Superimposing Controls of Acidic and Alkaline Dissolutions on Sandstone Reservoir Quality of the Paleozoic Xiashihezi and Shanxi Formations in Tabamiao Area, Ordos Basin

The main factors controlling the quality of the sandstone reservoirs of the Xiashihezi and Shanxi formations in Tabamiao area, Ordos basin, are discussed through the synthetic investigation of petrologic observation of thin sections, the restoration of burial history, fluid inclusion measurement, a comparison of formation water chemistry with clay minerals and porosity, and a statistical analysis of permeability. The study reveals that at about the end of the Early Cretaceous, the primary porosity of the sandstones seemed to be almost lost due to deep burial (about 4 500 m) and strong compaction, causing the soft debris deformation to fill the intergranular pores. However, the acidic dissolution of carbonate minerals and some feldspar, which occurred during the middle diagenesis, produced some secondary pores. During the late diagenetic stage, the alkaline dissolution raised in quartz grains and aluminosilic debris of the same sandstones to form a superimposed secondary porous belt, which has been thought to be the main controlling factor on reservoir quality. The profile of clay mineral contents coupled with formation water chemistry, porosity and permeability, contents of debris, calcite, authigenic quartz and matrix clearly indicates that there is a very important contact at about 2 670 m. At this depth, an acidic dissolution in sandstones developed, and beneath this depth, the sandstones underwent the late alkaline dissolution which superimposed on the early acidic dissolution. The strongest dissolution belt occurs between 2 670 and 2 740 m, and was filled with natural gases. Attention should therefore be paid to this belt in future exploration and development.