Cenozoic exhumation and shale-gas enrichment of the Wufeng-Longmaxi formation in the southern Sichuan basin, western China

https://doi.org/10.1016/j.marpetgeo.2020.104865Get rights and content

Highlights

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    New low-temperature thermochronological data was from the SE Sichuan basin.

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    An accelerated cooling in the late Cenozoic was found.

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    Post-gas-generation deformation and exhumation controlled the shale gas enrichment.

Abstract

Based on low-temperature thermochronological data (i.e., apatite fission-track (AFT) and (U–Th)/He (AHe)), structural evolution, burial and thermal history, this paper examines the relationship of multi-stage evolution between the upper Ordovician Wufeng and lower Silurian Longmaxi formations (i.e., the WL formation) and rapid exhumation at the Changning shale gas field in the southern Sichuan basin. AFT ages are generally young from southeast to northwest, decreasing from ca. 40 to 15 Ma while AHe single-grain ages range from ~35 to ~5 Ma. Inverse thermal histories show that a multi-stage cooling history with initial cooling began in the Late Cretaceous to early Cenozoic (middle Eocene), followed by accelerated rapid cooling in the late Cenozoic, with cooling rates of 2–4 °C/Myr across the Changning shale gas field. In particular, thermal models of samples in the western Changning area show a substantial increase in the cooling rates, i.e., from less than 1 °C/Myr to 3–5 °C/Myr in the Miocene. Integrated with inverse T-t models of low-temperature thermochronological data, the burial history indicates a four-stage thermal evolution of the WL Formation in the Changning shale-gas field: Early Mature from the middle Silurian to middle Permian, Middle Mature from the late Permian to Late Triassic, Late Mature from the Early Jurassic to Early Cretaceous, and Over mature since the middle Cretaceous, reaching a maximum burial depth of ~6000 m and maximum temperature of 190–200 °C at ca. 70–80 Ma. This evolution further suggests that a normal hydrostatic pressure index dominated the WL Formations in the Paleozoic. The gas generation and pressure index, however, have substantially increased since the Late Jurassic, particularly from the Early to middle Cretaceous, with a maximum pressure index of 2.1, which is indicative of the main shale gas accumulation and enrichment period in the WL Formation. Post-gas-generation structural deformation, uplift, and exhumation had a significant impact on shale gas enrichment, mainly through their influence on shale gas preservation. This suggests some relationships among the higher pressure coefficient of the WL Formation, the higher productivity and total gas content of the WL Formation, a weaker post-gas-generation structural deformation, and a weaker uplift, as indicated by the erosion and burial depth across the Changning shale gas field. Due to a significantly stronger cooling and uplift that occurred the in Late Cenozoic across the western Changning area, the pressure index decreased rapidly from ~2.0 to 1.0 with a normal pressure condition, indicating the destruction of the reservation condition in the WL Formation.

Introduction

With the discovery of several giant gas fields in the last ten years, a significant breakthrough in gas exploration has been made in the Sichuan Basin, becoming one of the most gas-productive sedimentary basins in China (Fig. 1), e.g., the Changning and Fuling shale-gas fields, with proven reserves of 4446.8 × 108 and 6008 × 108 m3, respectively (Ma and Xie, 2018; Zhao et al., 2019). The Changning and Fuling shale-gas fields have an annual production of ~100 × 108 m3, accounting for more than 25% of the total gas production per year in the entire basin. This demonstrates that the black shale from the upper Ordovician Wufeng (O3w) and lower Silurian Longmaxi (S1l) formations (referred to as the WL formation in this study) are the most advantageous producing reservoirs of shale gas in China (e.g., Ma et al., 2018; Ma and Xie, 2018; Yi et al., 2019; Lu et al., 2020), with estimated resources of over 10 × 1010 m3 across the Sichuan Basin.

The gas-bearing black shale from the WL formation, characterized as high-to over-mature, a high total organic content (TOC), and an effective thickness of 20–80 m, was deposited widely across the southeastern Sichuan depression with a depth of 3000–5000 m across the basin (Fig. 1c). Previous studies have widely examined the depositional conditions (e.g., Ma et al., 2016; Lu et al., 2020), pore characteristics (e.g., Jiao et al., 2018; Yang et al., 2016; Wang, 2019), and enrichment mechanisms (e.g., Guo, et al., 2014; Cao et al., 2019; Yi et al., 2019) of the shale gas in the Sichuan basin. However, the WL formation experienced multiple episodes of tectonic movement and deep burial from the Paleozoic to late Mesozoic, followed by intense uplift and exhumation since the Late Cretaceous (Deng et al., 2013; Liu et al., 2016a; Chen et al., 2017). Thus, the gas-bearing black shale from the WL formation in the Sichuan Basin is characterized by complex processes in the shale-gas accumulation and adjustment (e.g., Hao et al., 2013; Liu et al., 2016a), differently from the North America and other areas (e.g., Montgomery et al., 2005). In particular, the latest uplift and exhumation in the Cenozoic yielded significant changes to the temperature and pressure conditions of the black shale in the WL formation, which are vitally important for the accumulation and preservation of shale gas (Guo et al., 2014; Yi et al., 2019). The depth of the WL formation and deformation, as well as related the thermal history, are thus key parameters for the commercial evaluation of the WL formation across the basin, as indicated by the positive relationships among the WL formation depth, its gas content, and the pressure coefficients at the Changning and Zhaotong shale-gas fields (Yang et al., 2016; Ma et al., 2018; Xu et al., 2019). However, there is a lack of studies on the structural evolution of the WL formation, such that the role of a large-scale uplift and exhumation across the southern Sichuan basin remains unknown. This paper focuses on the WL formation in the Changning shale gas field of the southern Sichuan Basin, particularly on its Cenozoic uplift, structural evolution, and related shale gas thermal histories.

