Geology and shale gas resource potentials in the Sichuan Basin,China

Abstract

The organic-rich shales in the Sichuan Basin in China include Pre-Cambrian Sinian to Middle Permian marine shales deposited in passive margin to foreland settings, Upper Permian transitional shales deposited in a coastal swamp setting, and Triassic and Jurassic lacustrine shales deposited in a foreland setting. Regional shale property mapping, analysis of geochemistry, mineralogy and petrophysics based on sample tests, reservoir characterization of potential shale intervals, and recent exploration and production (E&P) results reveal that: Pre-Cambrian to Middle Permian marine shales, especially the Lower Silurian Longmaxi shale, deposited during transgressive systems tract to early highstand systems tract period have significant reservoir storage, high TOC, high maturity, high brittle mineral content and high gas content, and are similar to the Barnett shale in USA to hold a huge amount of shale gas. The coal-associated Upper Permian transitional shales and Triassic to Jurassic lacustrine shales are relatively clay-rich compared to marine shales, but possess some shale gas potential in organic-rich shales. The shale gas resource potentials and emerging production have been confirmed by the reservoir characteristics and test results of recent drilled wells targeting marine, transitional and lacustrine shale gas in the Sichuan Basin.

Keywords

Shale gas resource potential Sichuan Basin marine shale lacustrine shale transitional shale

Introduction and methodology

Sichuan Basin is a rich source of oil, gas, coal, and other mineral resources due to the basin’s multistage tectonic subsidence and orogenic movements (Huang et al., 2015; Ma et al., 2007, 2008; Pirajno, 2013). China is estimated to have the largest amount of shale gas potential in the world and the Sichuan Basin alone has 626 trillion cubic feet (TCF) of technically recoverable shale gas resource (EIA, 2013), which offers China’s most shale gas potential. Most offered blocks for shale gas exploration are located in the Sichuan Basin and its neighboring areas and nearly 400 shale gas wells have been drilled in the last 5 years (Figure 1).

Figure 1.

Geologic map of the Sichuan Basin, modified from Ryder et al. (1991). The sampling locations for marine and lacustrine shales and key wells used in this article are also shown on the map. The NW-SE oriented line is location of cross-section in Figure 2. Є: Cambrian; O: Ordovician; S: Silurian; D: Devonian; C: Carboniferous; P: Permian; TR: Triassic; J: Jurassic; K: Cretaceous; Q: Quaternary.

Active petroleum exploration have taken place in the Sichuan Basin for more than 50 years, and approximately 300 gas fields have been discovered (Ma et al., 2008). Conventional producing formations range from Pre-Cambrian marine strata to Jurassic lacustrine strata and the produced gas is mainly sourced from Pre-Cambrian Sinian, Lower Cambrian, Upper Ordovician, Lower Silurian, Permian, Upper Triassic, and Lower Jurassic shales (Ma et al., 2007, 2008; Zeng and Guo, 2015). Historically, some gas shows have been recorded from shale source rock intervals. In 1966, well Wei-5 in the Weiyuan area of southwestern Sichuan Basin experienced a 15–22 m high well-kick as it drilled into the Cambrian Qiongzhusi marine shale at a depth of 2797 m. A daily output of 24,600 m³ was measured at a formation pressure of 29.17 MPa. Reports show that gas shows have been observed in the Cambrian Qiongzhusi Formation (Fm) in 68 intervals in 41 wells (Li et al., 2007). In 1981, Yang-63 well reported a low-yield gas flow during a formation test in the Silurian Longmaxi marine shale. Historical records report gas shows in 32 intervals from 15 wells in the Longmaxi shale (Hachbarth et al., 2011; Li et al., 2007; Figure 1).

Sinopec announced the discovery of Fuling Shale Gas Field with production from Silurian Longmaxi marine shale in the southeastern Sichuan Basin in 2014, making China the only country outside of North America produce commercial shale gas. The Fuling area including Jiaoshiba Shale Gas Field has geological resources of approximately 2 × 10¹²m³ (74 TCF). A portion of this field covering an area of 106 km² was reported by Sinopec and China Ministry of Land and Resources (MLR) to have geological reserves in excess of 107 billion m³ (bcm; 3.8 TCF) and technically recoverable reserves of 26.7 bcm (0.96 TCF) in Silurian Longmaxi marine shale in Jiaoshiba area. In 2015, China has nearly tripled the size of proven reserves to 380.6 bcm (13.4 TCF) in its Fuling area according to Sinopec and Ministry of Land and Resources. By the end of August 2015, a total of 142 wells at the Jiaoshiba block had tested high-yield industrial gas flows and an additional 253 wells will be drilled according to Sinopec’s development plan. Jiaoye8-2HF horizontal well in Fuling shale gas field in east of Chongqiong was reported to produce 547,000 m³/day (Jiang et al., 2015). As examined and approved by China’s Ministry of Land and Resources recently, CNPC has added 207.87 km² of new shale gas bearing areas in the well blocks of Wei-202, Ning-201, and YS-108 in the southern Sichuan basin (Figure 1). The areas have been added with proven original gas in place of 163.53 bcm (5.78 TCF) and technically recoverable reserves of 40.88 bcm (1.44 TCF). All three well blocks are located in the national shale gas demonstration zone in the Sichuan basin. By August 2015, 47 wells have been put into production in the newly proved areas, producing 3.62 million cubic meters gas per day (MMcmgd). The rate from horizontal well Yang201-H2 in Luzhu, Sichuan was reported a peak gas rate of 430,000 m³/day.

To date, many shale gas blocks in the Sichuan Basin were reported to have good production test results after hydraulic fracturing, for example, the Triassic lacustrine shale in western Sichuan Basin, Jurassic lacustrine shale in Yuanba area in northern to northeastern Sichuan, Jurassic lacustrine shale in the Shizhu area in eastern Sichuan, and Cambrian to Silurian marine shales in Changning, Weiyuan, Zhaotong, Fushun and Yongchuan blocks, Fuling, and Pengshui in southern and southwestern Sichuan Basin (Figure 1). Based on recent shale gas exploration and production activities and shale studies in China, the Pre-Cambrian Sinian to Lower Silurian marine shales, Upper Permian transitional shale, Upper Triassic and Lower Jurassic lacustrine shale source rocks in Sichuan Basin represent the most potentially shale gas plays in China and the geological, geochemical and reservoir characteristics of these shales have been partially characterized (Cheng et al., 2009; Dong et al., 2014; Huang et al., 2012; Jiang, 2014; Jiang et al., 2016; Pu, 2008; Wang et al., 2009; Zhang et al., 2008; Zou et al., 2010, 2011a, 2015). But the integrated geology and shale gas resource potentials of marine, transitional, and lacustrine shales in the Sichuan Basin have not been systematically studied and summarized.

The purpose of this article is to characterize the geologic properties and shale gas resource potentials of marine, transitional, and lacustrine gas shales in the Sichuan Basin. We employed the reliable literature, historical industry data, test data of outcrop and subsurface core samples, and recently drilled shale gas wells as database to characterize the geology of shales in the Sichuan Basin. This article starts with regional geological review followed by regional shale property mapping, analysis of geochemistry, mineralogy, and petrophysics based on sample tests, regional to nano-scale shale reservoir characterization of potential shale intervals, and analyses on production tests of recent exploration and production wells. Robust results predict the distribution of potential shale gas plays and high quality gas bearing reservoir characteristics and shale gas resource potentials for marine, transitional, and lacustrine shales in the Sichuan Basin. This article will focus on the Lower Cambrian Qiongzhusi equivalent marine shale, the Lower Silurian Longmaxi marine shales, and the Upper Triassic Xujiahe5 lacustrine and the Lower Jurassic Zhenzhuchong or Daanzhai equivalent lacustrine shales.

Geologic setting of shales in the Sichuan Basin

The Sichuan Basin is a large basin on the Yangtze Platform’s “shale corridor” in southwestern China. This basin covers an area of approximately 230,000 km² (142,915 mi²) (Figure 1) and contains up to 6–12 km of Pre-Cambrian Sinian to Cenozoic sediments (Figures 2 and 3). Tectonic boundaries for the Sichuan Basin include the Longmenshan fold belt in the northwest, the Micanshan uplift in the north, the Dabashan fold belt in the southeast and the Emeisha-Liangshan fold belt in the southwest (Ma et al., 2007, 2008). The current tectonic units in the Sichuan Basin consist of a northeast oriented fold belt in the east, a dome structure in the center, and a subsidence belt in the west (Figures 1 and 2).

Figure 2.

Tectonic activities and basin development demonstrating the Sichuan Basin evolved from a passive margin marine setting (Cambrian to Ordovician) to a foreland marine setting (Silurian to Carboniferous) and finally to a lacustrine setting (Late Triassic-present) (stratigraphic column and tectonic events on the left modified from Ma et al., 2008). The marine, transitional, and lacustrine shales were developed penecontemporaneously. See Figure 1 for the location of the NW-SE oriented tectono-stratigraphic cross-section.

Figure 3.

The integrated stratigraphic column, tectonic evolution, and shale source rock intervals of Sichuan Basin (modified after Zou et al., 2010). TOC: total organic content; S2: the amount of hydrocarbons generated through thermal cracking of nonvolatile organic matter; T_max: the temperature at which the maximum release of hydrocarbons from cracking of kerogen occurs during pyrolysis (top of S2 peak).

The cratonic basement of the basin is a united craton, which was formed during the Jinning (850 Ma) and Chengjiang (700 Ma) orogenies and subsequently stabilized and metamorphosed prior to the Late Sinian (Wang et al., 2002). During its tectonic evolution, the Sichuan Basin evolved from a relatively stable craton dominated by marine deposits between the Sinian to Ordovician, to a marine-dominated foreland from the Silurian to Carboniferous, to passive margin environments with coastal swamps from the Permian to Middle Triassic, and then to foreland continental settings beginning in the Late Triassic (Ma et al., 2008, Figure 2). Regional subsidence resulted in depression structures occurring from the late Sinian to the Early Paleozoic. Thick siliciclastic sandstones, carbonates and shales were deposited in the associated passive margin settings. From Silurian through the Carboniferous, the Caledonian tectonic movement altered the Sichuan Basin to a foreland setting and while uplifted paleo-high areas were eroded, marine shale was deposited in a foreland setting. Volcanic eruptions occurred in association with rifting until the Middle Carboniferous and reached its climax in the Middle Permian creating large-scale basalt eruptions over much of the Sichuan Basin (Ma et al., 2008). During the Early to Middle Permian, marine deposits dominated the Sichuan Basin and the shales deposited in local rifts with limited extent are currently deeply buried (>4000 m, Dongsheng Zhou with China University of Geoscience at Beijing, personal communication, July 2014) and have little shale gas interest. During the Later Permian, eustatic sea level fell across most of China and a coastal swamp settings (transitional depositional setting) developed in north and south China. A transitional shale associated with sandstones, siltstones, and coal intervals was deposited widely across China including in the Sichuan Basin (Jiang, 2014).The current foreland Sichuan Basin began to take shape during the early Mesozoic Indosinian orogeny (Middle Triassic) when the northwestern margin of the Yangtze Platform collided with the Sonpan-Ganzi block (Bally et al., 1986), causing the northwestern Sichuan Basin to evolve from a passive margin environment into a peripheral foreland basin in response to the collision (Meng et al., 2005). At the late period of the Triassic, several subsequent episodes of the Indosinian orogeny resulted in rapid subsidence and thick lacustrine deposits in the northwestern and western Sichuan Basin. These subsequent Indosinian collisions of the Yangtze platform with the North China block to the north and Tibet to the west firmly amalgamated the Yangtze microcontinent to the Eurasian plate and completed the conversion of the stable marine carbonate platform to a continental basin (Ryder et al., 1991). From the Jurassic through the Early Cretaceous, the western basin margins were uplifted several times, and each uplift was accompanied by strong depressions in the mountain front allowing for lacustrine depositional environments to form. From the time of the Cenozoic Himalayan orogeny (Miocene-present), the Sichuan Basin has been experiencing uplifts and deformations (Ma et al., 2008). The predominant depositional environment for lacustrine shales is in foreland lacustrine settings. From the late Triassic, the multi-stage, large-scale orogenies readjusted the Paleozoic cratonic platform, superimposing lacustrine foreland basin sediments on the cratonic marine basin (Figures 1 –3).

