| This paper took the Wufeng-Longmaxi Formation shale of the Xuyong area of Southern Sichuan Basin as the research object.Based on the analysis of the regional geological background,the characteristics and evolution of geological structures of the study area,systematic analysis and comparative study were conducted on the macro-,micro-and supermicro structural deformation features of tectonically deformed shale(TDS)and high-temperature-high-pressure experimentally deformed shale,based on which we proposed a TDS classification scheme,summarized the identification marks and the microstructure deformation mechanisms of each type of TDS.Then,the evolution rules and mechanisms of pore-fracture structures and methane adsorption properties of different types of TDC,the formation and distribution rules of TDS and their control mechanisms on shale gas occurrence were revealed.The main conclusions and findings are as follows:(1)The structural patterns and superposition characteristics of the study area were revealed,and it is concluded that the near-E–W trending structures formed earlier than the near-N–S trending structures.The structural pattern is characterized by the superposition and reformation of two periods of structures trending in near-E–W and near-N–S direction,respectively.These two structure groups were separately finalized in the end of the Late Cretaceous(75–65 Ma)and the end of Oligocene(23 Ma),which are the two key tectonic periods of structural deformation of the target shale formation.The near-E–W trending structures were mainly a series of anticlines,synclines,and compressive faults,which significantly developed and finalized in the end of the Late Cretaceous(75–65 Ma)with the buried depth of the target shale formation ranging from 6 km to 7 km,the paleotemperature from 230℃ to 250℃,and the lithostatic pressure from 180 MPa to 210 MPa.As for the near-N–S trending folds,anticlines are relatively more closed than synclines,forming the well-developed wide-spaced anticlines(or ejective folds).The near-N–S trending structures were finalized in the end of the Late Oligocene(23 Ma),with the buried depth of the target shale formation ranging from 4 km to 5 km,the paleotemperature from 170℃ to 190℃,and the lithostatic pressure from 120–150 MPa.(2)A genetic-structural TDS classification scheme was proposed,and TDSs were classified into three deformation series,the brittle TDS series(including cataclastic and flaky subseries and four TDS types such as cataclastic-,porphyritic-,platy-,and flaky shale),the brittle-ductile transitional series(including two TDS types such as fractured-crumpled-and scaly shale)and the ductile TDS series(including two TDS types such as crumpled-and mylonitized shale).The microstructure deformation mechanisms of TDS were concluded into two types,the brittle and ductile deformation mechanisms.The brittle microstructure deformations that occurred in TDS include the tensile and compressive-shear fragmentation and fine granulation of shale blocks and mineral particles,the interlayer sliding of lamellar minerals,the rotation of mineral particles,the rounding and directional rearrangement of mineral particles and fragments,the mixing of minerals and organic matters.The ductile microstructure deformations include the flexible flow folding deformation of organic matters and clay minerals,the plastic compression and closure of organic pores and interlayer pores,and the extrusion of strong minerals to weak minerals.From brittle TDS series to brittle-ductile transitional series and then ductile series,the microstructure deformations gradually transit from brittle mechanisms to ductile mechanisms,and the brittle fragmentation transit from tensile type to compressive-shear type.(3)The influences of temperature,confining pressure,and differential stress on shale deformation behaviors were revealed through high-temperature-high-pressure shale deformation experiments.With the increase of confining pressure,the strength of shale samples gradually increased,the strain during differential stress loading and constant stages gradually decreased,and shale deformation transited from brittle mechanisms to ductile mechanisms.Under low confining pressure,mineral particles near main fractures gradually fell into fracture zones and consequently,the fracture zones broadened and the brittle deformation enhanced.With increasing confining pressure,fracture zones gradually narrowed,the size of shale and mineral fragments within the fracture zones decreased,the roundness increased,and the fragments began to arrange along their long axis.Under high confining pressure,the brittle fragmentation of shale near the main fractures was significantly restrained,and shale deformation mainly manifested in the solid rheological deformation of clay minerals and organic matters,the interlayer sliding of lamellar clay minerals,the directional rearrangement of mineral particles due to the action of“intragranular shear crushing & intergranular rotation”.Macroscopically,shale displayed crumpled shapes.Temperature rising mainly enhanced ductile deformation of shale samples to a certain extent.With the increase of differential stress,shale strain gradually increased.