Research On The Preservation Conditions Of Lower Paleozoic Shale Gas In Southern Sichuan Under Strong Transformation

It is characterized with high-quality Lower Paleozoic black shale across the Upper Yangtze area where has become a key-stone of exploration for the shale gas in the South China.Although the Changning and Fuling gasfileds at the Southern Sichuan are the National Bases for the shale gas exploration,there are still few breakthrough of exploration beyond the Sichuan Basin,in paricular the accumulation and preservation mechanism of the shale gas.The Sichuan basin is a superimposed basin controlled by the surrounding basin-mountain systems,and thus the Lower Paleozoic black shale has experimented multiply deformations.Based on the structural analysis,fractal analysis,geochemstry,fluid inclusion,low-temperature thermochronology and analogue experiments etc,it suggests there are different structural styles dominated with fluid activities,indicating of various preservation conditions for shale gas.(1)There were two-stages of deformation across the southern Sichuan Basin during Mesozoic to Cenozoic times,as the NW-SE-or NS-direction compression during Jurassic to Cretaceous times and the NE-SW-dircection compression in Cenozoic time.It accommodated different structural styles indicated by fieldworks and geophysical data etc.,as follows :Heft-type anticline(i.e.,Jiaoshiba anticline),High-steep anticline(i.e.,Daershan anticline),Box-shaped anticline with weak core deformation(i.e.,Jielongchang anticline)and Box-shaped anticline with strong core deformation(i.e.,Weixin anticline).(2)Four types of structures across the southern Sichuan Basin are dominated with different deformation,as well as faults and fractures that will significantly change fluid activities and preservation condition of Lower Paleozoic shale gas.The core of the Heft-type anticline(i.e.,Jianshiba anticline)is charaterized wth weaker deformation than that of limbs.It dominates with X-joint,and very tiny fractures in the core,in contrast,the limbs are charaterized with small faults,secondary folds and through-going joints.High-steep anticline(i.e.,Daershan anticline),the core is tightly closed,widespread with cleavages.The limbs are steep or overturned,dominated with small folds,bed-parallel faults,and multi-stage joints,indicating of much stronger deformation at limbs.For the box-shaped anticline with weak core deformation(i.e.,Jielongchang anticline),the core is inclined with low-angle,and the hinge is dominated with parasitic folds,axial cleavage and shear rupture-shear tension fractures,and the limb of the Permian is characterized with tensile fractures or bed-parallel faults.Box-shaped anticline with strong core deformation(i.e.,Weixin anticline),there are widespread faults,parasitic folds,cleavage,through-going shear joints and echelon veins across the core.Thus,it suggests that the deformation occurred at those four types of structure is with different strength,of which the Loushan fault-fold belt is greater than that of the Qiyueshan fault-fold belt,and both of them are substantially larger than that of the high-steep fold belt in the eastern Sichuan and the low-steep fold belt in the southern Sichuan Basin.(3)Low-temperature thermochronology data suggests that the time of rapid uplift and cooling becomes younger NW-wards from the Xuefeng orogen to the southern Sichuan Basin.In particular,there was a rapid cooling event occurred at 200 Ma across the Qiyueshan to Lichuan regions by the zircon fission-track data.It suggests that two-stage of uplift and cooling occurred at the eastern Sichuan basin from Late Cretaceous(80-70 Ma),followed by a slow uplift and cooling process during Early to Middle Cenozoic times.However,rapid uplift and cooling occurred at the Daloushan and the southern Sichuan basin around 50-40 Ma,followed by a slow uplift and cooling process.It should be noted that it is dominated with rapid uplift and cooling at 20 Ma,e.g.,rates of 5-6?/Ma,at the southwestern Sichuan Basin,in particular,a last 10 Ma rapid uplift and cooling across the southern Sichuan Basin.(4)There is a distinct coupling correlation between the fluid activities and tectonic-thermal event,indicated by the thermal history modelling and inclusion analysis etc.There are two-episodes of fluid activities occurred at the Daershan,the eastern Sichua Basin during Late Creacteous and Neogene,respectively.Furthermore,one major fluid activity that is characterized with lower salinity,occurred during Late Paleogene in the Changnning gas-field,the southern Sichuan Basin.