seismic reflection of the SQ2 and SQ3. See Figure 1a for the position of the seismic profile C–C′. Abbreviation: TWT = two-way travel time.
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Figure 8. Root mean square (RMS) seismic amplitude attribute map of a 40 ms sculpt from the SQ2 of the Mungaroo Formation, showing the dominant paleoflow direction, characteristics, and dispersal pattern of the shallow-water braided delta distributary channels. See the location of this seismic attribute map in Figure 1a.
Figure 8. Root mean square (RMS) seismic amplitude attribute map of a 40 ms sculpt from the SQ2 of the Mungaroo Formation, showing the dominant paleoflow direction, characteristics, and dispersal pattern of the shallow-water braided delta distributary channels. See the location of this seismic attribute map in Figure 1a.
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Figure 9. The lithology, GR wireline log feature, facies distribution and evolution, and the related three-dimensional conceptual model of the Triassic strata from the Locker Shale to the Brigadier Formation in the NCB. (a) The sedimentary facies maps of the SQ1 to SQ4 indicate the distribution pattern of the shallow water delta system. (b) The three-dimensional conceptual model of the the Locker Shale to the Brigadier Formation with typical lithologic distribution.
Figure 9. The lithology, GR wireline log feature, facies distribution and evolution, and the related three-dimensional conceptual model of the Triassic strata from the Locker Shale to the Brigadier Formation in the NCB. (a) The sedimentary facies maps of the SQ1 to SQ4 indicate the distribution pattern of the shallow water delta system. (b) The three-dimensional conceptual model of the the Locker Shale to the Brigadier Formation with typical lithologic distribution.
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Figure 10. Organic matter types, maximum pyrolysis peak temperature (Tmax), and hydrogen index (IH) characteristics of the Mungaroo Formation source rocks in different sedimentary facies or subfacies, including the (a) proximal delta plain, (b) distal delta plain, (c) delta front and (d) prodelta, littoral neritic sea, and carbonate platform.
Figure 10. Organic matter types, maximum pyrolysis peak temperature (Tmax), and hydrogen index (IH) characteristics of the Mungaroo Formation source rocks in different sedimentary facies or subfacies, including the (a) proximal delta plain, (b) distal delta plain, (c) delta front and (d) prodelta, littoral neritic sea, and carbonate platform.
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Figure 11. Total organic carbon (TOC) content and hydrocarbon generation potential (S1 + S2) of the Mungaroo Formation source rocks (mudstone, carbonaceous mudstone, and coal) in different sedimentary facies or subfacies.
Figure 11. Total organic carbon (TOC) content and hydrocarbon generation potential (S1 + S2) of the Mungaroo Formation source rocks (mudstone, carbonaceous mudstone, and coal) in different sedimentary facies or subfacies.
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Figure 12. (a) The three-dimensional dispersal pattern model of the typical shallow-water coal-bearing delta of the Mungaroo Formation in the NCB. (b) Two-dimensional conceptual profile of the distribution pattern of the terrigenous organic matter in the Mungaroo deltaic system, indicating the facies dispersal pattern and lithologies’ distribution and combination. See Figure 12a for the position of the profile D–D′.
Figure 12. (a) The three-dimensional dispersal pattern model of the typical shallow-water coal-bearing delta of the Mungaroo Formation in the NCB. (b) Two-dimensional conceptual profile of the distribution pattern of the terrigenous organic matter in the Mungaroo deltaic system, indicating the facies dispersal pattern and lithologies’ distribution and combination. See Figure 12a for the position of the profile D–D′.
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Table
Table 1. Statistics of the average content of the organic maceral compositions in different sedimentary facies of the Mungaroo Formation in the NCB.
Table 1. Statistics of the average content of the organic maceral compositions in different sedimentary facies of the Mungaroo Formation in the NCB.
Sedimentary
Subfacies		Average Content of Macerals (%)
InertiniteVitriniteExinite
Proximal delta plain46%23%12%
Distal delta plain30%54%16%
Delta front38%21%41%
Table
Table 2. Total organic carbon (TOC) values of the dark mudstone and coal seam in different delta subfacies of the Mungaroo Formation in the NCB.
Table 2. Total organic carbon (TOC) values of the dark mudstone and coal seam in different delta subfacies of the Mungaroo Formation in the NCB.
LithologyTOC
Value (%)		Proximal
Delta Plain		Distal
Delta Plain		Delta
Front		Prodelta/
Littoral Neritic Sea
Dark mudstoneAverage value1.16%4.11%1.05%0.96%
Minimum value0.5%0.52%0.43%0.32%
Maximum value4.16%26.8%4.26%1.74%
Coal seamAverage value33.3%37.4%//
Minimum value28.2%30.4%//
Maximum value35.2%40.5%//
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Zeng, Z.; Wang, W.; Zhu, H.; Yang, X.; Li, D. The Facies Analysis, Evolution, and Coal-Bearing Source Rock Features of the Middle–Late Triassic Shallow-Water Delta in the North Carnarvon Basin, Northwest Shelf of Australia. Energies 2023, 16, 2265. https://doi.org/10.3390/en16052265

AMA Style

Zeng Z, Wang W, Zhu H, Yang X, Li D. The Facies Analysis, Evolution, and Coal-Bearing Source Rock Features of the Middle–Late Triassic Shallow-Water Delta in the North Carnarvon Basin, Northwest Shelf of Australia. Energies. 2023; 16(5):2265. https://doi.org/10.3390/en16052265

Chicago/Turabian Style

Zeng, Zhiwei, Wei Wang, Hongtao Zhu, Xianghua Yang, and Dan Li. 2023. "The Facies Analysis, Evolution, and Coal-Bearing Source Rock Features of the Middle–Late Triassic Shallow-Water Delta in the North Carnarvon Basin, Northwest Shelf of Australia" Energies 16, no. 5: 2265. https://doi.org/10.3390/en16052265

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