Fine-grained carbonate formation and organic matter enrichment in an Eocene saline rift lake (Qianjiang Depression): Constraints from depositional environment and material source

doi:10.1016/j.marpetgeo.2022.105534

Marine and Petroleum Geology

Volume 138, April 2022, 105534

https://doi.org/10.1016/j.marpetgeo.2022.105534 Get rights and content

Highlights

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The multiple source materials participated in the formation of the inter-salt strata.
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Volcanic activity affected organic matter enrichment in saline lakes.
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Hydrothermal fluids affected the nucleation of inter-salt carbonate.

Abstract

Carbonate-rich fine-grained rocks in saline lakes have become potential targets for shale oil and gas exploration. However, few studies have investigated the processes of carbonate formation and organic matter enrichment in hypersaline environments. The Qianjiang Depression is a hypersaline rift lake basin that contains carbonate-rich fine-grained rocks in the inter-salt strata of the Qianjiang Formation. In this study, sedimentological observations and multiple geochemical analyses were conducted on the inter-salt strata in the sixth and eighteenth rhythmites of the Qianjiang 4 Member. The results showed that the middle inter-salt stratum in the sixth rhythmite is rich in dolomite, calcite, and organic matter, whereas the inter-salt stratum in the eighteenth rhythmite is rich in dolomite and sulfate minerals and low in organic matter. Comparisons among multiple redox parameters revealed that the middle inter-salt stratum in the sixth rhythmite was formed in a deep saline lake, while the inter-salt stratum in the eighteenth rhythmite was formed in a shallow saline lake. Further, sedimentary observations and analyses of δ³⁴S and trace and rare-earth elements revealed that the inter-salt stratum in the sixth rhythmite contained a significant amount of volcanic ash, whereas that in the eighteenth rhythmite was influenced by hot brine derived from older strata. Therefore, the differing environments and material inputs notably affected the carbonate crystallization and organic matter enrichment in these strata. This study suggests that the input of volcanic materials is conducive to improving biological productivity in saline lakes, and anoxic conditions with moderate salinity are conducive to the preservation of organic matter. These findings provide theoretical guidance for future self-sourced inter-salt shale oil reservoir explorations.

Introduction

In conventional petroleum reservoirs, evaporites and mudstones formed in evaporitic environments are solely regarded as cap rocks (Warren, 1986). Although shale oil and gas exploration has brought significant attention to fine-grained rocks, the traditional view that hypersaline environments are not conducive to biological survival (Clark and Philp, 1989) led to the consideration of inter-salt fine-grained rocks as inefficient exploration targets (Song et al., 2000). However, in recent years, many shale oil and gas resources have been successively discovered in continental saline formations in China (Kuang et al., 2012; Liang et al., 2017). The fine-grained rocks in these formations were mostly developed in saline-alkaline environments associated with evaporite and carbonate-rich minerals (Guo et al., 2020; Xia et al., 2020). Therefore, the roles of saline lake deposits in continental shale oil and gas exploration have been reevaluated in recent studies (Kong et al., 2020b; Li et al., 2020).

First, the influence of the evaporative environment on organic matter enrichment has been reviewed (Kong et al., 2020b). Previous studies have suggested that while biodiversity decreases with increasing salinity, the amount of life surviving in extreme environments does not necessarily change significantly (Clark and Philp, 1989). Warren (2016) further indicated that evaporative environments can have high primary productivity under suitable salinity conditions. An appropriate increase in salinity is also beneficial for enhancing water stratification and preventing the oxidation of organic matter (Kong et al., 2020a; Mansour et al., 2020a, b). Second, a new understanding of the reservoir types of shale oil has been gained (Jiang et al., 2021). Fine-grained rocks formed in saline lakes are rich in carbonate minerals rather than clay materials (Kong et al., 2017; Liang et al., 2017). These fine-grained rocks can develop intercrystallite pores between carbonate crystals, which can provide reservoir space for hydrocarbons (Kong et al., 2019; Zhang et al., 2019). Therefore, fine-grained rocks developed in saline lakes can have hydrocarbon generation and storage potential.

