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High Frequency Lake-Level Change of 3~(rd) Member of Shahejie Formation and Prediction of Lowstand Sand Body in Dongpu Depression

JI You-liang 1, FENG Jian-hui 2, WANG Sheng-lang 2, ZHANG Hong-an 2, WANG De-ren 2 (1. Department of Geology and Geophysics, Tongji University, Shanghai 200092; 2. Reserch Institute of Exploration and Development, Zhongyuan Oilfield Company, SINOPEC, Henan 457001, China)  
Recent study of lithology and sedimentary characters of the 3 rd member of Shahejie Formation of the Lower Tertiary in Dongpu Depression, has revealed many evidences indicating shallow water sedimentation and evaporation sequence, which could be used to interprete. The origin of the halite and gypsum interlayered in black colored mud located in the basin center. We is believe that the black colored mud is formed during high-level stage and that the halite and gypsum are deposited during low-level stage caused by evaporation. The paleowater depth is estimated from the data such as the character of sediments distribution, the species and diversity of Ostracode. According to the calculation and estimation, the maximum depth of lake water is 30 m. In addition, some evidences indicating frequently fluctuating of the lake level during the sedimentation of the 3 rd member of Shahejie Formatoin have been found, which are: (1) the frequent alternation of black colored mud and gypsum and halite; (2) 'rubble-made sand' in the sandstone, which indicates that the sand deposited during the highstand time was eroded into sand rubble and was transported, deposited with sand during the lowstand time; (3) In the sandstone interbeded with black mud of deep water environment, sandstong with cross bedding that was deposited in shallow water environment and current flow was found. The data of lithology, well log and seismic reflection character and the estimated paleowater depth were used to study the lake level fluctuation during the sedimentation of 3 rd member of Shaheje Formation. It is believed that in Dongpu Depression, during the sedimentation of 3 rd member of Shaheje Formatoin, the lake 1evel fluctuated frequently with large scale cycle and small scale cycle. According to the data of well log, response of resistivity and core, cycles of 6 scales were recognized, and the frequency of 6 th scale cycle is 1000/Ma. The minimum extent of lake level fluctuation is calculated according to the thickness of single halite bed or single gypsum bed. Combined with the palaeotopography character, the prossess of large transference of lakeshore line in landscape orientation has been explained. Basied on this study, a dynamic sedimentary modle was eastablished. As revealed in this modle, delta and bech sandbody were deposited along the shoreline and black mud with thin layer carbonate rock was deposited in the deposition center during the highstand time. During the lowstand time, the sandbody previously deposited along the shoreline was eroded and was prograded toward the sedimentation center of the basin, and during the weathering time, salt and gypsum rock was deposited in the center of deposition of the basin.
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