Contrasting diagenetic evolution patterns of platform margin limestones and dolostones in the Lower Triassic Feixianguan Formation, Sichuan Basin, China

Highlights

• Diagenetic patterns for time-equivalent oolitic dolostone and limestone were presented.

• Most primary porosity in limestone was occupied by early calcite cements.

• Dolostone has good porosity mainly because of dolomitization and sulfate reduction.

Abstract

Deeply-buried carbonate-reservoirs from the Lower Triassic Feixianguan Formation in the Sichuan Basin of China host extensive natural gas resources. These reservoirs are predominantly found in oolitic shoals, with the reservoir quality of dolomitized zones being higher than that of undolomitized limestone counterparts. Here we present a combination of petrographic, isotopic, fluid inclusion, and quantitative porosity data in order to understand and predict the diagenetic processes that have impacted the reservoir quality of dolostones and limestones. The porosity of limestones has been reduced to ∼7.5% due to calcite cementation, whereas the porosity in oolitic dolostones is not cemented with calcite and typically has ∼23.5% porosity. Dolomitization and concurrent early-diagenetic gypsum growth played crucial roles on the development and preservation of high porosity in the oolitic dolostone, first by stabilizing the rock fabric to inhibit loss of porosity during burial, and secondly through the generation of new porosity by dissolution of carbonate and anhydrite. A negative shift of δ¹⁸O and salinity values (<3.5 wt. %) measured from fluid inclusions in diagenetic calcite cement in limestones suggest that diagenesis associated with meteoric water played a key role in destroying limestone reservoir quality. Early oil charge seems to have had a positive effect on carbonate reservoir quality in the dolostones, since oil emplacement inhibited calcite cementation. Subsequently, thermochemical sulfate reduction (TSR) occurred, predominantly in the dolostones, as shown by TSR calcite cement with highly negative δ¹³C values (∼−20‰ VPDB) and δ¹⁸O (∼−10‰ VPDB) together with elevated calcite precipitation temperatures (>110 °C). It is likely that TSR was responsible for the formation of enlarged dissolution vugs that increased porosity by ∼2% in dolostones due to: i) anhydrite dissolution, ii) production of significant amounts of water resulting in formation water undersaturated with respect to calcite and dolomite, iii) generation of H₂S, and CO₂, and the consequent reaction of H₂S with the siderite (FeCO₃) component in calcite and dolomite. This study demonstrates the importance of diagenesis in the formation of deeply-buried, high-quality reservoirs in ooid-dominated grainstones influenced by the presence of evaporites. Our results should be useful for guiding future exploration and reservoir developments in similar paleogeographic and diagenetic settings.

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:

• What diagenetic processes have occurred and how have they affected pore evolution in coeval lime- and dolo-grainstone reservoirs?

• What are the factors that have controlled the presence of good reservoir quality in the deeply buried Feixianguan Formation?

• Can TSR improve reservoir quality, and if so, by what mechanisms?