NOTES Observation of hydrocarbon generation and migration of highly-matured carbonates by means of laser-induced fluorescence microscopy LIU Dehan, XlAO Xianming, JIA Rongfen, SHEN Jiagui, LIU Zufa, WANG Yunpeng & GENG Ansong State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China Correspondence should be addressed to Liu Dehan (email: Liudh@gig.ac.cn) Abstract Some important information on hydrocarbon generation, inclusion and migration in highly-matured carbonates of lower Palaeozoic age from the Ordos Basin and Tarim Basin has been analyzed by a newly-combined laser-induced fluorescence microscope (LFM) designed by our laboratory. The following information has been obtained from the lower Ordovician lamellar carbonates with equivalent vitrinite reflectance (R,) as high as 1.6%-1.7% and residual TOC of 0.14%--0.35% from the Ordos Basin: wide occurrences of oil and source macerals with strong fluorescence, including G.Prisca alginite, lamalginite, telalginite and algae-detrinite; fluorescing asphalt among mineral crystals; some groundmass and spheroid-like reservoir bitumen with high maturation levels in the pores of dolomites. Various kinds of fluorescing organic inclusions and asphalt have been found in the carbonates, calcareous shales and silt-shales with high maturation levels from the Cambrian-Ordovician strata in the Tarim Basin. All this helps us find and evaluate significant and excellent source rocks for large- and middle-scale gas fields. The net and micro-net systems for hydrocarbon generation, expulsion and migration in carbonates have been revealed by the highly-powered laser-induced fluorescence microscopy. Keywords: laser-induced f l u o r e ~ ~ ~ n c e microscopy, highly-matured carbonate, hydrocarbon generation and migration, organic inclusions. The fluorescence microscopy is an important tool for determining the organic matter of source rocks, and for investigating hydrocarbon generation and migration in strata for petroleum geology. It has been widely applied in oil and gas exploration['-31. However, great difficulties have been met for the lower Palaeozoic carbonates in basins such as Ordos, Tarim and Sichuan where much fluorescence information of the strata was lost because of the high maturity for most of the strata. In this paper, a newly combined laser-induced fluorescence microscope (LFM) is used to investigate the hydrocarbon generation and migration of the highly-matured carbonates in early Palaeozoic strata, and to identify and evaluate significant source rocks. 1 Distribution and characteristics of the studied samples Large- and middle-scale oil and gas fields related to lower Palaeozoic carbonate strata have been discovered recently in the Tarim, Ordos and other basins. However, there are different opinions about the hydrocarbon potential of the lower Palaeozoic Majiagou carbonates in the Ordos Basin since it has a high maturation level and a low residual total organic carbon (TOC). Thus, quite a few of Ordovician samples, including limestone, dolomite and lamellar argillaceous carbonates that formed in a shallow marine platform facies were selected. These samples have a relatively strong hydrocarbon potential, with TOC of 0.14%4.45%, and equivalent vitrinite reflectance (&) of 1.6%-1.74%. but little fluorescence information is available (samples HCl-HC3). The Ordovician carbonates in the Tarim Basin have a lower maturity than that in the Ordos Basin, so they have abundant fluorescence information in petroleum-producing sections, but the Cambrian carbonates and calcareous black shales and the other strata with higher maturation levels have very weak fluorescence information. The residual TOC for these samples varies widely, and different opinions exist about the discrimination between a significant source rock and an excellent source rock. In order to study the information of hydrocarbon generation and hydrocarbon inclusion, some samples of the Cambrian and Ordovician ages were selected, from high-mature to over-mature with equivalent 16 Chinese Science Bulletin Vol. 45 Supp. March 2000