1. Introduction
With the steady increase in the demand for oil and gas, shale gas exploration in deep and ultradeep strata has emerged as a primary research focus [
1], [
2], [
3]. Shale gas, which is currently one of the most important forms of unconventional petroleum resources [
4], reached a global annual production of 8.55 × 10
11 m
3 in 2022, with the United States being the major producer (8.07 × 10
11 m
3) [
5]. China is predicted to have shale gas reserves of approximately 2.00 × 10
13 m
3; however, the annual production is only 2.38 × 10
10 m
3 [
5], indicating the necessity for intensified efforts in the field [
6]. For shale gas reservoirs, depths of 3500-4500 and 4500-6000 m are defined as deep and ultradeep strata, respectively [
7]. The Sichuan Basin has considerable potential for deep and ultradeep shale gas resources [
6], [
8], [
9], and the lower Cambrian Qiongzhusi shale is estimated to have substantial shale gas geological resources of 8.86 × 10
12 m
3 [
10]. However, it is usually regarded as a source rock for conventional gas reservoirs in the Cambrian Longwangmiao and Ediacaran Dengying Formations [
11], [
12], [
13], [
14] rather than as an important shale gas reservoir. Over the past decade, some trials have been conducted to develop the shale gas from Qiongzhusi shale; however, most of them showed a low gas flow with a daily production of approximately 0.20 × 10
4−8.44 × 10
4 m
3, except for the well JS103HF (2.59 × 10
5 m
3 per day) [
15], [
16]. Low gas production makes the exploration potential of the Qiongzhusi shale highly debated.
Previous exploration practices for the Qiongzhusi shale have mainly focused on areas outside the Deyang-Anyue rift trough (DART) of the Sichuan Basin. Extensive studies have been conducted on the formation and depositional environment of the Qiongzhusi shale in these areas, including their organic matter (OM) enrichment, reservoir evaluation, and gas accumulation mechanisms (e.g., Refs. [
17], [
18], [
19], [
20]). In contrast, investigations of shales inside the DART are rare. Current studies suggest that highly over-mature shales may not exhibit optimal rock properties, pore structures, and gas contents compared with some high-quality shales [
21]. Therefore, whether overmature Qiongzhusi shale (equivalent vitrinite reflectance
Re > 2.5%) within the DART has the potential for industrial shale gas production is not clear. Recently, however, an industrial gas flow of 7.388 × 10
5 m
3·d
−1 was obtained from the Qiongzhusi Formation of well Z201 within the DART (see well location in
Fig. 1 [
22]), representing the highest gas production among the world’s oldest industrial gas reservoirs to date (
Table 1) [
8], [
23], [
24], [
25], [
26], [
27], [
28], [
29], [
30]. This exploration breakthrough showed the high shale gas potential of the deep-buried Qiongzhusi Formation inside the DART; however, a systematic comparison of the Qiongzhusi shale inside and outside the DART has not been conducted, especially regarding their OM enrichment mechanism (links with the Cambrian explosion), reservoir quality (the influence of high maturity), and gas preservation conditions. By resolving these issues, the shale gas enrichment pattern within the DART can be elucidated, and guidance for future deep shale gas development can be provided.
In this study, we first established a link between the high production interval of the Qiongzhusi shale and the main phase of the Cambrian explosion to investigate its OM enrichment mechanism. Subsequently, a comparative analysis of the differences between the Qiongzhusi shale inside and outside the DART was conducted, focusing on reservoir quality and gas preservation conditions. These findings enabled us to conclude a “ternary enrichment” model to understand the formation and accumulation of the Qiongzhusi shale gas within the DART and provide a “multilayer stereoscopic development” strategy for future shale gas development there.
2. Geological background of lower Cambrian shale in the Sichuan Basin
2.1. Tectonic and sedimentary characteristics
The Sichuan Basin is a first-class sedimental-tectonic unit in the western expanse of the Yangtze platform, covering an area of approximately 1.8 × 10
5 km
2 [
31]. The Sichuan Basin and its periphery were developed based on Archean-early Proterozoic metamorphic and magmatic rocks. The central Sichuan region features a solid crystalline basement with robust resistance to tectonic deformation [
31], [
32]. In contrast, the northwestern Sichuan region comprises a pliable depression basement prone to deformation. From the late Ediacaran to the early Cambrian, profound intraplate extensional rifting transpired along the periphery of the Yangtze platform due to the breakup of the Rodinia supercontinent and aggregation of the Gondwana continent. This led to a predominance of extension with mild compression in the Sichuan Basin, which controlled the formation of uplift and subsidence within the basin [
33].
Within the Deyang-Anyue area of the Sichuan Basin, intensive extension and erosion during the late Ediacaran to early Cambrian facilitated the emergence of an intracratonic rift trough known as the DART [
34]. The mechanisms underlying DART formation vary across regions. During the deposition of the lower Dengying Formation, the strong extension at the northern margin of the basin led to the first formation of a rift trough in the north [
35], whereas the extensional forces in central and southern Sichuan were insufficient to create a trough-platform geomorphology [
32]. During the deposition of the middle-upper Dengying Formation, the Tongwan movement led to regional uplift and erosion. Unlike the northern part of the basin, where erosion was limited as the rift trough was deep, the central-southern part of the basin experienced enhanced erosion because of the elevated exposure and its positioning in a drainage zone between the Kangdian ancient land and the central Sichuan uplift [
32], [
36]. Briefly, robust tectonic extension in the north and intensified erosion in the south caused the water environment to progressively deepen from south to north within the DART [
30] (
Fig. 1(a)).
