Geological features and hydrocarbon reservoir of unconformity structures in Jiyang Depression

The observation of core and micro research of mineral and element show that the unconformity structures in Jiyang Depression can be divided into three parts of weathered clay,semi-weathered rocks and un-weathered rocks.The lamination of weathered clay disappeared,and the feldspar and calcite within the rocks were altered thoroughly.The clay mineral and elements such as silicon,iron and aluminium are rich,but calcium and magnesium are leaching out.The porosity and permeability became worse because of post-diagenesis.In the semi-weathered rocks,some calcite and feldspar are still remained,and secondary clay minerals formed.The degree of element enrichment and loss in the semi-weathered rock is lower than that in the weathered clay.The dissolved pore and fractures are well developed,and the porosity and permeability conditions are better.The un-weathered clay and semi-weathered rock are the good reservoir-seal assemblage,which provides the favorable place for hydrocarbon migration and accumulation.