Recent studies have investigated the mechanism of organic matter enrichment and carbonate nucleation in saline lakes from the perspective of material input (Wright, 2012; Jiang et al., 2021). Multiple input sources, including influxes of weathered materials (e.g., carbonate rocks and basalt), volcanic ash falls, hydrothermal fluid releases, and seawater intrusion, can provide nutrient elements to increase biological productivity (Langmann et al., 2010; Wei et al., 2018; Xiao et al., 2021), and also carry Ca²⁺ and Mg²⁺ to support the crystallization of carbonate (Gierlowski-Kordesch, 1998; Capo et al., 2000; Liang et al., 2018; Zhu et al., 2019).

The Qianjiang Depression is an Eocene hypersaline lake basin (Huang and Hinnov, 2014). Previous studies have suggested that the Eocene Qianjiang Formation developed nearly 200 salt rhythmites composed of alternating layers of halite and carbonate-rich fine-grained rocks with variable organic matter content (Grice et al., 1998; Kong et al., 2020b). Therefore, the Qianjiang Formation is an ideal succession to analyze the differential enrichment of organic matter in saline lakes. Previous studies focused on the estimates of source rock potential (Li et al., 2018) and reservoir quality (Zhang et al., 2019) of the Qianjiang Formation. However, few studies have analyzed the common background and relationship between organic matter enrichment and carbonate crystallization in saline lakes from the perspective of the depositional environment and material source, limiting our understanding of inter-salt fine-grained rocks.

In this study, two inter-salt strata with different organic matter contents were selected for a comparative analysis of their sedimentological and geochemical characteristics. The main objective of this study was to analyze the influence of the depositional environment and material supply on carbonate mineral formation and organic matter enrichment. The results of this study provide a better understanding of the sedimentary genesis of fine-grained rocks in saline lakes with high contents of organic matter and carbonate.

Section snippets

Geological setting

The Jianghan Basin is a late Cretaceous–Oligocene rift basin located in Hubei Province, China, bounded by the Qinling-Dabie orogenic belt to the north and the Jiangnan orogenic belt to the south (Fig. 1A). Rifting in the basin started during the late Cretaceous (Dai, 1997). Since the late Cretaceous, the rifting intensity within the Jianghan Basin fluctuated, undergoing two main rifting phases: from the late Cretaceous to the early Eocene and from the middle Eocene to the Oligocene (Dai, 1997).

Sampling and methods

Core samples of the inter-salt strata in the sixth (3400.6–3405.4 m) and eighteenth (3645.2–3662.8 m) rhythmites in Well BYY2 (Fig. 1B) were selected for comparative analysis using thin section observations, whole rock X-ray diffraction (XRD) analysis, total organic carbon (TOC) and total sulfur (TS) contents, organic elemental analysis, vitrinite reflectance (Rr), major and trace elemental analyses, and isotope analyses of sulfur, carbon, and oxygen.

XRD analysis of 208 samples was conducted

Mineralogy and lithology

The XRD analysis results indicated that the inter-salt fine-grained rocks from the sixth and eighteenth rhythmites of Eq₄^L contained dolomite, calcite, clay, quartz, feldspar, pyrite, anhydrite, and glauberite (Fig. 2). However, mineral assemblages in each rhythmite differed. In the sixth rhythmite, the upper inter-salt stratum mainly contained evaporite minerals and dolomite, whereas the middle inter-salt stratum was composed of calcite and dolomite (Fig. 2A). The inter-salt stratum in the

Depositional environments in the saline lake

Depositional environments can be analyzed using redox- and salinity-sensitive trace metals (Tribovillard et al., 2006; Wei et al., 2018). For example, V, Mo and U are usually enriched in anoxic sediments (Jones and Manning, 1994; Algeo and Maynard, 2004). Compared with that of V, the accumulation of Ni in anoxic sediments is weaker (Hatch and Leventhal, 1992). Th is unaffected by redox conditions and is often enriched in clay-rich sediment (Wignall and Twitchett, 1996). Therefore, V/(V + Ni), Mo

Conclusions

The middle inter-salt stratum in the sixth rhythmite was formed in a deep saline rift lake with a stratified water column, whereas the inter-salt stratum in the eighteenth rhythmite was formed in a shallow saline rift lake under agitated water conditions. The elemental analyses showed that the development of the inter-salt strata in the Qianjiang Depression was affected by multiple source materials. The granites located around the basin might have provided the terrigenous weathering materials.