The objectives of this study are as follows: (1) reconstruct the Cenozoic uplift and exhumation process of the Changning shale gas field based on low-temperature thermochronology data (i.e., apatite fission track (AFT) and apatite (U–Th)/He (AHe)), (2) reconstruct the 1-D burial and thermal evolution of the WL formation via basin modelling techniques, and (3) interpret the enrichment and destroy process of the shale gas in the WL formation by reconstructing the 2-D structural and pressure evolution.

Section snippets

Geological setting

The Sichuan Basin, located along the western margin of the Yangtze block, is an oil- and gas-bearing sedimentary basin confined to the north by the Dabashan fold-and-thrust belt, to the northwest by the Longmenshan fold-and-thrust belt, and to the southeast and southwest by the Exi-Yudong and Daliangshan fold-and-thrust belts, respectively (Fig. 1b). Several tectonic events have affected the basin since the formation of the Yangtze basement (Guo et al., 1996; Liu et al., 2018), including the

Samples and methods

Apatite fission track (AFT) thermochronology, based on the spontaneous track fission of 238U, measures the density of fission tracks to determine an age while the length of the individual confined fission tracks is related to the thermal history experienced by the grain (Gleadow et al., 1986; Green et al., 1989). Apatite (U–Th)/He (AHe) dating measures an age based on the concentrations of the daughter product (4He) and parent isotopes (238U, 235U, 232Th, and 147Sm). Retention depends on the

AFT and AHe data

AFT ages from 13 outcrop samples range from 16 ± 1 to 42 ± 3 Ma, roughly increasing from west to east across the Changning shale gas field (Table 2, Fig. 2). All samples pass the chi-square probability test (χ2), suggesting that all grains are from a single age population given the singe-grain age variation (Galbraith and Laslett, 1993). All samples are characterized by a relatively short mean track lengths, varying from 12.2 to 13.3 μm, and an asymmetric unimodal distribution, with a standard

Burial, thermal history, and hydrocarbon generation of the WL formation

As shown in Fig. 6, the WL formation in the Changning shale gas field experienced three stages of burial (1) A slow subsidence with two minor uplifts (i.e., late Silurian and late Carboniferous). from the Late Ordovician to early Permian (i.e., ca. 460–280 Ma). During this stage, the rates of subsidence and sedimentation were slow, i.e., much smaller than 30–50 m/Ma, and the upper Silurian to Carboniferous sequences were eroded with exhumation of ~200–400 m (e.g., the #N-201, #NX-202, and #GS-1

Enrichment of shale gas across the changning area

Fig. 7 shows the structural evolution of the Changning shale gas field. The Caledonian orogeny at the end of the Silurian resulted in the uplift of the present southern Sichuan Basin (Song, 1996; Yuan et al., 2013). As a result, the Shangluo Fault and minor faults in the lower Paleozoic accommodated weak deformation (Fig. 7a) while upper Silurian to Carboniferous sequences were eroded in the Changning area. From the deposition to the middle Triassic, the WL formation was affected by a slow

Preservation and enrichment of shale gas in the changning shale gas field

There are more than 300 shale gas boreholes across the southern Sichuan Basin, where the drilling results indicate that the lower section of the WL formation is dominated by clayey siliceous shale and calcareous siliceous shale, with a high TOC, high proportion of organic pores, and a high porosity (e.g., Yang et al., 2016; Ma and Xie, 2018; Liang et al., 2020; Lu et al., 2020). These properties directly resulted in shale gas enrichment in the lower section from 20 to 80 m in the WL formation.

Conclusions

This study demonstrates how post-gas-generation uplift and deformation can have a significant impact on shale gas enrichment in the WL formation across the Changning shale gas field. AFT and AHe data indicate that late Cenozoic cooling in the Changning shale gas fields was characterized by rates of 2–4 °C/Myr. In particular, there is a substantial increase in the rates, i.e., 3–5 °C/Myr, that occurred in the Miocene at the western Changning area. This suggests that maximum erosion of

CRediT author statement

All co-authors have contributed to and agreed on the final version. Author contributions as follows, Bin DENG: Conceptualization, Methodology, Writing – original draft preparation. Wenping LIU: Fielding, Methodology, Writing – original draft preparation. Juan WU: Fielding, Methodology, Writing – original draft preparation. Jianfa WU: Conceptualization, Supervision. Zheng ZHOU: Fielding, Methodology, Formal analysis. Chao LUO: Fielding, Methodology. Wei WU: Fielding, Methodology. Kun JIAO:

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This study was supported by the Natural Science Foundation of China (Nos. U19B6003, 2017JQ0025), and National Science and Technology Major Project (Nos. 2016ZX05062, 2016E-0611).

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