Rare earth element and yttrium (REY) geochemistry in carbonate reservoirs during deep burial diagenesis: Implications for REY mobility during thermochemical sulfate reduction

Highlights

• A relatively closed diagenetic environment is indicated by⁸⁷Sr/⁸⁶Sr ratios.

• TSR calcite shows a prominent positive Eu anomaly and super-high-chondritic Y/Ho ratio.

• A new proxy to help indicate TSR calcite is proposed.

Abstract

The impact of burial diagenesis (especially deep burial-related processes such as thermochemical sulfate reduction, TSR) on the distribution of rare earth elements and yttrium (REY) in diagenetic minerals in carbonate reservoir has gained little attention even though they may be unique indicators of the diagenetic system being closed or open to external influx of material. Trace element and REY concentrations, ⁸⁷Sr/⁸⁶Sr, δ¹⁸O and δ¹³C have been determined for limestone, host dolomite, pore-filling calcite (calcite-2), and late stage fracture-filling calcite (calcite-3), barite (barite-2) and anhydrite (anhydrite-3) from Lower Triassic Feixianguan Formation reservoirs in northeast Sichuan Basin, China. Calcite-2, calcite-3, barite-2, and anhydrite-3 precipitated during deep burial (from 110 °C to 220 °C), demonstrated by petrology and fluid inclusion thermometry. Sr isotope analysis revealed that diagenetic carbonate minerals and anhydrite have largely identical ⁸⁷Sr/⁸⁶Sr ratios to Triassic seawater, indicating negligible input of Sr from terrigenous sources and probably a relatively closed diagenetic environment for precipitation of these minerals. Carbon isotope analysis showed that calcite-2 has relatively low δ¹³C values (down to − 18.9‰ V-PDB), suggesting that they are TSR calcites with the carbonate derived from oxidized, isotopically-light hydrocarbons. Unlike the seawater-like REY patterns of the limestone and dolomites, calcite-3 and anhydrite-3 are enriched in rare earth elements (ΣREE) and show light rare earth element enrichment and heavy rare earth element depletion, exhibiting a chevron-like pattern of shale normalized REY trends. In contrast, TSR calcite (calcite-2) has similar ΣREE to the host dolomite but a relatively flat REY_SN pattern, suggesting strong variations in elemental and REE compositions of the burial fluids. Significantly, TSR calcite shows a prominent positive Eu anomaly and an unusually high-chondritic Y/Ho ratio. Both yttrium versus holmium fractionation and Eu^2 + oxidation to Eu^3 + must have occurred during thermochemical sulfate reduction. Hence, a positive Eu anomaly and an elevated Y/Ho ratio may be used as effective proxies to differentiate calcite resulting from TSR from ordinary calcite cement. This is especially useful when carbon isotope analysis cannot be used to give an unambiguous interpretation of the origin of the calcite.

Introduction

It has been suggested that marine carbonates could incorporate rare earth elements and yttrium (REY) in ratios that are representative of the seawater in which they grew (Webb and Kamber, 2000, Nothdurft et al., 2004). The REY signatures in carbonates have been reported to remain stable over long periods of geological time (10s to 100 s of millions of years) as long as they do not undergo total dissolution and re-precipitation (Cherniak, 1998, Azmy et al., 2011). Recent studies have shown that meteoric diagenesis, and even dolomitization, might not change the initial (ancient) seawater REY pattern (e.g., Banner et al., 1988, Qing and Mountjoy, 1994, Webb et al., 2009, Nothdurft et al., 2004, Jones, 2013, Azmy et al., 2013). In some case studies, REY patterns in carbonates have been used to investigate ancient seawater REY distributions and to assess input of material via exotic water sources during early and burial diagenesis (e.g., Banner et al., 1988, Webb and Kamber, 2000, Kamber and Webb, 2001, Nothdurft et al., 2004, Webb et al., 2009, Azmy et al., 2011).

Due to the metastable nature of some depositional carbonate minerals (aragonite and high-Mg calcite), dissolution of parts of carbonate host rocks and re-precipitation of vug-filling or fracture-filling carbonate cements are routine during burial diagenesis. Thus it is possible that REY patterns can be altered in newly formed diagenetic minerals relative to the depositional minerals. Such a scenario requires large-scale fluid flow with waters interacting with silicate rocks that are rich in REY before they enter the carbonate rocks in question (Nothdurft et al., 2004, Azmy et al., 2011). The impact of burial diagenesis on REY in carbonate rocks remains largely unknown even though REY patterns have great potential for revealing the extent of mass flux and its effect on diagenesis and reservoir quality.

Thermochemical sulfate reduction (TSR), a reaction of petroleum with mineral or aqueous sulfate to produce calcite, sour gas (H₂S), organosulfur compounds and other byproducts, has been reported in many sedimentary basins worldwide (Orr, 1974, Machel et al., 1995, Worden et al., 1995, Cai et al., 2001, Cai et al., 2004, Cai et al., 2015a, Cai et al., 2015b). In cases where calcite has demonstrably replaced (pseudomorphed) anhydrite, TSR calcite has been shown to have fluid inclusion homogenization temperatures that are normally > 120 °C. As well as pseudomorphing anhydrite, TSR calcite can grow in the pore system, physically separate from the parent anhydrite that supplied the reacted aqueous sulfate. In this case it would be helpful to have a geochemical indicator that could unambiguously prove that calcite resulted from TSR. Carbon isotope data from proven examples of TSR calcite cover a wide range of values (from − 20‰ to 2‰ V-PDB) since some of the carbonate may come from pre-existing dissolved bicarbonate (i.e., from a marine source with high values of 2‰ V-PDB) as well as from oxidized petroleum compounds that contribute isotopically light carbon (Worden et al., 1995, Worden et al., 2000, Worden and Smalley, 1996, Cai et al., 2001, Cai et al., 2004, Zhu et al., 2005, Jiang et al., 2014a, Jiang et al., 2015a, Jiang et al., 2015b). TSR calcite with δ¹³C values as high as 2‰ V-PDB (Worden and Smalley, 1996, Zhu et al., 2005) cannot be isotopically differentiated from non-TSR calcite, suggesting that carbon isotopes may not always be the best way to discern the role of TSR in calcite growth. There have been no reports on whether rare earth elements and yttrium (REY) have any potential for identifying TSR calcite.

In this study, we present petrological and geochemical data (trace and rare earth elements, Sr, C and O isotope values and fluid inclusion homogenization temperatures; incorporating some geochemical data from previous studies; Jiang et al., 2014a, Jiang et al., 2014b, Jiang et al., 2015b) for depositional limestone, early diagenetic dolomites (dolomite-1, and dolomite-2), deep burial pore-filling calcite (calcite-2), and fracture-filling calcite (calcite-3), anhydrite (anhydrite-3) and barite (barite-2), in the Lower Triassic Feixianguan Formation, Sichuan Basin, China, to investigate the impact of burial diagenesis on marine carbonate REY distributions. Specifically, we address the following questions:

• What are the REY geochemical characteristics of late stage diagenetic calcite and sulfate minerals compared to their host Lower Triassic Feixianguan Formation dolostones?

• Does burial diagenesis have an impact on marine carbonate REY distributions?

• What mechanisms control REY mobility during TSR?

• Can REY signals be used as a proxy for identifying TSR calcite?