East Of Qaidam Basin Quaternary Biogenic Gas Formation Mechanism And Forming Rules

According to the observations and experimental analyses on the core and cutting samples, combined with the researches on the well logging and stratigraphy, the Quaternary strata in the east part of Qiadam Basin can be classified as eight sedimentary facies.In terms of the researches on the strata sedimentary characteristics and geochemical characters of gas source rocks, together with the laboratory analyses of bio-simulation and the analyses of isotopes helium/argon and helium/carbon, and the analyses on the composition and density of the natural gas, a viewpoint that the natural gas in the region derives from the methane biogas generated in Quaternary terrestrial facies is put forward and clarified.Based on the integrated analyses on the palaeontological sporo-pollen and ostracode, and the data from outcrop and boring samples, combined with the facts of much developed global Quaternary glacier, high altitude and high latitude but low temperature in the sedimentary area, the author suggests that the sedimentary condition during the Quaternary belong to a cold climate environment.In terms of bio-simulation experimental results and the palaeontological sporo-pollen analytical results, together with the analytical results of the minerals and the developing situation of halite in Quaternary strata (especially in Q₄), an argument that the climate during the Quaternary sedimentary period in the region belongs to a droughty climate is pointed out and fully discussed. In addition, an argument that the sedimentary rate is quick and the sulfate concentration in the sedimentary water body is high is fully discussed and clarified.An evolution profile of the Quaternary biogas in Qiadam Basin is set up firstly and systematically. Based on the bio-simulation experiment on core samples, a relationship between the compositions of microbe and organism and the production of biogas, with different depths of the strata, is determined. As a result, a lengthways evolution profile of the Quaternary biogas is set up.A new viewpoint that the formation of large mount of biogas in the region mainly attributes to a special sedimentary environment according with the geological situation of Qiadam Basin is first put forward and discussed systematically. Under the conditions of shallow burial depth (normally less than 2100m), short burial time ( not more than 3.2 millions years) and low ground temperature gradient (less then 3.6 °C/100m), together with a poor abundance of organic matter in the immature source rocks, no other kinds of gas but biogas can be formed.A quick sedimentary rate (0.8-1.0 mm/a) in the region is propitious to the preservation of organic matter, for the organic matter is covered rapidly by sediment and the decomposition is avoided. In addition, a quick burial is propitious to the prevention of the dissipation of natural gas.A higher porous and permeable formation is formed in the region due to shallow burial depth, short burial time, weak compaction and poor diagenesis.The porosity and permeability of the cap rock in the region is relatively high and its breakthrough pressure is relatively low due to above facts. The cap rock with high porosity and higher permeability and low breakthrough pressure (called as " two highs and one low") in the region cannot be called virtually as "cap rock", when compared to the cap rocks in other gas fields at home. However, this type of cap rock in the region really plays a sealing role and has a good function, so it can be called as a special cap rock. The study of this special cap rock is benefit not only to the exploration of the region, but also to the exploration of other areas with similar characters both at home and abroad.Based on two mechanisms of natural gas dissipation (seepage and diffusion) and the real geological characters of the region, both pressure closing theory (Darcy's Law) and diffusion theory (Fick's Law) are...