The tectonic development of the DART played a crucial role in shaping the sedimentary distribution of the early Cambrian Qiongzhusi shale and underlying Maidiping shale [
8], [
37] (
Fig. 1(b)). During the deposition of the Maidiping shale, the continental shelf of the South China Sea was exposed due to low sea levels, resulting in a sedimentation gap, especially outside the DART. Therefore, the Maidiping shale was primarily deposited within the DART, with thicknesses ranging from 50 to 200 m, and is mainly composed of siliceous shale and carbonaceous mudstone. Enhanced glacial melting triggered extensive transgressions with a warmer and more humid climate from the late Ediacaran to the early Cambrian [
38], [
39]. These environmental conditions facilitated the deposition of organic-rich Qiongzhusi shale in the Sichuan Basin [
40], [
41], [
42]. The Qiongzhusi Formation (Є
1q) can be biostratigraphically subdivided into 1st and 2nd members (Є
1q1 and Є
1q2), respectively), with small shell fossils
Lapworthella-Tannuolina-Sinosachites found in Є
1q1 and trilobite fossils
Eoredlichia-Wudinggaspis found in Є
1q2 [
43]. Based on total organic carbon (TOC) contents, gamma ray (GR) log, and lithology, the Є
1q1 can be further subdivided into 1st sub-member Є
1q1
1 (lower) and 2nd sub-member Є
1q1
2 (upper). The Є
1q1
1 shale was dominantly deposited during the seawater transgression, although most regions of the Sichuan Basin remained largely exposed due to the central Sichuan uplift. Therefore, the dark carbonaceous mudstone, shale, and argillaceous siltstone of Є
1q1
1 were mainly deposited within the DART and around the basin margins. The Є
1q1
1 shale contains several GR peaks with high but unsteady TOC contents (0.5-5.0 wt%). The expanded transgression led to the wide distribution of the Є
1q1
2 shale, characterized by black carbonaceous shale, medium-to-high GR values, and relatively high TOC contents (1-4 wt%). The Є
1q2 comprises grey or light grey argillite and siltstone (locally containing carbonate) with low GR values and low TOC contents (average < 1 wt%) [
44], [
45], [
46], [
47] (
Fig. 2). During the deposition of Є
1q2, the Sichuan Basin was largely covered by shallow-shelf and coastal facies, due to the uplift of the Kangdian land, the weakening of the rift trough activity as well as the declining sea level [
42]. Besides, the sedimentary difference between the DART interior and exterior at this time was inapparent owing to the filling and replenishment of Є
1q1.
2.2. Distribution of the Qiongzhusi shale in the Sichuan Basin
The distribution of the Qiongzhusi shale inside and outside the DART varies, with thicker shales observed within the DART (
Fig. 2;
Table 2). The Є
1q1
1 shale is mainly deposited within the deep-water shelf facies inside the DART (200-250 m in thickness) and around the basin margins (100-150 m). The Є
1q1
2 shale is widely deposited across the Yangtze platform, approximately 100-150 m inside and 80-100 m outside the DART. The Є
1q2 shale inside and outside the DART are similar, with thickness ranging from 100 to 250 m. Notably, due to the presence of the central Sichuan uplift, the Qiongzhusi shale on the western side of DART (wells JY1, W201, and W207) is thicker than that on the eastern side (well GS1;
Fig. 2).
Recently, well Z201 showed gas production of 7.388 × 10
5 m
3·d
−1, representing the highest shale gas production recorded in the Qiongzhusi Formation to date. This well is located within the DART and penetrates the Qiongzhusi Formation at depths ranging from 4292 to 4869 m. The thicknesses of Є
1q1
1, Є
1q1
2, and Є
1q2 in the Z201 are 250, 130, and 200 m, respectively. Based on characteristics of the GR curve and lithology, the Qiongzhusi Formation can be divided into eight sublayers, among which four high-GR intervals correspond to four potential shale gas reservoirs, including the 1st and 3rd sublayers in Є
1q1
1 (containing reservoirs 1 and 2), the 5th sublayer in Є
1q1
2 (containing reservoir 3), and the 7th sublayer in Є
1q2 (containing reservoir 4;
Fig. 2;
Table 2). The gas production of the well Z201 was produced from the reservoir 3 in Є
1q1
2 using hydraulic fracturing.
3. Samples and methods
TOC, reflectance, porosity, permeability, X-ray diffraction (XRD), gas content, pressure coefficient, optical microscopy, and field emission scanning electron microscopy (FE-SEM) data from 12 wells across the DART (wells HS1, GS17, GS1, JS103HF, JY1, W207, W201, W201-H3, Z201, ZY1, Z4, and ZG1) were provided by the PetroChina Southwest Oil & Gasfield Company. Over 100 samples were collected from well W207 for fossil investigations. The methods used for these analyses are described in the following sections. In this study, we also used previously reported TOC, trace element, and iron speciation data from wells W201, W207, and ZY1 (supplementary dataset in Appendix A) [
45], [
48], [
49]. This compilation of new and previously published data provided the basis of our discussion.
(1) TOC content. Approximately 200 mg of the powder was treated with HCl to remove carbonates and then washed with distilled water to eliminate residual HCl. Next, the samples were dried on a heating plate overnight at approximately 50-60 °C and analyzed using an Elementrac CS-i analyzer at the Research Institute of Petroleum Exploration & Development, PetroChina. The analytical precision, which was determined through repeated analyses of the Alpha Resources standard AR-4007 (with a total carbon content of 7.62%), exceeded 0.10%.
(2) Elemental proxies calculation. Enrichment factors (EFs) of the trace elements were calculated relative to the Post-Archean Australian Shale (PAAS) standard: X
EF = (X/Al)
sample/(X/Al)
PAAS, where X represents the trace element Mo or U. The Mo, U, and Al concentrations in PAAS were 1.00 parts per million (ppm), 3.10 ppm, and 10.01%, respectively [
50]. The biogenic elemental indicators were determined using the formula Y
bio = Y
sample − Al
sample × (Y/Al)
detr, where Y represents the Cu or Ni and (Y/Al)
detr is the Al-normalized detrital Y content. The value of (Y/Al)
detr was determined using an Al versus Y cross-plot, where the sample with the lowest Y/Al ratio was assumed to contain minimal biogenic Y [
51].
(3) Solid bitumen reflectance. Solid bitumen is the secondary microscopic component of high-maturity source rocks, and its reflectance can be applied to evaluate maturity [
52]. The rocks were embedded in thermoplastic epoxy and prepared petrographically [
53]. Solid bitumen reflectance was measured using a microscope. Calibration was performed using 0.908% Ro YAG crystal, 1.314% Ro glass, and 3.130% Ro cubic zirconia standards.
(4) XRD. XRD analysis was conducted on powder (5 g) using a Bruker D8 Advance X-ray diffractometer operated at 40 kV and 30 mA with Cu Kα radiation. Stepwise scanning measurement was performed at a rate of 4°·min−1 in the range of 3°-85°. Semi-quantitative measurements of the mineral composition were obtained using XPower software.
