The Oil Sources And Petroleum System In Eastern Part Of Salin Sub-Basin,Myanmar

The Salin sub-basin is a depression in the Central Basin of Myanmar in which many oil and gas fields developed and are rich in oil and gas resources.The sedimentary strata mainly include the Late Cretaceous to Cenozoic.The potential source rocks are developed in the Late Cretaceous,Eocene,and Oligocene strata.The currently commercially developed crude oil in the Salin sub-basin is mainly produced from Eocene to Miocene sandstone reservoirs.The main oil reservoirs include the Eocene(Tilin and Pondaung Formations),the Oligocene(Shwezettaw,Padaung,and Okhmintaung Formations),and the Miocene(Pyawbwe and Kyaukkok Formations).In the Kyaukkwet oil field,the Pondaung Formation is producing oil,but the reservoir properties of the Pondaung Formation are poor(low porosity and permeability).The fine to medium-grained sandstone of the Early Oligocene Shwezettaw Formation is producing oil in Sabe(North Yenangat),Thargyitaung,Tetma,Gwegyo,Ngahlaingdwin,Yenanma,and Lepando oil fields as well as the producing natural gas from the Shwezettaw Formation in the Ayadaw oil and gas field.The shallow marine facies,fine-grained sandstone of the Middle Oligocene Padaung Formation is the reservoir of the Chauk oil field.The Miocene and Middle Oligocene sandstone reservoirs are producing oil and gas in Yangyaung,Mann,Htaukshabin,and Kanni oil fields.Thin to thick-bedded,coarse-grained gypsiferous sandstones of Pyawbwe Formation are reservoirs in Peppi and Tagaing oil fields.This thesis takes the Salin sub-basin as the studied area and the Cenozoic source rocks and the forming factors of typical oil and gas play as the studied objects.Based on the theory and technology of oil and gas geology and petroleum geochemistry,using the research methods of petroleum system and basin modeling,41 mudstone samples were evaluated for the indexes of source rocks.14 crude oil samples were analyzed using gas chromatography(GC)and gas chromatography-mass spectrometry(GC-MS)and biomarker interpretation were carried out for oil-oil correlation.4 representative seismic profiles were interpreted for sedimentation and structures.One-dimensional(1D)geological models and modeling calculation of stratigraphic burial-thermal evolution-hydrocarbon generation histories were carried out for 3 typical wells.The comprehensive studies for the elements of petroleum system and hydrocarbon accumulation processes in the Cenozoic strata of Salin sub-basin have been developed.The studied results reveal the geochemical characteristics of source rocks and crude oils,clarify the development horizons of effective source rocks,explain the trap types and formation times of known oil and gas pools,analyze the direction of oil migration,and propose three petroleum systems(from oil source to trap)in this sub-basin.This study conducted a detailed investigation of the outcrops of argillaceous source rocks and sandstone reservoirs in the Salin sub-basin.The collected mudstone samples used testing techniques such as total organic carbon content(TOC),Rock-Eval rock pyrolysis,and Vitrinite Reflectance(R_o%)measurement method to evaluate the abundance of organic matter,kerogen type,and thermal maturity of source rocks.From the source rock evaluation data,it can be seen that the Late Cretaceous Kabaw(KB)Formation is a very good potential source rock,but it is in the over-mature gas generation stage;the TOC content of Eocene Laungshe(LA),Tabyin(TB),Pondaung(PO)and Yaw(YA)Formations are medium,and the kerogen type is type III-II,which is in the mature oil and gas generation stage.The samples of the Shwezettaw and Padaung(SZT,PD)Formations of the Early to Middle Oligocene show that the source rock potential is poor to medium,and the kerogen is type III,in the immature stage.The Late Cretaceous source rock contains more wood components organic matter.The sedimentary organic matter of the Eocene source rock has the characteristics of a mixed input of marine and terrestrial facies.The sedimentary organic matter of the Oligocene source rocks mainly comes from the input of higher plants.In this study,14 crude oil samples were collected from Early to Late Oligocene oil layers in the southern Yenangyaung(YNG),central Chauk(CHK),and northern Letpando(LPDO)oil fields in the Salin sub-basin of the Central Basin.The saturated and aromatic hydrocarbons of selected oil samples were analyzed by integration of gas chromatography(GC)and gas chromatography-mass spectrometry(GC-MS)methods to evaluate their origin,depositional environment,and thermal maturity.All analyzed crude oil samples have medium to high API degrees and are shown as a medium to light crude oils.According to the biomarker composition of crude oil samples,they can be divided into three groups:Group I crude oil(Late Oligocene Okhmintaung Formation,YNG oil field)are mainly deposited under weakly oxidizing environmental conditions(lacustrine-marine environment).Group II crude oil(Middle Oligocene Padaung Formation,CHK oil field)source rock organic matter mainly comes from plankton/algae,a small amount comes from terrestrial imported organic matter,and the sedimentary environment is aerobic/weakly oxidized(fluvial/delta facies-marine environment).Group III oils(Shwezettaw Formation in the Early Oligocene,LPDO Oilfield)source rock organic matter mainly comes from plankton/algae material and terrestrial organic matter deposited in an aerobic/weakly oxidizing