Effect Of Hydrocarbon Degradation On The Composition And Enrichment Of Shale Gas In High Maturity Shale Rocks

During the past few years the exploration and development of shale gas have achieved great success in China.The shale gas currently exploited in China is mainly featured by a high thermal maturity level and high dryness index concerning its hydrocarbon compositions.Among the gas molecules the methane concentration is generally more than 95%while C2^C5 hydrocarbon molecules are scarce.It is speculated that C2^C5 hydrocarbons may have been degraded in shale rocks.Up to date,the geochemical/physiochemical mechanisms of this kind degradation processes occurred inside shale rocks have not been fully understood,particularly the associated role and significance of minerals,transition metal elements,residual kerogen and water in the rocks should be probed during the degradation processes.Accompanying the small molecular hydrocarbon degrading,the change of the storage capacity of shale rocks is essential for the enrichment and development of shale gas resources.Therefore,firstly to characterize the trace elements and mineral composition and evaluate the hydrocarbon generation potential of residual kerogens in shale rocks,and then in this thesis some small molecule hydrocarbons were selected to perform series of thermal simulation experiments to study the characteristics of small molecular hydrocarbons degradation,their geochemical/physiochemical mechanisms of degradation,the role and significance of minerals and water in the degradation processes.Furthermore,a low mature and organic-rich shale rock was selected for series of pyrolysis experiments in different systems,to investigate the occurrence characteristics of residual kerogens and the pore evolutions in shale rocks during its thermal evolution.Some basic results and viewpoints were drawn from this thesis work as the following:The composition of trace elements in shale rocks and their residual kerogens from different depositional environments varies greatly.The shale rocks?Niutitang Formation and Longmaxi Formation?from deep-water shelf were rich in trace elements,and highly enriched in transition metal elements in their residual kerogens compared with the bulk shale rocks.The concentrations of the total trace elements in the shale rocks deposited in the salinity lake?Lucaogou Formation?and the coastal?Xiamaling Formation?were relatively low,and depleted in transition metal elements in their residual kerogens compared with the bulk shale rocks.The trace elements enriched in shale rocks and particularly in their residual kerogens can catalyze the formation of shale gas at high-over maturity stage.The results from Scanning electron microscopy?SEM?showed that the residual kerogens in shale rock can be divided into micron-scale intergranular,micron-scale intragranular and nano-scale intergranular organic matter?OM?,and the evolution of these OM are different from each other with the increasing thermal maturity.Among which the micron-scale OM has been pyrolyzed at EasyR_o 0.82%,and at EasyR_o 1.45%the micron-scale intergranular OM has been exhausted but the micron-scale intragranular OM still has hydrocarbon generation potential.It is estimated that the residual kerogens from Lucaogou Formation can generate methane about 1 m³/t rock at the high-over maturity stage,while under the actual geological conditions more methane can be produced from the small hydrocarbon molecules degradation in the shale rocks.Results from the simulation pyrolysis experiments showed that the yields and compositions of products from the hydrocarbon plus shale and water experiments were different from those from the control experiments.Among the propane simulation experiments under 360?,the yields of CH₄,C₂H₆ and the CH₄/C₂H₆ ratios from the propane plus shale?S1and S2 collected from Longmaxi Formation drilling core?experiments are generally higher than those from propane alone.Furthermore,the yields of CH₄,C₂H₆ and the CH₄/C₂H₆ ratios from the groups with S1 abundant in clay minerals were higher than those from the groups with S2.These results showed that the clay minerals in the shale can catalyze the degradation of propane and facilitate the production of methane.More CH₄,C₂H₆ and higher CH₄/C₂H₆ ratios were detected from the water added groups,indicating that water not only promote the degradation of propane,but also facilitate the enrichment of methane.The different results of propane pyrolysis between the aqueous and anhydrous systems indicated that the mechanism of propane degradation is not the same in different systems.In the anhydrous system,propane degraded through the free radical mechanism,while in the aqueous system mainly through carbenium ions mechanism.In addition,the addition of shale and/or water can promote the degradation of n-butane and n-pentane,but much more complicated for the products concerning their composition and distribution compared with the propane experiments.In general,the degradation of small molecule hydrocarbons in shale rock has positive effect on the dryness index and resource of shale gas.The clay minerals and water in shale have a positive effect on the degradation of small molecule hydrocarbons,especially when water is present,small molecular hydrocarbons can be degraded through carbenium ions mechanism.The carboxylic acids formed during the degradation process also have a positive effect on the development of shale gas.The carboxylic acids released by the OM pyrolysis process in the shale can react with the feldspar and carbonate,which can form dissolution pores on the mineral surface.In addition,some shrinking organic pores,OM pores,intergranular pores and intragranular pores can be developed in the shale rock.The spongy OM pores can be formed at EasyR_o 0.82%,but there is no obvious correlationship between this kind pores and the thermal maturity.