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.

Acknowledgments

The research presented in this paper was supported by the China Postdoctoral Science Foundation (grant number 2020M680624). We are grateful to the Jianghan Oilfield, SINOPEC, for support of this research. We thank editor and reviewers for their insightful comments, which have significantly improved our initial manuscript.

References (105)

T.J. Algeo et al.
Trace-element behavior and redox facies in core shales of Upper Pennsylvanian Kansas-type cyclothems
Chem. Geol.
(2004)
T.J. Algeo et al.
Environmental analysis of paleoceanographic systems based on molybdenum–uranium covariation
Chem. Geol.
(2009)
W.V. Boynton
Geochemistry of the rare earth elements: meteorite studies
R.L. Cullers
Implications of elemental concentrations for provenance, redox conditions, and metamorphic studies of shales and limestones near Pueblo, CO, USA
Chem. Geol.
(2002)
R.L. Cullers et al.
Geochemistry of the Mesoproterozoic Lakhanda shales in southeastern Yakutia, Russia: implications for mineralogical and provenance control, and recycling
Precambrian Res.
(2000)
Z. Doner et al.
Geochemical characteristics of the Silurian shales from the central Taurides, southern Turkey: organic matter accumulation, preservation and depositional environment modeling
Mar. Petrol. Geol.
(2019)
C. Dupraz et al.
Processes of carbonate precipitation in modern microbial mats
Earth Sci. Rev.
(2009)
E.H. Gierlowski-Kordesch
Carbonate deposition in an ephemeral siliciclastic alluvial system: jurassic Shuttle Meadow Formation, Newark supergroup, Hartford basin, USA
Palaeogeogr. Palaeoclimatol. Palaeoecol.
(1998)
E.H. Gierlowski-Kordesch
Lacustrine carbonates
P.A. Gonçalves et al.
The mesozoic–cenozoic organic facies in the lower tagus sub-basin (lusitanian basin, Portugal): palynofacies and organic geochemistry approaches
Mar. Petrol. Geol.
(2014)

P.A. Gonçalves et al.

Paleoenvironmental characterization of a Jurassic sequence on the Bombarral sub-basin (Lusitanian basin, Portugal): insights from palynofacies and organic geochemistry

Int. J. Coal Geol.

(2013)

P.A. Gonçalves et al.

Palynofacies and source rock potential of Jurassic sequences on the Arruda sub-basin (Lusitanian Basin, Portugal)

Mar. Petrol. Geol.

(2015)

K. Grice et al.

Molecular isotopic characterisation of hydrocarbon biomarkers in Palaeocene–Eocene evaporitic, lacustrine source rocks from the Jianghan Basin

China. Org. Geochem.

(1998)

P. Guo et al.

How to find high-quality petroleum source rocks in saline lacustrine basins: a case study from the Cenozoic Qaidam Basin, NW China

Mar. Petrol. Geol.

(2020)

J.R. Hatch et al.

Relationship between inferred redox potential of the depositional environment and geochemistry of the upper pennsylvanian (missourian) Stark shale member of the dennis limestone, wabaunsee county, Kansas

U.S.A. Chem. Geol.

(1992)

W.T. Holser et al.

Isotope geochemistry of sedimentary sulfates

Chem. Geol.

(1966)

B. Horsfield et al.

Organic geochemistry of freshwater and alkaline lacustrine sediments in the green river formation of the washakie basin, Wyoming

U.S.A. Org. Geochem.

(1994)

Z. Jiang et al.

Multi-source genesis of continental carbonate-rich fine-grained sedimentary rocks and hydrocarbon sweet spots

Petrol. Explor. Dev.

(2021)

B. Jones et al.

Comparison of geochemical indices used for the interpretation of palaeoredox conditions in ancient mudstones

Chem. Geol.

(1994)

X. Kong et al.

Organic matter enrichment and hydrocarbon accumulation models of the marlstone in the Shulu sag, bohai bay basin, northern China

Int. J. Coal Geol.

(2020)

X. Kong et al.

The tight oil of lacustrine carbonate-rich rocks in the Eocene Shulu Sag: implications for lithofacies and reservoir characteristics

J. Petrol. Sci. Eng.

(2019)

X. Kong et al.

Organic geochemical characteristics and organic matter enrichment of mudstones in an Eocene saline lake, Qianjiang Depression, Hubei Province, China

Mar. Petrol. Geol.

