Discovery of Tongwan unconformity in South Yellow Sea and its geological implications to oil and gas resources

The Tongwan unconformity marks the Cambrian-Sinian disconformity due to the combination of the compression at the end of the Sinian Period between the South China Block and the Paleo-Tethys lithosphere, and the global Baikonur glaciation. The unconformity is widely distributed in the Yangtze region, beneath which large and giant gas fields have been discovered in the weathered crust reservoir of dolomite at the top of the Sinian Dengying Formation in Sichuan Basin, making the unconformity very important in geoscience and petroleum industry. The Southern Yellow Sea represents the marine extension of the Yangtze Plate and constitutes the main body of the Lower Yangtze Block. In this article, we utilized the latest drilling data and years of seismic data collection and new research results from the "High coverage, Rich low-frequency, and Strong seismic source (HRS)" project in the Southern Yellow Sea, the Tongwan unconformity was recognize in multiple seismic profiles across the Laoshan fault uplift zone. Clear truncation of the strata below the unconformity was observed, and above the unconformity shows notable stratigraphic onlapping. A comprehensive interpretation of geological and geophysical data, along with marine-continental comparisons suggested that the geological evolution before and after the Tongwan unconformity produced a complete hydrocarbon reservoir-cap assemblage. Beneath the unconformity, the weathered crust reservoir of the Dengying Formation was developed, while the organic-rich mudstone of the Lower Cambrian as high-quality source rock and regional cap rock directly overlie the unconformity. Moreover, the unconformity itself acted as a crucial pathway for hydrocarbon migration. Therefore, the discovery of the Tongwan unconformity in the Southern Yellow Sea shall have significant implications to deep oil and gas exploration in the Sinian System of this marine area.