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摘要
Based on core observation and description, microscopic identification, analysis of carbon and oxygen isotope analysis, etc., combined with the sedimentary environments and tectonic background, this paper makes a detailed study on the genesis and mechanism of the type, sub-distribution, morphology and geochemical characteristics of carbonate cements from Jurassic to Neogene reservoir sandstone in the study area. The results show that the main cementation types of Meso-Cenozoic sandstone in the abdomen area of the northern margin of Qaidam is carbonate cementation, which can be divided into early, middle and late three stages. The early carbonate cementation includes calcite, dolomite and siderite, the middle stage are mainly calcite and iron-bearing calcite, and the late stage is iron-bearing dolomite. The abdomen of the northern margin of Qaidam Basin in Jurassic experienced a transition from closed salty lake to open lake, with δ13C values ranging from -16.77‰ to 8.01‰ and δ18O values ranging from -18.52‰ to -8.34‰, paleosalinity (Z) values ranging from 86.50 to 133.72 and paleo-temperature from 67.81 to 142.19 ℃, respectively. And the diagenesis stage was in the early diagenesis stage B to middle diagenesis stage B. In Paleogene (E1+2, E31, E32), the northern margin of Qaidam Basin was in an open water environment, and the carbon and oxygen isotopes were from -8.71‰ to -2.40‰ and from -16.62‰ to -8.77‰, respectively. The paleosalinity and paleotemperature were 102.85-116.16 ℃ and 63.03-117.28 ℃, with the diagenesis stage of the middle diagenetic stage A to the early diagenetic stage B. In Neogene (N1 and N21), it was also in an open water environment, with δ13C values of -6.81‰--3.80‰ and δ18O values of -12.73‰--6.13‰,The paleosalinity and paleotemperature were 107.65-114.89 ℃ and 25.54-88.93 ℃, whose diagenetic evolution stage was in the early diagenesis stage A to middle diagenetic stage B. The content of carbon and oxygen isotopes in different layers shows the same trend, but there are still some points showing the opposite trend, which may be related to atmospheric fresh water. The genesis mechanism of the carbonate cement includes both organic and inorganic origin. In the early stage, it was mainly affected by the leaching of fresh water. In the middle stage, it was mainly provided by the thermal release carboxylation of organic matter. And in the late stage, it was mainly biogenic carbonate, and also contained a small amount of organic carbon sources.
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