An analysis of favorable reservoir characteristics of Chang 10 Formation in the Ansai oilfield, Ordos Basin

The cements of feldspathic sandstone in Chang 10 formation mainly include such types as laumontite, quartz overgrowth, chlorite and calcite. Compaction was intense in early diagenesis because the formation water changed into acidic media and particles lacked the support of cement. Secondary pores were formed in Middle diagenesis because the acidic water derived from the mudstone of lake facies dissolved cements, feldspar, and lithic grains during the migration process from source rocks to sandstones, thus forming the favorable reservoir. In the vertical direction, the reservoir dissolution in unconformity and sequence boundary was intense, and secondary pores were developed. Upwards, the dissolution gradually became weaker, and hence secondary pores were not developed. In the plane, the dissolution became gradually weaker from the hydrocarbon center to the shore. The favorable reservoirs of Chang 10 formation in the Ansai oilfield are mainly distributed in high-energy distributary channel of the delta plain, and a large number of secondary pores formed by dissolution become beneficial facies of the favorable reservoir.