Research About Geological Conditions Of Ultra-shallow Biogenic Gas Accumulation In Hangzhou Bay Area

Biogenic gas is generated by organic matters through biochemical action under reduction condition, with characteristics of shallow burial depth, low exploration and exploitation cost, good benefits and great resource potential when compared with conventional natural gas. Therefore, research on the shallow biogenic gas has become a hot and key point of oil and gas resource exploration. Biogenic gas pools have been discovered in Qaidam Basin, Bohai Bay Basin, Songliao Basin, Subei Basin, Baise Basin and Hetao Basin in China. The accommodation space of Late Quaternary Qiantang River incised valley in Hangzhou Bay area provides a unique opportunity to study a relatively uninterrupted stratigraphic history. The Qiantang River incised valley is also associated with oil and gas resources. But the accumulation mechanism and exploration degree of ultra-shallow biogenic gas pools in Hangzhou Bay area are relatively poorly studied. In this thesis, combined a lot of core data with a large number of test results, we aim to 1) study accumulation conditions of ultra-shallow biogenic gas pools in Hangzhou Bay area in detail; 2) evaluate resource potential of ultra-shallow biogenic gas; 3) summarize the accumulation characteristics and influencing factors of biogenic gas pools in China. This study has great realistic significance to the exploration and exploitation of Late Quaternary ultra-shallow biogenic gas pools in Hangzhou Bay area.Four sedimentary facies, including fluvial channel, floodplain-estuary, estuarine-shallow marine, and estuarine sand bar, can be distinguished in Hangzhou Bay area. The composition of ultra-shallow biogenic gas in Hangzhou Bay area is dominated by CH4 (up to 90%), with minor heavy hydrocarbon, N2 and CO2, and the carbon isotope (δ13C1) ratios of methane is lower than-66‰, which belongs to the typical methane biogenic gas. The main gas source rocks in Hangzhou Bay area are Silty clay beds of floodplain-estuary facies and mud beds of estuarine-shallow marine facies. The gas source rocks of floodplain-estuary facies were deposited in cool and dry climate, and are abundant of herbal plants and aquatic organisms, such as algae and fungi. The insoluble organic matter can be recognized as humic type, which was formed in weak to strong reduction environment. The gas source rocks of estuarine-shallow marine facies were deposited in warm and moist climate, and contain more xylophyta, but few herbal plants and aquatic organisms, such as algae and fungi. The insoluble organic matter can be recognized as sapropelic-humic type, which was formed in weak to ordinary reduction environment. Organic matters of ultra-shallow biogenic gas source rocks are substantially at the immature stage due to their shallow burial depth, and ultra-shallow biogenic gas is mainly produced at the biochemical stage.Ultra-shallow biogenic gas in Hangzhou Bay area is mainly the production formed on reduction of CO2. The formation of ultra-shallow biogenic gas is controlled by many factors, which mainly include changes in sea level, crustal subsidence rate, depositional rate, sedimentary environment, depositional time, palaeoclimate and the property of water medium. The interactions among these factors are conductive to mass production and accumulation of ultra-shallow biogenic gas in Hangzhou Bay area. The total amount gas generated from the source rocks in Hangzhou Bay area is estimated to be 2647.99×108 m3, with total dissolved gas 1435.47×108m3, total adsorbed gas 832.29×108m3, total free gas 363.53×108m3, and total diffused gas 16.73－108m3. The amount gas generated from the source rocks in floodplain-estuary facies is estimated to be 1129.28×108 m3, with total dissolved gas 675.53×108m3, total adsorbed gas 275.61×108m3, total free gas 174.31×108m3, and total diffused gas 3.83×108 m3. While those from the source rocks in estuarine-shallow marine facies are 1518.71×108 m3,759.94×108m3,556.68×108m3, 189.22×108m3 and 12.90×108m3. These estimations show that despite part of ultra-shallow biogenic gas is adsorbed by rock and diffused to the surface, most of ultra-shallow biogenic gas is the form of water soluble gas and free gas in Hangzhou Bay area.Silty clay beds of floodplain-estuary facies serve as direct cap beds for ultra-shallow biogenic gas in Hangzhou Bay area, while mud beds of estuarine-shallow marine facies serve as indirect cap beds. Porosity, water saturation, liquid index and compression coefficient of cap beds are significantly high than those of the reservoirs, while permeability and volume density of cap beds are lower than those of the reservoirs, which shows that cap beds have stronger deformation-resistance, larger compressibility, lager volumetric deformation and better sealing ability than the reservoirs. The results of falling-head permeability test indicate that the permeability of reservoirs exceeds that of the cap beds over two or three orders of magnitude. Capillary pressure for gas to pass through pores in the cap beds is greater than that in the sand lenses, forming the capillary sealing. Pore-water pressure of clay and mud beds can exceed the total of pore-water pressure and original gas pressure of the underlying sand reservoirs, so the pore-water pressure sealing ability is formed. Hydrocarbon concentration in the cap beds is obviously higher than that in the reservoirs. The downward diffusion of gas in the cap beds can restrain the vertical flow of gas in the underlying reservoirs, thus a seal is formed by the gradient of hydrocarbon concentration, which is named hydrocarbon concentration sealing. Capillary sealing, pore-water pressure sealing, and hydrocarbon concentration sealing all contribute to the conservation of ultra-shallow biogenic gas pools in Hangzhou Bay area. Pore-water pressure sealing mechanism may be the most important factor for the formation of the sealing ability of cap beds. Disequilibrium compaction, swelling of clay minerals, and gas generation are the main reasons for the generation of high pore-water pressure. Large compressibility, low permeability, gas capillary seal, abundant organic matter, and volume expansion of clay minerals offer favorable conditions for the preservation of high pore-water pressure. Sealing ability of the direct cap beds is better than that of the indirect cap beds, because of the stronger capillary sealing, pore-water pressure sealing, and hydrocarbon concentration sealing ability.Three types of biogenic gas pools have been discovered in China, including low temperature-syngenetic diagenesis-shallow burial, middle temperature-early diagenesis A stage-medium burial and high temperature-early diagenesis B stage-deep burial. Favorable conditions of gas generation, properties and scale of reservoir, sealing and preservation conditions of cap beds, early trap and synsedimentary trap, continuous producing of biogenic gas all contribute to the accumulation of shallow biogenic gas in China.