Drilling �� Production Co�� of Subsidiary of CNOOC Energy Technology �� Services Limited��Tianjin 300452��China

By methods of thin section��scanning electron microscope and X- ray diffraction��the authors ana- lyzed the controlling effects of the early sedimentary water medium��CO₂ fluid��atmospheric water��and overpressure fluid��as well as the hydrocarbon fluid to clay mineral in reservoir�� Evidence showed that the sedimentary water me- dium played a decisive role in determining the constitution of clay minerals: the sand reservoirs deposited in brack- ish water lake��which often accompany with higher illite content; while the sand reservoir deposited in fresh water environment��which often accompany with higher kaolinite content�� The invasion of CO₂ fluid could bring obvious reservoir reconstruction and thermal fluctuation�� the S percentage content of I/S in the CO₂-bearing reservoir is low- er than mudstone at approximate depth��and the content of kaolinite in the reservoir is higher�� The clay evolution in under- compaction overpressure reservoir is obviously lag behind��indicating that the S% content in I/S is lower than the same depth of mudstone as well as normal pressure reservoir�� After hydrocarbon invasion��it is often accompanied by feldspar dissolution and the higher content of kaolinite�� The feldspar dissolution may result from the organic acids migrating with hydrocarbon��the higher content of kaolinite may result from the protection from the inert diagenesis environment influenced by the hydrocarbon invading��and the reservoir with high content of kaolinite layers often correspond to the oil- bearing layer and the high permeability layer�� ��eservoir influenced by atmospheric water is characterized by feldspar dissolution and kaolinite enrichment��and generally accompany with high porosity��