Rare earth element and yttrium (REY) geochemistry in carbonate reservoirs during deep burial diagenesis: Implications for REY mobility during thermochemical sulfate reduction
Graphical abstract
1) Different from dolomite and anhydrite, pore-filling TSR calcites-2 with low δ13C values (down to − 19‰) and high homogenization temperatures (from 115 °C to 220 °C) show positive Eu anomalies and an elevated super-high Y/Ho ratios, and thus the REY signatures are concluded to be used as a new effective proxy to help identify TSR 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:
- (1)
What are the REY geochemical characteristics of late stage diagenetic calcite and sulfate minerals compared to their host Lower Triassic Feixianguan Formation dolostones?
- (2)
Does burial diagenesis have an impact on marine carbonate REY distributions?
- (3)
What mechanisms control REY mobility during TSR?
- (4)
Can REY signals be used as a proxy for identifying TSR calcite?
Section snippets
Geological setting
The Sichuan Basin is located in the east of Sichuan Province, southwest China (Fig. 1). It is surrounded by the Longmenshan fold belt in the northwest, the Micangshan uplift in the north, the Dabashan fold belt in the northeast, the Hubei–Hunan–Guizhou fold belt in the southeast, and the Emeishan–Liangshan fold belt in the southwest.
The Lower Triassic Feixianguan Formation (T1f) carbonates sit within high energy facies belts and are dominated by oolitic carbonates (Zhao et al., 2005, Ma et al.,
Samples and analytical methods
More than 100 limestone and dolomite, fracture-filling calcite, anhydrite, barite, and pore-filling calcite core samples were collected from the lower unit of the Triassic Feixianguan Formation from the Puguang (PG), Maoba (MB), Luojiazhai (LJZ), Dukouhe (DKH), Po, Yuanba (YB), Longgang (LG), and Tieshan (TS) variably sour gas fields (Fig. 1) (Table 1). Polished, 25 μm thick and etched slabs and thin sections were stained with Alizarin Red S to distinguish calcite and dolomite. Selected samples
Petrography and mineralogy
The initial dolomite host rock was predominantly either micritic or granular (Jiang et al., 2013). There are two main types of oolitic dolomite in the Lower Triassic Feixianguan Formation reservoirs (Jiang et al., 2014b). The first type is a fabric-retentive dolomite (dolomite-1), composed of fine crystals (5–10 μm) containing a small proportion of fine, planar-e to planar-s dolomite crystals (Fig. 4A). The primary (replaced) ooids range in size from 150 to 500 μm and contain concentric
Paragenetic sequence
Following Jiang et al. (2014a), we have divided the diagenetic sequence of the dolomite hosted reservoir in the Feixianguan Formation into three stages defined by timing relative to TSR (Fig. 9).
The pre-TSR diagenetic processes included two stages of dolomite growth (Jiang et al., 2013, Jiang et al., 2014b); the dominant initial dolomitization (dolomite-1) of host rocks commenced at a temperature lower than about 50 °C and a depth less than 500 m, whereas recrystallized dolomites (dolomite-2)
Conclusions
- (1)
Deep burial diagenetic minerals, such as pore-filling calcite (calcite-2) and late stage fracture-filling calcite (calcite-3), anhydrite (anhydrite-3) have been defined by their petrological characteristics and high fluid inclusion homogenization temperatures. All these deep burial diagenetic minerals show coeval Feixianguan seawater 87Sr/86Sr values, suggesting a relatively closed diagenetic environment. However, the low carbon isotope values (down to − 18.9‰ V-PDB) of calcite-2 have
Acknowledgments
This work was financially supported by China Natural Science Foundation of China (Grant Nos. 41125009; 41402132; 40839906), 13th Five-Year National Key Petroleum Project (2016ZX05008003-040), and a scholarship under the International Postdoctoral Exchange Fellowship Program (grant No. 20150035) supported by the Office of China Postdoctoral Council (OCPC) (Issue No. 38 Document of OCPC, 2015).
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