<i>Using δD of methyl aromatics and oils to trace the reservoir charging characteristics of complex petroleum systems in Tarim Basin, northwestern China</i>

Abstract

Aromatization occurs steadily during the petroleum evolution stage of light oil/condensate (LOC) formation, for which the composition and delta D of aromatics are of considerable geochemical significance. This paper presents a study on the composition and delta D of monomethyl aromatic compounds (toluene, methylnaphthalene, methylphenanthrene, and methyldibenzothiophene) and related hydrocarbon compounds in LOCs from the Tazhong uplift area of the Tarim Basin in China. Thermal maturity, the source rock of the LOCs, and the effect of gas washing on the oils are discussed. The calculated reflectance of the oils using methyldibenzothiophene and dimethylnaphthalene parameters is approximately 1.15%-1.5%. The thermal maturity of the oils in the Tazhong I fault-slope zone is slightly higher than that of the oils in the Tazhong 10 structural belt due to the more intense gas washing of the oils in the former. The varying degrees of gas washing also caused enrichment of low-carbon-number aromatic hydrocarbons and their H-2 isotopes in the oils of the Tazhong I fault-slope zone relative to those of the Tazhong 10 structural belt. In addition to the contributions from lower Cambrian and Middle-Upper Ordovician source rocks, these crude oils may have received contributions from upper Cambrian Furongian source rocks deposited within locally shielded environments. Crude oils such as TZ62(Silurian) and ZG432(Ordovician) contain heavy delta C-13 but light delta D isotopic compositions and may represent end member oils derived from Furongian source rocks