Distribution characteristics of carbonate cement of nearshore subaqueous fans in Upper Es4 of paleogene of Dongying depression

Key words:  carbonate cement  reservoir  nearshore subaqueous fan  Paleogene  Dongying depression

Abstract:

Distribution characteristics of carbonate cement were studied by the analysis of cores, thin-section,well logging et al in sand bodies of nearshore subaqueous fan of Upper Es₄ of Paleogene Shahejie formation in Yongan area of the northern steep slope belt in Dongying depression. The results show that high carbonate cement sandstones (>15%) are common in the thin fine sandstones of the outer fan of nearshore subaqueous fan and the slump turbidite fan. The low carbonate cement sandstones (<10%)are common in the thick sandstones of the mid-fan channel of nearshore subaqueous fan. Cement layers of the outer fan are of well continuity, while those of middle fan only distribute on the top and bottom of the channel sandstones with weak continuity and rich ankerite. The cement layers in middle fan and outer fan are generally thin, but those in inner fan are thick. The content of the limestone gravel and calcareous heterobase in conglomerate is high, and the compaction is strong. Feldspar in sand-bodies can buffer the acid formation water, while calcite cement layers are formed in the edge of the sand body and nearby regions. Cement layers usually correspond with the layer sections where plagioclase is dissolved and the current content of plagioclase is low. The strong heterogeneity of reservoirs of nearshore subaqueous fan is caused by calcite cementation layers and physically blocked lithologic reservoirs are formed.