(5) Porosity and permeability. The total porosity of the shale was determined based on the difference between the bulk and grain densities. The grain density of the cylindrical shale blocks was measured using helium pycnometry. Fully refined paraffin wax was used to isolate the shale plugs from water. The bulk density was measured by weighing the shale block in air before and after paraffin coating. The coated samples were then weighed in water to calculate their total volume (shale and paraffin). Consequently, the volume of the shale block was equal to the difference between the total volume and the paraffin volume. Subsequently, the bulk density was obtained from the relationship between weight and volume.
The permeabilities of the core plugs were measured using a Corelab CMS300 system under effective stress of 500 psi (1 psi = 6894.757 Pa). A chamber with a known volume was filled with nitrogen at a certain pressure. The gas was released into the atmosphere through the core sample. The decrease in pressure over time was recorded, which allowed the permeability to be determined. A combination of the Darcy, Klinkenberg, and Forchheimer equations was applied to determine the permeability of the core samples [
54], [
55].
The combined uncertainties of the porosity and permeability measurements were obtained from all relevant sources, including variations in sample preparation (plugging, cutting, and grinding), core shapes, core weighing, density assumption of paraffin wax, testing environment (temperature and humidity), and standard volume calibration. The maximum standard deviations of the porosity and permeability calculated from three repeated measurements were 0.1% and 0.02 μD (1 μD= 1 × 10−6 μm2), respectively.
(6) Optical microscopy and FE-SEM. For optical microscopy, thin-section samples perpendicular to bedding were prepared by cutting the drill samples, impregnating them at approximately 7-8 mm long with epoxy, and polishing them down to a thickness of approximately 30 μm after hardening. They were observed under a Zeiss HAL100 optical microscope using magnification of 40× under cross-polarized light.
Small shale blocks (1.0 cm × 0.5 cm × 0.5 cm) were polished by Ar ion milling (IM4000, Hitachi High-Tech, Japan) with an accelerating voltage of 3 kV and a milling time of 2 h. FE-SEM images were obtained using a Hitachi S8010 system associated with an energy-dispersive spectroscopy (EDS) system. The two-dimensional FE-SEM images evidently showed the nanopores in the shales and were used to identify the pore types, locations, and arrangements. EDS measurements with an accelerating voltage of 15.0 kV and a resolution ratio of 130.2 eV were conducted to obtain the elemental compositions.
(7) Gas content. To ensure that the parameters were comparable, the gas content of the shale in this study was exclusively acquired using the on-site desorption method, which is considered the most direct approach for measuring gas content. The shale core was promptly loaded into a sample tank once it was lifted to the wellhead during drilling and coring. Subsequently, the total gas content released from the shale was measured under conditions that simulated the formation temperature.
(8) Fossil investigations. Macroscopic fossils in well W207 were observed and photographed by the PetroChina Southwest Oil & Gasfield Company. Black shale samples were collected every 5 m from the well and sent to the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences (NIGPAS) for palynological analyses. Small carbonaceous fossils were extracted using the following protocol: First, they were macerated in 10% HCl for 12 h and then in 40% HF for one week. Subsequently, the solution was diluted to neutral pH and boiled in 30% HCl. After another round of dilution to neutral pH, the remains were processed using a double-layer sieve with mesh sizes of 100 and 10 μm. The sieved samples were observed, selected, and photographed using a Nikon Eclipse Ni optical microscope coupled with a Nikon DS-Fi1c charge-coupled device.
4. Temporal correlations between the deposition of the Qiongzhusi Formation and the Cambrian Explosion
The lower Cambrian Qiongzhusi shale is the dominant source rock with a high hydrocarbon generation potential [
8], [
56]. Across the Ediacaran-Cambrian transitional interval (560-520 Ma), the rise of metazoans in the Earth’s ecosystem, referred to as the Cambrian Explosion, represented an unprecedented and unique key biological evolution event in Earth history [
57], [
58], [
59]. During this evolutionary event, a diverse range of skeletalized animal fossils (small shelly fossils (SSFs)) first burst at the base of the Fortunian and thrived until the end of Age 2, followed by Burgess-shale-type fossil fauna such as the Chengjiang fauna, which marked the peak of the Cambrian Explosion and demonstrated the intricate complexity of the early Cambrian marine ecosystem [
59]. Despite its evolutionary significance, the influence of the Cambrian Explosion on the OM enrichment of the early Cambrian Qiongzhusi shale remains unclear, partially because of the limited resolution of the Cambrian stratigraphic framework.
A precise chronostratigraphic framework is a prerequisite for understanding the relationship between the deposition of the organic carbon-rich Qiongzhusi shale and the occurrence of major biotic events. In comparison to other chronostratigraphic approaches, such as biostratigraphy and carbon isotope chemostratigraphy [
60], cyclostratigraphic analysis has the advantage of providing continuous temporal calibration for major events with resolutions as fine as 20-400 kiloyears (kyr) [
61], [
62]. Liu et al. [
47] conducted a cyclostratigraphic analysis using GR log data from three Qiongzhusi shale wells (ZY1, W207, and JY1) across the DART from west to east and established an absolute astronomical time framework using an anchor U-Pb age of (526.86 ± 0.16) Ma from the base of the Qiongzhusi Formation. According to this stratigraphic correlation, the interval exhibiting high gas production in reservoir 3 (4585-4605 m) of Z201 corresponds to a stratigraphic layer of approximately 3130 m at W207 and 3400 m at JY1 (
Fig. 2). These layers are located in lower stage 3 with an age of approximately 520 Ma, corresponding to the main phase of the Cambrian Explosion (
Fig. 2; cf. Ref. [
59]). Although comprehensive fossil studies of the Qiongzhusi shale in well Z201 are currently lacking, abundant diagnostic fossils of lower Cambrian stage 3 are found in the Qiongzhusi shale of the adjacent well W207 (
Fig. 3), supporting the cyclostratigraphic correlation. The synchronicity between the Cambrian animal colonization and deposition of the organic-rich Qiongzhusi shale indicates a potential link between major evolutionary events and hydrocarbon generation. This hypothesis is further discussed in Section 5.4.
