| There are a lot of Paleozoic-Mesozoic paleo-oil reservoirs which are mainly distributed at the northern margin of the central Guizhou-Xuefeng-Jiangnan uplift and the Nanpanjiang-Shiwandashan area in South China. The existence of these paleo-oil reservoirs demonstrate that there was large-scale oil-gas accumulation and migration in geological history, but later suffered destruction. Majiang Paleo-oil reservoir is one of the largest marine Paleo-oil fields, which is a structural traps-block complex lithology reservoir, and the original area was of 2450 km2, but the current area is only 800 km2. It has been estimated that the original oil reserves were more than 1.5 billion tons, but the remnants are only 350 million tons of bitumen. The main reservoir beds are the Lower Ordovician Honghuayuan formation and Middle-Lower Silurian Wengxiang group, and the bitumen mainly occurs in dissolved holes, holes, cracks, interlayer cracks and intergranular pores and crevices. Obviously, the understanding of the formation and evolution history for theses paleo-oil reservoirs has great importances not only for the marine oil and gas exploration, but also for the comprehension of tectonic evolution of sedimentary basins in South China. Nevertheless, there are still a lot of important basic geological issues need to be resolved, eg. the accumulation and destruction age of these paleo-oil reservoirs. The most important barrier is the lack of dating of organic substances that are related to paleo-oil reservoirs.Recently, with the development of analysis test and mass spectrum technology, and the understanding of pro-organic attribute for Re-Os isotope system, Re-Os isotope has been successfully applied in geochronology of hydrocarbon accumulation. However, because of the low concentrations of Re and Os (from ng/g to pg/g in the order of magnitude), analytical method of the pros and cons have important implications for the experiment results, which limit the development of this method. Therefore, the establishment and development of organic rich material Re-Os isotope analytical techniques is of great importance to the chronology of oil and gas reservoir, also this method can provide constraints on the destruction time of paleo-oil reservoir.Majiang paleo-oil reservoir was selected as the main study area, based on thorough field survey and related sample analyses, the time of hydrocarbon accumulation and destruction are discussed. New developments are achieved in the following two aspects:(1) A Re-Os isotope analytical method suitable for organic rich materials like bitumen is established for the first time in ChinaBased on a comparison and evaluation of the documented Re-Os isotope techniques, combined with the special nature of the bitumen samples and the facility available in the State Key laboratory of Geological Processes and Mineral Resources (GPMR), the Re-Os isotope analytical method suitable for organic rich materials like bitumen is established.This analytical method includes the following important steps:sample digestion, separation and purification of Os, separation and purification of Re, and Re-Os isotope measurement with mass spectrometry. The sample digestion plays an important role in the whole processes, employing the Carius tube method, no more than 200mg samples were dissolved and equilibrated with a known amount of185Re and 190Os spikes, in inverse aqua regia (2:1 14N HNO3 and 12 N HCl,9mL) for 24h at 190℃with the stage heating method; The dissolved samples are always presented in light red color with a large amount of needle like material, and the phase of this material has been studied. Osmium was separated using solvent extraction (CCl4 and HBr) and purified using micro-distillation method, Rhenium was separated and purified using anion chromatography techniques. The purified Os was loaded onto Pt filaments and measured using negative ion thermal ionization mass spectrometry (NTIMS) using an electron multiplier in peak-hopping mode, the reduction technique in vacuum condition is of great importance for improving the measuring accuracy. The purified Re was measured using inductively coupled plasma mass spectrometry (ICP-MS) method, in order to correct the mass fractionation during the analysis processes, the standard sample-sample-standard sample "sandwich-like" sequence was employed during the measurement; 5% HNO3 and purified water were employed to wash the pipe to avoid cross-contamination. The measured Osmium isotope data should be processed with mass fractionation correction and oxygen isotope interference correction. After the mass fractionation of the original Re isotope data, combined with the isotope dilution formula, the Re isotope data was obtained. The quality of the data was controlled by the known sample and international standard sample. Following the above method, the data show good consistence with the documented and recommended value.(2) The Re-Os isotope features of the bitumen from Majiang paleo-oil reservoir and its indication for the timing of hydrocarbon accumulation and destructionBased on the fluid inclusion analysis of reservoirs rocks, combined with the tectonic evolution of Majiang paleo-oil pool and the reservoir burial history-temperature history curve, the hydrocarbon accumulation time was limited at Indosinian period. Employing the bitumen Re-Os isotope analytical method which is established by this research, the bitumen in the interlayer was analyzed, and showed high concentration of Re (21.12-373.9 ng/g) and low concentration of Os (0.1086-0.9081 ng/g), the 187Re/188Os ratio ranges from 777.0 to 3587, 187Os/188Os ratio ranges from 1.313 to 4.913, exhibiting highly radiogenic Os isotopic signature. According to these features, the data processing method of the low level and highly radiogenic sulfide was employed, and a Re-Os isochron age of 87.0±3.3 Ma is obtained. This age is reckoned as the formation time of this bitumen, which is been reckoned as the destruction time of Majiang paleo-oil reservoir and the mineralization age of Danzhai mercury-golden deposit, which corresponding with the regional fluid flow events.Employing the bitumen Re-Os isotope analytical method which established by this study, we obtained meaningful data. The results showed that the hydrocarbon was accumulated at Indosinian period, and the reservoir was damaged at Late Yanshanian period ca.87 Ma. The research also showed that the Re-Os isotope method may have great importance in crude oil and oil sandstone with single sourced, weak tectonic movements influenced petroliferous basins.The features and innovations of this research:(1) the first report of Re-Os isotope isochron age for bitumen from paleo-oil reservoir, which provided the direct chronological information for the destruction of Majiang paleo-oil reservoir; (2) the Re-Os analysis method for organic rich material like bitumen was first established in China.
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