Distribution of naphthalenes in crude oils from the Java Sea: Source and maturation effects

Amounts and distributions of C1-C3 naphthalenes and cadalene were determined by capillary gas chromatography in sixty crude oil samples from the Java Sea, southeast Asia and in a sample of Talang Akar (Oligocene) resinite supposed to be a major contributor of hydrocarbons to oils in that area. Maturity of the oils was assessed by the Methylphenanthrene Index (MPI-1). Source effects discovered by scatterplots were scrutinized by Principal Component Analysis (PCA). Based on empirical relationships with C27-C29 steranes (measured by gas chromatography-mass spectrometry), dimethylnaphthalenes in oils from the Ardjuna and Jatibarang (sub-)basins are mainly derived from terrestrial sources. Transition to a marine depositional environment in the Sunda basin, as indicated by a decrease in C29 sterane relative abundance from 70 to 30%, results in a decrease of total dimethylnaphthalene (DMN) concentrations from 20 to less than 3 mg/g oil. This is due, mainly, to a decrease in the concentration of the dominant 1,6-DMN. Thus, terrestrial Ardjuna or Jatibarang oils rich in 1,6-DMN (2–15 mg/g oil) are readily discriminated from marine Sunda oils (0.2–2 mg 1,6-DMN/g oil). Resinite of the Talang Akar type (1.4 mg 1,6-DMN/g total organic matter [TOM]) appears to be a significant contributor of 1,6-DMN to oils in the Ardjuna basin. Terrestrial affinities are inferred also for cadalene from concentration levels that are higher in terrestrial oils (1–3 mg/g oil) and Talang Akar resinite (6 mg/g TOM) than in the marine oils (0.1–1 mg/g oil). Ardjuna basin oils have the highest concentrations of cadalene, hence supporting resinite of the Talang Akar type as a possible source. However, 1,6-DMN and cadalene are likely to be derived from different terrestrial organic matter sources, because they load on different factors in PCA. Maturity levels of the oils in the range of 0.6–0.9% calculated vitrinite reflectance (Rcb), as inferred from the Trimethylnaphthalene Ratio (TNR-2), are in reasonably good agreement with MPI-1 measurements and geological constraints, e.g., highest maturities detected near the depocentre of the Ardjuna basin. The TNR-2 parameter hence proved useful for maturity assessment of Java Sea oils.