5. Source rock evaluation and OM enrichment mechanism
5.1. OM abundance
OM serves as the basis for hydrocarbon generation [
63], [
64]. Throughout the Є
1q1 period, the deep-water shelf facies were widely distributed in the Sichuan Basin. Owing to the uplift of the Kangdian land, infilling and leveling of the DART, and a gradual decrease in sea levels, the Sichuan Basin experienced increasing oceanic oxygen levels and an enhanced influx of terrestrial debris during the deposition of the Qiongzhusi Formation [
65]. Therefore, the Qiongzhusi Formation exhibits a trend of increasing siltstone content, lighter rock color, and lower TOC content from bottom to top (
Fig. 2) [
44], [
45], [
46], [
47]. In well Z201, the average TOC contents of the 1st, 3rd, 5th, and 7th sublayers were 4.4, 2.8, 2.3, and 1.1 wt%, respectively, showing a decreasing trend stratigraphically upwards.
Horizontally, the TOC content within the Qiongzhusi shale varied from 0.1 to 7.6 wt%, where areas with TOC exceeding 1.0 wt% accounted for over 70.0% of the basin (
Fig. 4(a)) [
66]. The deep-water continental shelf is conducive to the deposition of organic-rich shale; therefore, the contour map showing the TOC content of the Qiongzhusi shale is consistent with the distribution of source rock thickness (cf.
Figs. 1(a) and 4(a)) [
66]. High TOC content (typically 3-7 wt%) in the northern part of the DART may indicate good hydrocarbon generation potential. In the southern part, the TOC content was within the range of 2.0 to 3.5 wt%, whereas these values were 2.0-3.0 wt% in the central part. The shales on the platforms contained TOC contents of less than 2.0 wt%, showing limited hydrocarbon generation potential compared to that inside the DART.
5.2. OM maturity
OM maturity is a crucial indicator of the hydrocarbon generation potential of source rocks [
67], [
68]. Within the basin, the
Re values of the Qiongzhusi shale samples ranged from 2.4% to 4.2% (
Fig. 4(b)) [
69]. The
Re variations were attributed to differences in regional subsidence and magmatic activity. Affected by the Caledonian movement, the northwestern margin of the basin experienced an uplift during the late Silurian. Consequently, the Qiongzhusi Formation was mainly distributed around the ancient uplift and gradually deepened toward the eastern part of the basin, where kerogen maturity continued to increase from the early Paleozoic through the Mesozoic. Thermal evolution ceased during the Mesozoic, resulting in the high maturity of the OM, with a maximum
Re at approximately 4.0%. In the southwest of the basin, hydrocarbon generation of the Qiongzhusi shale accelerated due to the heating of the Emeishan mantle plume during the late Permian [
70], leading to
Re values generally exceeding 3.5%. However, in the central-northwest of the basin, the Qiongzhusi shale located within the DART was delayed in hydrocarbon generation, with rapid maturation commencing only during the middle to late Permian. Therefore, compared with other regions of the basin, the maturity of the Qiongzhusi shale within the DART is relatively low, but it has entered a high-maturity stage (
Re > 2.5%) (
Fig. 4(b) [
69]).
5.3. OM type
The hydrocarbon generation potential of source rocks is profoundly influenced by OM type; however, this is not a critical problem for the Qiongzhusi shale, as the high plants had not developed in the Cambrian, and OM is predominantly derived from algae and phytoplankton forming type I kerogen. This was verified by the experimental results. The average carbon isotope of organic matter (δ
13C
org) of the Qiongzhusi shale is approximately −33‰ [
71], suggesting the abundance of type I kerogen, which typically has a δ
13C
org below −28‰ [
72]. Additionally, the macerals of the Qiongzhusi shale are mainly composed of sapropelinite, which accounts for 95% [
73], and hydrocarbon-generating organisms are dominated by amorphous flocculent organic aggregates (ostracods, foraminifera, and algal microbial mats [
42]), with a minimal difference between the aggregates inside and outside the DART.
5.4. OM enrichment mechanism in the Qiongzhusi shale
The formation mechanism of organic-rich sediments is linked to the interplay of productivity, preservation, and dilution. High primary productivity provides more OM, part of which is deposited and preserved on the seafloor, while most OM is demineralized through aerobic respiration while sinking in the seawater column [
51], [
74]. OM preservation benefits from water column stratification and associated bottom water anoxia [
74], [
75], [
76]. Relatively high sedimentation rates facilitate the rapid burial of OM without strong degradation, whereas an excessively high rate can increase the flux of detrital debris into the basin, consequently diluting the sedimentary OM [
77]. In this study, we analyzed the sedimentary environment of the Qiongzhusi shale around the DART and discussed the controlling factors for OM enrichment in Є
1q1
1, Є
1q1
2, and Є
1q2 shales, respectively.
The OM enrichment of Є
1q1
1 shale potentially benefits from the higher primary productivity and euxinic bottom water. The biogenic Cu and Ni (Cu
bio and Ni
bio) abundances in marine sediments are robust productivity indicators, with higher values indicating higher productivity [
78]. The Є
1q1
1 shale from multiple wells showed high Cu
bio and Ni
bio (
Fig. 5) [
40], [
45], [
48], [
49], [
79], [
80], [
81], [
82], [
83], [
84], [
85]. High primary productivity is unlikely to be caused by enhanced nutrient uptake from hydrothermal activity. Hydrothermal activity, mainly occurring in the marginal area of the Yangtze platform [
86], [
87], did not develop within the DART, as shown in the Al-Fe-Mn diagram (
Fig. 6) [
40], [
45], [
49], [
84], [
88], [
89]. This is more likely attributable to the upwelling of nutrient-rich deep waters during the early Cambrian transgressive [
41], [
90]. Additionally, the Є
1q1
1 shale was deposited in euxinic condition, as reflected by high Mo
EF/U
EF ratios ((1-3) × present-day seawater) [
40], [
45], [
49], as well as iron-speciation parameters Fe
HR/Fe
T > 0.38 and Fe
py/Fe
HR > 0.7 (where Fe
HR is highly reactive iron, Fe
T is total iron, and Fe
py is pyrite iron) (
Fig. 5,
Fig. 6) [
48]. The euxinic condition was explained by a slightly poor hydrographic connection between the DART and open ocean because the Mo/TOC ratios of Є
1q1
1 shale were 3.3-44.6 (average 16.0), located between values for modern moderately (Cariaco Basin) and strongly restricted (Framvaren Fjord) environments (
Fig. 6). The moderate restriction during the deposition of Є
1q1
1 shale is linked to robust tectonic development [
37], [
39]. Although the sea level increased at this stage, the development of the DART and Sichuan central uplift potentially hampered water circulation.
