OIL-BEARING CHARACTERISTICS IDENTIFICATION IN SURFACE SANDSTONES——TAKING CUOQIN, KEKEXILI, BIRU AND CHANGDU BASINS IN TIBET FOR EXAMPLE

It is very important to find oil shows in surface samples for a new oil and gas exploration area. Rock-Eval, chloroform extract, fluorescence observation, and element analysis of filling matter in pores can be used to determine whether surface sandstone is oil-bearing .If the fluorescence exist and carbon content is 75% and O/C atom ratio is 0.5 in pore filling matter, the surface oil-bearing sandstone will be characterized qualitatively in Tibet area. Meanwhile, pyrolysis hydrocarbon content can be used to calculate original oil-bearing saturation. In Tibet area oil-bearing level originally was from oil saturated to oil-showing if original oil-bearing saturated is 45% and chloroform extract is 120×10^-6. It was oil immersed to oil spot while oil-bearing saturation is 20%-45% and extracts is(60-120)×10^-6.It was only oil trace if they respectively are<20% and<60×10^-6. Surface oil-bearing sandstones have been found in 4 basins of Tibet, that is, 3 in Cuoqin, 4 in Kekexili, 3 in Biru, and 4 in Changdu basins. Among them the best is C163 sandstone in Cuoqin basin. The pyrolysis hydrocarbon is 28.36mg/g and saturation is 51.8% and chloroform extracts is 123×10^-6. Finally, Cuoqin and Kekexili basins are considered as oil-bearing basins, and Biru and Changdu basin natural gas-bearing ones.