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Abstract 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 (T h) of aqueous inclusions in reservoir rocks shows a bimodal distribution pattern, indicating that the Ordovician reservoir has been charged twice. Coupling the measured T h (°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.

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 ( T ) of aqueous inclusions in reservoir rocks shows a bimodal distribution pattern, indicating that the Ordovician reservoir has been charged twice. Coupling the measured T (°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. [ABSTRACT FROM AUTHOR]

In this study, 12 crude oil samples were collected and analyzed from the Ordovician reservoir in the Halahatang Depression, Tarim Basin, China. Although the density of oil samples varies considerably, based on saturated hydrocarbon gas chromatographic (GC), saturated and aromatic hydrocarbon gas chromatographic-mass spectrometric (GC/MS) and stable carbon isotopic composition analyses, all the samples are interpreted to represent a single oil population with similar characteristics in a source bed or a source kitchen, organic facies and even in oil charge history. The co-existence of a full suite of n-alkanes and acyclic isoprenoids with UCM and 25-norhopanes in the crude oil samples indicates mixing of biodegraded oil with fresher non-biodegraded oil in the Ordovician reservoir. Moreover, according to the conversion diagram of double filling ratios for subsurface mixed crude oils, biodegraded/non-biodegraded oil ratios were determined as in the range from 58/42 to 4/96. Based on oil density and oil mix ratio, the oils can be divided into two groups: Group 1, with specific density >0.88 (g/cm) and oil mix ratio >1, occurring in the north of the Upper Ordovician Lianglitage and Sangtamu Formation pinchout lines, and Group 2, with specific density <0.88 (g/cm) and oil mix ratio <1, occurring in the south of the pinchout lines. Obviously, Group 2 oils with low densities and being dominated by non-biodegraded oils are better than Group 1 oils with respect to quality. It is suggested that more attention should be paid to the area in the south of the Upper Ordovician Lianglitage and Sangtamu Formation pinchout lines for further exploration. [ABSTRACT FROM AUTHOR]