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Sichuan Basin

Geologic Setting:

Area of the Sichuan Basin

The Sichuan Basin is located in the Sichaun Province and the Chongquinq Municipality in Southwest China^[1]. The Sichuan Basin is one of China's four Largest basins and accounts for 46% of the Sichuan Province and covers a total area of 260,000 square kilometers^[1]. The basin comprises of two main parts. The first is mountainous regions on the edges which approximate 100,000 square kilometers and ranging in heights from 1,000 to 3,000 meters above sea-level. The second part is lowlands in the middle which make up approximately 160,000 square kilometers with a central part ranging 400 to 800 meters above sea level^[1]. The Sichuan Basin is bordered on all sides by the Quinhai-Tibet Plateau in the west, the mountainous regions of Hunan and Hubei in the east, the Qunling Mountains and the Loess Plateau in the north, and the Yunnan-Guizhou Plateau in the south^[1].

Tectonic map of Sichuan Basin and the surrounding areas highlighting main points.

Tectonic Setting:

Placement of Sichuan Basin on Yangtze Craton

The Sichuan Basin belongs to the Sichuan Terrance which is a major part of the Yangtze Terrace^[1]. The Sichuan Basin consists of a basement from the Yangtze Craton which is formed from Proterozoic rock^[2]. The basin is surrounded on all four sides by fold-thrust belts^[3]. Two of the major tectonic occurrences that had a large effect on the basin is the the Qinling orogene and the Longman Shan Belt. The Qinling orogene on the north experienced a protracted tectonic history and was regarded to have undergone a two-phase collision along two suture zones in the middle Paleozoic and Triassic times, respectively^[3]. The Longmen Shan belt, lying between the Songpan-Ganzi terrane to the west and the Sichuan basin to the east, evolved into an intracontinental transpressional orogen in the Late Triassic^[3].

Key for the geologic cross section

Stratigraphy:

As previously stated the Sichuan Basin consists of a Proterozoic basement. The basement is covered by a thin, incomplete succession of Sinian shallow-marine rocks^[2]. On top of that, the rock strata ranges from Paleozoic to Cenozoic sedimentary rocks. The rocks here are basically reddish sandstones and purple shales which are rich in calcium, phosphorus, potassium and other nutrients. Since the Sinian Period, the basin has been a large depression with a relatively strong, stable crust^[1].

Geologic cross section of Sichuan Basin (including Longman Shan)

Basin Evolution:

Pre-Mesozoic Evolution:

Pre-Sinian:

The rocks from this time period are high grade metamorphic igneous rocks and low-grade meta sedimentary rocks. These rocks represent the Protozoic basement. Basin development for this time period is classified as metamorphic basement.^[2]

Sinian:

During the Sinian period, the Yangtze Craton underwent significant rifting. There was deposition of very thick terrigenous and volcanic deposits, Over the first half of the Sinian Period, the basin development was early continental rift. In the second half of the Sinian the basin development was a passive margin^[2].

Cambrian and Ordovician:

During the Cambrian and the Ordovician a layer of rock with varying thickness was deposited in shallow-marine and non-marine environments. We continue to see the passive margin development of the basin which began in the Late Sinian through the Cambrian and Ordovician^[2].

Silurian, Devonian, and Carboniferous:

Strata from the Silurian, Devonian, and the Carboniferous are missing over a large area of the subsurface for the basin, However, by using evidence from the Longmen Shan belt, it can be suggested that at least the western part of the Sichuan Basin experienced an extensional process. During the Silurian, the passive margin development comes to and end, and marginal extension develops the basin through the Devonian and the Carboniferous^[2].

Permian:

Rocks from the Permian consist mainly of shallow-water limestone and dolomite. At the very beginning of the Permian the marginal extension of the basin comes to an end. During the Middle Permian, there was a major eruption of basalt, known as Emel basalt, which occurred over a broad region of the southern part of the basin^[2].

Mesozoic Evolution:

Triassic:

Lower and Middle Triassic rocks are shallow marine to locally non-marine. During the Late Triassic, the Sichuan Basin became a flexurally loaded foredeep filled with non-marine clastic rocks with a development of prominent coal deposits. These coal deposits were the first sign that convergent activity had begun. The basin development for most of the Triassic was a passive margin, but development changed to a peripheral foreland basin toward the end of the Triassic^[2].

Jurassic and Cretaceous:

Rocks from the Lower and Middle Jurassic sit unconformably on deformed upper Triassic and older rocks. Upper Jurassic and Lower Cretaceous sedimentary rocks thicken towards the northern parts of the basin; with patterns of disruption and indicate the foredeep subsidence of this period. Exposure of Upper Cretaceous rocks is more restricted than that of lower Cretaceous units. During the early Jurassic Period, we see and end of the peripheral foreland basin. Intracontinental subsidence occurs from the Middle Jurassic thru the Cretaceous^[2].

Cenozoic Evolution:

Paleogene, Neogene, and Quaternary:

Most Cenozoic rocks are restricted to the southern part of the basin. Paleogene rocks conformably overlie Cretaceous rocks and consists of fluvial-lacustrine red beds. Neogene rocks are very uncommon. Quaternary strata are mainly sub horizontal and unconformably deposited on older rocks. The basin Development for the Cenozoic era is a rejuvenated foreland basin^[2].

Main stratagraphic chart including petroleum potential

Petrology:

The Sichuan Basin is one of China's most important petroliferous basins. Know source rocks for petroleum in the basin include: Triassic Strata, Upper Triassic carbonaceous mudstone, Paleozoic Strata, and Lower Silurian marine shale. Known hydrocarbon reservoirs consists of: Permian dolomites, Middle Triassic limestones, Upper Triassic sandstones, and Middle Jurassic sandstones. The known seals for the reservoirs are Lower Triassic evaporates, Upper Triassic coal seams, and Jurassic silty mudstones^[2]. Generation of hydrocarbons occurred after the last phase of thrusting in early Cretaceous forming small magnitude anticline traps^[2].

References:

^ ^a ^b ^c ^d ^e ^f China National Petroleum Corporation. "Sichuan Basin" (PDF).
^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j ^k ^l Dong, J., Wei G., Chen Z., Li B., Zeng Q., and Yang G., 2006. Longmen Shan fold-thrust belt and its relation to the western Sichuan Basin in central China; new insights from hydrocarbon exploration. AAPG Bulletin, v. 90, Issue 9, p. 1425-1447.
^ ^a ^b ^c Meng Q. Wang E. Hu J., 2005. Mesozoic sedimentary evolution of the northwest Sichuan Basin; implication for continued clockwise rotation of the South China Block. Geological Society of America Bulletin, v. 117, Issue 3-4, pp. 396-410.