The Coupling Relationship Between Paleo-uplift Evolution And Hydrocarbon Accumulation Of Ordovician,Southwest Tarim Basin

Paleo-uplift is the information carrier of basin evolution and the key area of oil and gas exploration.The paleo-uplift in southwestern Tarim Basin is an important oil and gas exploration area.However,due to the multiple structural superpositions and previous data limitations of southwestern Tarim Basin,the integrality research of spatio-temporal coupling relationship between paleo-uplift evolution and hydrocarbon accumulation is limited,which restricts the integrality study of the tectonic evolution of the Tarim Basin and the deepening of hydrocarbon exploration.Based on a summarization of previous research results,guided by the Sedimentology,the Structural Geology,and the Petroleum System Theory,this paper focuses on the relationship between paleo-uplift evolution in southwestern Tarim Basin and hydrocarbon accumulation of Ordovician,and systematically analyzes the latest drilling,seismic,isotopic dating,and exploration data.Major advances and innovations are as follows:1.The spatio-temporal distribution and scale of regional unconformities since the late Proterozoic have been defined in the southwestern Tarim Basin.A new classification scheme of "4 periods and 8 stages" of the paleo-uplift formation and evolution has been proposed.The evolution of the fault system has been reconstructed.Based on reprocessing of the high quality seismic data across southwestern Tarim Basin,the isotopic dating and interpretation of well-seismic calibration,the spatiotemporal distribution,and scale of the unconformities,in response to the formation and subduction of the Tethys tectonic domain since the Late Proterozoic,have been defined.The research proposes a new classification scheme of "4 periods and 8 stages" of the paleo-uplift formation and evolution,in southwestern Tarim Basin.The paleo-uplift rose and fell from north to south and then back to north.The formation period of the north basement uplift corresponds to the Nanhua-Sinian Bachu basement uplift stage.The development period of the south paleo-uplift includes four stages:(1)the formation of the south NW-trending paleo-uplift in southwestern Tarim in Middle Cambrian,(2)the inherited development of the paleo-uplift in Middle Ordovician,(3)the trend change of the paleo-uplift in Late Ordovician,(4)the formation of NEtrending Hetian paleo-uplift from Silurian to Early-Middle Devonian.Besides,the paleo-uplift in Middle and Late Ordovician controlled the platform and basin differentiation,and the platform being submerged by deposition of the Ordovician.The burial period of the south paleo-uplift and formation of the north paleo-uplift includes two stages:(1)the oblivion of Hetian paleo-uplift and the formation of Keping-Bachu paleo-uplift in Early Carboniferous,(2)the subsidence of Hetian paleo-uplift and development of Keping-Bachu paleo-uplift in Late Permian.The period of south Hetian paleo-uplift extinction and intensive uplift of north Bachu paleo-uplift in MesozoicCenozoic corresponds to the southern paleo-uplift extinction and the northern Bachu uplifting stage.Under the paleo-uplift transition,the growth process of the fault system consisting of basement involvement,caprock detachment,and composite superposition pattern,and three types of fracture systems consisting of early active deep burial,late active shallow burial,and late active double-layer superimposed shallow burial type,have been re-established.2.The spatio-temporal distribution heterogeneity of "south oil and north gas" and "early oil and late gas" of Ordovician reservoir,in southwestern Tarim basin,have been illustrated.The origin and single homology of oil and gas have been clarified.Ordovician oil and gas in southwestern Tarim Basin are distributed in the deepburied to shallow-buried fault system in the South and North.By analysis of accumulation characteristics,karst fracture-cave reservoir and Carboniferous mudstone caprock are developed in the southern reservoir,which contains the early mediumheavy oil formed by the mixing of early biodegradable oil and medium-term normal crude oil,with a low filling degree and constant volume.Karst fracture-cave-pore type reservoir and Carboniferous mudstone caprock are developed in the northern gas reservoir,which is filled with crude oil and kerogen pyrolysis dry gas,forming dry gas with a small amount of light oil in the late stage and high filling degree.In Bachu uplift,there are cave-fracture reservoirs,Upper Ordovician mudstone caprock and kerogen pyrolysis gas filling,and the scale is limited.By systematic analysis of organic geochemistry data,it is clear that the origin of oil and gas mainly includes marine sapropelic type I,high-salinity and strong-reduced environment,algae and lower-order plants.The oil and gas underwent Thermochemical Sulfate Reduction(TSR)with Middle Cambrian gypsum rocks,and only come from the source rocks of the Lower Cambrian Yuertusi Formation,which is different from the previous understanding of multiple sources.3.The formation and evolution of paleo-uplift,the spatio-temporal distribution of Ordovician reservoir forming elements,and the coupling relationship between the accumulation and adjustment of Ordovician reservoirs,have been established.The exploration direction of "looking for oil in the south and gas in the north" has been pointed out.The paleo-uplift evolution in different stages provided various basic geological conditions for petroleum.The formation of the north Bachu basement paleo-uplift and the south rift provides conditions for the development of lower Cambrian source rocks.In Middle Cambrian,the gypsum-salt rock controlled by the development of the south paleo-uplift in the southwestern Tarim Basin blocks later hydrocarbon migration and accumulation.The southwest Tarim paleo-uplift in Middle Ordovician,the southwest Tarim paleo-uplift trend change in Late Ordovician,and the Hetian paleo-uplift in Silurian to Early-Middle Devonian,control the development of multi-stage karst fracture-cave reservoir in Ordovician.During the south burial and north uplift period,the oblivion of Hetian paleo-uplift and the formation of Keping-Bachu uplift controls the overlying Ordovician paleo karst of the Carboniferous regional caprock.During Silurian to Early-Middle Devonian Hetian paleo-uplift stage,the source rock responded and generated the early initial liquid hydrocarbon,light oil escaped and remained through fault dredging,which inhibited the further development of karst reservoirs.During the stage of the subsidence of the southern Hetian paleo-uplift and the development of the northern Keping-Bachu paleo-uplift in Late Permian,the source rocks corresponded and generated middle secondary hydrocarbon,forming mediumheavy reservoirs through the gypsum-salt rock fault drainage,light oil charging and mixing,and the dissolution caused by the migration of acidic fluid containing hydrocarbon formed by TSR reaction improves the porosity and permeability of dredging system and trap.In the stage of the south Hetian paleo-uplift reversal extinction and the intensive uplift of north Bachu uplift in Mesozoic-Cenozoic,later gas pyrolysis of both kerogen and crude oil filled traps in the form of water-soluble formed by ion replacement charge.At the same time,the Ordovician paleo-reservoir adjusted,with multi-type oil and gas convergence,rock-water-gas material exchange,dynamic imbalance of fluid and solid,"oil in south and gas in the north" differentiation position.The Yudong fault with strong activity in the early stage and deep burial in the south,the NE-trending strike-slip fault system with strong activity in the early stage and deep burial near the north,and the Haimiluosi-Selibuya south fault with shallow burial superimposed by strong activity in the late stage,are favorable exploration directions of "looking for oil in the south and gas in the north".