Origin and mixing ratio of crude oils in different charging episodes of Yangjiang Sag of Pearl River Mouth Basin

Abstract

The oil and gas exploration in the Yangjiang Sag of Pearl River Mouth Basin has successfully carried out and breakthroughs have been achieved in the past two years. The generation mechanisms of crude oils of multiple charging episodes were studied and the sources, mixing ratios were quantitatively calculated by the using of analytical results of gas chromatography-mass spectrometry (GC-MS) for the saturated hydrocarbon fractions, fluid inclusion system analysis and whole-oil gas chromatography (GC-FID) for mixed-source oil proportion experiment. Results showed that there were two episodes of crude oil charging in the Yangjiang Sag, 12.0-7.5 Ma and 6.5-0 Ma. In the first stage, crude oils came from the medium-shallow deposited lacustrine source rocks in the third member of Wenchang Formation in the Enping 20 sub-sag, and mainly were accumulated in the Zhujiang Formation. In the second stage, crude oil came from the medium-deep deposited lacustrine source rocks of the first and second members of Wenchang Formation, and accumulated in the Zhujiang and Hanjiang formations. A template for the calculation of mixed propotion of oils was established through quantitative experiments. It was concluded that the Hanjiang reservoirs in the B20-1 and B20-2 oil fields were mainly charged during the second episode, while the Zhujiang reservoirs in the B20-1 oil field were charged during both the first and the second episodes, accounting for 39% and 61%, respectively. The coupling relationship among fault activity intensity, trap development interval and effective source rock controlled the degree of enrichment.

Keywords

• proportioning experiment
• crude oil origin
• crude oil charging history
• quantitative calculation of mixed-source proportion
• yangjiang sag
• pearl river mouth basin