Base level cycle and reservoir macroscopic heterogeneity of sangonghe formation in Baolang oilfield

Sangonghe formation of Jurassic in Baolang oilfield of Yanqi basin is a braided river delta depositional system with shallow gentle slope,and it is characterized by strong reservoir heterogeneity,obvious cyclicity and well -bedded feature.By applying the base-level cycle theo- ry,oil-bearing intervals are classified as four short- term cycles with retrograding superposition feature.The research finds out that the base-level cycle and the corre- spondent containable space variation are key factors to af- fect the macroscopic heterogeneity.Controlled by base- level cycle,the characteristics of interlayer heterogeneity such as the channel pattern,reservoir thickness,sand- body scale,interbed distribution and connection manner of sandbody,are all changed regularly.With the increase of the containable space,channel character is converted from braided river course to meandering river course.As a re- sult,the reservoir properties and the degree of interlayer heterogeneity are also changed.