(2020)

L. Kuang et al.

Formation conditions and exploration potential of tight oil in the Permian saline lacustrine dolomitic rock, Junggar Basin, NW China

Petrol. Explor. Dev.

(2012)

M.J. Leng et al.

Palaeoclimate interpretation of stable isotope data from lake sediment archives

Quat. Sci. Rev.

(2004)

M. Li et al.

Expelled oils and their impacts on rock-eval data interpretation, Eocene Qianjiang Formation in Jianghan Basin, China

Int. J. Coal Geol.

(2018)

W. Li et al.

Shale oil in saline lacustrine systems: a perspective of complex lithologies of fine-grained rocks

Mar. Petrol. Geol.

(2020)

T.K. Lowenstein et al.

The Green River salt mystery: what was the source of the hyperalkaline lake waters?

Earth Sci. Rev.

(2017)

A. Makhnach et al.

Comparative analysis of sulfur isotope behavior in the basins with evaporites of chloride and sulfate types

Sediment. Geol.

(2000)

A. Mansour et al.

Depositional and organic carbon-controlled regimes during the Coniacian‒Santonian event: first results from the southern Tethys (Egypt)

Mar. Petrol. Geol.

(2020)

A. Mazumdar et al.

Sulfur and strontium isotopic compositions of carbonate and evaporite rocks from the Late Neoproterozoic–Early Cambrian Bilara Group (Nagaur-Ganganagar Basin, India): constraints on intrabasinal correlation and global sulfur cycle

Precambrian Res.

(2006)

F.-W. Meng et al.

The major composition of a middle–late Eocene salt lake in the Yunying depression of Jianghan Basin of Middle China based on analyses of fluid inclusions in halite

J. Asian Earth Sci.

(2014)

A. Michard et al.

The REE content of some hydrothermal fluids

Chem. Geol.

(1986)

J.T. Murphy et al.

Preservation of primary lake signatures in alkaline earth carbonates of the Eocene Green River Wilkins Peak-Laney Member transition zone

Sediment. Geol.

(2014)

I. Nyirő-Kósa et al.

Nucleation and growth of Mg-bearing calcite in a shallow, calcareous lake

Earth Planet Sci. Lett.

(2018)

K.E. Peters et al.

Petroleum systems in the jiangling-dangyang area, Jianghan Basin

China. Org. Geochem.

(1996)

D.A. Petrash et al.

Microbially catalyzed dolomite formation: from near-surface to burial

Earth Sci. Rev.

(2017)

C. Pflumio et al.

Hydrothermal activity in the northern tanganyika rift, east africa

Chem. Geol.

(1994)

X. Qiu et al.

High salinity facilitates dolomite precipitation mediated by Haloferax volcanii DS52

Earth Planet Sci. Lett.

(2017)

I.E. Quijada et al.

Challenges to carbonate-evaporite peritidal facies models and cycles: insights from Lower Cretaceous stromatolite-bearing deposits (Oncala Group, N Spain)

Sediment. Geol.

(2020)

D.A. Sverjensky

Europium redox equilibria in aqueous solution

Earth Planet Sci. Lett.

(1984)

N. Tribovillard et al.

Trace metals as paleoredox and paleoproductivity proxies: an update

Chem. Geol.

(2006)

K.H. Wedepohl

Environmental influences on the chemical composition of shales and clays

W. Wei et al.

Identifying marine incursions into the Paleogene Bohai Bay Basin lake system in northeastern China

Int. J. Coal Geol.

(2018)

L. Wu et al.

Deciphering the origin of the Cenozoic intracontinental rifting and volcanism in eastern China using integrated evidence from the Jianghan Basin

Gondwana Res.

(2018)

L. Xia et al.

Unsynchronized evolution of salinity and pH of a Permian alkaline lake influenced by hydrothermal fluids: a multi-proxy geochemical study

Chem. Geol.

(2020)

D. Xiao et al.

Neoproterozoic postglacial paleoenvironment and hydrocarbon potential: a review and new insights from the Doushantuo Formation Sichuan Basin, China

Earth Sci. Rev.

