Predicting Source Rock Distribution of the Ninth Member of the Upper Triassic Yanchang Formation Based on Well-Logging Parameters and TOC Values in the Longdong Area, Ordos Basin, China

In recent years, new oil reservoirs have been discovered and exploited in the ninth member (Chang 9 Member, T₃y₉) of the Upper Triassic Yanchang Formation (T₃y) in the Longdong area, Ordos Basin. Some studies have shown that the crude oils of the Chang 9 Member may originate from the Chang 9 source rock in some areas, which may be related to the distribution of the source rock. However, the distribution of the Chang 9 source rock in the Longdong area is still unclear, which hinders further exploration and development of petroleum. In this study, we established a multiple linear regression model for predicting total organic carbon (TOC) based on the relationship between well-logging parameters and measured TOC values of shale core samples from 30 wells in the study area. The results show that the Chang 9 shale is mainly composed of gray and dark mudstones, which mainly belong to the interdistributary bay and front delta depositional subfacies. The TOC values of the shale core samples from this member vary in a range of 0.11-4.8%, with an average value of 0.96%. Compared with traditional and improved Δlog <i>R</i> models, our model shows a higher accuracy of TOC prediction with <i>R</i> ² = 0.9181, which meets the requirements for predicting the distribution of the Chang 9 source rock. In the map of the Chang 9 source rock predicted by our model, the thickness of the source rock (TOC ≥ 1.0%) varies in the range of 1-12 m, showing a decreasing trend from northeast to southwest in the Longdong area. The crude oil in the northeastern areas enjoys a high ratio of 17α(H)-<i>C</i> ₃₀ rearranged hopane and <i>C</i> ₃₀ hopane (<i>C</i> ₃₀*/<i>C</i> ₃₀), and the thickness of the Chang 9 source rock is also greater than in other areas. It is speculated that the Chang 9 Member tight oil in the northeast area is mainly from the Chang 9 source rock, while the oil in other areas is from the Chang 7 source rock. In our study, we presented a method for predicting the source rock distribution, which can be widely used for exploring the tight oil of the Chang 9 Member in the study area.