Genetic Identification And Generation Models Of Deep Natural Gas In Jiyang And Linqing Super-depression

Genesis of natural gas in deep-burial sequences such as Forth Member of the Shahejie Formation and Carboniferous-Permian buried below 4000m in the Jiyang and Linqing Super-depressions is rather complicated, which consist of coal-derived gas, oil-type gas and mixed-gas. The coal-derived gases are generated in primary burial history (primary gas for short) and in reburial history (secondary gas for short) of the Carboniferous-Permian in studied areas. The oil-type gases include kerogen-degraded gas and oil-cracked gas. Investigation on the generation plays an important role in natural gas exploration. The author studied genesis of the natural gases found in Gubei, Minfeng and East Linqing areas, selected special indexes for identification between the primary gas and secondary gas, as well as between kerogen-generated gas and oil-cracked gas, by comprehensive geological and geochemical analysis in the studied areas. Furthermore, generating models are established for the different genetic types of gases based on investigations on the gas source rock evolution and the gas generation kinetic modeling.On the bases of the analysis of source rocks characteristics, the ternary geochemical-tracing system of natural gas and research achievement of geologic condition of gas pools were used in order to clarify the geochemical characteristics and genesis of deep natural gas. The result shows that the natural gas in Gubei-Bonan area shifts from oil-typed gas to coal-derived gas from west to east. Natural gas in Bonan deep sag is high-matured oil-type gas originated from Es4 source rocks in Bonan sag, and gas in Gubei buried hill is mainly composed of coal-derived gas generated by C-P source rocks. Gases found between the two zones are mixtures with different proportions of coal-derived and oil-type gas. Oil and gas produced from C-P sandstones are generated from the same sequence in east part of the Linqing depression. Gas found in the Well Gaogu-4 is generated from C-P coal source rocks at mature stage, and the associated oil is generated in similar process. In the Minfeng area of the Dongying depression, gas and light oil produced from Es4 sandstones are generated from Es4 source rock, mixing with the inorganic CO2.In addition to geological and geochemical investigation on the coal-derived gas generation, a series of thermal stimulation experiments and analysis on gas-generation kinetics are conducted on primary gas generation and secondary gas generation from coals. It is the first discover that there are differences in the gasous hydrocarbon compositions between the primary and secondary generated gases. Therefore, the indexes are proposed for identifying the primary gas and secondary gas and are verified by the genesis-known natural gases. The primary gas is characteristics of iC4/nC4 > 0.9, n(C6+C7)/i(C6+C7) < 0.95, nC6/(2-MC6+3-MC6) < 1.6, nC5/(2-MC5+3-MC5) <1.0 and methal-benzol/benzol < 1.25, and the opposite is the secondary gas. It concludes that the natural gas reserved in the Gubei buried-hill and produced from the Well Gaogu-4 is the secondary gas from C-P coal sequence.Thermal stimulation experiments and analysis on hydrocarbon-generation kinetics are also conducted on source rocks and crude oils to find the composition parameters to distinguish the kerogen-degraded gas and oil-cracked gas. It shows that the kerogen-degraded gas represents as ln(CC5/nC6) < -1, ln(CC5/nC5) < -2,(MCC5+CC6)/nC6 < 0.8, CC6/nC5 < 0.1,(CC5+MCC5+CC6)/(nC5+nC6) < 0.5, (MCC5+CC6+MCC6)/(nC6+nC7) <1.0,DMCC5/CC5 > 0.6 and methal-benzol/benzol >1.1, in contrast, the oil-cracked gas. Therefore, it is believed that the natural gas produced from the Minfeng area is mainly sourced from kerogen-degraded gas, others from oil-cracked gas.Investigation of distributions and burial histories of the gas source rocks, as well as the geologic application of gas-generation kinetics parameters, the coal-derived gas generation models from C-P source rocks are established as five types: (1) the up-rise no-gas generating model; (2) the up-left primary generating and no secondary generating model; (3) the buried up-left secondary high-generating model; (4) the depression-slope secondary high-generating model and (5) the deep sag over-matured generating model. The author believe that the forth model is the best for natural gas generation and accumulation, the third and fifth are good for gas generation and accumulation. The south and north areas of the East Linqing, the south slope of the Huimin depression, the Luojia slope of the Zhanhua depression, the east slope of the Chexi depression and the north slope of the Chengdong structure are best favorable for coal-derived gas generation and accumulation. The north slope of the Dongying depression, the Bonan area and the steep-slope of the Weibei structure is perspective for exploration of high-matured kerogen-derived gas and oil-cracked gas.