Cretaceous Paleo-uplift Restoration And Its Implications On Hydrocarbon Accumulation In The Kuqa Depression

Kuqa Depression has favorable petroleum geological conditions.In recent years,with the discovery of Keshen 2,Keshen 8,Dabei 201 and other large gas reservoirs,the geological reserves of natural gas in Kuqa Depression exceed trillion cubic meters.With the deepening of exploration and development,several paleo-uplifts have been identified in the Kuqa Depression.Though a few researches have been conducted on the paleo-uplift in the southern margin of Kuqa depression,the study on the Cretaceous paleo-uplift in the Kelasu structural belt is still scarce.This dissertation takes the Dabei-Tubei paleo-uplift and the Kelasu paleo-uplift which located in the Kelasu structural belt as the research objects.Using the seismic,drilling and well logging data to depict the thickness distribution pattern of the Bashijiqike formation in the paleo-uplift region.The Bashijiqike formation were denuded in the late Cretaceous,and the denudation intensities were different between Dabei block and KelaKeshen block obviously.In the Dabei block,the first member of the Bashijiqike formation is completely denuded,the second and third members of the Bashijiqike formation are remained.The Kela-Keshen block preserved the first member of the Bashijiqike formation while it suffered erosion.The apatite fission track analysis of Mesozoic sandstone samples collected from Kuqa river,Kapushaliang river and Kelasu river profiles shows that many samples have mixed age characteristics.Statistics indicated that the samples recorded four groups of age,namely 270~230 Ma,170~150 Ma,120~75 Ma and 50 Ma.Based on previous studies,these ages represent the remote effects of the Kunlun-Qaidam collision,the Lhasa collision,the Kohistan-Dras collision and the Cenozoic Indian-Eurasian collision,respectively.According to the thermal simulation results,the Cretaceous paleo-uplifts in the Kuqa Depression mainly underwent two uplift events after their formation.The first uplift event which occurred in the late Cretaceous(110~60 Ma)resulted in the absence of the upper Cretaceous strata and the erosion of the lower Cretaceous Bashijiqike formation.The second uplift occurred in the Neogene(~20 Ma),which is the reflection of the long-distance effect of the India-Eurasia plate collision since the Cenozoic.The spectral analysis of drilling log curves in the paleo-uplift region suggests the sedimentation of the Bashijiqike formation was controlled by Milankovitch cycle.According to the number of cycles in the residual stratum,the denudation amount of the Bashijiqike formation in the Late Cretaceous was calculated.Combining with the results of fission track analysis,the denudation amounts of the Dabei-Tubei paleo-uplift and the Kelasu paleo-uplift were calculated.The results indicate that the denudation amount of the Dabei block is larger than that of the Kela-Keshen block.The original layer thickness in the paleo-uplift region was obtained by superimposing the calculated denudation amount of the Bashijiqike formation and the residual layer thickness,and the Dabei-Tubei paleo-uplift and the Kelasu paleo-uplift were restored.The results show that the scope of the Dabei-Tubei paleo-uplift was roughly the same as the scope of the Dabei block,the core of the paleo-uplift was located in the Dabei 1 well and Tubei 2 well area.The position of the Kelasu paleo-uplift was in the north of the middle-Kuqa Depression,and the core of the paleo-uplift was located in the Kelasu river-Kuche river area.The Kela 5-Keshen 2-Kela 3 well area was the slope of the Kelasu paleo-uplift.Based on the balanced cross-section restorations of classic profiles,the evolution processes of the Dabei-Tubei paleo-uplift and the Kelasu paleo-uplift were depicted.It is concluded that the evolution of the Cretaceous paleo-uplifts included three stages: formation and development stage,burial stage and destruction stage.Using thin section and inclusion test analysis,the influence of the paleo-uplift evolution on hydrocarbon accumulation elements was elaborated.We concluded that the whole evolution process of the paleo-uplifts controlled the reservoir reconstruction,restricted the types of traps,and affected the hydrocarbon accumulation periods of the study area.Finally,the control of the formation and evolution of the paleo uplifts on hydrocarbon accumulation is discussed.The early crude oil accumulated in the Cretaceous paleo-uplifts,then these paleo-oil reservoirs were destroyed and adjusted to become the present large-scale gas reservoirs in the case of late tectonic change and natural gas filling.According to the evolution of the paleo-uplifts and the law of hydrocarbon accumulation in the region,we considered that the hydrocarbon accumulation pattern in the paleo-uplift area belongs to "the early accumulated oil was destructed and adjusted,then natural gas accumulated in late stage".