Selected Figures

	Tectonic map of Sichuan basin area as index map for study area.
	Stratigraphic Stratigraphic column for Sichuan basin area, with indication of tectonic movement and position of source rocks.
	Fault map of study area, with locations of gas fields and gas-bearing structures.
	Seismic line down-plunge of structure, showing position of Puguang 1.
	Paleogeographic map of Lower Triassic Feixianguan Formation, containing the main reservoir rocks.
	Log of First Member, Feixianguan Formation, showing the postion of abundant bitumens.
	Burial history profile, Puguang gas field.
	Structure map of Puguang gas field.
	Schematic cross-sections, 180 Ma – present, showing the development of Puguang gas field.

  

Geological Setting

• The Sichuan basin, with an area of about 190,000 km² (Figure 1), is one of the most important gas-producing basins in China.
• This basin is tectonically bounded by a number of fold belts.
• The Sichuan basin experienced several important tectonic movements. The movements at the end of the Triassic resulted in the conversion from marine to continental sedimentation.
• Five possible source rock intervals are overmature (Figure 2).

 

 

Puguang Field 

Exploration History of Sichuan Basin

Petroleum exploration in the Sichuan basin began in 1950s. Before the discovery of the Puguang gas field, almost all gas reservoirs found have gas/water contact shallower than 4900m, which was previously considered the lower limit for gas accumulation.

Discovery

The exploration area is in mountain region with complicated structural configuration and high surface relief (up to 1200m) (Figure 3). All structural highs had been drilled, and no commercial oil/gas accumulations had been found.

Geological and Evolution History

The Puguang Gas Field was discovered by Well Puguang-1, in which the depth to top of the Feixianguan Formation is 1400m deeper than that in Well Chuyue 83, previously drilled on the structure high (Figure 4).

Proven gas reserves are greater than 3,812x10⁸ m³(13.47 TCF). Gas in this field is mainly contained in the Lower Triassic and Upper Permian. The best reservoirs are oolitic dolomites of platform margin shoal/reef facies.

The Lower Triassic Feixianguan Formation is the main reservoir rocks , which developed mainly in the platform-margin shoal and evaporitic platform environments (Figure 5).

Abundant solid bitumens were found in all cored wells in marine carbonate reservoirs. The maximum cumulative thickness of solid bitumen-bearing carbonate reservoirs in a single well is greater than 300 m, suggesting that the Puguang gas field evolved from a large paleo-oil reservoir (Figure 6).

The hydrocarbon gases display relatively constant C₁/C₂ values and significant increases in C₂/C₃ ratios; they should be interpreted as secondary cracking products. The formerly accumulated oil in the marine carbonates from which the solid bitumen and hydrocarbon gases originated was confirmed to have generated from Upper Permian source rocks.

Gases in the field have relatively high CO₂ and H₂S contents. CO₂ and H₂S were derived from thermochemical sulfate reduction (TSR). TSR in the NE Sichuan basin significantly altered the chemical composition of petroleum hydrocarbons by forming sulfur-rich pyrobitumen and increasing the non-hydrocarbon gas contents, the dryness of hydrocarbon gases, and the d¹³C values of light hydrocarbons.

The marine carbonate reservoirs in the Puguang gas field were buried to a depth of about 7000 m and experienced maximum temperature up to 210°C before uplift to the present-day depth (Figure 7). Oil accumulation took place mainly in Early Jurassic, and thermal cracking of formerly accumulated oils occurred mainly in Late Jurassic and Early Cretaceous (160 to 120 Ma).

As a result of different uplift and erosion during the late tectonic movements, the morphology of the sealing surface changed significantly, which resulted in fluid re-migration in this field to the present configuration (Figure 8).

During the Late Triassic and Early Jurassic, oil generated from the Upper Permian source rocks migrated through faults and dolomites and accumulated in the Puguang-Dongyuezhai structure (Figure 9(A)). From about 160 to 145 Ma, increasing reservoir temperature resulted in thermal cracking of accumulated oils and thermochemical sulfate reduction (Figure 9(B)). From about 140 Ma, changes in three-dimensional morphology of the cap rocks, gases in the field re-migrated and finally re-accumulated, forming the Puguang gas field (Figure 9(C)).

 

New Discoveries

LangfordThe exploration concept and technique have been used in the Dawan and Yuanba areas, which led to the discoveries of the Dawan ,Tonngnanba, Yuanba gas fields.

 

Conclusions

• The discovery of the Puguang gas field indicates that huge potential reservoirs still remain in the mature basin and at a depth of more than 5000m.
• Giant oil/gas fields could be preserved even after complex post-accumulation alteration in areas with highly overmature source rocks and complicated tectonic evolution history.
• New play concepts and advanced technologies will play key roles in finding deep energy reserves.

 

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