Research paperContrasting diagenetic evolution patterns of platform margin limestones and dolostones in the Lower Triassic Feixianguan Formation, Sichuan Basin, China
Introduction
Porosity and permeability of carbonate successions generally decrease with burial depth (Schmoker and Halley, 1982, Lucia, 1995, Lucia, 2004, Sun, 1995, Ehrenberg et al., 2006). The reduction of porosity in limestones is caused predominantly by calcite cementation as a result of mechanical compaction, pressure-solution (chemical compaction)(Heydari, 2000). In contrast, deeply buried dolostone reservoirs normally show higher porosity compared to limestone because of reduced calcite cementation (Neilson and Oxtoby, 2008). The porosity of shallow-buried (<3500 m) dolostones (e.g., Pliocene-Pleistocene and Miocene dolostone) is typically equal to, or less than, the porosity in age-equivalent limestones (Lucia, 1995, Ehrenberg et al., 2006). However, there are some exceptional, shallow dolostone reservoirs that have higher porosity than their equivalent non-replaced limestones. Examples include the First Eocene reservoir at the giant Wafra Field (Saller et al., 2014), and the Miocene carbonate platforms of the Marion Plateau (Ehrenberg et al., 2006). As burial depth increases to more than 3500 m, reservoir quality of dolostones tends to be better than that of limestone equivalents (Sun, 1995, Heydari, 1997, Ehrenberg et al., 2006, Jiang et al., 2014b, Jiang et al., 2016). This has been interpreted as a result of dolostones being more resistant to porosity-loss than limestones (i.e., more resistant to mechanical and chemical compaction and cementation) during progressive burial (Schmoker and Halley, 1982). Moreover, dolostone has been commonly reported to contain enlarged dissolution pores in deep burial environments (>3500 m) (Hugman III and Friedman, 1979). However, pores in deeply buried dolostone may have commonly been lined or plugged by late stage cements during deep burial diagenesis (Heydari, 1997, Loucks, 1999, Worden et al., 2000, Worden et al., 2004, Machel and Buschkuehle, 2008, Neilson and Oxtoby, 2008, Jiang et al., 2014a).
The preservation of porosity in deeply buried dolostone reservoirs is predominately controlled by (i) the amount of remaining primary porosity (Choquette and Pray, 1970), (ii) the formation of secondary porosity due to replacement of calcite with dolomite (Sun, 1995, Machel, 2004a), although some authors have suggested that dolomite cementation could significantly reduce the reservoir quality (Lucia, 1995, Warren, 2000, Machel, 2004b), (iii) the dissolution of calcite or aragonite during dolomitization (Jiang et al., 2014b, Saller et al., 2014), as well as (iv) the preservation of the remaining early diagenetic porosity during burial (Choquette and Pray, 1970). Secondary pores generated by dissolution during deep burial diagenesis are unlikely to significantly contribute to reservoir quality, because pore fluids in sedimentary basins are typically saturated with carbonate and thus cannot dissolve carbonate minerals (Sun, 1995, Machel, 2004a, Ehrenberg et al., 2006, Ehrenberg et al., 2012, Dickson and Kenter, 2014). However, some case studies have reported that substantial porosity could be created during deep burial and/or uplift in carbonate reservoirs due to thermochemical sulfate reduction (TSR) (Ma et al., 2008a, Cai et al., 2014) and oxidation of the sulfate reduction-produced H2S (Hill, 1995).
TSR is the abiological oxidation of hydrocarbons by sulfate at elevated temperatures (generally greater than 110 °C), resulting in significant alteration of petroleum and the generation of a variety of reduced forms of sulfur (i.e., native S and H2S) and oxidized forms of carbon (carbonate minerals and CO2) as well as a combination of water, sulfide minerals, organosulfur compounds and bitumen (Machel, 1987, Machel et al., 1995, Worden et al., 1995, Worden et al., 2000, Bildstein et al., 2001, Cai et al., 2003, Jiang et al., 2015c).
A general reaction can be written as follows:sulfate + petroleum → calcite + H2S ± H2O ± CO2 ± S ± altered petroleum
Recent studies have confirmed that TSR can generate substantial amounts of low salinity water (Worden et al., 1996, Jiang et al., 2015c). Consideration of the addition of this TSR water to deeply buried carbonate reservoirs may shed new light on mesogenetic secondary porosity generation and reservoir quality improvement (Worden et al., 1996, Jiang et al., 2015c).
Moreover, processes such as hydrothermal dolomitization, fluid mixing, fluid cooling, fracture system formation and brecciation, may also play important roles in causing mesogenetic dissolution in deep burial environments (Qing and Mountjoy, 1994, Sun, 1995, Machel, 2004a, Davies and Smith, 2006, Saller and Dickson, 2011, Hiemstra and Goldstein, 2015, Jiang et al., 2015b, Zhu et al., 2015).
