Geochemical characteristics and exploration significance of ultra-deep Sinian oil and gas from Well Tashen 5, Tarim Basin, NW China

Abstract

The Well Tashen 5 (TS5), drilled and completed at a vertical depth of 9017 m in the Tabei Uplift of the Tarim Basin, NW China, is the deepest well in Asia. It has been producing both oil and gas from the Sinian at a depth of 8780–8840 m, also the deepest in Asia in terms of oil discovery. In this paper, the geochemical characteristics of Sinian oil and gas from the well were investigated and compared with those of Cambrian oil and gas discovered in the same basin. The oil samples, with Pr/Ph ratio of 0.78 and a whole oil carbon isotopic value of −31.6‰, have geochemical characteristics similar to those of Ordovician oils from the No. 1 fault in the North Shuntuoguole area (also named Shunbei area) and the Middle Cambrian oil from wells Zhongshen 1 (ZS1) and Zhongshen 5 (ZS5) of Tazhong Uplift. The maturity of light hydrocarbons, diamondoids and aromatic fractions all suggest an approximate maturity of 1.5%–1.7% Ro for the samples. The (4-+3-) methyldiamantane concentration of the samples is 113.5 μg/g, indicating intense cracking with a cracking degree of about 80%, which is consistent with the high bottom hole temperature (179 °C). The Sinian gas samples are dry with a dryness coefficient of 0.97. The gas is a mixture of kerogen-cracking gas and oil-cracking gas and has Ro values ranging between 1.5% and 1.7%, and methane carbon isotopic values of −41.6‰. Based on the equivalent vitrinite reflectance (Reqv = 1.51%–1.61%) and the thermal evolution of source rocks from the Cambrian Yu'ertusi Formation of the same well, it is proposed that the Sinian oil and gas be mainly sourced from the Cambrian Yu'ertusi Formation during the Himalayan period but probably also be joined by hydrocarbon of higher maturity that migrated from other source rocks in deeper formations. The discovery of Sinian oil and gas from Well TS5 suggests that the ancient ultra-deep strata in the northern Tarim Basin have the potential for finding volatile oil or condensate reservoirs.