The OM enrichment in the Є
1q1
2 shale is potentially controlled by the alteration of the biological pump. During the deposition of Є
1q1
2 shale, the oxygen level of seawater increased, which was reflected by decreased Mo
EF/U
EF, Fe
HR/Fe
T, and Fe
py/Fe
HR (
Fig. 5,
Fig. 6) [
48], consistent with the continuous increases of δ
98Mo to modern seawater levels during the Cambrian Explosion [
91]. Considering that the deposition of Є
1q1
2 shale coincided with the main stage of the Cambrian Explosion (
Fig. 2,
Fig. 3), where the proliferation of metazoans potentially promoted OM enrichment by enhancing the efficiency of the biological pump [
92]. Specifically, the OM generated by algae and plankton is characterized by its small volume and slow settling in the water column, with only a small fraction preserved in sediments. However, animals occurring during the Cambrian Explosion could feed on plankton and generate larger organic fragments and fecal pellets, thereby accelerating their physical settling rate to sediments (cf. Refs. [
93], [
94]). Although no significant changes in Cu
bio and Ni
bio were observed from the Є
1q1
1 to Є
1q1
2 period (
Fig. 5), the biological pump could largely benefit the preservation of OM [
94], [
95], [
96]. Therefore, as compensation for the oxic bottom water condition, the enhanced biological pump in the Є
1q1
2 likely contributed to the enrichment of OM. In addition, the deep DART hampered the exchange between bottom water and surface water [
97], which created better preservation conditions, thus explaining the higher TOC content of the Є
1q1
2 shale within the DART than that outside the DART (
Fig. 2).
The TOC content of the Є
1q2 shale was significantly low (< 1 wt%), which was likely affected by a combination effect of lower primary productivity, higher sedimentation rates, and oxic bottom water. A major regression event occurring at this stage resulted in a weakening of nutrient-rich water upwelling, thus limiting the primary productivity, as reflected by low Cu
bio and Ni
bio in the Є
1q2 shale (
Fig. 5). In addition, the low sea level led to extensive exposure of the continental shelf and intensified the weathering influx of terrigenous detritus into the DART, as reflected by the enhanced Al content (
Fig. 5). Enhanced weathering caused more damage to OM preservation by diluting the OM in the sediments rather than enhancing primary productivity by bringing abundant nutrients to the basin [
45], [
97]. Additionally, the lower sea level allowed better bottom water ventilation, which was also unfavorable for OM preservation of the Є
1q2 shale.
Overall, the OM enrichment of the shales in the different stages of the Qiongzhusi Formation showed a close relationship globally with the coevolution of the biological and environmental spheres and regionally with the local geological settings and tectonic movements, especially the different features inside and outside the DART.
6. Evaluations of shale reservoir properties
Exploration practices in north America have demonstrated that the most productive shale gas reservoirs exhibit excellent physical properties, including considerable thickness, favorable brittleness, high TOC content, porosity, permeability, and gas saturation [
98], [
99]. These parameters also performed well in some reservoir layers of the Qiongzhusi Formation [
100], possibly leading to the current high shale gas production in well Z201.
6.1. Thickness
The continuous thickness of organic-rich shales serves as a crucial parameter for assessing the quality of shale gas resources and estimating the production potential of the target strata, particularly for the black shales with a TOC content ≥ 2 wt% [
101]. Exploration practices in north America have demonstrated that a minimum thickness of 15 m is necessary for large-scale shale gas production [
102].
The Qiongzhusi Formation inside the DART exhibits thicker organic-rich shale reservoirs and contains four potential reservoirs, 1, 2, 3, and 4, from bottom to top (
Fig. 2;
Table 3). However, not all reservoirs can be found outside the DART. For example, reservoir 2 is not present in well JY1, and reservoirs 1, 2, and 4 are absent from well GS1 (
Fig. 2). Reservoir thicknesses vary as a result of sea-level changes, tectonic uplift, and/or erosion. The basal part of the Є
1q1
1 shale was deposited during the early stage of marine transgression, which likely resulted in relatively thinner reservoirs 1 and 2, with thicknesses of approximately 5-20 and 0-30 m, respectively. The Є
1q1
2 shale was widely deposited on the deep-water shelf, resulting in reservoir 3 exhibiting the maximum thickness both inside and outside the DART of 18.0-30.0 and 2.5-18.0 m, respectively. The Є
1q2 shale formed during a regressive period marked by shallow-water shelf sedimentation and experienced uplift and erosion during the late Caledonian period [
70], resulting in reservoir 4 being relatively thinner, ranging from 1 to 7 m (
Table 3).
6.2. Brittleness
Brittleness, determined by the content of brittle minerals, such as quartz, feldspar, and carbonate, is a crucial parameter in determining shale reservoir porosity (detailed depiction in Section 6.3) and engineering feasibility, both exerting a direct impact on shale gas production [
103], [
104], [
105]. For example, shale gas reservoirs commercially developed in North America typically have quartz contents over 20%, and some can even reach 75% [
102]. The shale gas industry in China has established a brittleness standard in which the brittle mineral content exceeds 40% and the clay mineral content is less than 30% [
106].
Reservoirs 3 and 4 had higher brittleness than the above standard and were more favorable inside the DART (
Fig. 7(a);
Table 3). Both were targeted for horizontal hydraulic fracturing in three shale gas wells: Z201, W201-H3, and JY1 (locations are shown in
Fig. 7(a)). Specifically, reservoir 3 in the Z201 (inside DART) and W201-H3 (outside DART) had brittle mineral contents of 80.6% and 66.6%, respectively, while reservoir 4 in JY1 had 57.0% (
Table 4). The higher brittleness of the reservoirs inside the DART is conducive to hydraulic fracturing.
6.3. Pores and porosity
The storage capacity of shale gas reservoirs is a quantitative index for measuring reservoir quality and is primarily controlled by storage space, porosity, and permeability [
107]. Shale primarily contains two types of storage space: matrix pores and fractures. Matrix pores can be divided into OM and inorganic mineral (IM) pores, which constitute the primary spaces for shale gas storage. Open fractures, bedding fractures, and microfractures are the main pathways for shale gas flow. Collectively, a higher reservoir porosity corresponds to a larger storage space for shale gas, whereas increased reservoir permeability enhances the flow capacity of shale gas [
101], [
108]. The recommended standards for favorable marine shale reservoirs in China are porosity exceeding 4% and permeability greater than 0.1 mD [
106].
