| The basin-mountain systems and its geodynamics is one of the importantcomponents of the continential dynamics and was subject of tectonic studies inwestern China. The Sichuan basin located the eastern margin of the Tibetan Plateau ata transfer zone between the Tethys-Himalaya domain and the western Pacific domainis a complex sedimentary, tectonic and topographic basin. The basin and its peripheralmountains comprise composite basin-mountain system. This study uses structures,lower-temperature thermochronology, geochemistry etc., to document the relationshipbetween the basin-mountain system and the oil/gas distribution in the Sichuan basin.(1) The composite basin-mountain system in the Sichuan basin and its peripheralmountains can be divided into two types-margin-plate systems and interior-platesystems.The margin-plate basin-mountain systems include the Sichuan basin and itssurrounding Longmen Mountains, Micang Mountains and Daba Mountains. All ofthose are located at the western and northern marginal area of the Sichuan basin,representing the western margin of the Yangtze plate (South China Block). Themargin-plate basin-mountain systems，with binary units of large-scale thrust belts andforeland basins, have different deep lithospheric structures, abrupt boundaries andlarge contrast in today’s geomorphology. The coupling of the foreland basins andadjacent mountains is chiefly controlled by the diversity in the lithospheric structure.Sedimentary data from field and borehole investigations allow reconstruction of thesouthwesterly variations in proximal sedimentary processes along the Longmenshanthrust belt during Late Triassic and Early Jurassic and their relation to thedevelopment of a transpression basin.In contrast, the interior-plate basin-mountain systems represented by the Sichuan basin and its adjacent interior-plate Qiyue Mountains, Dalou Mountains and DaliangMountains; located within the Yangtze plate, represent the eastern and southernmarginal area of the Sichuan basin. Due to gradual boundaries in shallow structureand similar deep lithospheric structure, the interior-plate basin-mountain systems lackof large-scale thrust belts and thrust-loaded foreland basins. The coupling of the basin(Sichuan basin) and the interior-plate mountains largely depends on the degree ofdeformation of the interior-plate mountains and multi-layer detachments in thesedimentary cover of the basin. For example, the superimposed structures indicate thatthe Daloushan basin-mountain system has experienced multistage build-up episodesunder the control of Xuefeng intercontinental orogen, accompanied by the northwardpartition of the Paleo-Yangtze basin.(2) Due to different models between folding and deflected isotherm withinlow-relief and long-wavelength topography in the interior-plate basin-mountainsystems, it could unravel the processes of uplift/exhumation and folding deformationby multi-system lower-temperature thermochronology at a fold or anticline.New apatite and zircon (U-Th)/He thermochronometry results using the age vs.elevation/structural depth relationship in the Daliang Mountains, support that,(1)cooling and exhumation with apparent rates of~0.15mm/yr from~30to~10Ma, waspervasive across the region after the build-up time of the orogen,(2) a differentexhumation occurred at the Daliang Mountains and southwestern part of the Sichuanbasin; the former has~3-5km exhumation magnitude while the latter~1-2kmexhumation, and (3) Late Cenozoic eastward growth of the Tibetan Plateau controlledthe rapid post~10Ma cooling and denudation, with rates of~0.4-0.8mm/yr in theDaliang Mountains.(3) Based on the lower-temperature thermochronology and thermal models, it issuggested that there were different uplift and exhumation processes across theSichuan basin, controlled by a coupling between basement and compositebasin-mountain systems.Modeled temperature-time histories and the Boomerang plot of AFT datasetacross the Sichuan basin suggest two-stage cooling and exhumation processes. Afterreaching their maximum burial depth during120~80Ma, it was followed significantacceleration in cooling rate during last20~10Ma. In particular, nested old-age centerseparated by Huayun Mts. was found in the center-to-northwest part of the Sichuanbasin, coupled with the low-grade deformation in the basin center. It indicatessignificant change in exhumation (at least~2000m) across the Huayun Mts. The magnitude of exhumation across the Sichuan basin was more than~2000-3000m since the Late Cretaceous, while more