Abstract: Recently, breakthroughs have been made in oil and gas exploration of Fengcheng Formation in the south slope of Mahu Sag, the conditions of hydrocarbon accumulation such as source rock and reservoir rock need to be re-recognized, and the main controlling factors of pure oil reservoir in lithologic reservoir zone need to be identified. In view of the above problems, geochemical and sedimentary reservoir testing were carried out for exploration wells in recent years, and logging, and seismic data were integrated, and the results show: 1) recently drilled wells in the study area in the outer front of fan delta facies mature and good type II hydrocarbon source rock reach of certain thickness, revealing that Fengcheng Formation source rock also exist in study area. 2) the plain facies of fan delta develops massive glutenite, and there is no mudstone cap layer inside, and the front facies of fan delta develops mudstone cap layer inside, which can be used as local cap layer; reservoir rocks are extra-low porosity and extra-low permeability reservoirs. 3) four hydrocarbon accumulation belts are identified, and massive glutenite is distributed in plain facies reservoir belt, and downdip direction water hose is formed due to the unconformity of the plain facies reservoir zone cap layer sealing; pure oil layers in fan delta front reservoirs are controlled by regional mudstone cap rock development, and outer frontal shale cap rock develops generally. We thus select a 289 km² area favorable for exploration, guiding further exploration and deployment.

Key words: south slope of Mahu Sag, Fengcheng Formation, hydrocarbon source rock, cap rock, accumulation conditions