Application of C7 hydrocarbons technique to oil and condensate from type III organic matter in Northwestern Taiwan

The purpose of this study was to examine the potential of C7 light hydrocarbons as biomarkers for petroleum exploration. The oil and condensate reservoirs of the sedimentary basins in northwestern Taiwan generally are in the deltaic–littoral–neritic sandstones of Upper Oligocene to Upper Miocene strata. Oils and condensates produced from these basins are terrigenous, with a variable δ13C‰ ratio − 30.07 to − 27.50‰/PDB, and Pristane/Phytane ratio from 4.79 to 9.22. A large set of condensate and oil samples were collected from major gas fields across the Hsinchu–Miaola basin, and the Kovat Index was used to identify C7 light hydrocarbons in the samples. The isomeric parameters of C7 included single-branched and multiple-branched heptanes and isomeric pairs, such as nC7/Methylcyclohexane ratio vs. Toluene/nC7 ratio, 2-MH + 2, 3-DMP vs. 3-MH + 2, 4-DMP, n-Heptane ratio vs. iso-Heptane ratio. The ratios for parent (P1), mono-branch (P2) and poly branch (P3) heptane isomers ranged from 37 to 64%, 29 to 45% and 6 to 19%, respectively. The ratios for nC7/Methylcyclohexane and Toluene/nC7 ranged from 0.16 to 0.60 and 1.52 to 10.58, respectively. The n-Heptane value and iso-Heptane value ranged from 10 to 26 and 0.8 to 1.8 respectively and the iso-alkanes (3RP), cyclopentanes (5RP), cyclohexanes (6RP) ratios ranged from 6 to 29%, 5 to 13% and 53 to 90%, respectively with the 2-MH + 2,3-DMP vs. 3-MH + 2,4-DMP found to be constant for the linear equation Y = 1.1458X − 0.0755. The use of this technique as a qualitative tool shows that all the oils from the northwestern Taiwan were mature, free of biodegradation, affected by evaporative fractionation and derived from the same petroleum system. The source rock of the oils should be terrigenous.