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Characteristics of Changxing-Feixianguan Reservoirs of Panlongdong Outcrop in Northeastern Sichuan Basin, China

Zhai, Xiufen *1; Luo, Ping 1; Qing, Hairuo 2; Xu, Ana 1; Wang, Zecheng 1
(1) Research Institute of Petroleum Exploration & Development, Beijing, China.
(2) Regina University, Regina, SK, Canada.

Panlongdong outcrop lies in Xuanhan County, Sichuan Province in China. In this outcrop, both organic reefs in Upper Permian Changxing Formation, and oolitic shoals in Lower Triassic Feixianguan Formation are well exposed. According to combined lithology study and thin section identification, three depositional facies, basin margin slope, open platform, and restricted platform are identified, and Permian and Triassic stratigraphic boundary is redefined in this outcrop.

The thickness of Changxing Formation is about 130m, with 2 cycles of reef complex, each cycle divided into reef-base, reef-core and reef-cap. Reef-base is mostly composed of bioclastic limestones. Mainly bafflestones and framestones formed the reef-core facies, with sponges, hydroids and algae being the main reef-building organisms, and brachiopods, bivalves, crinoids, gastropods, foraminifera, etc. and micritic or sparry cements filling the spaces between skeletons. On top of the reef-core are dolomitized bondstones and bioclastic dolostones. Grain banks and oolitic shoals deposited in Feixianguan Formation member 1, with a thickness of about 35m. Upward micrite dolostones interbedding with marlites of tidal-flat or lagoonal facies are found.

Both reef-core skeleton and bioclastic rocks in Changxing Formation and grain bank and oolitic shoal facies in Feixianguan Formation are intensively dolomitized. Favorable reservoirs are found, with a lot of dissolution pores and cavities filled or half-filled with calcite or bitumen, where dolomitization most develops, like in the 5th, 6th, and 11th layers of the ourcrop. Intercrystalline and intergranular dissolution pores are the most important reservoir space, followed are vugs, moldic pores and dissolved fractures. Dolomitized sponge framestones in Changxing Formation and oolites in Feixianguan Formation member 1 form the best reservoirs, which is different from previous studies of this region, and adds Feixianguan Formation member 1 to the exploration target layers.

Dolostones in both Changxing and Feixianguan formations show similiar petrographic properties of burial dolomite, and geochemical characteristics of high 87Sr/86Sr isotopes, lower Sr content, higher Fe2+ content, relatively higher δ13C, and very negative δ18O. The homogenization temperature and salinity of fluid inclusions are very high, indicating that the dolomitization fluid of both formations comes from the same marine-origined saline formation water in Feixianguan Formation.


AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California