Abstract

Late Quaternary shallow biogenic gas reservoirs have recently been discovered and exploited in the coastal Hangzhou Bay 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 Qiantangjiang 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: fluvial floor facies (IV), flood-plain facies (III), sublittoral-marine bay facies (II), and estuarine facies (I).

All commercial gas fields occur in flood-plain sand bodies of incised valleys. The bodies are buried 30–60 m (98–197 ft) deep and are 3.0–7.0 m (9.8–23 ft) thick, with a maximum thickness of more than 10 m (33 ft). They are surrounded by impermeable clays. Rapid deposition of overlying sublittoral-marine bay sediments supplied not only abundant gas sources, but also good preservation conditions. The main hydrocarbon sources are dark gray clays of the flood-plain facies and gray muds of the sublittoral-marine bay facies. Sediments of both facies have organic carbon content generally more than 0.4%. Shallow biogenic gas fields and deep gas fields require vastly different drilling and completion techniques. Drilling and completion costs are much lower for the biogenic gas fields.

Quaternary incised valleys and flood plains other than Hongzhou Bay in coastal areas of eastern China are promising targets of further exploration for shallow biogenic gas.

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