Potential shale gas intervals in the Sichuan Basin

A number of organic-rich Pre-Cambrian to Cenozoic shale intervals (especially Paleozoic shales) are widely distributed on the Yangtze Platform in South China including the Sichuan Basin on the Upper Yangtze. The Pre-Cambrian to Mesozoic organic-rich shales in Sichuan Basin are proven source rocks. The conventional gas production in Sichuan Basin was sourced from seven shale source rock intervals: 1) Pre-Cambrian Sinian, 2) Lower Cambrian, 3) Upper Ordovician, 4) Lower Silurian, 5) Permian, 6) Upper Triassic, and 7) Lower Jurassic (Ma et al., 2007, 2008). These shale source rock intervals are generally organic-rich with total in-situ organic content (TOC) >2%. The low S2 (<3 mg/g) and high T_max (mostly >500℃) of the Paleozoic shales indicate they are generally overmature and in the dry gas window (see Figure 3 for the meanings of S2 and T_max). The maturity of 31 marine and 3 lacustrine representative shale samples from the Sichuan Basin was measured at Weatherford Labs, Houston, TX. Measurements for the Lower Cambrian marine Niutitang shale, its age equivalent Qiongzhusi shale, and the Lower Silurian marine Longmaxi shale were made on pyrobitumen since there was no vitrinite during the Early Paleozoic and the values were then converted to vitrinite-equivalent reflectance values (%Ro) using the Jacob equation (1989). Results indicate that the Cambrian to Silurian marine shales are in the dry gas window with Ro >1.3% and a portion of the marine shales are overmature as indicated by Ro >3% (Figure 4). The measurements of vitrinite for Jurassic Ziliujing lacustrine shale indicates this lacustrine shale is in oil to wet gas window with Ro ranging from 0.7% to 1.1% (Figure 4).

Figure 4.

Vitrinite reflectance (%Ro) of Lower Cambrian marine Niutitang shale and its age equivalent Qiongzhusi shale, Silurian marine Longmaxi shale, and Jurassic lacustrine Ziliujing shale in Sichuan Basin. For the Cambrian to Silurian shales, the measurements were made on pyrobitumen and values converted to vitrinite-equivalent %Ro values using the Jacob equation (1989). Weatherford Laboratories in Houston, TX measured the samples. See Figure 1 for sampling locations.

Pre-Cambrian Sinian Doushantuo marine shale

The first formation with shale gas potential is the Sinian Doushantuo shale in eastern Sichuan Basin and the surrounding area. It was deposited in passive margin settings from 635 to 551 Ma, which coincided with the end of the Rodinia rifting and the beginning of thermal passive-margin subsidence (Korsch et al., 1991). The thickness of the Doushantuo Fm varies from several hundred meters on the shelf to tens-of-meters in the Basin. Black shale of the Doushantuo Fm represents the oldest potential hydrocarbon source and reservoir rocks in the Yangtze Platform. The high TOC (generally >2%) and high maturity to over maturity (%Ro > 2%) of the Doushantuo shale indicate a shale gas potential (Yang et al., 2012, Table 1).

Table 1.

Geological parameters and resource potentials of Pre-Cambrian to Jurassic shales in Sichuan Basin.

Age	Shale	Tectonic setting	Depositional setting	Lithofacies	TOC% range/ average	Ro% range/ average	Kerogen type	Depth/m	Thickness/m	Geologic resource potential (10¹²m³)	Technically recoverable resource (10¹²m³)
Early Jurassic	Ziliuding	Rift	Lacustrine semi- to deep-lake	Carbonate-rich shale, quartz-rich shale and clay-rich shale	0.5–2.4	0.8–1.8	Type I	Up to 3800	20–220/80	10.82 (Dong et al., 2014)	2.14 (Dong et al., 2014)
Late Triassic	Xujiahe	Foreland	Lacustrine lake to swamp	Coaly shale, silty shale, quartz-rich shale	1–4/2.7	1.4–2.8	Type 3	2000–5200	10–600/350	8.47 (Dong et al., 2014)	1.67 (Dong et al., 2014)
Permian	Longtan (mainly)	Rift to passive margin	Marien to continental (transitional)	Clay-rich shale	2–12/4%	1.3–3/2.5%	Type III	1000–5000	20–120/50	20.25 (EIA, 2013)	6.07 (EIA, 2013)
Silurian	Longmaxi	Foreland	Intra-shelf low	Siliceous shale	1.5–10/2.6	1.3–5	Type I	900–4500	20–100	9.9 (Dong et al., 2014)−17.5 (Zou) 32.45 (EIA)	1.73 (Dong et al., 2014) 3.46 (Zou) 8.11 (EIA)
Cambrian	Qiongzhusi	Passive margin	Shelf to slope	Siliceous shale	2–10/3.5	1.5–5	Type I	1500–5000	20–80	4.85–10.83 (Dong et al., 2014,2015) 8.86 (Zou et al., 2015) 14.15 (EIA, 2013)	0.91 ((Dong et al., 2014) 0.886 (Zou et al., 2015) 3.54 (EIA, 2013)
Pre-Cambrian Sinian	Doushantuo	Passive margin	Shelf to slope	Siliceous shale	1–6	2–6	Type I	1500–5500	15–150/50	Unassessed	Unassessed

The resource potentials modified from EIA (2013), Dong et al. (2014, 2015), and Zou et al. (2015).

