Petroleum accumulation diversity of Funing Formation in Weima area, northern slope belt of Gaoyou Sag

The Weima area is one of the richest oil fields in the northern slope of the Gaoyou Sag, and has a large amount of residual oil resources. Some problems, such as petroleum migration pathways and the factors controlling accumulation, are not solved yet. Based on the data of the oil shows in well logs, reservoir parameters and crude oil geochemical index, we analyzed the characteristics of source rocks, reservoirs, cap rocks, tectonics, carrier systems and oil and gas pools. Three types of crude oils (mature, low maturity and extra-low maturity) were found in the study area. According to the correlation of maturity between oil and source rock, it was pointed out that hydrocarbon in the study area was generated from the same source rock with different maturity levels in the north slope: mature oil source in the southeast of Majiazui area, mixed oil source with low maturity from the middle shallow sag and Cheluo saddle, and extra-low maturity oil source mixed with different maturity levels in the north. The geochemical parameters such as carbazole and thiophene, together with oil show features, were applied to study petroleum migration pathways, accumulation direction and hydrocarbon supply radius. The characteristics of hydrocarbon enrichment and fluid distribution were determined. The cause for a new reservoir type composed of both uniform and comb-like oil-water systems was described. The same source rock with different maturity levels was the material basis for the variation of oil enrichment in Weima area. Fault carrier systems were the main controlling factor for the direction and diversity of petroleum accumulation. In the end, two prospective areas were identified for potential petroleum resources.