Elsevier

Precambrian Research

Volume 350, November 2020, 105897
Precambrian Research

Dating and characterizing primary gas accumulation in Precambrian dolomite reservoirs, Central Sichuan Basin, China: Insights from pyrobitumen Re-Os and dolomite U-Pb geochronology

https://doi.org/10.1016/j.precamres.2020.105897Get rights and content

Highlights

  • Present-day gas reservoirs evolved from paleo-oil reservoirs via thermal cracking.

  • Gas generation took place at ca. 154 ± 21 Ma as suggested by pyrobitumen Re-Os dating.

  • Dolomite U-Pb age is correlated in timing with eruption of Emeishan flood basalts.

  • Paragenesis, coupled with fluid inclusion, constrains the time of paleo-oil filling.

Abstract

Primary gas accumulation with trillions of cubic meters of gas volumes was discovered recently in the Precambrian Dengying Formation of the Gaoshiti-Moxi block, Central Sichuan Basin, China. In this study we investigated the pervasive saddle dolomite cements and solid bitumens within karst vugs in the Dengying dolomite reservoirs. Using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) in situ U-Pb dating, we obtained a high-precision absolute age of 259.4 ± 3.0 Ma for the saddle dolomite cementation, consistent in timing with the eruption of Emeishan flood basalts (262–251 Ma). Combined with high homogenization temperatures (Th) and salinities of primary fluid inclusion, as well as rare earth element (REE) compositions, we consider that the saddle dolomite precipitation was likely related to hydrothermal activity driven by the Emeishan mantle plume.

The identification of rare surviving oil inclusions and extensive solid bitumens within the dolomite vugs is indicative of the presence of paleo-oil reservoirs. These bitumens are characterized by high equivalent vitrinite reflectance (Requ), no fluorescence, and deformed fibrous textures, typical of highly mature pyrobitumens. Gas geochemistry data indicates that only minor thermochemical sulfate reduction (TSR) has occurred. Thus the pyrobitumen was interpreted to result mainly from the thermal cracking of paleo-oils, and its associated Re-Os isochron age of 154 ± 21 Ma, representing the timing of dry gas formation. Petrographical evidence suggests that the paleo-oil charge must have taken place after sparry dolomite cementation but prior to pyrobitumen generation. This, combined with coeval aqueous inclusion Th data and published thermal-burial models, constrains the time of paleo-oil emplacement to ca. 222–205 Ma, suggesting an oil-filling event related to the Indosinian Orogeny. This study documented the first chronological data constraining the complex evolution history of a petroleum system in one of the oldest primary gas reservoirs in the world.

Introduction

Recently, a giant gas field was discovered in the Sinian Dengying Formation of the Gaoshiti-Moxi block, Central Sichuan Basin, China. This field is the first economic primary gas field encountered in Precambrian reservoirs anywhere in the world (Zhu et al., 2015). The gas reservoirs are deeply buried and have experienced a protracted geologic evolution, including multiple tectonic episodes. The evolution of the associated petroleum system was equally complex. Hydrocarbons produced from this field consist of dry gas dominated by methane (Wei et al., 2014), considered to be derived from oil (Zhu et al., 2015), and primarily sourced from the Cambrian Qiongzhusi (Є1q) shales or black mudstones in the third member (Z2dn3) of the Dengying Formation (Zou et al., 2014). However, constraining the formation age of the dry gas remains challenging. Within the gas-bearing dolomite reservoirs, bitumen is commonly encountered, occurring as vug- and fracture-filling material, implying the presence of a paleo-oil reservoir. However, the timing of paleo-oil charge and the processes leading to the emplacement of the present-day dry gas are not clear. At present, few accurate chronological data are available for the temporal evolution of dry gas associated hydrocarbon systems in Precambrian reservoirs (McKirdy and Imbus, 1992).

