Keywords: 
• early reservoir forming /
• hydrocarbon accumulation period /
• geological implication /
• Bonan sag /
• saline lacustrine basin

Abstract: It was the traditional view that Paleogene of Bonan Sag had three hydrocarbon accumulation periods and the main period was later. With the proposed new theory that source rock in saline lacustrine basin was generated and expulsed early and the quantity of generation and expulsion is various. Early reservoir forming process of Bonan Paleogene was re-evaluated using the analysis and testing data of drilling and seismic exploration. The research shows that source rock formed under saline environment is well developed in the upper part of 4^th member of Shahejie Formation of Bonan sag, and geochemical parameters indicate that source rocks can generate a great number of oil and gas and that is the material basis of early reservoir forming. Fluorescent color and homogenization temperature of fluid inclusion indicate that early reservoir forming is developed in Luojia area of Bonan sag and Sanhecun sag. Oil and gas of Paleonege at Sanhecun sag was transferred from Luojia area at early sedimentary stage of Dongying group when Gunan fault throw was small. Re-recognition on early reservoir forming not only made breakthroughs in oil and gas exploration of Paleonege in Sanhecun sag, but also built complete reservoir forming period sequence. Rock wettability alternation caused by early reservoir forming helped successive development of dominant migrating pathway, organic acid filling through the process of reservoir forming helped development of the reservoir space, and, more importantly, re-recognition of early reservoir forming will help transforming the exploration strategy and expand the exploration area, which will bring significant geological and exploration implication.