The Lower Triassic Feixianguan Formation, present on the platform margin of the Kaijiang-Liangping Bay in the Sichuan Basin, offers a good opportunity to study the impact of both shallow and burial diagenesis on pore evolution (with depth of up to 7500 m). Previous studies have shown that good quality reservoirs in this area are predominantly found in oolitic shoal facies present both at the platform margins and interiors, while dolomitized grainstones have much better reservoir quality compared to their limestone counterparts (Ma et al., 2008a, Jiang et al., 2014b, Chen et al., 2015, Wang et al., 2015, Qiao et al., 2016). Interparticle and dissolution-enhanced porosity (e.g., dissolution vugs, solution-enlarged pores or moldic pores) are the main pore types in these reservoirs (Ma et al., 2008a, Chen et al., 2015, Qiao et al., 2016). Most reservoirs in the northeast side (NE), and a few located in the southwest (SW) side, of the Kaijiang-Liangping Bay have been extensively dolomitized (Zhao et al., 2005, Jiang et al., 2014b). In contrast, most of the reservoirs in the SW side of Kaijiang-Liangping Bay are non-replaced limestones that have been heavily cemented by calcite and saddle dolomite (Cai et al., 2014, Jiang et al., 2014b, Zhou et al., 2014). This paper focuses on documenting and understanding the different diagenetic processes that have affected oolitic limestone and oolitic dolostone reservoirs from platform margin shoal and platform interior shoal facies in the Feixianguan Formation. We aim to determine the effects of dolomitization and TSR on the rock properties in deeply buried carbonate gas reservoirs. Specifically, this study seeks to address the following research questions:
- 1.
What diagenetic processes have occurred and how have they affected pore evolution in coeval lime- and dolo-grainstone reservoirs?
- 2.
What are the factors that have controlled the presence of good reservoir quality in the deeply buried Feixianguan Formation?
- 3.
Can TSR improve reservoir quality, and if so, by what mechanisms?
Section snippets
Geological setting
The intracratonic Sichuan Basin is located in the east of the Sichuan Province, southwest China and has an area of about 230,000 km2 (Fig. 1A). The Sichuan Basin is tectonically-bounded by the Longmenshan fold belt to the northwest, the Micangshan uplift in the north, the Dabashan fold belt the northeast, the Hubei-Hunan-Guizhou fold belt to the southeast, and by the Emeishan-Liangshan fold belt to the southwest.
The Lower Triassic Feixianguan Formation was deposited in a tidal-dominated,
Methodology
218 core samples from 23 wells, containing various diagenetic phases and carbonate host rocks (e.g. limestone, dolostone), were collected from cores of the Lower Triassic Feixianguan Formation from the Puguang, Maoba, Luojiazhai, Dukouhe, Longgang, Tieshan, and Yuanba sour gas fields (Fig. 1A). 168 thin sections (30 μm thick) were stained with Alizarin Red S to differentiate calcite and dolomite and their ferroan versions (Dickson, 1966). Selected polished thin samples were examined by scanning
Petrography and paragenetic sequence
The entire paragenetic sequence in the studied Feixianguan Formation consists of 23 distinct events. The relative timing of these phases is based on superposition and cross-cutting of various features, as well as homogenization temperatures derived from various diagenetic minerals (see details below). It should be noted that information about limestone represent new data generated during this study, which has here been compared to the paragenetic sequence in dolostone by summarising and
Interpretation of diagenetic history
The paragenetic sequences for the two lithologies, limestone and dolostone, both show similarities and differences to those reported from earlier studies of Lower Triassic carbonates in the Sichuan Basin (Cai et al., 2004, Cai et al., 2014, Hao et al., 2008, Jiang et al., 2014a, Hao et al., 2015, Jiang et al., 2015c). Three overall stages have previously been defined (Jiang et al., 2014a) that represent the diagenetic history in the Feixianguan Formation (Fig. 15): (i) pre-TSR diagenesis,
Conclusions
- (1)
Integration of petrographic, isotopic, fluid inclusion, and porosity point counting data, reveals discrete diagenetic and porosity evolution patterns in limestone and dolostone reservoirs in the Lower Triassic Feixianguan Formation.
- (2)
Early calcite cementation and mechanical compaction, pressure solution, and late stage calcite cementation have reduced porosity in limestone to ∼2%. Negative trend of δ18O and the low salinity data (<3.5 wt. %) of some calcite-2 and calcite-3 in limestone suggest
Acknowledgements
This work has been financially supported by the Natural Science Foundation of China (Grant Nos. 41402132 and 41672143), the Youth Innovation Promotion Association of the Chinese Academy of Sciences, the National Science and Technology Major Project (Grant No. 2017ZX05008-004), a joint project between PetroChina and Chinese Academy of Sciences (Grant No. RIPED-2015-JS-272), and scholarships under the China Postdoctoral Science Foundation award for International Postdoctoral Exchange Fellowship
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2022, Sedimentary GeologyCitation Excerpt :Investigating the dolomitization mechanism(s) responsible for the massive dolostone body across diverse depositional environments of the GBG builds a foundation towards assessing global, regional, and local controls on the basin-wide distribution of the massive dolostone. Because the Lower Triassic platform margin of the GBG is largely composed of dolomitized oolitic shoals (e.g., Rongling section in Fig. 4), the results of this study may be particularly relevant to dolomitized reservoir facies in the Feixianguan Formation in the Sichuan Basin of China and the Khuff and Dalan formations in the Middle East (Amel et al., 2015; Ehrenberg et al., 2007; Jiang et al., 2018). The GBG has an advantage over the targets of previous case studies where dolomite bodies are attributable to multiple episodes of dolomitization likely related to different mechanisms (Garaguly et al., 2018; Nader et al., 2004; Wang et al., 2015).