Sedimentary characteristics of the Shahejie Formation in the Zhangdong district, Chenghai Oil Field, Hebei

The Zhangdong district, Chenghai Oil Field, Hebei is very complicated in geological structures and thus less explored in petroleum exploration. The authors in the present paper give a detailed division and description for the Shahejie Formation as main oil-bearing rock series in the Zhangdong district on the basis of grain sizes, lithology, sedimentary structures, sedimentary microfacies and well logs. The third member of the Shahejie Formation displays the typical normal graded structures, and belongs to the delta front subfacies including the channel mouth bar, subaqueous distributary channel and delta front sheet sands microfacies. The sandstone reservoirs are characterized by moderate porosity to moderate permeability, moderate porosity to low permeability, low porosity to extra-low permeability, and intergranular, granular and intragranular solution openings generated during the A stage of late diagenesis. The second member of the Shahejie Formation is organized in the delta (littoral to shallow-lake)environment in the lowstand tracts system, and occurs as the best favourable hydrocarbon reservoirs and target area in the Zhangdong district. The first member of the Shahejie Formation is built up of the dolostone-marl-oil shale couplets, locally with thin-bedded siltstones as good cap rocks and source rocks deposited in the relatively enclosed stationary lagoon environment.