Paleoenvironment and origin of heavy oil of the lower Cretaceous-Oligocene: a case study

Abstract

ABSTRACT Many heavy oil wells have been found in the Jilantai shallow depression in the Hetao Basin. The origin of heavy oil in the study area was analyzed comprehensively according to the physical properties and geochemical characteristics of heavy oil. Two sets of source rocks were detected in the Guyang and Linhe groups, in the Hetao Basin. During the Guyang Formation, the differences of sedimentary environments resulted in brackish water sedimentary environment in the south of the study area and freshwater reducing environment in the north of the study area. The high content of boron in the source rocks of Linhe Formation indicates its saltwater environment. By the method of oil source comparison, it is found that the Pr/Ph of the crude oil in Jilantai area and Linhe Formation is lower than 0.50, and the Pr/nC17 is between 0.7 and 3.5, which should come from the source rock of the first segment of the southern moderate lacustrine Guyang Formation. The C29 sterane 20S/(20S+20 R) and C29 sterane ββ/(αα+ββ) parameters indicate that the low maturity of crude oil is the main cause of heavy oil formation. The research on the genesis and geochemical characteristics of Jilantai crude oil indicates that the northern and central depression of the Hetao Basin is a rich area of favorable resources.