Paleosols in the Upper Guantao Formation (Miocene) of the Gudong Oil Field and Their Application to the Correlation of Fluvial Deposits

Abstract

The main pay zone of the Upper Guantao Formation of the Gudong oil field, North China basin, is a 300-m-thick fluvial sequence. Due to lack of bio- and lithostratigraphic markers, correlation of the main pay zone has been difficult, especially in the beginning of the oil-field development. However, paleosols are well developed in this fluvial sequence. Core investigation indicates that the paleosols can be identified by a set of characteristics including color of sediment, original plant roots, caliche nodules, sedimentary structures, distribution of facies, and mineral composition. Log responses of paleosols include lower gamma-ray activity and higher sonic ^Dgrt value, relative to non-paleosol sediments. An analysis of the evolution of paleosols and controlling geological factors illustrates that paleosols present time-stratigraphic markers and can be used to solve correlation problems. A stratigraphic framework, in which the Upper Guantao Formation was divided into three paleosol sets and two non-paleosol sets, was built using paleosols as correlation markers. The results prove that the paleosol method of correlation is viable and very helpful for geological study of similar oil fields in the early evaluation and development design phase.