Figures

	Figure 1. Regional columnar section, schematic cross-section of the Upper Permian and Lower Triassic strata, and location map.
	Figure 2. Paleogeographic map of the Changxing Formation reefs in the Late Permian, deposited on the carbonate ramp or platform around the troughs. Some larger sponge-hydrozoa reefs are developed along the margin of carbonate ramp and platform around the trough, with the single field area over 10 km2 and gas reserves > 30x108m3. Photomicrographs of reservoirs in the Permian Changxing Formation.
	Figure 3. Paleogeographic map of Feixianguan stage of the Early Triassic, with outcrop photo, core photos and photomicrographs of reservoirs in the Triassic Feixianguan Fm. The oolite shoals were widely developed on both platforms, and on the open platform there are many shoaling upward cycles. The discovered gas pools are small scale and belong to the limestone reservoirs with low porosity, while the more massive ooid limestone reservoirs with medium-sized gas pools were developed along marginal facies around the troughs. On the evaporite carbonate platform the countless shallowing-upward sedimentary cycles with sabkha sequences consisted of peloidal mudstone, oolitic grainstone, oolitic dolomite, anhydrite and dolomicsite. The discovered gas pools are the dolomite reservoirs with high porosity on the evaporite platform. The leeward marginal facies along the evaporite carbonate platform margin around Kaijiang-Liangping trough is a favorable belt with large and medium-sized reservoirs, up to 200-300 m thick.
	Figure 4. Schematic cross-section of platform to troughs (upper left). Stratigraphic cross-section from platform to Chengkou-Exi trough (lower), accompanied by illustrations of the various facies; location of cross section is shown on Figures 2 and 3.
	Figure 5. Seismic section from carbonate platform to trough; the lower profile is flattened on T1f.. Location is shown on Figures 2 and 3.
	Figure 6. An interpreted segment of a 3-D seismic section through well PX 1 (PX), located on the platform to well CF 85 (CFX) in the Kaijiang-Liangping trough; and section of log of CFX well with core photos and photomicrograph from basal part of Feixianguan Formation. Location is shown on Figures 2 and 3.
	Figure 7. The Dalong Formation of the Upper Permian is typical of the deep-water trough facies. Its lithologies are dark siliceous shale and limestones with siliceous radiolarians and microforaminifers; ranges from 6.1 to 50 m in thickness. The characteristics of the Dalong Formation indicate that it is the condensed section of the maximum flooding surface in deep-water starved basin and an important source rock of the Upper Permian with an average TOC of 3.88% in the north Sichuan Basin.

 

Outcrop Study 

The outcrop study for Chengkou-Exi region during Late Permian to Early Triassic in Northern Sichuan Basin provides for a more complete synthesis of the evolution of the Upper Permian to the Lower Triassic depositional systems, and it also contributes to the knowledge of subsurface seismic facies and to concepts useful in petroleum exploration for the reservoirs of reefs and oolitic shoals found in northern Sichuan Basin. In the dominantly outcrop cross-section from platform to trough (Figure 4) the typical deep-water trough facies is recorded in (Chengkou-) Miaoba profile. The thickness of the Upper Permian is less than 100 meters, and in its uppermost part the Dalong Formation consists of dark shale, only 6.1 m in thickness, and is regarded as typical of deep-water deposition.  

The deep-water sediments of Feixianguan Formation, Lower Triassic, are composed of dark micritic limestone and carbonate gravity flow deposits, about 150 meters in thickness. The (Xuanhan-) Panlongdong profile is recognized as a typical shallow-water depositional facies, in which the complex of reefs of the Upper Permian and the oolitic shoals of the Lower Triassic represent the marginal facies of the carbonate platform. The (Xuanhan-) JiChang profile consists of dark siliceous limestones of the Late Permian, gray nodule limestone and mudstone with rare microfossiliferous clastics, interbedded calcareous shales and carbonate gravity flow deposits of the Early Triassic, where the distance from the platform margin is not more than 2 kilometers. Study of both profiles indicates that there was a steep slope between the platform margin and the trough in the Late Permian and Early Triassic, as suggested by a rapid sedimentary facies change and rapid changes in thickness as a result of the differential subsidence of basement fault blocks.