than1000m in the Neogene. Ingeneral, the lowest exhumation area is located at the margin-plate basin-mountainsystems, while the largest exhumation area is at the interior-plate basin-mountainsystems. The exhumation in the basin center have magnitude averaging betweent thethe latter two. A simplified one-dimensional, steady-state solution model wasdeveloped to calculate the mean exhumation rate, which is0.05~0.2mm/yr in mostof the basin. It suggests slow uplift and exhumation across much of the basin. Theregional pattern of AFT age, length and erosion rate support a progressive changefrom the nested old-age center towards the southwest. This pattern supports theprolonged, steady-state uplift and exhumation across the basin, controlled by cratonicbasin structure. The eastern growth of the Tibetan Plateau has exerted significantcontrol on the rapid exhumation of the southwestern part of the Sichuan basin, but noton all of the basin during the Late Cenozoic.(4) Considering the different features such as structure, deformation, evolutionand dynamics etc., the Sichuan basin can be subdivided into five units.Unit I-North Sichuan margin-plate basin-mountain system (under the control ofQinling orogen), Unit II-West Sichuan margin-plate basin-mountain system (underthe control of Tibetan Plateau), Unit III-East Sichuan interior-plate basin-mountainsystem (under the control of Xuefeng orogen), Unit IV-SW Sichuan interior-platebasin-mountain system (under a composite control of Xuefeng orogen, TibetanPlateau and basement structures), Unit V-Central Sichuan interior-plate uplift (underthe control of basement structures). Due to different dynamics, these five units havedifferent build-up time; the Unit I has been formed during the Late Yanshanian, theUnit II and III during the Himalayan period, and the Unit IV during Late Cenozoic.Thus, the evolution of the basin and its topography are controlled by couplingprocesses between basement structures and composite basin-mountain systems of theSichuan basin.(5) The basin-mountain systems in the Sichuan basin exert major control ontoday’s oil/gas distribution across the basin chiefly by their controlling influence onthe fluid flow and petroleum reservoirs preservation. It is controlled by the couplingprocesses between the steady-state uplift and exhumation and the thrusting andfolding deformation.The margin-plate basin-mountain system with strong thrusting deformation, ischaracteristic by vertically throughgoing fluid flow in shallow system, and lateral fluid flow in the deep system. Whilest the interior-plate basin-mountain system ischaracteristic by throughgoing fluid flow closely correlated with a percolating clustermodel of veins. In general, the fluid flow is controlled by the thrusting and folding.However, the fluid flow shows a close correlationship with the steady state uplift andexhumation in the basin center under control of a basement.The strong uplift and exhumation with a magnitude larger than~3000-4000m,exerts major control on the energy change and accumulation of fluid and oil/gas.During the progressive uplift and exhumation, the shallow strata attained differenttypes of fractures and joints, etc. that formed under different stress-strain regimewithin shallow and/or deep depth. The connection among different types of fractureand joint resulted in multistage fluid flow with boiling features, and finally in adestraction of petroleum reservoirs. Presence of fractures and joints have significantinfluence on the fluid flow, its connectivity and accumulation. There is a goodcorrelation betweent the gas field and fractures in the Sichuan basin, with fieldspreferentially located in an areas of a weak uplift and low exhumation.The tectonics, terrestrial strata distribution and the late Mesozoic-Cenozoic upliftaccompanied by denudation, control the regional conditions for natural gas reservoirpreservation in the Sichuan basin. The foreland basins of the margin-platebasin-mountain systems contain (more than3000m) thick T3—K terrestrialsedimentary strata, controlled by tectonics and formation of the basin-mountainsystems. These strata were subjected only to a low degree of uplift and denudationand to weak tectonic deformation, which resulted in good preservation of oil/gasreservoirs. Therefore, today’s natural gas proven reserves and medium-large sizenatural gas fields in Sichuan basin are mainly located within the foreland basins at themargin-plate basin-mountain systems. |
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