C5–C13 light hydrocarbons of crude oils from northern Halahatang oilfield (Tarim Basin, NW China) characterized by comprehensive two-dimensional gas chromatography

Abstract

The Halahatang Oilfield in the Tabei Uplift of the Tarim Basin has substantial petroleum potential and contains many prolific commercial oil reservoirs. In this study, a total of 15 DST oils were sampled from the northern Halahatang oilfield, and their geochemistry of C5–C13 light hydrocarbons (LHs) is characterized by two-dimensional gas chromatography (GC × GC). The concentrated clusters in the LH triangular diagrams and parent-daughter plots of the n-Heptane (%C7) vs. ln (Cps/CHs) corroborating with the similar K1 values (1.01–1.23), imply a high degree of compositional similarity and source affinity. The C7 LHs show a predominance of n-heptane (30.07%–42.75%) and a nearly identical abundance of MCyC6 (35.92%–45.85%), low IMCyC6 (methylcyclohexane index) value (35.92%–45.57%) and a moderate ICH (cyclohexane index) value (25.92%–45.66%), demonstrating that they were typical marine oils mainly originated from a substantial amount of sapropelic organic matter mixed with a nearly equivalent amount of humic one. The LH maturity indexes, such as the heptane (H) ratio (17.80%–29.28%), isoheptane (I) ratio (1.25–2.15), °Ctemp (113.72 °C −124.63 °C),methyladmantane index (MAI: 52.15%–63.65%) and equivalent vitrinite reflectance (1.1%–1.3% Ro) indicate a high thermal maturity. The values of 3-ethyl-2-methylheptane to 3-methylnonane (MT1: 0.91–3.65), 1,1,2,3-tetramethylcyclohexane to propylcyclohexane (MT2: 0.85–1.37), 2-methyl-3-ethylheptane/2,6-dimethyloctane (MTR: 0.59–1.25), and Pr/Ph (0.87–1.62) observed for the northern Halahatang oils consistently suggest a marine carbonate lithology for the source rock deposited in an anoxic, reducing environment. Geochemical and geological correlation imply the Halahatang oils are possibly derived from the O2+3 potential source rocks.