In the Qiongzhusi shale, a higher proportion of OM pores was observed in the reservoir located inside the DART than that outside, whereas IM pores were abundant both inside and outside. OM pores inside the DART were predominantly found within kerogen or bitumen, formed during the thermal maturation of OM, and exhibited diverse shapes such as elliptical, elongated bubble-like, and irregular polygons (
Figs. 8(a) and
(b)). They showed significant heterogeneity, with pore sizes ranging from approximately 20 to 300 nm. IM pores inside the DART primarily comprised brittle mineral pores and clay mineral interparticle pores, with pore sizes varying from 100 nm to 1 μm (
Fig. 8(c)). The brittle mineral particles created sturdy frameworks through mutual contact, resulting in irregularly shaped pores (
Fig. 8(d)). Outside the DART, the OM pores were less developed or smaller than those inside (
Figs. 8(e) and
(f)). In addition, the sizes of the IM pores ranged from 100 to 300 nm, which were smaller than those inside the DART. The clay mineral interparticle pores exhibited random orientations, generally in the form of triangular or flake-like structures (
Fig. 8(g)), and brittle minerals were frequently observed near the clay mineral interparticle pores (
Figs. 8(g) and
(h)). This is attributed to the relatively solid nature of brittle minerals, which usually provide substantial rigid support for soft clay minerals, thereby reducing the damage to clay mineral interparticle pores caused by compaction and diagenesis [
102], [
105].
Reservoirs 3 and 4 exhibited favorable porosity characteristics, with higher porosity observed inside the DART than outside (
Fig. 7(b)). Reservoir 4 had higher porosity (2.4%-5.7%) than that of reservoir 3 (1.1%-4.1%;
Table 3). The porosities of the aforementioned wells (Z201, W201-H3, and JY1) were 5.6%, 5.5%, and 4.7%, respectively (
Table 4), exceeding the porosity threshold of 4% for favorable marine shale gas plays in China.
Overall, the reservoir porosity inside the DART surpassed that outside (
Table 3), which is likely attributable to the differences in OM abundance, thermal maturation, and brittle minerals. Although the OM of the Qiongzhusi shale inside and outside the DART is mainly derived from aquatic organisms and algae, with type I kerogen as the dominant source (Section 5.3), the OM content of the reservoirs inside the DART (mostly TOC > 3 wt%) was more abundant than that outside (mostly TOC < 2 wt%;
Fig. 4(a)), causing the formation of more OM pores. In addition to the lower TOC content, the OM outside the DART is subject to higher thermal maturation than the inside (
Fig. 4(b)), possibly resulting in graphitization and the collapse of the OM pores [
109]. The reservoirs inside the DART contained more brittle mineral content (61%-82%) than those outside (47%-82%), which provides more rigid frameworks to support OM and reduces the possibility of their collapse due to compaction [
102].
6.4. Gas content
The gas-bearing properties can be quantitatively evaluated using an array of parameters, such as the formation pressure coefficient, gas saturation, and gas content, of which the gas content is the most crucial and direct parameter for evaluating the quality of the shale gas reservoir and gas production potential [
102]. Commercially developed shale gas reservoirs in north America exhibit gas content ranging from 1.10 to 9.91 m
3·t
−1, while a gas content threshold of > 2 m
3·t
−1 is used for favorable marine shale gas reservoirs in China [
106].
Shale reservoirs 3 and 4 exhibited good gas-bearing properties, with the gas content of the reservoirs inside the DART being higher than that outside (
Fig. 7). This trend can potentially be attributed to a combination of factors, including reservoir thickness, OM abundance, brittle mineral content, porosity, thermal effects, and gas preservation conditions. As mentioned above, the reservoirs inside the DART exhibited greater thicknesses and porosities, providing ample storage space for shale gas (
Table 3). Importantly, the tight shales above and below the Qiongzhusi Formation inside the DART are well-developed, hindering the escape of shale gas [
110]. Additionally, the reservoirs inside the DART are located away from significant faults. Therefore, they experienced minor tectonic uplift and exhibited limited microfractures, both promoting the preservation of shale gas (Ref. [
100]; details in Section 7).
Well Z201 inside the DART and W201-H3 well outside the DART both achieved industrial gas flows after fracturing reservoir 3, with the test gas production of well Z201 being 26 times higher than that of well W201-H3 (7.388 × 10
5 vs 2.83 × 10
4 m
3·d
−1;
Table 4). However, they had comparable OM abundances (TOC 4-6 wt%), porosity (approximately 5.5%), and brittle mineral content (66.6%-80.6%). The Z201 has some favorable characteristics that may benefit gas production (
Table 4), including 6.4 times greater reservoir thickness (13.5 vs 2.1 m), three times higher gas content (9.7 vs 3.2 m
3·t
−1), 2.5 times longer in the horizontal length of hydraulic fracturing (1800 vs 738 m), three times higher in the fluid pressure (91 vs 27 MPa) and better preservation conditions (“upper capping and lower plugging” configuration, Section 7.1 for details). Overall, these factors collectively contributed to the higher shale gas content and better fracturing conditions for reservoir 3 inside the DART than those outside, enabling the higher shale gas production of well Z201.
7. Shale gas preservation
The commercial production potential of shale gas relies heavily on its preservation conditions, which are usually favored by a relatively stable tectonic background, as in the case of north American shale gas plays [
111]. However, pronounced tectonic activities in southern China have complicated the preservation of shale gas in the Sichuan Basin [
112]. Previous studies have clarified that the preservation conditions of shale gas in southern China are controlled by a combination of factors, including the sealing ability of the roof and floor, self-sealing capability, and development of faults and microfractures [
113], [
114].
