Geochemical Characteristics and Genesis of Natural Gas in Qikou Sag

Qikou sag is the most enrichment area of natural gas in the Huanghua depression.Till the end of 2007,the proved gas reserve has been up to 673.95×108 m3,accounting for 98% of the whole depression.Natural gas was mainly distributed in the structural belt of Banqiao fracture and northern Dagang buried hill.The gas-produced interval is in range of 2 000-4 500 m.Based on geochemical data,we consider that most of mature natural gas is of humic origin in the Qikou sag.According to thermal stage of natural gas,four types of natural gas are classified as follows: microbial gas,mixing gas of microbial-thermal gas,thermal biogenic gas and abiogenic gas.The thermal biogenic gas as a main component is the future exploration target.In comparison with light hydrocarbon absorbed in the rock gas and natural gas,we consider that natural gas in the Qikou sag would be mainly originated from the Es3 source rock,and a certain contribution of Es1 source rock and Es1 oil biodegraded play an important role.Thus,natural gas originated from the Es3 source rock is the main goal for future exploration as well as the Es1 source rock.