Formation And Preservation Of Carbonate Reservoirs Under Neotectonism

The surface seepages and bitumens are widely distributed under strong neotectonism in the Kalpin fold-and-thrust belt,northwestern Tarim Basin.The Kalpin fold-and-thrust belt was considered a forbidden zone for oil-gas exploration.The success of the Ketan1 well in 2019 shows the potential of oil-gas exploration in the Kalpin fold-and-thrust belt.However,the oil-gas distribution and enrichment law is unclear so far,and the oil-gas potential is still unknown.In this dissertation,the formation and preservation of carbonate reservoirs in the Kalpin fold-and-thrust belt under neotectonism were comprehensively researched by integrating geological,geochemical and geophysical approaches.The accumulation conditions of hydrocarbons were clarified,and the formation and preservation mechanisms of ancient oil-gas reservoirs were revealed.The main understandings obtained are as follows.(1)In the Kalpin fold-and-thrust belt,4 sets of source rocks developed in the Lower Cambrian and Middle-Upper Ordovician: the Yuertusi,Xiaoerbulake,Saergan and Yingan Formations vertically,among which the argillaceous carbonate rocks in the Xiaoerbulake Formation is the newly discovered source rocks.It is found that source rocks in the Kalpin block are widely distributed with high organic matter abundance and good types.The 4 sets of source rocks were deposited in a predominantly reductive environment with variable palaeo-climatic conditions.The Yuertusi Formation source rocks have the highest organic matter abundance,which is attributed to the input of flourishing planktonic algae,nutrients offered by strong volcanic/hydrothermal activities and upwelling currents,and preservation conditions provided by the sulfide environment during the depositional period.Biodiversity and more reducing water columns determine the higher organic matter abundance in the Xiaoerbulake and Saergan Formations.The more oxidized water mass resulted in lower organic matter content in the Yingan Formation.(2)The Xiaoerbulake and Wusonggeer Formations are characterized by microbial rocks and diamict,respectively.The reservoir space contains intercrystalline(dissolved)pores,intergranular or intragranular dissolved pores,dissolved pores and caves,algae framework pores,coelomic pores and fractures.The reservoirs in the Wusonggeer Formation are poorer,but the development of fractures improves the reservoirs.In the Kalpin block,the gypsum-salt caprocks are widely distributed and thick.The sealing intensity of caprocks in the outcrop area is poor due to weathering and leaching,while the sealing conditions in the downhole are better.The plastic-brittle transition nature of the gypsum and salt rocks indicates the better sealing intensity of the deep strata.(3)The higher dryness coefficient and lower carbon isotope difference suggest that the natural gas in the Kalpin block is oil-cracked genesis in the over-mature stage(Ro～2.5%),and the generating temperature is ～200°C.The trace elements and carbon isotopes of source rocks reveal that the helium-rich natural gas in the Kalpin block originates from Cambrian source rocks,and helium is dominated by crustal sources.The surface bitumens and seepages have reached the over-mature stage(the reflectivity of vitrinite-like maceral is ～2.0%).However,large differences exist between their geochemical characteristics,which are owed to different biodegradation degrees.The higher 25-norhopanes in the samples are inherited from source rocks rather than biodegradation.Triaromatic steroids and carbon isotopic evidence suggest that the oil source is the Lower Cambrian source rocks.(4)The Kalpin fold-and-thrust belt is characterized by strong neotectonism,with arc-shaped fractures and folds.Although the strong neotectonism caused oil and gas leakage,it also improved reservoir conditions and formed new traps.The high-quality gypsum and salt caprocks,the traction anticlines and hidden anticlines in the hanging wall and footwall of the fold-and-thrust faults provide good preservation conditions for oil-gas accumulation and preservation.(5)During the geological history of the Kalpin fold-and-thrust belt,one phase of hydrocarbon generation and two phases of accumulation occurred.Continuous burial led to sequential hydrocarbon generation in the Lower Cambrian source rocks: from the Late Ordovician to Late Devonian,the source rocks evolved from low-maturity to over-maturity,with dry gas dominating the later stages.Neotectonism between the Oligocene and Miocene caused secondary migration and accumulation of hydrocarbons.The Re-Os isotopic ages of bitumen samples in the study area perfectly recorded the generation of natural gas in the Middle-Late Devonian(～374 Ma)and hydrocarbon adjustment in the Late Paleogene-Early Neogene(24.0～30.5 Ma).