Lower Cambrian Qiongzhusi equivalent marine shale

The Lower Cambrian Qiongzhusi equivalent marine shale is a thick organic-rich shale interval, which is widely distributed across the Sichuan Basin. During the worldwide sea level transgression period (“Niutitang/Badaowan Event” in southern China), the Qiongzhusi and equivalent formations developed extensively across the shelf-slope-basin environments of the Ediacaran Yangtze block margin (Xue and Jiang, 2012). Our study indicates an average TOC for the whole Cambrian shale interval (40–400 m) ranges between 0.5% and 3% and areas with the highest TOC (>2%) are situated in west-southwest and northeast of the Sichuan Basin (Figure 5). The average vitrinite reflectance (%Ro) of the Lower Cambrian shale ranges from 1.5% to 5% and is in the dry gas window in the Sichuan Basin (Figures 4 and 5, Table 1). The hydrocarbon generation intensity ranges from 25 to 175 × 10⁸m³/km² (Figure 5).

Figure 5.

Thickness, TOC, %Ro and hydrocarbon generation intensity (HydCarbGenInt, unit: 10⁸m³/km²) overlay map of Cambrian Qiongzhusi equivalent shale in the Sichuan Basin (modified from Pu, 2008).

Based on the shale properties, we predict the Cambrian shales located in west to southwest and northeast of the Sichuan Basin are probably the two most prospective areas for shale gas exploration in China. These two areas have the 20–80 m hot shales deposited in an intra-shelf low depositional setting. The prospective shales in these two areas in general have rich organic matter content (generally TOC > 2%, average 3.5%), medium to high maturity (%Ro > 1.5%), high hydrocarbon generation intensity (>75 × 10⁸m³/km²), and a reasonable burial depth of <4500 m (Figure 5, Table 1).

Upper Ordovician Wufeng to Lower Silurian Longmaxi marine shale

Shale gas potential of the Ordovician Wufeng shale is supported by high TOC (>2%) and high maturity values with Ro >1.3%, but Wufeng shale is only a few meters thick. The Lower Silurian Longmaxi shale directly overlies the Wufeng shale, so the Upper Ordovician to Lower Silurian Longmaxi shale is considered a single potential play. Focus in this article is on the commonly called “Longmaxi shale play.”

The thickness of Longmaxi shale in the Sichuan Basin ranges from 50 to 550 m (Pu, 2008; Wang et al., 2009) with depocenters located in south to southwest of the basin, as well as east to southeast and north of the basin. The thickness in the first two depocenters are approximately 550 m. The average %Ro of Lower Silurian Longmaxi shale ranges from 1.7% to 5% throughout most regions of the Sichuan Basin (Figures 4 and 6). This shale was generally highly to overly mature (%Ro > 2.0%) and is now in the dry gas window.

Figure 6.

Overlay map of thickness of hot shale interval with TOC >2%, hydrocarbon generation intensity and %Ro of Silurian Longmaxi shale, showing southern Sichuan Basin and eastern to southeastern Sichuan Basin (areas circled in pink color) are prospective areas for Silurian shale gas development (modified from Pu, 2008 and Pu et al., 2010).

According to Pu (2008) and Wang et al. (2009), the average TOC of the entire Lower Silurian Longmaxi shale interval in the Sichuan Basin ranges from 0.5% to 2.6%. Most regions in southern, and southeastern Sichuan Basin have 20–100 m of Silurian siliceous hot shale with TOC >2% in the Lower Silurian section (Table 1 and Figure 6). The overlay map of shale interval thickness with TOC >2%, facies distribution and depth, points to the southern Sichuan Basin and east to southeast of the basin to be the most prospective areas for Silurian shale gas development (Figure 6).

Upper Permian Longtan transitional shale

The typical characteristics of the Upper Permian Longtan transitional shale deposits are a close association with coal, sandstone, and siltstones. The thickness of the Upper Permian Longtan shale ranges from 20 to 120 m and the depocenters are located in the west, south, and east sections of the Sichuan Basin (Chen, 2011; Ma et al., 2004). Upper Permian shales (mainly from Longtan Formation) have thickness ranging from 20 to 150 m, TOC of 2–14%, and gas window maturity of 1.3–3% (Table 1).

Upper Triassic lacustrine shale

In the late Triassic, the Sichuan Basin was dominated by a foreland continental setting. The Upper Triassic shales, coals, and tight sands were deposited in the foredeep depozone in lake to lake-swamp settings in the western Sichuan Basin (Ma et al., 2008). At least five shale intervals from the Xujiahe1 to Xujiahe5 members in the Upper Triassic Xujiahe Fm have shale gas potential (Ye and Zeng, 2008). These shale layers with gross thicknesses up to 600 m have an average TOC ranging from 1% to 4% and average maturity values (vitrinite reflectance) ranging from 1.4% to 2.8% (Table 1). These shales are mainly gas prone due to Type III kerogen with terrestrial origin and high maturity (>1.4%). The Upper Triassic Xujiahe5 member (T₃X⁵) is considered the best shale gas play of the Upper Triassic interval and is current exploration focus due to a high TOC compared to other members. To date, the Upper Triassic Xujiahe5 lacustrine shale in the western Sichuan Basin is a top exploration target area due to its high gas content and high hydrocarbon generation intensity (>25 X 10⁸m³/km²) (Figure 7).

