In 2021,high production gas flow was obtained in tight limestone of Lower Permian Taiyuan Formation in Well A,a risk exploration well.Through integrated exploration and development,Hengshan gas field,the first limestone gas field in Ordos Basin,was discovered with a submitted reserve of over 200 billion cubic meters.It is a replacement field of reality for increasing reserves and production in the Changqing Oilfield.Based on data of cores,casting thin sections,scanning electron microscopy,physical properties,logging,and gas testing,comprehensive research on reservoir characteristics such as sedimentary facies,petrology,reservoir space and diagenesis is conducted to analyze the formation mechanism and the distribution pattern of favorable reservoirs.The results show that:(1)The lithology of limestone reservoir is mainly composed of bioclastic very finely crystalline limestone,algal bound limestone and bioclastic micritic limestone.The storage space is mainly composed of dissolution pores,residual organism cavities,intergranular pores and microcracks.Dissolution pores,nano-submicron scale intergranular micropores and fractures form a three-dimensional network storage space,which is the key to form a tight limestone gas reservoir in the Taiyuan Formation.(2)The limestone reservoir is mainly formed in the intermittent exposure of bioclastic shoals under the influence of karst during the quasi contemporaneous period.The bioclastic shoal are the material basis for reservoir formation,determining the type and planar distribution of reservoir space.Quasi syngenetic karstification is conducive to the formation of dissolution pores and can effectively improve the reservoir storage performance.Cracks play a crucial role in improving the permeability of tight limestone reservoirs.(3)Controlled by sea level cycles and paleogeomorphology,the favorable limestone reservoirs of Taiyuan Formation are vertically developed in Xiedao Member,and horizontally developed with a pattern of"around depression and along highland",especially with larger reservoir thickness at the slope due to lateral stacking under the background of regression.This study will help to promote a further understanding of biogenic limestone of Taiyuan Formation in North China,and have important implications for the exploration and development of ancient carbonate rock in China.