Abstract: Drilling in Dongdaohaizi sag started in the 1990s, but little progress has been made in oil and gas exploartaion due to the complex structural evolution and sedimentary characteristics for a long time. Recently, oil and gas indications have been  found repeatedly in the exploratory wells deployed in the Dongdaohaizi Sag, which has rekindled the hope of oil and gas exploration in this area. However, the understanding of the origin of oil and gas in this area is still poor, causing significant difficulties in selecting an exploration direction. Based on systematical analyses of stable carbon isotope composition, isoprenoid alkane, sterane terpane biomarkers, and other geochemical characteristics of crude oil and source rocks in the Carboniferous and Permian strata of Dongdaohaizi sag and its surrounding areas, the genetic types and oil sources were discussed. The results showed that the crude oil in the study area has entered the main oil generation window, which belongs to mature crude oil and can be divided into three different types. Type I oil is characterized by heavy stable carbon isotope composition, high Pr/Ph ratio, abundant C19 tricyclic terpanes, C20 tricyclic terpanes and C24 tetracyclic terpanes, and low gammacerane index. These features indicate that  it was derived from humic source rocks in the Lower Carboniferous Songkarsu formation, which was deposited in a freshwater environment. Type II oil is characterized by relatively light stable carbon isotope composition, relatively low Pr/Ph ratio, abundant β-Carotene, high gammacerane index, and low abundance of C19 tricyclic terpane, C20 tricyclic terpane, and C24 tetracyclic terpane. These features indicate that it was derived from the saline lacustrine source rocks in the middle Permian Pingdiquan formation. Geochemical characteristics of Type III oil are between those of Types I and II crude oil, showing a mixed source. The above findings provided a type case of oil source correlation in a multi-source superimposed area and also pointed out the direction for future petroleum exploration in the Dongdaohaizi sag and its surrounding areas.

Key words: Junggar Basin, Dongdaohaizi sag, oil source correlation, Carboniferous, Permian, stable carbon isotope composition;
biomarker fingerprints