Figure 7.

Gas generation intensity of Middle to Upper Triassic source rocks. The gas generation intensity of the Upper Triassic Xujiahe5 lacustrine shale is modified from Ma et al. (2004).

Lower Jurassic lacustrine shale

The Lower Jurassic Daanzhai member or Dongyuemiao member equivalent shale of the Ziliujing Formation was deposited in shallow lake to deep lake settings in the Sichuan Basin. The TOC of the Ziliujing shale ranges from 0.5% to 2.4% and shale thickness ranges from 20 to 220 m (Table 1). The depocenters of organic-rich shale with TOC of >1% are located in the eastern and northeastern Sichuan Basin (Figure 8). The organic-rich Jurassic lacustrine shale is usually associated with bioclastic shoals (Zou et al., 2011b). Our sample analyses from Jurassic cores in eastern Sichuan define TOC in the range of 0.75–1.67% and vitrinite reflectance in the range of 0.7–1.1% (Figure 4). These data suggest that the preliminary reports of shale gas produced from Jurassic lacustrine shales in eastern Sichuan Basin (e.g., Jian111 vertical well and J1HF horizontal well in Jiannan Gas Field in Shizhu area of West Hubei) is most likely wet gas from kerogen cracking (Figure 8). Dry gas can be expected from the Jurassic shale in northern and northeastern Sichuan Basin, where the shale is buried deep in the current foredeep setting and is currently in the dry gas window (%Ro up to 1.8%).

Figure 8.

Depositional systems and TOC of shale source rock in the Jurassic Lacustrine Ziliujing shale interval in Sichuan Basin (modified from Zou et al., 2011).

Mineralogy of shales in the Sichuan Basin

X-ray diffraction (XRD) is a commonly used to determine the mineral composition of shales. Data gathered from these analyses allows an estimate to be made regarding the brittle response (or fracability) for specific formation intervals. We conducted XRD analysis on Pre-Cambrian to Silurian marine shales, Permian transitional shale, and Triassic to Jurassic lacustrine shales from the Sichuan Basin and compared the shale mineralogy in the Sichuan Basin with typical shale mineralogies of US shales (Figure 9). The mineral composition of the marine shales in the Sichuan Basin places them in the quartz and feldspar rich zones of the ternary diagram and their mineralogical composition is similar to the biogenic quartz-rich Barnett shale in the USA from this point of view, the Pre-Cambrian to Silurian marine shales in the Sichuan Basin are expected to have favorable brittle responses and to respond favorably to hydraulic fracture techniques since they generally have brittle quartz and feldspar content greater than 50% and clay mineral content less than 40% . The mineralogy of Upper Permian transitional shale is plotted in the clay-rich zone. The mineral composition of the Upper Triassic Xujiahe5 lacustrine shale and Jurassic lacustrine shale (Daanzhai equivalent shale) in the Sichuan Basin falls in a transitional zone (indicated by several data points between quartz- and feldspar-rich zone and clay-rich zone) (Figure 9). The main issue for lacustrine and transitional shales regarding shale gas development is that they are less prone to stimulation by hydraulic fracturing due to its clay-rich nature (Jiang, 2014), which makes the marine shales in the Sichuan Basin are current exploration priority.

Figure 9.

Ternary diagram of mineralogy from XRD for Precambrian marine to Jurassic lacustrine shales in Sichuan Basin. The overlay outlines represent the mineralogy of major shale plays in the USA. See Figure 1 for the sampling locations for shales in the Sichuan Basin.

Petrophysics of shales in the Sichuan Basin

Most of the prospective marine shale rocks in the Sichuan Basin have undergone mechanical and chemical compaction and have been deeply buried (>4000 m); they generally have very low porosity and permeability. Tests by TerraTek-Schlumberger using the Tight Rock Analysis (TRA) method for 34 marine shale samples from the Sichuan Basin indicate the effective matrix porosity of subsurface cores is usually less than 6% (generally less than 9% for core and outcrop samples) and the pressure-decay permeability is most often less than 0.001 mD (millidarcy) or 1 µD (microdarcy) (Figure 10). This data indicates that the reservoir quality of marine shales in the Sichuan Basin is very tight considering most US shales generally have porosity >5% (Roth, 2010). When comparing to the successful Barnett shale with average porosity of 6% and average permeability of 250 nD (0.25 µD) (Roth, 2010), it is still good even though marine shales in China have lower porosity than most US shales.

Figure 10.

Porosity and permeability for marine shales in Sichuan Basin. The porosity is often less than 9%. Low porosity (<6%) samples are primarily from shale gas exploration wells and higher porosity (>6%) samples are primarily from outcrops in the Sichuan Basin margin. Samples were tested by TerraTek-Schlumberger in Salt Lake City using Tight Rock Analysis (TRA) method. See Figure 1 for sampling locations.

