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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.