Modification Effects of Bioturbation on Reservoirs: A Case Sudy of the Member 4 of Ordovician Majiagou Formation in Ordos Basin

The bioturbated reservoirs in Member 4 of the Middle Ordovician Majiagou Formation (Ma 4) in the Ordos Basin represent one of the primary carbonate reservoir types in the study area. Key challenges include assessing the impact of bioturbation on reservoir properties and developing a refined classification framework. This study conducted a systematic ichnological and sedimentological analysis of Member 4 of the Ordovician Majiagou Formation in the Ordos Basin. By integrating lithofacies analysis with reservoir geological characteristics, comparative and synthetic analyses were performed, leading to the following findings: (1) The Glossifungites ichnofacies is developed in Member 4 of the Majiagou Formation, which exhibits a strong correlation with reservoir development. The reservoirs in Ma 4 can be further into three categories: Class I, Class II and Class III, based on integrated ichnofacies, ichnofabric data, and petrological analyses. Specifically, Class I reservoirs are located in the upper part of the Glossifungites ichnofacies, Class II reservoirs are in the middle part, and Class III reservoirs are in the lower part. (2) Multiple transgressive-regressive sequences are identified within the stratigraphic succession. In regressive phases, intense bioturbation generates complex burrow systems that enhance dolomitization and promote the development of effective reservoir porosity, showing a strong association with high-quality Class I reservoirs. In contrast, with simpler burrow structures and reduced bioturbation intensity, transgressive phases are marked by limited reservoir space development and lower reservoir quality. (3) By integrating the bioturbation index with the refined ichnofacies classification, high-quality Class I reservoirs are predominantly located in the middle interval of sub-member Ma42 and near the boundary between the Ma42 and Ma43 sub-members. Class II reservoirs occur primarily in the middle to lower part of sub-member Ma43 and the middle portion of sub-member Ma41. This research can provide certain reference value for the classification work of bioturbation reservoirs.