The Geological-geochemical Study On The Main Petroleum Migration Pathways In The Pearl River Mouth Basin

The Pear River Mouth (PRM) basin is one of the main petroleum resources bases in China, and most petroleum discoveries are occurring in the two structure ridges. Petroleum migration and accumulation in the basin is one of the focus problems for explorers. In combination with the development of the basin, this work will try to reveal the characteristics of the main petroleum migration pathways in the structure ridges with new geochemical methods.The data of GC/MS of the whole oils show that there are three oil groups in the basin: group A with abundant 4-melhylsterancs. group B with relative low 4-methylstcrane contents, and group C with relative lowest maturity. In combination with the source characteristics, the three oil groups can be extrapolated to three end-member oils, or three oil populations: population A derived from Wenchang source rock, population B derived from Enping formation with absence of 4-methylsterane, and population C might be sourced from the Wenchang source rocks in the leftover sags with lowest maturity as a typical feature. Therefore there are at least three phases of petroleum migration in the PRM basin and the contributions to the mixed oils are quantitatively evaluated.The analyses of the nitrogen compounds show that the ratio of l,3-/l,6-dimethylcarbazole (DMC) displays a remarkable invariance (about unity) in the oils in the PRM basin. In combination with the characteristics of the molecular structure of carbazole, a possible genetic mechanism of DMCs is proposed: l,x-DMCs may be derived from 1-methylcarbazole by methylation. The methylation at the cyclocarbon positions 3 and 6, with equal chemical reaction activity, would result in equal products; at the same time, competitive methylations would occur between cyclocarbon positions 3,6 vs 4,5; different conditions of reactions will result in different preferences of 1,3-+1,6-DMC or 1,4-+1,5-DMC. In the geosphere, the different conditions may indicate various depositional environments. In other words, the differences of the ratios of 1,3-+1,6-/1,4-+1,5-DMC may imply various depositional environments.Based on the distributions of nitrogen-compounds in the reservoirs, there might be two individual petroleum carrier systems along the HLSR: the Upper and the Lower systems, which happened to be bordered by a thin layer of marls widely spreaded in the Dongsha Massif. The Lower carrier system may be the main petroleum migration pathway for the accumulations in the LH oil fields.The maturities of the oils display that the oils in the depression areas show relative high thermal maturity and the oils in the east part of the Huiliu Structure Ridge (HLSR) have relative low maturity, which consists with that the more migrated oils bear less mature and the less migrated oils more mature. That may reveal the main petroleum migration pathway from Huizhou sag through HZ26-1 structure to the LH oil fields is the HLSR.The molecular compositional heterogeneities in the oils may indicate that the west part of the HLSR gathered "branched oils" migrating direct from the Huizhou sag to form "a main petroleum river" migrating eastward to LH oil fields, and the east part of the HLSR show clearly the characteristics of main petroleum migration pathway. And the oils with abnormally low maturity in the HLSR may be attributed to the contributions from the Wenchang source rock in some small sags occurring the Dongsha Massif at a latest phase.