Study On Hydrocarbon Accumulation Dynamics In The Lunpola Basin,Tibet

The Lunpola Basin,located in the Baingoin County,Tibet Autonomous Region,is a Cenozoic continental basin developed on the marine basement of Yanshanian folded period.The tectonic evolution of the basin has gone through three stages:fault depression,depression and structural uplift.The latter stage of tectonic uplift has a strong adjustment and alteration to the primary oil and gas reservoirs of the basin.The Lunpola Basin has a low degree of exploration,and its actual data and theoretical knowledge are relatively small.However,as the only basin in Tibet to obtain industrial oil and gas flow,it has important research value in both exploration practice and theoretical research.Based on the oil and gas geological characteristics and data status of the Lunpola Basin,this paper makes full use of the existing drilling,logging,seismic and analytical testing data in the study area,absorbs the latest research results and exploration progress of predecessors,supplements the field geological investigation,core observation and related testing and analysis work,guided by the theory of hydrocarbon accumulation dynamics and combining point?drilling?-line?profile?-surface?plane?,systematically analyses and evaluates the geological conditions of hydrocarbon accumulation such as source rocks,reservoirs,preservation and temperature and pressure in the Lunpola Basin,quantitatively and dynamically depicts the process of hydrocarbon generation,migration and accumulation,and discusses the process of reservoir adjustment and transformation.Finally,on the basis of comparative anatomy of typical reservoirs,oil and gas accumulation models are established,main controlling factors of oil and gas accumulation are summarized,and favorable oil and gas exploration zones in the basin are predicted.The main achievements and understandings are as follows:1.Geological conditions of hydrocarbon accumulationThe Niubao Formation is the main source rock of the Lunpola Basin and the results show organic matter abundance of source rocks of the Eocene Niubao Formation in the Lunpola Basin shows great differences in different strata of different sags.Among them,organic matter abundance is generally characterized by"high in the west,low in the East and higher in the north than in the south".Good hydrocarbon source rocks?TOC>0.8%?are mainly developed in the Jiangriacho and Jiangjiaco sags in the central and western part of the basin.Elemental analysis,maceral analysis,rock pyrolysis analysis and chloroform asphalt"A"group composition discrimination methods show that the organic matter type of source rocks of the Niubao Formation in the Lunpola Basin is mainly type II₁,followed by type I.Nowadays,most of the source rocks in the second member of the Niubao Formation are in the middle mature stage?0.7%<Ro<1.3%?and most of the source rocks in the third member of the Niubao Formation are in the low mature stage?0.5%<Ro<0.7%?.Oil--source correlation confirms that the crude oil stored in the second member of the Niubao Formation mainly comes from its own source rocks and belongs to“self-generation and self-storing”,while the crude oil stored in the third member of the Niubao Formation comes partly from the second member of the Niubao Formation and partly from itself,both“self-generation and self-storing”and“bottom-generating and top-storing”.The Lunpola Basin developed various reservoir bodies such as delta,fan delta,braided river delta and sublacustrine fan.Its lithology is dominated by sandy conglomerate,fine sandstone,dolomitic siltstone,limestone siltstone and dolomite.Reservoir properties are characterized by low porosity and permeability,and there are some differences among different sags.Among them,the reservoir of middle sub-member of section 3 of the Niubao Formation has the best physical properties in the western Jiangriaco Sag and the eastern Paco Sag,while the reservoir lower sub-member of section 2 of the Niubao Formation in the central Jiangjiaco Sag has the best physical properties.Among the caprocks of the Four sets,the Second and Third members of Niubao Formation,and the Second and Third members of the Dingqinghu Formation,the upper sub-member of section 2 of the Niubao Formation and lower sub-members of section 3Niubao Formation are the most important.Its single mudstone thickness is up to 600 m,shale/stratum value is up to 0.9,displacement pressure is up to 13 MPa,which has strong regional sealing ability and provides favorable sealing conditions for oil and gas preservation.The water type of the Niubao Formation in the Lunpola Basin is mainly NaHCO₃.The PH is generally higher than 7.Ca²⁺is positively correlated with Mg²⁺and the salinity of water is low.The change of water salinity with depth tends to decrease.It reflects that the formation water is in an active alternating zone,mostly in an open,oxidation-weak oxidation hydrogeological environment,and the oil and gas preservation conditions are mostly in a poor level.The Lunpola Basin was strongly reformed by late fault activities,and a large number of tense-shear faults developed in the middle and eastern part of the basin,which played an adjusting and reforming role in primary oil and gas reservoirs.2.Dynamic process of hydrocarbon accumulationOn the basis of field outcrop,drilling core,logging and seismic data identification,the erosion thickness of the top interface of the Niubao Formation is restored by means of fluid inclusion,mud interval transit time method and stratigraphic contrast method.On the plane,thickness is characterized by small erosion in the middle,large erosion in the north and south,and greater erosion in the north than in the south.The erosion thickness of the top interface of the Dingqinghu Formation is about 600m in the West,but only 300m in the east which is estimated by stratigraphic contrast method.The results show that the burial history curve of a single well presents a"two-stage"feature,the total subsidence rate and tectonic subsidence rate in the faulted period are obviously higher than those in the