Original Sedimentary Pattern of an Inverted Basin:A Case Study from the Bozhong Depression,Offshore Bohai Bay Basin

The third member of Shahejie Formation （Sha-3 member; 42–38Ma of Eocene） in the Bozhong Depression, offshore Bohai Bay Basin was subject to multiple post-depositional modifications. The present structural framework of the Bozhong Depression, which is characterized by sags alternating with uplifts, does not reflect its original sedimentary pattern. Previous studies have not discussed the post-depositional modification of this succession, including the sedimentary pattern variations and the depositional geodynamic setting. This work determined the characteristics of the post-depositional modification and original sedimentary pattern of the Bozhong Depression through analysis of seismic data, well-log data and fission-track ages. The results demonstrate that the Shijiutuo rise, a major structural feature of the current basin, did not exist during the major depositional stage of the Sha-3 member, when the Qinnan sag was largely connected to the Bozhong sag to form a single contiguous deposition area within the basin. By contrast, the Shaleitian and Chengbei rises, located in the western part of the Bozhong Depression, have existed before the depositional period of the Eocene Sha-3 member; these features were manifested as syn-depositional tilted fault blocks, the uplifted footwall blocks of which provided sediments for the neighboring Shanan and Chengbei sags. The western part of the Bonan low rise, located in the southern part of the Bozhong Depression, did not experience uplifting during the depositional phase of the Eocene Sha-3 member. The Huanghekou sag was connected with the Bozhong sag in the western part of the Bozhong Depression. The original sedimentary boundary of the southern Miaoxi sag possibly extended eastward about 10 km and connected with the Bozhong sag at its northern part. The present-day Bodong low rise, which is bounded by the Tan–Lu fault zone, also formed after the depositional period of Eocene Sha-3 member. It is thus concluded that the Bozhong Depression formed a connected large-scale sub-basin during the depositional stage of the Eocene Sha-3 member. Several neighboring sags that are now separated by rises, including the Qinnan, Shanan, Chengbei, Huanghekou, Miaoxi and Bodong sags, formed a single contiguous depositional area during the Eocene. The significant differences between the present and original basin patter and framework provide valuable information for better understanding the history of basin inversion and its impact on related hydrocarbon-system evolution.

Correlations between Shortening Rate,Uplift Rate,and Inversion Rate in Central Inversion Zone of Xihu Depression,East China Sea Basin

Late Miocene shortening rate, uplift rate, and inversion rate in the central inversion zone of the Xihu （西湖） depression, East China Sea basin, were independently determined from a large number of 2D reflection seismic data and methods. Shortening rate was estimated from the 2DMOVE balanced cross-section technology, uplift rate was calculated after absolute erosions were determined from seismic data during the uplifting time, and inversion rate was measured using the improved calculation method by Song （1997）. The cross correlations among shortening rate, uplift rate, and inversion rate show a good positive relationship, with some differences existing in local areas. This article analyzes the cross correlation between these structural rates and discusses the dynamics of mechanisms for basin inversion and their influence on hydrocarbon accumulation.

Geochemical Modeling and Statistical Analysis for Groundwater Evolution Assessment in Wadi Qasab, Sohag, Eastern Desert, Egypt

Qasab basin is one of the most promising areas for the sustainable development in the Eastern Desert fringes of the Nile Valley, Egypt. The integration between statistical analysis, stable isotopes as well as geochemical modeling tools delineated the geochemical possesses affecting groundwater quality and detected the main recharge source in Qasab basin. The most of groundwater samples are brackish (88%), while the minority (12%) of the samples are fresh. The electrical conductivity of groundwater ranged from 1135 to 10,030 μS/cm. The statistical analysis and hydrochemical diagrams suggest that the groundwater quality is mainly controlled by several intermixed processes (rock weathering and agricultural activities). The mineralization of the Pleistocene groundwater is regulated by the rock weathering source, evaporation processes and reverse cation exchange. The isotopic signatures (δ2H and δ18O) represent two groundwater groups. The first group, is enriched with the isotopic signature of δ18O, which ranges from 0.9‰ to 5.5‰. This group is mostly affected by the recent meteoric recharge from the surface water leakage. The second group, is relatively depleted with the isotopic signature of δ18O, reflecting a palaeo recharge source of colder climate. The δ18O‰ varies from -10.1‰ to -6.4‰, indicating upward leakage of the Nubian sandstone aquifer through deep seated faults. The inverse geochemical model reflects that the salinity source of the groundwater samples is due to the leaching and dissolution processes of carbonate, sulphate and chloride minerals from the aquifer matrix. This study can demonstrate the hydrochemistry assessment guide to support sustainable development in Qasab basin to ensure that adequate groundwater management can play to reduce poverty and support socioeconomic development.