Development models of two types of source rocks in the composite half-graben basin：a case study of Yueguifeng Formation in the Lihsui Sag

To clarify the development model of source rocks in the composite half-graben basins, we studied the Yueguifeng Formation in the Lishui Sag. The tectonic background and sedimentary facies were analyzed, and representative wells were selected from different tectonic units and sedimentary facies zones. The source rocks in organic petrology and geochemistry were compared. Results show that the enrichment of organic matter in the source rocks of Yueguifeng Formation was directly related to the amount of organic matter input and preservation conditions. Two enrichment models were developed, namely, the prodelta mixed input type and the mid-deep lacustrine in-situ preservation type. The enrichment degree of the two types of source rocks in half-graben basins was different in each sub-sag due mainly to the variation of the sedimentary “source-to-sink” systems around different sub-sags. In the mixed input type, the degree of organic matter enrichment under weakly oxidizing environments depended on the amount of terrestrial organic matter in the delta, and the dominant kerogen type was II₂-III. This type of source rock is generally distributed in the western Lishui Sag in prodelta facies. In the in-situ preservation type, the accumulation of organic matter in a mildly reduction environment was controlled by the growth of local aquatic organisms and preservation conditions. The kerogen type is mainly type II₂, and the source rock of this type is distributed in the central area of the sag under the control of the growth of syn-rift faults. The western part of the Lishui Sag faced a vast Zhejiang-Fujian uplift to the west, with braided river delta development and large sediment injection, resulting in a small scale of development of type II₂ source rocks, while the eastern part of Lishui Sag was in-between the Lingfeng Low uplift and Yandang uplift, receiving small-scale input from point-sourced rocks, thus the development scale of this type of source rock is relatively large.