 

Permian Carbonate Deposition 

The outcrop study for the Chengkou-Exi region reflecting the lithology and thickness change is effectively applied to a subsurface seismic facies analysis of the platform-trough system (Late Permian to Early Triassic) of Kaijiang-Liangping region in northern Sichuan Basin (Figures 5 and 6). An interpreted segment of a 3-D seismic section through well PG. 1 (PX) located in the platform to well CY. 83 (CFX) in the Kaijiang-Liangping trough (Figure 6) shows that there was a steep slope of about 20 degrees from the platform margin to the trough. This geometry suggests an estimated water depth of the trough from 400 meters to 600 meters. This sedimentary model is very similar to the modern sedimentary environment of the Bahama platform. 

In the trough the thickness of the Upper Permian was roughly 50-200 meters; conversely, on the shallow-water carbonate shelf the sequence thickness is about 400-500 meters (Figure 7). The lower part of the Upper Permian is composed of gray thin-thick beds of bioclastic limestone with chert nodules or thin chert beds. The top of the Dalong Formation is a typical deep-water trough facies. The dominant lithologies of the Dalong Formation of the Upper Permian are dark siliceous shale and siliceous limestones, with siliceous radiolarians and microforaminifers (Figure 7), and ranges from 6.1 m to 50 m in thickness. These characteristics of the Dalong Formation indicate that it is the condensed sections of the maximum flooding surface in deep-water starved basin; it is an important source rock of the Upper Permian with an average TOC of 3.88% in the north Sichuan Basin. The sedimentary environment of the lower Feixianguan Formation of the Triassic succeeded the Late Permian. The trough facies of the Feixianguan consists of dark micritic limestone with fine laminations, mud shale, and limestone debris with 0.2-2m-thick interbeds containing carbonate lithology characteristics of the carbonate platform, such as ooids and lithoclasts; these lithologies are respectively interpreted as pelagic sediments and carbonate gravity flow sediments (turbidites and debrite flows) in the deeper water basin and slope environment. Their thickness is from 50 to 380 meters. The FMI data and cores in many wells in Kaijiang-Liangping trough region show some unmistakeable evidence of the deep-water carbonate deposition. The cores of well CF 83 (CFX) (4857.3-4857.5 m) of Kaijiang-Liangping trough region show a carbonate turbidite with a complete Bouma sequence and a thickness of 14.5 cm. The turbidite exibits normally graded bed and five internal units T_a-e (Figure 6). The carbonate turbidite has some ooid lithoclasts that came from the shallow carbonate platform. In addition, the FMI data of another well reveal a breccia limestone thought to be a carbonate debris flow. 

During the depositional period of the highstand systems tract carbonate production was greater on the platform margins. Those carbonate deposits could be rapidly transported to the deeper basin settings; consequently the trough regions were filled and changed to the shallow-water carbonate deposition environments late in the Early Triassic. Porous reservoirs are quite unusual due to lack of the reef and oolitic deposition in the Kaijiang-Liangping deep-water trough region, where so far only small gas pools have been discovered. These fields consist of limestone with low porosity and low permeability.

The Changxing Formation reefs in the Late Permian were deposited on the carbonate ramp or platform around the troughs (Figure 2). The carbonate ramp setting containing bioclastic packstone with chert nodules was in East Sichuan, and the carbonate-platform setting, rich in bioclastic grainstone, was in Northwest and the Northeast Sichuan. Some larger sponge-hydrozoan reefs developed mainly along the shallow-water carbonate deposition margin around the troughs, whereas some smaller patch reefs could be widely spaced, at random, on the carbonate ramp and platform. These reefs, which were commonly dolomitized, became important gas reservoirs of the Changxing Formation of the Late Permian in the study area. As a result, the larger reef reservoirs are developed along the margin of carbonate ramp and platform around the trough, with the single field area over 10 km² and gas reserves > 30×10⁸ m³. In contrast, the small patch reef reservoirs occur on the wide carbonate ramp and platform, with individual reservoir being smaller than 5 km² and gas reserves < 10×10⁸ m³.

