Organic matter enrichment mechanism of different types of source rocks in the Baiyun sag

The source rocks of the Paleogene Wenchang Formation and the Enping Formation in the Baiyun sag are complex. Although the semi-deep lacustrine, shallow lacustrine, terrigenous marine, and deltaic facies have been identified, the mechanism of organic matter enrichment is unknown, leading to long-standing disagreement about the main source rocks in the area and their potential. This study analyzed the main trace elements and the characteristics of strawberry-shaped pyrite grain size in different types of source rocks that have been drilled and analyzed the development and preservation conditions of organic matter in different types of source rocks. It revealed the organic matter enrichment mechanism of different types of source rocks in the Baiyun sag for the first time. The results show that the primary productivity level of shallow lacustrine source rocks in the Baiyun sag is high. The distribution range of strawberry-shaped pyrite grain size in the Wenchang Formation is 0. 4 14. 3 rim with a wide distribution range and an average particle size of 6. 8 rim, while the distribution range of strawberry-shaped pyrite grain size in the source rock of the Enping Formation is 4. 9-15. 9 rim with an average particle size of 8. 7 rim; the overall environment is sub-oxidation. The primary productivity level of the semi-deep lacustrine source rocks is lower than that of the shallow lacustrine source rocks, and the distribution range of the strawberry-shaped pyrite grain size is narrow, ranging from 5. 6 to 8. 0 rim with an average particle size of 5. 8 rim, which is in a sulfide-sub oxidation environment with good preservation conditions for organic matter. The primary productivity level of terrigenous marine source rocks is relatively low, the preservation conditions of organic matter are good, and they receive terrigenous nutrient inputs, resulting in the enrichment of organic matter. The deltaic source rocks received a large amount of nutrients from terrigenous clastic inputs, resulting in high sediment accumulation efficiency and rapid burial. The average grain size of strawberry-shaped pyrite is > 7. 0 rim and is characterized by euhedral pyrite in an oxidized-strongly oxidized environment with poor preservation conditions for organic matter. With a large amount of source input and accumulation efficiency, it also forms a favorable area for organic matter enrichment.