7.1. Favorable self-sealing and the roof-floor configuration for the Qiongzhusi shale inside the DART
Roofs and floors with excellent sealing capacities play a key role in preventing the vertical migration and dissipation of shale gas. The sealing capacity is linked to the lithology, thickness, and physical properties [
115]. For example, in the Silurian Longmaxi shale gas reservoir of the Sichuan Basin, the roof is silty shale, and the floor is marl and limestone, both showing low porosity, low permeability, and large thickness. This “upper capping and lower plugging” configuration efficiently inhibits the vertical escape of Longmaxi shale gas (
Fig. 9(a)) [
56], [
114]. A similar configuration was also observed in the Qiongzhusi shale inside the DART, where the Canglangpu and Maidiping Formations are the roof and floor, respectively. The Canglangpu Formation consists of fine dolomitic sandstone interbedded with shale, whereas the Maidiping Formation consists of siliceous shale and argillaceous dolomite (
Fig. 9(a)). Both formations inside the DART are characterized by large thicknesses of 100-250 m, low porosity and permeability, and an excellent sealing capacity. However, outside the DART, the Maidiping Formation is substantially thinned or even undeveloped because of the interplay of tectonic and/or sedimentary factors, causing the Qiongzhusi shale to be directly underlain by the Dengying Formation (
Fig. 9(a)), which is a weathering layer at its top with well-developed fractures dominated by dolomite. The permeable feature of Dengying Formation outside DART limits its potential as a sealing floor, forming an “upper capping and lower permeating” configuration. This drives the Qiongzhusi shale gas outside the DART to escape to the Denying or Longwangmiao reservoirs (well GS1 in
Fig. 2), which may explain the repeated instances of exploration failures within the Qiongzhusi shale.
In addition, dense shale facilitates gas preservation through its inherent sealing capacity. The escape of shale gas must overcome adsorption resistance, which is related to shale thickness and composition [
116]. In general, organic-rich shale with a large thickness and high TOC content can enhance the adsorption resistance and self-sealing capacity. From the outside to the inside of the DART, the thickness of the organic-rich shale generally increased from 120 to 170 m (
Fig. 2), and the TOC increased from an average of < 2 wt% to >3 wt%. In combination with an excellent roof-floor configuration, the self-sealing of shale likely facilitates better shale gas preservation inside the DART.
7.2. Less faults and microfractures inside the DART facilitate shale gas preservation
The influence of faults on shale gas preservation varies according to their scale, spatially superimposed patterns and active periods. The faults are divided into four grades based on their lateral extension and vertical separation distances. The first- and second-order faults primarily control the entire basin and secondary structural unit, with an extension distance exceeding 40 km and a separation distance exceeding 0.5 km. These faults often provide channels for gas to escape and destroy shale gas reservoirs within 4 km [
112]. Considering the Dingshan area in the Sichuan Basin as an example, the Longmaxi shale gas reservoirs near the Qiyueshan first-order faults show poor preservation conditions and low gas contents (1-3 m
3·t
−1), whereas those situated further away generally exhibit better preservation conditions and higher gas contents (6-9 m
3·t
−1) [
117], [
118]. The third- and fourth-order faults control the local structures with extension distances of < 40.0 km and separation distances of < 0.5 km. The development of these faults is powerful for enlarging the reservoir space, improving seepage ability, and improving the fracturing effect. Our study revealed that third- and fourth-order concealed faults are limited to the DART and surrounding areas, which may promote shale gas preservation and accumulation. In contrast, many first- and second-order faults developed along the margin of the Sichuan Basin, which likely destroyed the shale gas reservoirs, as evidenced by the absence of gas findings in the nearby ZG1 well and HS1 wells (
Fig. 9(b)) [
119].
Microfracture features, including their type, quantity, and active period, can also affect shale gas preservation. Microfracture activity at high frequencies has the potential to accelerate gas escape. For example, the gas contents of Longmaxi shale gas reservoirs are relatively low (1.2-4.4 m
3·t
−1) in the Nanchuan area, where four periods of microfractures occurred. In contrast, the gas content in the Jiaoshiba area was higher (6-10 m
3·t
−1), partially because of only two periods of microfracture activity [
120]. In the study area, the microfracture conditions were distinct between the Qiongzhusi shale inside the DART and the western platform. Microfractures are limited to the Qiongzhusi shale inside the DART, facilitating gas preservation and accumulation with a gas content as high as 9.7 m
3·t
−1. On the western platform, there are numerous high-angle shear and bedding fractures, often filled with veins composed of calcite, dolomite, and barite grains. The result of vein cathodoluminescence indicates two periods of microfracture activities [
121], which may explain the low gas content in the reservoirs there (1.7-3.2 m
3·t
−1).
7.3. High formation pressure inside the DART indicating better shale gas preservation
Well-preserved gas in shale reservoirs can cause abnormally high formation pressures, making it an effective quantitative tool for assessing shale gas preservation conditions. The formation pressure can be quantitatively assessed using the pressure coefficient, which is the ratio of the formation pressure to the hydrostatic pressure at the same buried depth. Pressure coefficients falling within the ranges of < 0.9, 0.9-1.3, and > 1.3 represent low pressure, normal pressure, and overpressure, respectively [
122]. Overpressure typically arises from the generation of hydrocarbons during deep burial [
123]. The maintenance of the overpressure is closely linked to gas preservation [
114]. In the study area, the pressure coefficients for the Qiongzhusi shale gas reservoirs gradually increased from values below 1.0 on the western platform to values exceeding 2.0 inside the DART (
Fig. 9(c)). In particular, the pressure coefficients of Z201 and GS17 (both inside the DART) were 2.01 and 2.05, respectively, indicating excellent fluid sealing and preservation conditions of the Qiongzhusi shale gas reservoirs.
8. Qiongzhusi shale gas enrichment model inside the DART and its exploration implications
Lower Paleozoic marine shale gas in the Sichuan Basin exhibits differential enrichment characteristics, initiating two shale gas enrichment models, namely the “binary enrichment” and the “ternary enrichment.” The “binary enrichment” model applies to the margin and exterior regions of the Sichuan Basin, characterized by significant tectonic alterations. This model argues that organic-rich siliceous shale deposited on the deep-water continental shelf lays the foundation for shale gas generation and accumulation and that the sealing abilities of the roof-floor and tectonic activity are crucial to shale gas enrichment [
124]. In contrast, the “ternary enrichment” model applies to shales within the Sichuan Basin, where tectonic alteration is less pronounced but thermal maturation is high. This model highlights the significance of a deep-water shelf environment, an optimal degree of thermal evolution, and excellent preservation conditions for promoting shale gas enrichment [
125]. Considering the tectonic stability of the interior DART (
Fig. 9(b)), the Qiongzhusi shale gas enrichment in this area is more like the “ternary enrichment” model.
