Influence of Feldspar Dissolution on the Pore Structure and Characteristics of a Tight Sandstone Reservoir: A Case Study From the Northeast Margin of Ordos Basin, China

The Ordos Basin is the main tight gas-producing basin in China. Under the background of strong heterogeneous physical property, there is still high porosity and permeability "sweet spot" in tight reservoirs. Feldspar dissolution exerts a significant effect on porosity/permeability, while its genetic mechanism is unclear, which restricts the prediction of effective reservoir. In this paper, genesis of feldspar dissolution and its effect on reservoir heterogeneity were studies. Carboniferous-Permian Formation, which is the main gas-producing strata in northeast margin of Ordos Basin, is taken as the target. Based on the methods of thin section observation, physical property test, inclusion, isotope and productivity analysis, mechanism of feldspar dissolution, and its positive modification to tight sandstone reservoir are studied. The results show that the target sandstone is dominantly made up of litharenite and feldspathic litharenite. About 98.6% of the samples possess permeability less than 1 mD, while 89.7% of the samples possess porosity less than 10%. The high porosity/permeability space within tight reservoirs is dominated by dissolved pores, accounting for more than 80% of the total pore space. The dissolution minerals are mainly associated with feldspar, and the amount of feldspar dissolution is positively correlated with porosity and permeability. According to the fluid chemical information of quartz overgrowth and ankerite (related to feldspar dissolution), feldspar dissolved pores are caused by organic acid derived from the thermal maturity of organic matter. According to production data, no daily production can be achieved in single well, when the porosity and permeability are less than 7% and 0.1 mD, respectively. However, once it exceeds this porosity/permeability threshold value, the production capacity is exponentially improved. Furthermore, when surface porosity of feldspar dissolution exceeds 7%, most values of the porosity and permeability exceed the threshold values of the allowable productivity, indicating that extensive feldspar dissolution is favorable factor for effective reservoir space development in tight reservoirs.