--> Abstract: Features and Sealing Mechanism of Shallow Bbiogenic Gas in Incised Valley Fills (the Qiantang River, Eastern China): A Case Study, by Chun-Ming Lin, Yan-Li Li, Zhi-Phi Zhang, and Hong-Chun Zhuo; #90078 (2008)

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Features and Sealing Mechanism of Shallow Bbiogenic Gas in Incised Valley Fills (the Qiantang River, Eastern China): A Case Study

Chun-Ming Lin, Yan-Li Li, Zhi-Phi Zhang, and Hong-Chun Zhuo
Nanjing University, Nanjing, China

Late Quaternary shallow biogenic gas reservoirs have recently been discovered and exploited in the Qiantang River estuary area, northern Zhejiang Province, eastern China. The river in this area strongly incised the underlying old beds during a period of glacial maximum, which resulted in the formation of the Qiantang River and the Taihu incised valleys. These incised valleys were filled with fluvial sediments and buried by marine sediments during the postglacial period. Late Quaternary strata of the incised valley area are composed of four sedimentary facies in ascending order, river channel facies (IV), floodplain-estuarine facies (III), estuarine and shallow marine facies (II), and estuarine sand bar facies (I).

All commercial gas fields occur in floodplain-estuarine sand bodies of incised valleys. The deeply incised valleys in the study area provided favorable conditions for the generation and accumulation of shallow biogenic gas. The sand bodies of incised valleys are buried 30-80 m deep, and are 3.0-7.0 m thick, with a maximum thickness of 26 m. They are surrounded by impermeable clays, are lenticular in shape, and may be represent a tidal ridge system in the incised valley. Rapid deposition of overlying estuarine and shallow marine facies sediments supplied not only abundant gas sources, but also good preservation conditions. The main hydrocarbon sources are dark gray clays of the floodplain-estuarine facies and gray muds of the estuarine and shallow marine facies. Sediments of both facies have organic carbon content generally more than 0.4%.

The pore-water pressure of the clay and mud beds can exceed the total pore-water pressure and gas pressure of underlying sand reservoirs. The maximum of the pore-water pressure occurs at the top of the shallow gas reservoirs, just as the interface of gas reservoirs and clayey cap beds

 

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas