Geology and formation mechanism of late Quaternary shallow biogenic gas reservoirs in the Hangzhou Bay area, eastern China

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.

Characteristics and accumulation model of the late Quaternary shallow biogenic gas in the modern Changjiang delta area, eastern China

“…Lin et al 2004;Zhang et al 2013). H/C and O/C ratios of kerogen vary in a range of 0.89-1.25 and 0.27-0.38, respectively, and plot in the type III kerogen area (gas prone; cf.…”

“…However, the TOC contents and chloroform bitumen values for the argillic sediments in the study area are significantly lower than those from the nearby Qiantang River incised-valley fill (cf. Lin et al 2004;Zhang et al 2013), which may be caused by the remarkably different amount of terrigenous sediment inputs. The mean annual suspended sediment load is *4.8 9 10 8 t/yr for the Changjiang, generally two orders of magnitude higher than that of the Qiantang River (*0.09 9 10 8 t/yr).…”

“…Lin et al 2004;Gao et al 2010Gao et al , 2012. As a result, most gas in the study area is regarded as being transported from gas-source beds to sand beds by formation water with the differential compaction between sand and mud beds.…”

“…Shallow biogenic-gas reservoirs are principally distributed along the eastern and southern coasts of China, especially the Jiangsu-Zhejiang coastal plain area, where the postglacial Changjiang and Qiantang River incised valleys are located (Wang 1982;Zheng 1998;Lin et al 2004;Liu et al 2008;Li et al 2010a;Zhang et al 2013Zhang et al , 2014. Incised valleys generally have high preservation potential for their fill deposits (Dalrymple et al 1994) and provide a fundamental and important background for the formation of the shallow biogenic-gas reservoirs therein, i.e., gas-source beds, cap beds, and sand reservoirs can occur together in close geographic and stratigraphic proximity (Xu et al 2009;Lin et al 2004;Zhang et al 2014).…”

“…Incised valleys generally have high preservation potential for their fill deposits (Dalrymple et al 1994) and provide a fundamental and important background for the formation of the shallow biogenic-gas reservoirs therein, i.e., gas-source beds, cap beds, and sand reservoirs can occur together in close geographic and stratigraphic proximity (Xu et al 2009;Lin et al 2004;Zhang et al 2014). Considerable biogenic-gas accumulations with depth \120 m have been discovered and well documented in the late Quaternary Qiantang River incised valley with a predicted total gas amount of 244.5 9 10 9 m 3 (Lin et al 2004Zhang et al 2013). Also, the produced gas was used as gas supply for local villages and factories .…”