Source and thermal maturity of crude oils in the Junggar Basin in northwest China determined from the concentration and distribution of diamondoids

Abstract

Here we discuss three types of diamondoid parameters for oils from the Junggar Basin. These are: absolute concentrations, concentration ratios, and isomerization ratios. According to the absolute diamondoid concentrations, the oils collected from different areas of the basin were broadly divided into three maturity stages: (1) low-mature (<100 ppm adamantanes; <5 ppm diamantanes); (2) mature (100–1000 ppm adamantanes; 5–50 ppm diamantanes); and (3) highly mature (>1000 ppm adamantanes; >50 ppm diamantanes). The oils in the northwestern region of the Junggar Basin are in the low-mature to mature stages. Based on a combination of diamondoid concentration ratios and biomarker indices, these oils can be divided into three groups, i.e., Group I oils in the Wuxia Zone derived from the lower Permian Fengcheng Formation (P1f), Group II oils in the Kebai Zone sourced from middle Permian Lower Wuerhe Formation (P2w), and Group III oils in the Mahu Depression generated from more mature source rocks of Jiamuhe Formation (P1j) or the P1f. Diamondoid concentrations and isomerization ratios were used to precisely evaluate the thermal maturity of mature oils and highly mature condensates, respectively. Our results indicate that the oils in the central part of the basin have decreasing thermal maturity from south to north, whereas the oils in the Kelameili area display increasing thermal maturity from east to west. In this study we found that different diamondoid indices are useful only in certain thermal maturity ranges. Therefore, at least for the Junggar Basin, it is crucial to know which thermal region one is in before using diamondoid ratios for maturity assessment.