(2021)

Standard Test Methods for Sulfur in the Analysis Sample of Coal and Coke Using High-Temperature Tube Furnace Combustion Methods

(2008)

Standard Test Method for Microscopical Determination of the Reflectance of Vitrinite Dispersed in Sedimentary Rocks

(2011)

K.M. Bohacs et al.

Production, destruction, and dilution—the many paths to source-rock development

SEPM (Soc. Sediment. Geol.) Spec. Publ.

(2005)

M.A. Bustillo et al.

Dolomitization, gypsum calcitization and silicification in carbonate-evaporite shallow lacustrine deposits

Sedimentology

(2017)

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Citation Excerpt :
The Ts/(Ts + Tm) values in our samples ranged from 0.12 to 0.37 (Table 2) but varied with lithology (Fig. 9). Tm and Ts are sensitive to clay catalysis, and Ts/(Ts + Tm) values are typically low in carbonate rock (Moldowan et al., 1986; Kong et al., 2022). Paleoclimatic characteristics reflect the temperature and humidity during deposition and affect the supply of sediments, weathering conditions, biological species, and OM enrichment (Bush et al., 2011; Pan et al., 2021; Witkowski et al., 2014).
Jianghan Basin is the largest terrestrial saline lake basin in China, with the largest salinization area in the Paleogene. Research on salt rhythmite formations is crucial for petroleum exploration in salt lake basins. With alternating dry and wet climates, the difference in the organic matter (OM) provenance and enrichment remain controversial. In this study, petrology, element geochemistry, and molecular geochemical analyses were performed for the Qianjiang Formation to determine the depositional environment and provenance of its OM. The response of OM enrichment in the saline lake to the alternating dry and wet climates was reassessed. Three lithofacies associations (LAs) were designated: (1) LA1 was rich in argillaceous dolomite and argillaceous limestone with high total organic carbon (TOC); (2) LA3 was interbedded argillaceous glauberite and argillaceous dolomite with low TOC; (3) LA2 was a mixture of the above minerals with moderate TOC values. LA1 was deposited during desalination with a reducing environment and water stratification. The ∑C21-/∑C22+, C29/C30 hopane and maceral indicate the OM of LA1 derive from algae. Compared with LA1, LA2 was deposited with higher salinity and less algae. LA3 was deposited during intense evaporation, high salinity, and a disturbed environment. The OM sources of LA3 include higher plants and low aquatic organisms. Dry and wet climates control changes in organic enrichment. Hypersalinity is unfavorable to algae but beneficial to preservation. The stratified deep water is conducive to the accumulation of OM. The desalination is a crucial period for organic matter enrichment in the saline lake.

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Fine-grained carbonate formation and organic matter enrichment in an Eocene saline rift lake (Qianjiang Depression): Constraints from depositional environment and material source

Highlights

Abstract

Introduction

Section snippets

Geological setting

Sampling and methods

Mineralogy and lithology

Depositional environments in the saline lake

Conclusions

Declaration of competing interest

Acknowledgments

Chem. Geol.

Chem. Geol.

Chem. Geol.

Precambrian Res.

Mar. Petrol. Geol.

Earth Sci. Rev.

Palaeogeogr. Palaeoclimatol. Palaeoecol.

Mar. Petrol. Geol.

Int. J. Coal Geol.

Mar. Petrol. Geol.

China. Org. Geochem.

Mar. Petrol. Geol.

U.S.A. Chem. Geol.

Chem. Geol.

U.S.A. Org. Geochem.

Petrol. Explor. Dev.

Chem. Geol.

Int. J. Coal Geol.

J. Petrol. Sci. Eng.

Mar. Petrol. Geol.

Petrol. Explor. Dev.

Quat. Sci. Rev.

Int. J. Coal Geol.

Mar. Petrol. Geol.

Earth Sci. Rev.

Sediment. Geol.

Mar. Petrol. Geol.

Precambrian Res.

J. Asian Earth Sci.

Chem. Geol.

Sediment. Geol.

Earth Planet Sci. Lett.

China. Org. Geochem.

Earth Sci. Rev.

Chem. Geol.

Earth Planet Sci. Lett.

Sediment. Geol.

Earth Planet Sci. Lett.

Chem. Geol.

Int. J. Coal Geol.

Gondwana Res.

Chem. Geol.

Earth Sci. Rev.

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