Shale gas resource potentials

For the shale gas resource assessment, a broad range of parameters covering geology, geochemistry, mineralogy, petrophysics, geomechanics, depth, thickness, structure, pressure are required. Considering the exploration and development activities are still at an early stage in the Sichuan basin and variations in shale distribution, thickness, organic matter content, porosity, and gas content are not yet fully understood or evaluated based on the limited number of exploration wells. Realistic estimate for shale gas resource potentials in Sichuan Basin is challenging. The EIA did the first pass China shale gas resource assessment based on analogs to US shales and used simple extrapolations for shale extent and thickness based on limited data in 2011 and 2013. Since 2008, China Ministry of Land & Resources (MLR) has been working with regional geological surveys, research institutes, universities and oil companies to investigate China shale gas and shale oil potential. In terms of shale gas resources assessment, the probabilistic volumetric method combining analog and statistical calculations (using Monte Carlo simulation) is recommended by MLR and China University of Geosciences at Beijing to use for China shale gas resource assessment because shales heterogeneous reservoirs and many unknown reservoir parameters, uncertainties, and risks exist for Chinese shales deposited in complex geologic settings (Zhang et al., 2012). In March 2012, the MLR released estimates for China shale gas resources. The total potential geologic shale gas resource is 134.4 trillion cubic meters (TCM) (4748 TCF). Based on the North America’ s shale gas recovery factors and analogs of China’ s shale geology to US shales, it was estimated that 25.08 TCM (886 TCF) of the reserves were technically recoverable shale gas resources contained in marine, transitional, and lacustrine geologic settings in onshore China (excluding the Qinghai-Tibet Plateau area). Sichuan Basin alone has 40.02 TCM geological shale gas resources and 6.45 TCM technically recoverable shale gas resources and accounts for about 30% of China’s total shale gas resources (Zhang et al., 2012).

In terms of shale gas resources of different stratigraphic intervals with various geologic ages ranging from Cambrian to Jurassic in the Sichuan Basin, the Lower Cambrian Qiongzhusi marine shale is estimated to have 0.886–3.54 × 10¹²m³ of technically recoverable shale gas resources out of 4.85–14.15 × 10¹²m³ of risked geologic shale gas resource in-place; the Lower Silurian Longmaxi marine shale is estimated to have 1.73–8.11 × 10¹²m³ of technically recoverable shale gas resources out of 9.9–32.45 × 10¹²m³ of risked geologic shale gas resource in-place; the Upper Permian Longtan transitional shale is estimated to have 6.07 × 10¹²m³ of technically recoverable shale gas resources out of 20.25 × 10¹²m³ of risked geologic shale gas resource in-place; the Upper Triassic Xujiahe lacustrine shale is estimated to have 1.67 × 10¹²m³ of technically recoverable shale gas resources out of 8.47 × 10¹²m³ of risked geologic shale gas resource in-place; the Jurassic lacustrine shale is estimated to have 2.14 × 10¹²m³ of technically recoverable shale gas resources out of 10.82 × 10¹²m³ of risked geologic shale gas resource in-place (Table 1; Dong et al., 2014, 2015; EIA, 2013; Zhang et al., 2012; Zou et al., 2015,). Considering the shale gas resources in the Upper Triassic to Jurassic lacustrine shales and especially the Upper Permian transitional shales to are currently difficult to be exploited due to clay-rich nature, the Lower Silurian Longmaxi siliceous marine shale deposited in intrashelf low setting dominates the shale gas potential in the Sichuan Basin, followed by the Lower Cambrian Qiongzhusi marine shale.

Examples of shale gas exploration wells recently drilled in the prospective areas in Sichuan Basin and shale reservoir characteristics

For the Pre-Cambrian Sinian Doushantuo marine shale, the Zidi 1 well drilled in the Zigui County in Hubei Province in prospective eastern Sichuan Basin indicated the gas content reached 2–4 m³/ton from the Doushantuo shale with thickness of 145 m and the flare success of this shale gas well showed the potential.

The promising Cambrian shale resource plays areas including Jingyan-Jianwei block in west of Sichuan Basin and Chengkou block in northeast of the basin have been exploration targets since 2007. In 2007, PetroChina and Newfield teamed up to drill the Wei001-2 well in the Weiyuan area of southwestern Sichuan Basin to jointly study the shale gas potential of the Cambrian Jiulaodong/Qiongzhusi marine shale. The reported TOC is less than 2% from this well and gas content is also low (0.23–1.03 m³/ton). The mineralogical results indicate this Cambrian shale is brittle in terms of insensitivity to drilling fluids and less than 40% clay content. Between 2009 and 2010, PetroChina and Shell jointly drilled and hydraulically fractured the first shale gas well Wei201 in the Weiyuan area of southwestern Sichuan Basin to target the Lower Cambrian, and the Upper Ordovician to the Lower Silurian marine shales. The daily production rates from vertical Wei201 well after hydraulic fracturing from Cambrian Qiongzhusi marine shale and Silurian Longmaxi marine shale are 10,800 m³ (381,398 ft³) and 3000–17,000 m³ (105,944–600,349 ft³), respectively. Another type well targeting Cambrian shale is the Jinshi1 well in the southwest Sichuan Basin, this vertical well was initially tested and produced at 36,000 m³/day. The horizontal Jinye HF-2 well in the Weiyuan area in western Sichuan Basin was reported to produce 80,000 m³/day after hydraulic fracturing.

