Reef Composition and Their Forming Models of Changxing Formation in Jiantianba Section of Lichuan,Western Hubei

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Abstract: Jiantianba of Lichuan, western Hubei, is located in platform margin facies between carbonate platform facies in the eastern Sichuan and Western Hubei Trough. In the study area, the water gradually shallowed from the bottom to up in the stage of Changxing Formation of Late Permian, and developed a huge scale platform marginal reef deposition in aggradation progradation. Based on the multidisciplinary analysis of measured profiles, the authors discussed the reef composition and forming model of Changxing Formation reef in Jiantianba section. The study demonstrated that the study area developed two thirdorder sequences in Changxing depositional stage of late Permian and the body of reef in the Jiantianba of Lichuan was located in HST of sequence 1of Changxing Formation. And its internal constituted by the four parasequence sets. Pss1 mainly composed by dark grey lamellar or lamellar  middle bedded cryptite, clastizoic cryptite, middle bedded bioclastic cryptite, the internal could be further divided into three parasequences namely PS2, PS3 and PS4. PS2 and PS3 were located in PSS1 lowermiddle part which are composed by a set of slope facies of lamellar dark gray mudstone deposit, and PS4 was located at the top of PSS1 and developed a set of gray middle lamellar bioclastic limestone with the shoal facies deposition and the reef of Jiantianba was developed on the basis of the above bioclastic bank. PSS2 was a platform edge bioherm deposition, developed a set of grey massive algae bonding, algal bondingbaffle and baffle bioconstructed limestone, this parasequence set internal developed three parasequences, and PS5 mainly was algae bondstone, the lower part of PS6 and PS7 mainly were algae bondstone, the upper part of PS6 and PS7 was algae bondbafflestone, which reflects the water shallow gradually, hydrodynamic stronger and bioherm prosperous gradually. PSS3  is mainly composed by a set of gray massive baffleframework reef limestone, sponge organic framework reef and partly limestone breccia, the internal can be further divided into three parasequences namely PS8 PS9 and PS10; With relative sea level further drops, frame builders extremely flouring and mainly are Sphinctozoa, Inozoa and little Sclerospongiae, Hydrozoa and Bryozoan, this period was flourishing period of reefs, this parasequence set constitutes the main deposition of platform margin reefs. PSS4 constituted by a mediumbedded and  light gray bioclastic limestone with dolomite and remains of bioclastic dolomite, and belonged to platform edge and restricted platform deposition. As the relative sea level fell gradually and hydrodynamic force strengthening progressively, the reef in the Jiantianba of Lichuan experienced four stages of evolution, including a foundation, the initial flourishing, prosperous, and the largest decline of organic reef. Reefs in Jiantianba of Lichuan reservoir system mainly distributed at the top of the HST, the environment for the formation include platform edge bioherm, platform edge reef and restricted platform dolomite deposition at the top of reef, high quality reservoir is closely related to the reef composition and deposition conditions.