 

Triassic Carbonate Cycles 

During Feixianguan stage of Early Triassic, the Late Permian carbonate ramp was transformed into an open carbonate platform, whereas the carbonate platform of the Late Permian was transformed into a carbonate platform with tidal flat evaporites (Figure 3). The oolite shoals were widely developed on both platforms. At the end of Feixianguan stage of the Triassic, the entire Sichuan area was covered by a widespread tidal flat environment during the late stage of a sea level high stand. 

On the open platform of the Feixianguan stage of the Triassic there are many cycles; each cycle has two types of sediment sequences which can be identified: high-energy shoal facies, which may consist of oolite grainstones and peloid grainstones; and low-energy lagoon or tidal-flat facies, which may consist of wackestones and mudstones--shoaling upward cycle (parasequence) from relatively deep water to tidal flat with grainstone shoal facies just beneath sabkha. These sedimentary sequences have little or no dolomitie. The discovered gas pools are small scale and belong to the limestone reservoirs with low porosity on the open platform, while the more massive ooid limestone reservoirs with medium-sized gas pools were developed along marginal facies around the troughs.  

The countless shallowing-upward sedimentary cycles with sabkha sequences on the evaporite carbonate platform of Feixianguan stage of the Triassic consisted of peloidal mudstone, oolitic grainstone, oolitic dolomite, anhydrite and dolomicrite. The thick oolitic grainstones commonly occur along the platform margin around the troughs and were strongly dolomitized and form the excellent reservoirs of the Feixianguan Formation of the Triassic in the northern Sichuan Basin. Oolitic dolomites mainly developed on the evaporite platform are the most important reservoirs in this area. So far, the discovered gas pools show sulfur content and are the dolomite reservoirs with high porosity on the evaporite platform. The leeward marginal facies along the evaporite carbonate platform margin around Kaijiang-Liangping trough is a favorable belt with large and medium-sized reservoirs, where high quality gas reservoirs of massive dolomites of Feixianguan Formation are developed, up to 200-300 meters in thickness.

 

Conclusion 

In particular, the reefs and oolitic shoals on the carbonate platform margin along the Kaijiang-Liangping trough have favorable characteristics to become the best dolomite reservoirs in the northern Sichuan Basin. They are adjacent to a trough that is a hydrocarbon-generating sag; they have been strongly dolomitized and exhibit burial dissolution.  An important giant gas accumulation around the Kaijiang-Liangping trough has been identified on the platform. The zone extending over an area of 10,000 km², about 500 kilometers long, where 8 marginal reef and 15 oolitic shoal gas fields have been discovered; Puguang gas field has proven individual reserves approaching 3560×10⁸ m³.

 

Selected Bibliography 

Calver, F., M.E. Tucker and J.M. Henton, 1990, Middle Triassic carbonate ramp systems in the Catalan Basin, northeast Spain: facies, systems tracts, sequences and controls, in M.E. Tucker, J.L. Wilson, P.D. Crevello, J.R. Sarg and J.F. Read (Editor), Spec. Publs. Ass. Sediment., p. 79-108.

 

Esteban, M. and C.F. Klappa, 1983, Subaerial exposure environment, in P.A. Sckolle, D.G. Bebout and C.H. Moore (ed.), Carbonate depositional environments, AAPG Memoir 33, p. 1-95.

 

Fanghao, Hou, Fang Shaoxian, Zhao Jingsong, et al, Atlas of carbonate rock reservoirs of Ordovican in Ordos Basin, Chendu: Sichuan people’s publish house, p. 58-77 (in Chinese).

 

Haitao, Hong, Wang Yigang, Yang Tianquan et al., 2008, Sedimentary facies of Changxing Formation and distribution of organic reef gas reservoirs in northern Sichuan basin, Natural Gas Industry, 2008, v. 28, no. 1, p. 38-41.

 

Kinsman, D.J.J., R.K. Park, 1976, Algal belt and coastal sabkha evolution, Trucial Coast, Persian Gulf, in M.R. Walter (ed.), Stromatolites, Elsevier, Amsterdam, p. 421-433.

 

Martin-chivelet, J. and R. Gimenez, 1992, Palaeosols in microtidal carbonate sequences, Sierra de Utiel Formation, Upper Cretaceous, SE Spain, Sediment. Geol., p. 81:125-145.

