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Discovery of Giant, Ultra-deep Paleo-Reservoirs in the Tarim Basin and Geochemical Origin of Crude Oil


The Tarim Basin is one of the most petroliferous basins in China. The basin contains some deeply buried lower Paleozoic carbonate reservoirs holding rich oil and gas resources derived from highly mature deep source rocks. In December 2015, the Fuyuan-1 (FY1) well located in the Fuyuan block of the northern Tarim Basin produced commercial oil flow from a depth of 7,711 m. The daily crude oil and gas production was 214.62 m3 and 28,970 m3, respectively, giving a gas-oil ratio (GOR) of 135 m3/m3. The hydrocarbons was produced from the Ordovician Yingshan Formation. This is the deepest oil reservoir ever discovered in China. The FY101 and FY102 wells were successfully completed and also yielded commercial oil flow, confirming the oil enrichment in the deep geological formations of the Tarim Basin. Detailed analysis of hydrocarbon compounds of the oil was carried out using a 2D gas chromatography/time-of-flight mass spectrometry (GC×GC-TOFMS). It was revealed that a small varieties and a low content of adamantanes are present in the oil. No compounds found in high temperature pyrolysis, such as thiaadamantanes, and dibenzothiophenes. The crude oil has matured normally and has yet been cracked. The carbon isotope data indicates that the gas produced in this area is dominated by oil-associated gas type, further confirming the oil in the paleozoic reservoir at a depth of 7,711 m has not been cracked. The reservoir oil has been preserved due to a low regional geothermal gradient with the reservoir temperature being as low as 172°C. Since its formation during the Permian, the reservoir has been progressively buried deeper by the deposition of the overlying sedimentary strata. The basin underwent a rapid subsidence starting at 5 Ma, increasing its burial depth by 2,000 m, leading the paleo-reservoir to achieve its current maximum burial depth. The oil has not been cracked because of the relatively short duration in reaching its current high temperature. This implies that there is a huge potential of finding deep liquid petroleum through exploration in the region. The hydrocarbon discovery in the FY1 well will undoubtedly lead to large-scale oil exploration in the deep sedimentary strata of the Tarim Basin in future.