| Lake water salinization is one of the important aspects for studying terrestrial hydrocarbonrich sags.In the Paleogene,there distributed lots of saline lakes in Chinese Mainland,some of which later developed as the famous small-but-rich sags.In this kind of lacustrine basins,high-quality source rocks and halite all developed in lake centers and they are generally interbedded.Because of their spatial and temporal co-existence,some researchers proposed that lake salinization was conductive to the formation of high-quality source rocks.However,although this proposition has been made for a long time,details about why and how remain unclear.Primarily,causes of water salinization and genesis of salts in terrestrial petroliferous basins are controversial.In these basins,when main source rocks deposited,were the interbedded salts evaporatic genesis as conventional wisdom says or hydrothermal origin as recent popular opinion says? What are the difference between source rocks developed in different saline degree waters and what kind of lakes is best for the development of highquality source rocks.All these questions restrict the theoretical perfection of terrestrial hydrocarbon-rich sags.The Qaidam Basin is one of the Paleogene saline petroliferous basins in China.Its lake water became saline just as the beginning of its Cenozoic sediments deposition.Due to the differentiation of structure and sedimentation and lake isolation,different degrees of saline lakes can all found during the long time-span of the Cenozoic.Simultaneously,dark fine-grained rocks occurred in every Cenozoic formation,but with distinct hydrocarbon potential.Therefore,the Cenozoic Qaidam Basin is a perfect location to study the coupling relationships between lake water salinization and source rocks formation.The paper is aimed to study the saline environment of high-quality source rocks formation.We collected ca.80,000 data of chloride and carbonate contents from over 180 boreholes spreading widely in the Qaidam Basin and firstly built a crossplot to present the relationships between chloride-carbonate contents and saline facies.We study the spatial and temporal coupling relationships of organic matter abundance and palaeosalinity based on ca.20 boreholes and concluded the saline condition and environmental settings of high-quality source rocks formation.Based on chloride contents,we divided water palaeosalinity into five categories: fresh water(0-500 mg/kg),brackish water(500-2000 mg/kg),mesosaline water(2000-6000 mg/kg),penesaline water(6000-10000 mg/kg),and supersaline water(>10000 mg/kg).During the Eocene and early Miocene,the Qaidam palaeolake was characterized by widespread brackish water and one local supersaline water,while the supersaline center migrated northeastward.During the late Miocene and Pliocene,the Qaidam palaeolake was characterized by widespread meso-penesaline water and several supersaline centers,while the supersaline centers migrated southeastward.In the Quaternary,the Qaidam palaeolake was characterized by widespread supersaline water.Based on carbonate and chloride contents,the Qaidam palaeolake was divided into brackish lake,saline deep lake,saline shallow lake and salt lake.The perennial lake in the Qaidam Basin was actually small in size.The evolution of the Qaidam palaeolake can be divided into three stages.(1)During the Eocene and early Miocene,the perennial lake was mainly situated in the Western Depression and characterized by small size and deep water with salinity lamination.In the late Eocene,there occurred a small but deep salt lake in the Shizigou area.(2)During the late Miocene to Pliocene,the palaeolake partly migrated to the Yiliping Depression,with larger size and shallower water.In the late Miocene,there occurred a salt lake in the Dashaping area;while in the middle-late Pliocene,there occurred a playa in Xiaoliangshan-Nanyishan area,in front of the Altyn Tagh.(3)During the Quateranry,the main palaeolake migrated stepwise to the Sanhu Depression and the lake shrunk continually,ending up with several small separated salt lakes.Halite precipitation occurred early in the east and late in the west.The formation of the three pre-Quaternary salt lake and playa was closely related with the movement of the three first-order faults in the Qaidam Basin,which suggesting that tectonism can play a crucial part in the formation of salts when climate is not that arid.In the late Eocene,the western Qaidam Basin was mainly in extensional setting and the firstorder XI Fault controlled the subsidence and sedimentation of the Shizigou supersaline sag.The tilting of faults and rising of blocks resulted in subaqueous uplifts.The late Eocene doubted climate made the water in the Shizigou sag and water in the larger Qaidam palaeolake separated during arid climate and collected during humid climate.This salt lake was small but deep with multiple halite beds.In the late Miocene,the western Qaidam Basin was in compressional setting and the first-order Chainan Fault controlled the subsidence of the Dashaping supersaline sag.The foredeep caused by the fault's thrusting offered accommodation for halite precipitation and preservation.The extension of this salt lake was subparallel to the strike of the fault.This salt lake was larger but shallower than the earlier Shizigou salt lake.In the middle-late Pliocene,the western Qaidam Basin was in a compressional setting and the first-order Chaixi Fault controlled the integral uplift of the western Qaidam Basin and local subsidence in the Xiaoliangshan-Nanyishan,resulting in the formation of widespread playas.The distribution of playas and percentage of halite were larger than the previous two salt lakes but the thickness of the strata was much smaller.Salt in different tectonic settings has different size,depth and halite percentage,which results in different quality of source rocks.The coupling of organic matter abundance with chloride and carbonated contents in spatial and temporal scale indicates that Cenozoic source rocks of the Qaidam Basin can be divided into three categories.Source rocks developed in brackish lakes are characterized by high carbonate contents(30-60%),medium organic carbon contents,I-II-type organic matter and high-superhigh hydrocarbon conversion.Source rocks developed in deep saline lakes are characterized by medium carbonate contents(20-40%),medium organic carbon contents,III1-type organic matter and medium hydrocarbon conversion.Source rocks developed in shallow saline lakes are characterized by medium carbonate contents(15-35%),highsuperhigh organic carbon contents,III-type organic matter,and very low hydrocarbon conversion rates.Therefore,high-quality source rocks mainly developed in brackish lakes and can be recognized by high carbonate contents.Brackish lakes with abundant carbonate precipitation and low detritus influx generally have high primary productivity.This carbonate-bearing source rocks can develop in pre-salt layers or can develop in extra-salt layers due to salinity lamination. |
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