Shale Gas in China's Sichuan Basin: USGS Assessment of Paleozoic Shales, 2015

Summary

The U.S. Geological Survey estimated a mean 23.9 trillion cubic feet of undiscovered, technically recoverable shale gas — plus 343 million barrels of natural gas liquids — in three Paleozoic shale intervals of the Sichuan Basin, China.

The shales

Three organic-rich intervals are the main source rocks for the basin's conventional oil and gas fields:

Simplified stratigraphic column for the Sichuan Basin; ages in millions of years. USGS, modified after Hao and others (2008), the USGS Geologic Names Committee (2010) and Zou and others (2010).

• The Qiongzhusi, Wufeng and Longmaxi shales were laid down in deep marine basins on the continental shelf of the South China block, near the early Paleozoic margin of Gondwana. They hold up to 8 weight percent of Type II organic matter, averaging 1 to 5.5 percent.
• The Longtan is usually seen as a coal-bearing coastal and delta rock, but it also has a substantial deep marine organic-rich facies, and has sourced major conventional fields. The overlying Dalong also contains organic-rich marine shale.

Shale gas testing began in 2008; commercial production from the Longmaxi in the southeastern foldbelt, east of Chongqing, has run since late 2012.

Method

The USGS uses a well-performance approach. Outside the United States it assesses shales that:

1. hold more than 2 weight percent total organic carbon;
2. are in the thermal maturity window for gas;
3. are more than 15 meters thick;
4. contain Type I or II oil-prone organic matter.

Recoveries per well and drainage areas came from U.S. shale gas analogs. Six assessment units were defined — a relatively undeformed platform unit northwest of the southeastern foldbelt and a foldbelt unit within it, for each of the Cambrian, Silurian and Permian shales.

How the gas formed — and leaked

• The Cambrian and Silurian shales were buried in deep basins on a passive continental margin and generated oil.
• After the North and South China blocks collided in the Triassic, rapid sedimentation buried them deeper, and heat cracked the oil to gas. The Permian shales generated oil, then gas, during Mesozoic burial.
• Much oil and gas migrated into shallower conventional traps, but substantial gas stayed in the shales.
• Late Cretaceous and Tertiary uplift, linked to the India–Eurasia collision, fractured and faulted the basin, letting gas leak. Leakage was likely greatest in the southeastern foldbelt, with its folds and thrust faults, so foldbelt reservoirs should succeed only where least deformed — like the Silurian wells of the Jiaoshiba gas field east of Chongqing.
• Elsewhere, shales above and below seal the reservoirs, except near unconformities (at the base of the Cambrian and around the Permian shales), which raise the risk of leakage. All the assessed shales are brittle enough for hydraulic fracturing.

Results

Natural gas liquids: mean 343 million barrels (range 50–920). These are technically recoverable volumes, not economically recoverable ones.

Sources

Based on "Assessment of Paleozoic Shale Gas Resources in the Sichuan Basin of China, 2015," U.S. Geological Survey fact sheet; a work of the United States government in the public domain. The stratigraphic column is taken from the fact sheet's PDF; its location map, on an Esri imagery base, and a photograph licensed CC BY-SA are left out.