Shallow Gas Genesis and Reservoir Forming Mechanism in the South of Daqing Placanticline

Shallow gas in the south of Daqing placanticline is mainly biogas originated from K₁n₁₊₂, mixed with a small amount of thermogenic gas of K₁qn and inorganic CO₂. The main biogas generation and expulsion period of K₁n₁₊₂ is the end of K₂m sedimentary period, when fault activities caused the formation of anticline and fault trap of Heidimiao oil layer and induced vertical migration of biogas. The composite of the three periods made large amount of biogas migrate into Heidimiao oil layer. Active faults are all boundary faults in fault-condensed belt, along which biogas migrated vertically. The position of biogas accumulation depends on allocation relation between fault and strata of two walls. The wall whose dip is contrary to that of the fault is the main position for biogas accumulation. Generally the southern part of Daqing placanticline is an asymmetric anticline that is gentle in the west and steep in the east. Biogas is mainly accumulated in the traps of the east boundary of fault-condensed belt. There are 5 sets of reservoir-seal assemblages. Cap rock thicknesses of 3 sets of assemblages in the lower part are greater and the probabilities of them to be leaped by faults are relatively small. They are the main target beds. Most of the traps in Heidimiao oil layer are fault traps. The lateral sealing of fault determines trapping potential. The lower limit of SGR required by lateral seal of fault is 0.375. There are 3 kinds of situations when we analyze trapping potential by use of Allen graphic to calibrate lateral sealing of fault or not taking lateral sealing of fault into consideration. The first is that fault is sealing in the lateral and the area and amplitude of trap don't change (type Ⅰ). The second is that the lateral sealing of fault is sectionalized and the area and amplitude of trap become smaller (type Ⅱ). The third is that fault isn't sealing in the lateral and there is no trap (type Ⅲ). Among them, traps of Ⅰand Ⅱare favorable targets for biogas accumulation. The south of Daqing placanticline shows that the key factors of shallow gas accumulation in Songliao basin are integrity of traps controlled by faults that can connect gas source and regional distributed cap rocks.