Alkyl Naphthalenes and Phenanthrenes:Molecular Markers for Tracing Filling Pathways of Light Oil and Condensate Reservoirs

Condensates and light oils are generally characterized by high maturity, low concentration of sterane and terpane biomarkers and low content of non-hydrocarbon fraction. As a result, some commonly-used sterane, terpane and carbazole migration parameters in conventional oil reservoirs may have a certain limitation in condensate and light oil reservoirs for their poor signal-noise ratios in the gas chromatography-mass spectrometry （GC-MS）. Naphthalene, phenanthrene and their methylated substituents, however, are present in significant concentrations in condensates and light oils. Taking the Fushan depression （in the Beibuwan Basin, Northern South China Sea） as an example, this paper attempts for the first time to use polycyclic aromatic hydrocarbon （PAH）-related parameters to trace migration directions and filling pathways for condensate and light oil reservoirs. The result shows that TMNr （i.e. 1, 3, 7-TMN/（1, 3, 7-TMN ＋ 1, 2, 5-TMN）, TMN： trimethylnaphthalene））, MPI-1 （i.e. 1.5×（2-MP ＋ 3-MP）/（P ＋ 1-MP ＋ 9-MP）, P： phenanthrene MP： methyiphenanthrene）, MN/DMN （∑methylnaphthalene/∑dimethylnaphthalene, %） and MN/TMN （∑nethylnaphthalene/∑trimethylna- phthalene, %） can be used to trace the filling pathways of condensate and light oil reservoirs. These parameters, together with geological consideration and other bulk oil properties （e.g. the gas to oil ratio and density）, suggest that the condensates and light oils in the Huachang oil and gas field are mainly sourced from the Bailian sag that is located to the northeast of the Huachang uplift in the Fushan depression.

Oil charging history and pathways of the Ordovician carbonate reservoir in the Tuoputai region,Tarim Basin,NW China

Geochemical analysis of 64 oils sampled from an Ordovician carbonate reservoir in the Tuoputai region was undertaken to study the composition of molecular markers. All the oils have similar geochemical characteristics and belong to a single oil family. They are presumed to derive from the same source kitchen and have similar oil charging history. A histogram of homogenization temperatures(Th) of aqueous inclusions in reservoir rocks shows a bimodal distribution pattern, indicating that the Ordovician reservoir has been charged twice. Coupling the measured Th(°C) with the burial and geothermal histories reconstructed using 1D basin modeling, we relate the homogenization temperature to the relevant geological ages: i.e.,425–412 and 9–4 Ma, corresponding to the Middle to Late Silurian and the Miocene to Pliocene, respectively. The oil filling orientation and pathways are traced using molecular indicators related to alkyldibenzothiophenes and benzo[b]naphthothiophenes. The oil charging orientation is from south to north generally. It can be predicted that the Ordovician reservoirs were sourced from a kitchen located to the south of the Tuoputai region, most probably between the Awati and Manjiaer Depressions. Traps located in the southern side of the Tuoputai region, along the oil charging pathways, should therefore be preferred oil exploration targets.