The encouraging test results of Silurian Longmaxi marine shale gas in other areas, for example, Fushun-Yongchuan area in southwestern Sichuan Basin, Pengshui, and Jiaoshiba areas in southeastern Sichuan Basin reveal a high confidence for shale gas reserves in the Longmaxi shale. The recent commercial shale gas production from Sinopec’s Jiaoshiba Shale Gas Field in Fuling in southeast Sichuan Basin confirmed the shale gas potential of the Longmaxi shale. The typical wells drilled have daily production rates ranging from 10 million to 19.1 million ft³ from Silurian Longmaxi marine shale. The first commercial shale gas well, Jiaoye-1 well, in Sinopec’s Jiaoshiba Gas Field in Fuling is used as example to characterize the Silurian Longmaxi marine shale reservoir in terms of sequence stratigraphy, facies, geochemistry, mineralogy, rock fabrics, gas content, and heterogeneity (Figure 11). The current exploration target—the Lower Silurian Longmaxi shale was deposited in a third order depositional sequence. During a transgressive systems tract (TST), the siliceous organic-rich shale with TOC of 3–5% and high gamma ray log was deposited in an intra-shelf low setting. Followed by a highstand systems tract (HST), the early HST (EHST) is dominated by organic-rich, black to grey shale with TOC of 2–4% and the late HST (LHST) is characterized by grey shale interbedded with silty shale and siltstone with low TOC content (<2%) and low gamma ray. The TOC, gamma ray value, quartz content, and gas content correspondingly decease upward in this Longmaxi shale interval (Figure 11). The stratigraphic variations in lithology, gamma ray, TOC, gas content, quartz content, lithofacies, and rock fabric indicated by core images and QEMSCAN® images all clearly show the large degree of heterogeneity of the this marine shale. Based on test results, the shale gas production of Jiaoye-1 well (at a rate of 200,000 m³/day) is primarily from the TST and EHST intervals containing a high content of brittle quartz minerals and organic matter (Figure 11). Jiaoye8-2HF horizontal well in Jiaoshiba area of Fuling Shale Gas Field in Chongqiong was reported to have a commercial output rate of 547,000 m³/day (19.1 million ft³/day) from similar reservoir interval as that of Jiaoye-1 well in October 2013, which is the highest daily production rate recorded for marine shale in China. Ning201 and Ning203 wells in the Changning block of southern Sichuan Basin were reported to have daily productions of 7500 m³ (264,860 ft³) and 7500–15,000 m³ (264,860–529,720 ft³) from Silurian Longmaxi shale, respectively. Vertical Yang101 and horizontal Yang201-H2 wells in Shell and PetroChina’s Fushun-Yongchuan joint-exploration block were tested and produced shale gas from Silurian Longmaxi shale at a rate of 60,000 m³/day (2,118,880 ft³/day) and 430,000 m³/day (15,185,307 ft³/day), respectively, in 2012.

Figure 11.

Jiaoye-1 marine shale gas well in the Sichuan Basin, showing the heterogeneity of the Silurian marine shale reservoir. See Figure 1 for the well location. Well logs, TOC, quartz content (indicated by both XRD test and QEMSCAN® analysis) and gas content are from Guo (2013) and Guo and Liu (2013). Production data was gathered from various public news reports in China. QEMSCAN® analysis was used to determine rock fabric and was run in EGI at the University of Utah.

For the Upper Permian transitional shale gas in the Sichuan Basin, several shale gas wells were also drilled recently in the eastern Sichuan Basin. The mud logging of L-801 well (see Figure 1 for the location of the well) in the Shizhu area of eastern Sichuan Basin indicates the Upper Permian transitional shale associated with coal interbed has significant gas shows. The other shale gas wells showing shale gas resource potential of the Upper Permian transitional shale include the Baye 1 well in the Badong County in eastern Sichuan Basin, indicated by 1–2 m³/ton gas content in the shale and flare after test. This Upper Permian transitional shale is very clay rich comparing to marine and lacustrine shales (Figure 9). So far, no production was reported from this transitional shale.

Testing to determine the potential of Triassic to Jurassic lacustrine shale gas in Sichuan Basin has been conducted by Sinopec in the last several years, a couple of wells have been drilled in the western, northern, northeastern, and eastern Sichuan Basin and preliminary test results show that the Triassic Xujiahe5 shale and Jurassic Dongyuemiao or Daanzhai equivalent shale is able to produce shale gas. The Xinye HF-2 well and Xinchang32 well in western Sichuan Basin were reported to produce at 46,000 and 12,000 m³/day from Triassic Xujiahe5 shale formation, respectively. The Yuanba21 well in Yuanba area in northeastern Sichuan Basin and Jian111 well in the Shizhu area in the eastern Sichuan Basin were reported to produce at 507,000 m³/day (17.9 million ft³/day) and 3925 m³/day (138,610 ft³/day) from the Jurassic Ziliujing shale formation, respectively. JY-HF-1 well in Jiannan Gas Field in Shizhu area in the eastern Sichuan Basin produced more than 500,000 m³ at an initial rate of 12,000 m³/day in 2011. The Jurassic Dongyuemiao (Daanzhai member equivalent) lacustrine shale gas reservoir in Ziliujing Fm is characterized by heterogeneous shale interbedded with thin carbonate and marl beds and most productive interval has characteristics of good hydrocarbons show (based on total hydrocarbons anomaly by mud logging), high GR, high AC (acoustic travel time), high resistivity and high TOC, The production is contributed from both the organic rich shale and associated carbonate interbeds (Figure 12).

Figure 12.

JY-HF-1 well in east Sichuan Basin targeting Jurassic Dongyuexiao lacustrine shale gas in Ziliujing Fm (modified from Xu et al., 2013). See Figure 1 for the well location. The shallow lake facies is characterized by alternating carbonate and shale lithofacies and the semi-deep lake facies is characterized by thick shale interbed with thin marl.

The above-mentioned historical well records and recent E&P activities indicate the potential of shale gas in the Sichuan Basin.

Conclusions and discussions

The Sichuan Basin is the most important shales gas basin on the Yangtze Platform shale corridor with shales ranging in age and depositional setting from Precambrian Sinian to Lower Silurian and to Middle Permian marine shales, Upper Permian transitional shales, to Triassic and Jurassic lacustrine shales. Since there is no production and good test results reported from Upper Permian transitional shale with very high clay content so far, the primary shale opportunities for shale gas indicated by shale geologic property, shale gas resource potentials and recent exploration results in the Sichuan Basin include: Lower Cambrian marine shale, Upper Ordovician-Lower Silurian marine shale, and Upper Triassic to Lower Jurassic lacustrine shales. These prospective gas shales are thick, have high TOC and are mature to overmature. Historical records, parameters from recent exploration well, encouraging production tests of these four shale intervals in southwestern, southern, southeastern, eastern and northeast Sichuan Basin. For the currently focused marine shale gas, commercial shale gas production is from the priority Silurian Longmaxi marine shale in Fuling area in southeastern Sichuan Basin so far; For the potential lacustrine shale gas, some production is reported from Jurassic Dongyuemiao lacustrine shale in Jiannan Gas Field in eastern Sichuan Basin.