depression period and the adjustment and alteration period.The Lunpola Basin is a typical"hot basin"with high geothermal gradient and paleo-geothermal gradient.The evolution of geothermal field shows two stages of"warming-cooling"process with two crustal uplifts at the end of the Niubao Formation and the end of the Dingqinghu Formation.On the thermal maturation history of source rocks,the time of hydrocarbon source rocks entering the threshold of oil generation and gas generation in the first number of Niubao Formation and lower sub-sections of the section 2 of Niubao Formation in the Jiangriaco Sag is earlier than that in the Jiangjiaco and Paco sags,and most of the source rocks are in high maturity stage?Ro=0.7%-2.0%?while the time of hydrocarbon source rocks in the upper sub-section of the second member of the Niubao Formation and above is reverse,and most of the source rocks are in low maturity stage?Ro=0.5%-0.7%?.In the same section of the Niubao Formation in the Lunpola Basin,the time of oil generation and expulsion is earlier than that of gas generation and expulsion,and the oil generation and expulsion rate is higher than that of gas generation and expulsion,but the exhaust efficiency?mostly greater than 50%?is higher than that of oil expulsion?mostly less than 50%?.Constrained by the thermal evolution degree of source rocks,the time of hydrocarbon generation and expulsion of deep source rocks is earlier than that of shallow source rocks,oil and gas generation and expulsion rates are also relatively high.Up to now,there is no large-scale natural gas generation and expulsion in the middle sub-member of section 2 of Niubao Formention and above.Paleo-pressure field and Paleo-Fluid potential in the Lunpola Basin have good inheritance in the main reservoir-forming period.Paleo-pressure is"high in the middle and low on both sides",and paleo-fluid potential field is"high in the south,low in the northwest and east"in the plane.On this basis,combined with sedimentary facies,thermal maturity of source rocks and sealing capacity distribution,the secondary migration path and traps of oil and gas are predicted,which shows lithologic traps in the central depression zone of the basin are favorable accumulation areas for primary gas reservoirs.Microscopic petrographic observation of the reservoirs of the Niubao Formation in the basin are characterized by three types of fluid inclusions:salt water,hydrocarbon-bearing brine and hydrocarbons,which are mainly distributed in the quartz micro-cracks and calcite cement in the form of linear and banded.Microscopic observation of fluid inclusions is rich in fluorescence,showing a variety of fluorescent colors such as blue-white,blue,yellow and orange,qualitatively discriminating the presence of different maturity of crude oil in this region.The fluorescence spectra of hydrocarbon inclusions have two peaks of 495 and 540 nm,which quantitatively reveal that there are two stages of hydrocarbon filling in Niubao Formation of the basin.The homogenization temperature of the brine inclusions associated with the oil and gas inclusions is projected onto the history map of refined single well,and it is determined that there are two phases of hydrocarbon filling in the Niubao formation reservoir in the Lombopo Basin.There are 23-21Ma and 30-27Ma respectively,and the second phase is the main.The Lumpola Basin has strong fracture activities,stratigraphic folds,erosion and stratigraphic cutting.After the formation of primary oil and gas reservoirs,it has undergone physical adjustment and transformation.On this basis,most of the primary oil and gas reservoirs have undergone chemical adjustment and transformation such as biodegradation,water washing and oxidation.The effect is that the light components of the crude oil are reduced,the heavy components are increased,the density is increased,and the viscosity is increased,thereby the present heavy oil reservoir has been formed.3.Model and main controlling factors of hydrocarbon accumulationSeven of the eight discovered reservoirs in the Lunpola Basin are heavy oil reservoirs and one is light oil reservoir.The discovered reservoirs are mainly distributed in the fault development zone of the central sag zone,and have the characteristics of many vertical distribution layers,long lateral migration distance and active reservoir adjustment and transformation.Through detailed dissection of typical oil and gas reservoirs,two types of reservoir-forming models in the Lunpola Basin are established,such as late-stage destructive model and late-stage preservative model.Late-stage destructive models mostly form heavy oil reservoirs,including the Hongxingliang?early overpressure-fault combined sealing and late fault-destructive?,the Hongshantou?near-source fault vertical conduction late fault-destructive?and the Changshan paleo-reservoir?middle-source sand body lateral conduction late uplift-destructive?.Late-stage preservation models mostly form conventional reservoirs,such as the Romaniuk reservoir?Late preservation of lateral transport of distant sand bodies?.Based on the analysis of geological conditions for hydrocarbon accumulation,the reconstruction of reservoir-forming dynamics and the fine dissection of typical reservoirs,the main controlling factors of hydrocarbon source-reservoir-preservation ternary complex reservoir formation in the Lunpola Basin are put forward.Then the exploration zones in Lunpola Basin are classified into three types according to the evaluation of reservoir-forming conditions?Class I?Class II and Class III?:Class I zones have two zones,including the Jiangriaco Sag and Paco Sag dominated by structural-lithologic traps,its reservoir-forming conditions are the most superior.Class II zones have two zones:the Dayushan thrust belt dominated by structural-lithologic traps and the Jiangjia fault dominated by lithologic traps.There are three zone in Class III,including the Ejiazu thrust fold belt and the Lunpori-Changshan fold uplift belt dominated by tectonic traps,and the south Jiangriaco thrust fold belt dominated by tectonic-lithologic traps.