Diamondoids in oils from the ultra-deep Ordovician in the North Shuntuoguole area in the Tarim Basin, NW China

Abstract

The ultra-deep Ordovician Yijianfang-Yingshan Formations from the North Shuntuoguole region in Tarim Basin, NW China, have produced industrial oil and gas. The reservoir fluid phases differed between faults. Using the GG-MS and GC × GC-TOFMS methods, the study focuses on the distribution and concentrations of diamondoids of the Ordovician oils in the North Shuntuoguole area. In terms of relative diamondoids content, oil samples from F1, including splay and sub-faults, and F3 faults contain more adamantanes than oils from F5 and F7 faults. The concentrations of diamondoids in oils from the North Shuntuoguole area showed a positive correlation with the 4-/1-MDBT ratio, implying that the maturity dominated the concentrations of diamondoids in oil samples. The varying relative content of adamantanes and diamantanes in oils from different faults may be due to different hydrocarbon filling histories and preservation conditions. In the late Himalayan period, the F1 reservoirs were charged with natural gas of a relatively higher maturity, obtained more adamantanes from the deep strata, and led the oils rich in adamantanes. The preservation condition of the F5 reservoirs was weaker than that of the F1, as shown by some light hydrocarbon loss and a decrease in adamantanes in oil samples from F5. The (4-+3-) methyldiamantane concentrations of 22 μg/g and 33 μg/g were used as the baseline for the oil samples from F1 and F5, respectively, due to difference in the relative contents of diamondoid. According to the method proposed by Dahl et al., the degree of oil-cracking of the oils from the F1 (including splay and sub-fault), the middle and the south sections of F5 are 0–42 percent, 20 to 33 percent, and 54 percent, respectively.

Keywords

• Diamondoids
• Oil-cracking
• North Shuntuoguole area
• Ultra-deep
• Ordovician
• Tarim Basin