 

Moore, C.H., 1984, The Upper Smackover of the Gulf Rim: Depositional systems, diagenesis, porosity, evolution and hydrocarbon production, in W.P.S. Ventress, D.G. Bedout, B.F. Perkins and C.H. Moorey (ed.), Publ. Soc. Econ, Paleont. Miner, Gulf Coast Section, p. 283-308.

 

Powers, R.W., 1962, Arabian Upper Jurassic carbonate reservoir rocks, in W.E. Ham (ed.), Classfication of Carbonate Rocks, a Symposium, AAPG Memoir 1, p. 122-192.

 

Sellwood, B.W., 1978, Shallow-water carbonate environments, in H.G. Reading (ed.), Sedimentary environments and facies, London: Blackwell science publications, p. 259-313.

 

Tucker, M.E., V.P. Wright, 1990, Carbonate Sedimentology, Oxford London: Blackwell Scientific Publications, p. 101-164.

 

Wang, Xingzhi, Zhang Fan, Ma Qing et al., 2002, The Characteristics of Reef and bank and the fluctuation of sea-level in late Permian to Feixianguan Period of early Triassic East Sichuan Basin, Acta Sedimentologica Sinica v. 20, no. 2, p. 249-253 (in Chinese with English abstract).

 

Wang, Yigang, Wen Yingchu, Zhang Fan et al., 1998, Distribution Law of the organic reefs in Changxing Formantion of Upper Permian in East Sichaun, Natural Gas Industry, v. 18, no. 6, p. 10-15 (in Chinese with English abstract).

 

Wang, Yigang, Liu Huanyi, Wen Yingchu et al., 2002, Distribution law, exploration method and prospectiveness prediction of the oolitic beach reservoirs in Feixinguan Formation in northeast Sichuan Basin, Natural Gas Industry, v. 22 (supplementary issue), p. 14-19 (in Chinese with English abstract).

 

Wang, Yigang, Wen Yingchu, Hong Haitao et al., 2004, Exploration target of the deep oolitic beach gas reservoir of Triassic Feixianguan Formation in Northeast Sichuan basin, Natural Gas Industry, v. 24, no. 12, p. 5-9 (in Chinese with English abstract).

 

Wang, Yigang, Wen Yingchu, Hong Haitao et al., 2007, Diagenesis of Triassic Feixianguan Formation in Sichuan Basin, Southern China, Acta Sedimentologica Sinica, v. 25, no. 6, p. 831-839 (in Chinese with English abstract).

 

Wang, Yigang, Hong Haitao, Xia Maolong et al., 2008, Exploration of reef-bank gas reserviors surrounding Permian and Triassic Troughs in Sichuan basin, Natural Gas Industry, v. 28, no. 1, p. 22-27.

 

Warren, J.K., 1991, Sulfate dominated sea-marginal and Platform Evaporative Settings: Sabkhas and salinas, mudflats and salterns, in J.L. Melvin (ed.), Evaporites, Petroleum and mineral resources, Developments in Sedimentology v. 50, p. 69-187.

 

Wen, Yingchu, wang Yigang, Chen Xubin et al., 1988, Extension Of carbonate platform in Feixianguan Age of Early Triassic and the distribution of favorable reservoir facies belts in Sichuan, Natural Gas Industry, v. 8, no. 2, p. 18-23 (in Chinese with English abstract).

 

Wen, Yingchu, Wang Yigang, Zheng Jiafeng et al., Carbonate rock reservoirs of paleoweathering crust, Chendu: Press of University of electronic science and technology of China, p. 94-113 (in Chinese).

 

Yongsheng, Ma, Xusheng Guo, Tonglou Guo et al., 2007, The Puguang gas field: New giant discovery in the mature Sichuan Basin, southwest China, in AAPG Bulletin, v. 91, p. 627-643.

 

Yue, Guangyu, Du Siqing, Huang Tijun et al., 1996, Principle of structural compounding-combine, Chengdu, Press of Cheng du University of Science and Technology, p. 119-151 (in Chinese).

 

Zhang, Jing, Wang Yigang, 2003, Characteristics of carbonate evaporation platform edge deposition in Early Feixianguan Epoch at Hekou region of Xuanhan in Sichuan, Natural Gas Industry, v. 23, no. 2, p. 19-22 (in Chinese with English abstract).

  Return to top.