Organic-rich Lower Cambrian Qiongzhusi equivalent and Upper Ordovician Wufeng to Lower Silurian Longmaxi marine shales were deposited extensively across the ancient passive margin shelf-slope (Cambrian) to foreland marine (Silurian) settings in the Sichuan Basin. The Lower Cambrian shale is thick (50–400 m) and its lower hot shale section with TOC of >2% and %Ro >2% has a thickness of 20–100 m. This lower section hot shale interval also contains the high percent of brittle minerals (quartz and feldspar) and is the main target for shale gas production. Based on the available data for our study, the northeastern Sichuan Basin and southwestern Sichuan Basin are the most favorable areas for future exploration of Cambrian shale play. Upper Ordovician to Lower Silurian Longmaxi shales range in thickness from 20 to 500 m. The lower 20–100 m interval with TOC of >2%, %Ro >1.7% and the high percent of brittle minerals is the best interval for shale gas exploration. In the Sichuan Basin, the predicted favorable regions for these Upper Ordovician Wufeng to Lower Silurian Longmaxi shales are in northern, eastern, and southern Sichuan Basin. Upper Triassic and Lower Jurassic lacustrine shales deposited in a foreland setting in the Sichuan Basin are emerging shale gas plays. Triassic shales ranging in thickness from 10 to 600 m include the shale members Xujiahe1 to Xujiahe5. They are characterized by an average TOC of 1–4% and average Ro between 1.4% and 2.8%. Upper Triassic Xujiahe5 shale is considered as a potential shale play and it has some good production test results in western Sichuan Basin. Jurassic shales with gross thicknesses of 20–220 m and TOC range of 0.5–2.4% include the Daanzhai equivalent member of Dongyuemiao Fm. This Jurassic shale distributed in the eastern Sichuan Basin is moderately buried and has a moderate maturity placing this interval primarily in the wet-gas window. The Jurassic shale distributed in the northern Sichuan Basin is deeply buried and is currently in dry gas window.

Lower Cambrian and Ordovician–Lower Silurian marine shales in the Sichuan Basin are similar to the Barnett shale (USA) in terms of mineralogy. The best reservoir interval has higher TOC (>2%) and brittle mineral content (>40%). Maturity (%Ro) of these China shales is generally higher than in the Barnett. Mineralogy of Jurassic shales in the Sichuan Basin are positioned in the overlay zone of Barnett, Marcellus, Haynesville, and Eagle Ford in ternary diagram. Generally, the lacustrine shale is less brittle than marine shale and more brittle than transitional shale in the Sichuan Basin. The porosity of marine shales in the Sichuan Basin is generally lower than that of most US shales, but the permeability is comparable to that of successful Barnett shale.

Even though the huge shale gas resource potential exists in marine, transitional to lacustrine shales in the Sichuan Basin and recent locally commercial production from Silurian Longmaxi shale has confirmed the potential. China still faces the significant challenges related to complex geology, water shortage, environmental issues, etc. for shale gas development in the Sichuan Basin. Despite the rich shale gas reserves in the Sichuan Basin for Palezoic marine shales with similar mineralogy to US Barnett shale, the shales in the Sichuan Basin have more complex geologic settings than US shales in terms of depositional and tectonic settings and geological history. The marine shales in the Sichuan Basin are either buried deeper than 3000 m or faulted and folded or outcropped. They have much more complex geologic history, high maturity level, complex stress setting and karst and cave occurrence, which makes simple copy of US shale gas technologies does not work well for marine shales in China (Jiang et al., 2014). For example, the Pengye1 in SE Sichuan Basin margin and Zhao104 well and Zhao103 well in the Southern Sichuan Basin margin only got 15,000 m³/day, 10,400 m³/day and no production tested from Silurian Longmaxi shale, respectively, these reduced production test results comparing the Sichuan Basin inside reflects shale gas leaking caused by complex tectonic disruptions. The complex stress fields and presence of karst and cave resulted in high drilling and completion cost in China (twice than that of US shales). For example, the Pengye HF-1 well in SE Sichuan Basin margin encountered serious drilling fluid loss in karst and cave intervals and it took 2 months to solve the problem (Wang, 2013). For the lacustrine in the Sichuan Basin, it is clay rich and has rapid property change spatially. Many fractured wells have no commercial production test results and many wells targeting Triassic to Jurassic lacustrine shales (e.g., Yuanba21, Xinye HF-2, etc.) with reported good shale gas production test results may mainly produce from tight carbonate and sandstone interbeds (Wang et al., 2013). Whether it is tight gas dominated or shale gas dominated need to be further studied. Also, rugged ground condition, the land with high density population, water resource shortage, environmental issues, etc. pose further challenges for shale gas development in the Sichuan Basin.

Footnotes

Acknowledgements

The international collaboration between PetroChina, Sinopec, CNOOC, China University of Geosciences at Beijing, China University of Geosciences at Wuhan, China Geological Survey and EGI made the research and paper possible. We are grateful to Sinopec, CNOOC and China University of Geosciences at Beijing for providing partial shale samples. Special thanks are also extended to Schlumberger–TerraTek and Weatherford for their help on petrophysics and geochemistry tests.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This study was supported by China National Giant Oil and Gas Program (2012ZX05018), CNPC Special Project (2012F-47-02), Open fund from Petroleum Exploration & Production Research Institute of Sinopec (G5800-15-ZS-WX038) and EGI’s multi-year China Shale Plays consortium (100980) sponsored by 20 multi-national oil companies.

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