The Cambrian and Ordovician carbonate reservoirs in Dongying Sag of the Bohai Bay Basin are highly heterogeneous due to multiple stages of tectonic activities and diagenesis they have experienced.Pore space and its development patterns in the Lower Paleozoic buried hills in Dongying Sag were systematically studies through an integration of logging

core

thin section

carbon and oxygen isotope

fluid inclusion as well as seismic data.The pore space is dominated by fracture

solution cavities and dissolved pore.The solution cavities are mostly of lime mud type and inter-breccia type.The fractures are mainly tectoclases and solution fractures.And the pores include inter-breccia dissolution pores

intra-vein dissolution pores and intercrystalline dissolution pores.The Lower Paleozoic buried hills in Dongying Sag are largely of residual mound-type and fault-block type.The former is distributed in Caoqiao and Zhengjia-Wangzhuang areas and nurtured weathering crust reservoirs with its vertical vadose zone and horizontal phreatic zone caused by unconformities.Pore space there is mostly high-angle fracture

inter-breccia solution cavity and horizontal layered solution cavity.They usually have massive epigenetic karst belt and are high-quality reservoirs for hydrocarbon with well-connected large fractures and pores.The hills dominated by limestone have epigenetic karst belts as thick as 170 m

while those controlled by dolomites have thinner epigenetic karst belts of only 70 m in thickness.The latter is distributed in Gaoqing-Pingnan and Wangjiagang areas.Influenced by multi-stage faulting and deep hydrothermal activities

this type of hills contains mostly reservoirs with pore space dominated by fractures near faults and on crest of folds

as well as dissolution pores and vugs developed along the fractures.