(4)The evolution rules and mechanisms of pore structure of different types of TDSs were revealed.Due to the relative reduction of organic matters and clay minerals caused by carbonate mineral filling tectonic fractures,the super-micropores,micropores,and small transitional pores of weakly brittle TDS significantly decreased compared with primary structure shale.The strongly brittle TDS displayed a slight reduction in super-micropores-small transitional pores and a significant increase in large transitional pores-micro fractures,which is the comprehensive result of the mineral filling tectonic fractures,the fragmentation of shale blocks and mineral particles,and the irreversible compression of organic pores and the interlayer pores within clay minerals.As for the ductile TDS,it is the compression of pore structures caused by the plastic rheological deformation of organic matters and clay minerals that resulted in the significant reduction of super-micropores ～ small transitional pores.The significant increase of large transitional pores and mesopores is mainly due to the folding deformation and interlayer sliding of clay minerals,the rotation and intergranular sliding of mineral particles,the mixing of clay minerals and organic matters.(5)The methane adsorption properties of TDSs and the influencing factors were systematically explained.With the increase of temperature,the methane adsorption capacity of shale gradually decreased while the Langmuir pressure increased as a whole.With the enhancement of shale deformation,the Langmuir volume gradually decreased while the Langmuir pressure gradually increased.The methane adsorption capacity showed significant positive correlations with the development of super-micropores ～transitional pores,TOC content and quartz content,and negative correlations with the content of clay minerals and carbonate minerals,indicating that the relative reduction of organic matters due to the increase of mineral contents is an essential reason of the decrease of methane adsorption capacity of TDS.The Langmuir volume also showed a certain positive correlation with the development of super-micropores ～transitional pores of kerogen,indicating that the plastic compression of organic pores also contributes to the reduction of methane adsorption capacity of TDS.(6)The controlling effects of deformation environment and geological structures on the development and distribution of different types of TDS were analyzed.The high-temperature-high-pressure shale deformation experiments showed that the Wufeng-Longmaxi Formation shale could show ductile microstructure deformation mechanisms under the temperature of 200℃ and the confining pressure of 100 MPa.Therefore,it is suspected that the target formation was in ductile or brittle-ductile transitional and brittle or brittle-ductile transitional deformation environments in the two key tectonic periods,respectively.Geological structures of different types and properties have more significant controlling effects on shale deformation.Shale near normal faults and strike-slip faults mainly show cataclastic structure,while shale near the fault planes of reverse faults mainly developed strongly deformed shale such as crumpled,scaly,and flaky shale.The thin-bedded shale near the fold cores usually showed strong deformation features and displayed crumpled,fractured-crumpled,scaly,and porphyritic structures.The structural deformations of shale in core limbs were generally weak and mainly developed platy and flaky shale.(7)The shale gas occurrence rules in TDS distribution areas and the tectonic controlling effects were revealed.In the end of the Late Cretaceous,the target shale formation mainly underwent ductile or brittle-ductile deformation;tectonic fractures were less developed and mostly filled with carbonate minerals.After that,the study area experienced slow uplift and denudation from the end of the Late Cretaceous to the end of Oligocene.As the uplift amplitude was relatively limited,the impact on the preservation conditions was not significant,and there was no continuous shale gas seepage dissipation during this period.In the end of the Oligocene,the target shale formation experienced brittle or brittle-ductile transitional deformations.Then,the study area experienced continuous and rapid uplift,the openness of tectonic fracture structures gradually increased,and the seepage capacity of shale reservoirs was enhanced.Continuous seepage dissipation took place in the areas with overlying fractured caprock,resulting in the significant gas content reduction of the target shale reservoir.The period of uplift(from the end of the Oligocene to the present)is the key period of shale gas migration,dissipation,and gas content adjustment of the target shale formation.Based on methane adsorption tests and the adsorption potential theory,it is proposed that the conversion depth of adsorbed gas content from increasing to decreasing is roughly 2300-2400 m in the target shale formation of the study area.In the slow uplift period from the end of the Late Cretaceous to the end of the Oligocene,the target shale formation was below the conversion depth,the shale gas adsorption capacity of the target formation gradually increased,accompanied by the relative migration of adsorbed shale gas from the distribution areas of strongly TDSs to the areas developed with weakly TDSs.During the continuous rapid uplift from the end of the Oligocene to the present,the shale gas adsorption capacity of the target formation experienced the conversion from increasing to decreasing.In the depth shallower than the conversion depth,the shale gas adsorption content gradually decreased with the decrease of buried depth,resulting in the overall occurrence characteristics of adsorbed shale gas content higher in the basin structure areas(with larger buried depth)and lower in the dome structure areas(with smaller buried depth).Meanwhile,the free shale gas storage space and content of the target shale reservoir gradually increased while the content and proportion of adsorbed shale gas decreased.The dissertation contains 123 figures,41 tables and 247 references. |
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