environment(fluvial/delta facies-marine environment).The maturity of Group I and Group II crude oil has reached the peak of the oil generation window,while Group III oil is in the early stage of the oil generation window.The stable carbon isotope composition of all oil samples indicates that the parent material of the source rock comes from the mixed input of marine algae and terrestrial higher plants.The oil source comparison shows that the crude oil of Group I(YNG oilfield)and the crude oil of Group II(CHK oilfield)may come from the black mudstone of the Yaw Formation of the Late Eocene and the coal shale/resin coal of the Pondaung Formation of the Middle Oligocene.Group III crude oil(LPDO field)is mainly derived from the coal shale/resin coal of the Late Eocene Yaw Formation and/or the Mid-Oligocene Pondaung Formation,and may also be contributed by the black mudstone of the Middle Eocene Tabyin Formation.This study explained in detail the seismic profile of the Eocene to Pliocene strata in the Salin sub-basin and analyzed the tectonic and sedimentary evolution of the Salin sub-basin.The study found that there are two different mechanisms for fault sealing,namely,(1)primary juxtaposition seals and(2)secondary fault rock seals(membrane seals).When the organic matter of the source rock layers(Eocene and Oligocene stratigraphic units)begins to mature,the generated oil and gas will migrate to the reservoirs(Pondaung PO,Shwezettaw SZT,Padaung PA,and Ohkmintaung OHK)through these fault channels.Generally,oil and gas migrate along faults or fracture surfaces or pore systems in reservoirs.Most of the anticline structures in the northern Salin sub-basin are related to thrust faults.There are hydrocarbon accumulations in the structural traps on the upper and lower sides and flanks of the thrust fault zone.Two large thrust-related synclinal structures are,the Yenangyat-Chauk thrust anticline belt(the first structural line)in the west formed earlier,and the Ngashandaung Gwegyo-Tunyintaung thrust anticline belt(the second structural line)in the east formed late.These two large structure belts could be considered as the main sources area feeding the uplifted areas in the region.The 1^st structural trend is relatively older than the outer 2^nd structural trend.During the formation of the 2^nd structural trend in the Late Miocene-Pleistocene,the 1^st structural trend might have acted as a barrier separating between the main proven kitchen of the Salin sub-basin and 2^nd structural trend.Main oil and gas producing fields are located along with the 1^ststructural trend,so these oil fields can receive hydrocarbon directly charged from the main proven kitchen to the west of the structure.Due to the long-distance migration distance,less than 30 km from 1^st structural trend and limited amounted of hydrocarbon that might have charged into the Miocene-Oligocene supra-thrust along with the 2^nd structural trend,will not be a large quality of oil extracted from oil fields located on the 2^nd structural line.There are three types of traps in the study area.The oil and gas reservoirs in the Letpando oil field are dominated by stratigraphic traps,and the oil and gas reservoirs in the Yenangyaung and Chauk oil fields are anticline structural traps and structural-stratigraphic composite traps.Typical anticline structural trap oil and gas reservoirs are Chauk anticline trap and Yenagyaung anticline trap oil and gas reservoirs.The modeling results of 1D,the burial history,thermal history,mature hydrocarbon generation history show that differences in the oil and gas generation time of different horizons and burial depths in the Salin sub-basin.Select three representative wells are Well-A in the north,Well-B in the middle,and Well-C in the south of the sub-basin.The 1D modeling results show that the mature oil generation window of the Laungshe source rock began in the Middle Eocene(about 48 Ma)and ended in the Early Oligocene(about 30 Ma),that the gas generation time of the Laungshe source rock began in the Middle Eocene(about 45 Ma)and ended in the Early Pliocene(about 5 Ma);the mature oil-generating window of Tabyin source rocks began in the Late Eocene(about 35 Ma)and ended in the Early Miocene(about 20 Ma);the mature oil-generating window of Pondaung source rocks began in the Early Oligocene(about 30 Ma)ended in the Middle Miocene(about 10 Ma);the mature oil-generating window of Yaw source rocks began in the Late Oligocene(about 25 Ma)ended in the Early Pliocene(about 5 Ma).The 1D modeling results also showed that the oil generation time in the northern Salin sub-basin was early,the oil generation time in the southern part of the basin was late,and the oil generation time in the central part of the basin was between the above two.According to the geochemical characteristics and correlation of crude oil and source rocks collected from YNG,CHK,and LPDO oil fields and the study of the formation time of structural traps,three petroleum systems for oil accumulation can be identified in the Salin sub-basin,namely(1)Middle Eocene-Late Eocene petroleum system;(2)Late Eocene-Oligocene petroleum system and(3)Late Eocene-Miocene petroleum system.Incidentally,the petroleum system for gas accumulation can be named Early Eocene-Late Oligocene petroleum system.The preservation time starts from the Late Oligocene-Pliocene time.The critical moments for hydrocarbon accumulation are the Late Oligocene(?26 Ma),the Early Miocene(?18 Ma),and the Late Miocene(?8 Ma).