Although a combination of burial history modeling and fluid inclusion homogenization temperature data (Th) allows indirect dating of petroleum charge (Bhullar et al., 1999, Tseng and Pottorf, 2002), within a single reservoir, multiple equivocal age estimates can be derived from the same Th value due to non-monotonic variations in formation temperature related to uplift (Guo et al., 2016). Absolute isotope dating has shown potential for precisely constraining the timing of geological events (Parnell and Swainbank, 1990, Pevear, 1999; Mark et al., 2010, Qiu et al., 2011). For instance, the rhenium-osmium (Re-Os) isotopic systematics has been successfully used to constrain the timing of early oil generation and migration (Lillis and Selby, 2013), and oil thermal cracking related gas generation during deep burial (Ge et al., 2016, Ge et al., 2018). More recently, the application of laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) in situ uranium-lead (U-Pb) geochronology provided a new approach allowing direct dating of carbonate materials such as veins and cements in hydrocarbon bearing reservoirs (e.g., Roberts and Walker, 2016, Hansman et al., 2018, Nuriel et al., 2017, Nuriel et al., 2019). Here, we use the combined Re-Os isochron dating of bitumens and LA-ICP-MS U-Pb dating of carbonates, coupled with petrography and fluid inclusion analysis, to document the first geochronological evidence relating to the formations of pyrobitumens and saddle dolomite cements in the ancient reservoirs of Gaoshiti-Moxi block, allowing the reconstruction of the paleo-oil charge and dry gas formation history.

Section snippets

Geological setting

The Sichuan Basin, located in southwest China, is structurally complex. It evolved from a relatively stable cratonic basin during the Sinian-middle Triassic into a foreland basin during the late Triassic-Cretaceous. The basin developed on the northwest of the Upper Yangtze Craton, covering an area of approximately 1.8 × 105 km2, with a structurally controlled NE-SW trending diamond shape (Zhai, 1992) (Fig. 1A). The Gaoshiti-Moxi gas field was discovered in a paleo-uplift zone in the central

Petrographic observations

A detailed description of Z2dn4 drill cores in the Gaoshiti-Moxi region was undertaken to allow the classification of lithology, grain size, and mineral phases. Forty-nine cores from eleven wells were sampled to create 30 μm thick, doubly polished thin sections for petrographic investigations, in order to examine cross-cutting relationships between different mineral phases using a Nikon 80I microscope equipped with transmitted, reflected, and ultraviolet light. Cold cathodoluminescence (CL)

Petrography

Hand specimen and thin section examinations of the Z2dn4 dolomites indicated that there are two main dolomitic textures: matrix dolomite and coarse, sparry cement dolomite. The matrix dolomite phase predominantly consists of algal dolomites and fine to coarse crystalline grain dolomites. The coarse crystalline dolomite phase was observed as cements lining the walls of dissolution vugs and fissures in matrix dolomites (Fig. 3A-B). They account for approximately 10–20% of total dolostone volume.

Bitumen origins

Bitumen, as a residual by-product of organic matter, could have been formed throughout the entire process of hydrocarbon evolution. Several natural secondary alteration processes may also lead to the formation of bitumen.

During biodegradation, small molecular components of crude oil are preferentially consumed, resulting in the enrichment of asphaltenes (Milner et al., 1977). However, biodegradation can only occur at temperatures below 80 ℃ (Head et al., 2003), while the minimum trapping

Conclusions

This study reconstructs several key moments in the petroleum system evolution of a gas field hosted in Precambrian sedimentary sequences in the Gaoshiti-Moxi block, Central Sichuan Basin, China, using pyrobitumen Re-Os and dolomite cement U-Pb geochronology, coupled with detailed petrographic and fluid inclusion analysis. The following conclusions can be drawn:

  • (1)

    Pervasive, vug-filling bitumens in the Dengying dolomite reservoirs are typically high-maturity pyrobitumens, derived from the thermal

CRediT authorship contribution statement

Ao Su: Conceptualization, Methodology, Software, Writing - original draft. Honghan Chen: Project administration, Supervision, Funding acquisition. Yue-xing Feng: Investigation, Writing - review & editing. Jian-xin Zhao: Supervision, Writing - review & editing. Ai Duc Nguyen: Data curation. Zecheng Wang: Funding acquisition. Xiaoping Long: Investigation.

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

We would like to thank the chief editor (Prof. Guochun Zhao) and the two reviewers (Prof. Tonguc Uysal and an anonymous reviewer) for their constructive reviews. This research was funded by the Key Project of Natural Science Foundation of China (No.41730421) and National Science and Technology Major Project (No: 2016ZX05004-001). Su’s visit to the University of Queensland was supported by an overseas joint-training Ph.D scholarship from China Scholarship Council. Zhao and Feng acknowledge the

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