First, the flourishing life, stable tectonics, and favorable sedimentary environment lay the foundation for shale gas generation of the Qiongzhusi Formation within the DART. The early Cambrian Explosion and deep-water continental shelf environment collectively controlled the deposition of the Qiongzhusi shale. Within the DART, the spatial distribution and quality of Qiongzhusi shale were further influenced by the development of the rift trough and the presence of euxinic water conditions. From the outside to the inside of the DART, noticeable increases occurred in the shale thickness and OM abundance, both indicating excellent hydrocarbon generation potential.
Second, an appropriate thermal maturity of the Qiongzhusi shale within the DART ensured sufficient hydrocarbon supply and high-quality reservoirs. Substantial gas generation and OM pore formation during shale maturation are generally regarded as prerequisites for shale gas enrichment; however, excessively high thermal maturity (
Re > 3.33%) may hamper shale gas enrichment for two reasons. ① The gas starts to diffuse freely once it is generated and escapes from the reservoirs; thus, the gas content in the shale reservoir results from the balance between gas generation and loss. Once the shale is over-matured, its kerogen may lose the ability to generate enough gas to compensate for its loss outside the shale, resulting in a decrease in gas saturation. ② Overmaturation may cause the OM in shale to undergo fibrosis and even graphitization, which can compress the OM pores, thereby reducing porosity and even losing the most useful pores for gas storage, thus damaging shale gas enrichment [
113]. As previously mentioned, the Qiongzhusi shale within the DART is characterized by delayed hydrocarbon generation owing to its shallow burial and limited heating by the Emeishan mantle plume, which ensures a steady supply of hydrocarbons. Moreover, the abundant porous OM (
Figs. 8(a) and
(b)) creates storage spaces for gas accumulation. These two factors enabled Qiongzhusi shale gas enrichment within the DART, with a gas saturation of approximately 80%.
Third, the favorable sealing capacity of the roof and floor ensured the effective preservation of the Qiongzhusi shale gas. Within the DART, the tight Maidiping shale serves as an excellent floor that is mostly absent outside the DART. In addition, the impermeable roof and inherent gas sealing of the reservoir further limit the vertical escape of shale gas. In addition, the limited occurrence of large faults and microfractures in the DART contributes to the preservation of shale gas in this area, as evidenced by the pressure coefficients reaching as high as 2.0.
The success of Z201 in the Sichuan Basin marks an exploration breakthrough, as it produces the highest daily gas volume among the oldest shale gas reservoirs. This discovery has significant implications for further exploration of shale gas in the Sichuan Basin. Using Z201 as a representative case, a comprehensive analysis of four sets of vertical shale gas reservoirs within the DART was conducted. Reservoirs 1, 2, and 3 showed favorable characteristics across multiple parameters, including shale thickness, TOC content, brittle mineral content, porosity, and gas content (
Fig. 10). Therefore, in addition to reservoir 3 in Є
1q1
2 that has shown high gas production, there may be promising exploration potential for reservoirs 1 and 2 in Є
1q1
1. Based on comparative analysis, we proposed a “multilayer stereoscopic development” for the future development of the Qiongzhusi shale gas in the DART, targeting shale reservoirs 1, 2, and 3 simultaneously. This suggestion is expected to yield promising gas production in the ongoing early Cambrian exploration efforts in the Sichuan Basin.
9. Conclusions
The recent achievement of high shale gas production of 7.388 × 105 m3·d−1 from the Qiongzhusi shale in well Z201 marks a breakthrough in the development of the world’s oldest industrial shale gas reservoir. This study revealed the shale gas enrichment mechanism within the DART of the Sichuan Basin and offered insights applicable to shale gas exploration and development efforts worldwide.
The following conclusions were drawn:
(1) The high-production interval (4585-4605 m, lower Є1q12) in well Z201 correlates with the bottom of the Cambrian stage 3 (∼520 Ma). The abundant fossils associated with the precise time framework indicate a link between the main phase of the Cambrian Explosion and the formation of the organic-rich Qiongzhusi shale. Together with the abundant algae that allowed high primary productivity, early animals likely accelerated the physical settling rates of OM by feeding on planktonic organisms and generating larger organic fragments and fecal pellets. This underscores the importance of biotic turnover in shaping the sedimentary environments and hydrocarbon generation.
(2) The Qiongzhusi shale reservoirs inside the DART are characterized by greater thickness, higher porosity, more abundant brittle minerals, and elevated gas content than those outside the DART. Inside the DART, abundant brittle minerals provide robust frameworks supporting both clay minerals and OM pores, and a high gas content facilitates the development of overpressure, which indicates the excellent fracability of the shale within the DART.
(3) A “ternary enrichment” model was developed to understand the Qiongzhusi shale gas enrichment within the DART. The flourishing of life, combined with the unique tectonic setting and deep-water shelf environment, laid the foundation for the generation of shale gas. Appropriate thermal maturity allows sufficient hydrocarbon supply and the development of high-quality OM pores. Favorable self-sealing, “upper capping and lower plugging” configuration, and the limited faults and microfractures collectively contribute to the effective preservation of shale gas. This model provides a predictive tool for identifying prospective shale gas reservoirs in similar geological settings globally.
(4) A “multilayer stereoscopic development” strategy was proposed for optimal Qiongzhusi shale reservoirs 1, 2, and 3 within the DART. This approach maximizes gas recovery as well as minimizes risks and costs associated with exploration and production activities in the Sichuan Basin.
Acknowledgments
This study was supported by the National Natural Science Foundation of China (U23B20155 and 42303004), China Postdoctoral Science Foundation (2023M730038), the Science and Technology Research Project for the China National Petroleum Corporation (2021DJ1802 and 2021YJCQ03), and the National Postdoctoral Researcher Program of China (GZC20233111). We also appreciate the assistance by PetroChina Southwest Oil & Gasfield Company.
Compliance with ethics guidelines
Caineng Zou, Zhengfu Zhao, Songqi Pan, Jia Yin, Guanwen Lu, Fangliang Fu, Ming Yuan, Hanlin Liu, Guosheng Zhang, Cui Luo, Wei Wang, and Zhenhua Jing declare that they have no conflict of interest or financial conflicts to disclose.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.eng.2024.03.007.
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