(5)The fractal analysis,geochemsty and inclusion analysis etc.,suggest that coupling relationship among the vein fractal,paleofluid flow and infilling,and decollment of shale.In the hedged structure,the fracture system is dominated with isolated narrow fractures that are with weak to moderate activity of paleofluid,indicating of the endogenous source of paleofluid.In high-steep structure,the fracture system is dominated with diverging and random narrow fractures that are with medium activity of fluid,indicating of endogenous source for fluid.For the box structure with weak core deformation,the fracture system in the core is dominated by concentrated narrow fractures,and wide fractures at the steep limbs.The fluid activity is moderate,with multiple sources of fluid that are from both endogenous and exogenous sources,in particular,there is some fresh-water.In the box structure with strong core deformation,the fracture system at the core is dominated by concentrated narrow fractures,with medium fluid activity and exogenous source of paleofluid.Most of paleofluid at the Permian-Lower Triassic in the box-shaped anticline region are endogenous(endogenous heterotactic),while a few areas(e.g.,the erosion area of the Longmaxi formation)have the throughgoing fluid activities,indicating the Permian-Lower Triassic as a positive caprock across the Sichuan Basin.It should be noted that the fluid activities and geochemistry are quite different beyond and below the Lower Silurian formation that is dominated with decollement in the eastern Sichuan Basin,indicating of a stratification of the fluid activity separated by the Lower Silurian Longmaxi formation.Thus,there are two fluid circulation systems,as follows: the upper system(Silurian-Permian)and the lower system(Upper Cambrian-Ordovician).It is dominated with endogenous(or endogenous ectopic)fluid migration in the upper system,and a throughgoing fluid migration in the lower stystem in southern part and beyond of the Sichuan Basin.(6)Based on analogue experiments and shale-gas explorations etc.,it suggests that there are four types of structure with potential trap for Lower Paleozoic shale gas,as follows: Weak superimposed structure,initial welding and senior welding structures,and overturned welding structure.Among which the first two structures have positive preservation conditions of Lower Paleozoic shale gas,whilst the others have much negativity in the preservation conditions.It should be noted that the preservation condition of Lower Paleozoic shale gas deteriorates with increased superimposed deformation,in particular,there are about 15% in area of the overturned welding structure without positive preservation conditions.It further suggests that there is a distinct decrease in the preservation conditions from the Heft-type anticline(i.e.,Jianshiba anticline),High-steep anticline(i.e.,Daershan anticline),Box-shaped anticline with weak core deformation(i.e.,Jielongchang anticline)and Box-shaped anticline with strong core deformation(i.e.,Weixin anticline),across the southern Sichuan Basin.(7)Shale gas production is closely related to the self-sealing property of shale reservoir.Self-sealing failure may be an important factor of low production,and densely developed Cutting layer fractures and fragmentation of shale reservoirs are important reasons for damaging the self-sealing property of shale.Due to the mechanism of buckling fold,the strong compressive deformation has a destructive effect on the shale reservoir,the roof and the floor,and the cap rock.Generally,the structural area of the overall lifting movement are underdeveloped in strong fold structure and thrust structure,which is beneficial to Shale gas preservation and is often the best area for shale gas preservation.(8)Favorable exploration areas have been identified.The shale gas preservation conditions of the Wufeng Formation-Longmaxi Formation are generally effective beyond the fault belt in the basin.It is pointed out that in-basin exploration should focus on the self-sealing of shale reservoir and focus on the research of tectonic fracture damage.The favorable structure for exploration is the gentle slope of anticline and the monoclinic structure around the paleohigh.The box-shaped anticline in the box-like fold area outside the basin generally does not have the bottom plate condition and caprock sealing condition of Qiongzhusi Formation shale gas of the Lower Cambrian.The shale reservoir is poorly self-sealing.It is recommended to look for the paleohigh periphery and the weakly deformed anticline structure formed in the late stage during exploration,such as the initial fold stage of the fault-related folds,and the positive structures formed by the late stage in the syncline.