Quantitative prediction of mixed-source crude oils and its significance for understanding oil accumulation in subtle pools in the Dongying Depression, Bohai Bay Basin

Abstract

Conventional geochemical approaches were utilized in the quantitative prediction of the proportions of mixed-source crude oils derived from the Es 3 and Es 4 members of the Paleogene Shahejie Formation. The mixed-source oils are accumulated in the middle interval of the Es 3 member (Es 3 M ) in the Niuzhuang Sag, which is one of the sags where subtle traps are primarily of lenticular basin-floor turbidite sands within mudstones mostly developed in the Dongying Depression, Bohai Bay Basin. The result showed that about 18-92% of the mixed-source oils were derived from the Es 4 source rocks with an average of 55-60%. Reservoirs associated with deep faults appear to have much more Es 4 genetic affi nity oils. A high proportion of the Es 4 -derived oils discovered in the Es 3 M subtle lithological traps in the Niuzhuang Sag have long migration distances. This suggests that surrounding source rocks might not necessarily control the hydrocarbon supply for subtle traps. Subtle migration pathway may play an important role in the vertical oil migration. The traditional concept of hydrocarbons accumulation in these lenticular turbidite sandstone traps within a short migration distance from the surrounding source rocks is not supported by this study. The present result is also consistent with our previous fi ndings that immature oils in the Bamianhe Oilfi eld in the south slope of the Niuzhuang Sag were actually mixed-source oils mostly sourced from the Es 4 in the depocenter of the Niuzhuang Sag, and the petroleum potential of the Es 4 member in the Dongying Depression should therefore be re-evaluated.

Correlation of Neogene Oil and Paleogene Source Rocks in the Middle Huanghekou Sag, Bohai Bay Basin

“…Although there is a lack of carbon isotope data lower than n-C 15 in the study samples, the curve trend shows that there is a high possibility of a max carbon peak of n-C 15 . There is a max peak carbon of n-C 32 (Figure 8), is basically consistent with previous research results [36], indicating that the crude oil samples in the study area are more likely to be derived from the Es 3 source rock.…”

“…In Figure 8, it can be seen that there is a certain similarity between three crude oils, indicating a high correlation between the crude oils, and the sedimentary environment also has a certain similarity. The Es 3 formation in the study area is brackish or a freshwater delta facies deposition, which is highly consistent [36] with the carbon isotope values. In addition, previous studies in neighboring areas found that the crude oil generated from the Es 3 source rock has relatively heavy carbon isotopes and shows a double-peak curve with the max carbon peaks of n-C 15 and n-C 32 [37].…”

“…The crude oil samples are roughly distributed in a similar area, basically with the Es3 source rocks. The −28‰ of the carbon isotope value of saturated hydrocarbon monomers is used to divide salt water and fresh water environments [36,37]. In Figure 8, it can be seen that there is a certain similarity between three crude oils, indicating a high correlation between the crude oils, and the sedimentary environment also has a certain similarity.…”

Geometry, kinematics and dynamic characteristics of a compound transfer zone: the Dongying anticline, Bohai Bay Basin, eastern China

“…The second episode was relatively late, allowing oil to accumulate in a variety of traps formed prior to and during source rock maturation. Many reservoirs are found in the Es U 3 and Ed members [27].…”

“…A transfer zone is a kinematic and dynamic transition zone between major faults, and the zone transfers strain by transmitting the displacements of major faults or secondary faults [27,29]. The fault expansion index and throw can be used to characterize the strain transition.…”