Pore genetic types and their controlling factors in sandstone reservoir of Yanchang Formation in Longdong Area, Ordos Basin

The reservoir of Yanchang Formation in Longdong Area is of typical low porosity and low permeability. The analysis of pore types and their controlling factors is instructive for the prospecting of relatively high porosity and permeability reservoir. The pore types of Yanchang Formation in Longdong area are divided into primary pores and secondary pores based on thin section identification. Primary pores include post compaction residual porosity and post cementation residual porosity, and secondary pores include miscellaneous matrix solution pores, cement solution pores, solution pores within feldspar or detritus, mouldic pores and intergranular kaolinite and chlorite matrix crystal pores, and besides, there are small amout of microfractures of tectonic origin. The present pores are almost mixed pores consisted of primary pores and secondary pores. Pore genetic type is closely related to the sedimentary conditions and diagenesis. Sedimentary conditions control the hydrodynamics during the deposition, and further control the characteristics of the rock and the development of primary pores. Compaction process is decisive for the remnant pore sizes and pore volume left by the compaction, and cementation controls the content of residual pores after cementation. Dissolution controls the development of all kinds of dissolution pores, such as miscellaneous matrix solution pores, carbonate cement solution pores, solution pores in feldspar or detritus grain or of grain margin. Primary porosity are important in some coarser grain size members in Chang 8-9 and 4+5, while secondary porosity are important in finer grain size members as in Chang 6-7 and the top of Chang 1-3 near to the unconformity. On the basis of large amount of informations from rock thin sections and petrophysic studies, the present paper firstly proposed that the pore genetic type of the sandstone reservoir of the Yanchang formation in Longdong area is of mixed porosities, and the secondary porosity is probably superior to the primary porosity.