Factors controlling the occurrence, distribution and physical properties of the Ordovician palaeokarstic reservoir in the Ordos Basin Daniudi gas field

Abstract

The Daniudi gas field in China's Ordos Basin is located in the transitional zone between palaeo‐topographic highs and slopes, whereby dissolution‐collapse breccias are widespread within the palaeokarstic reservoir. Abundant cores, well logs and thin sections were used to study palaeo‐topography, palaeokarstic breccia types, diagenesis and reservoir pore characteristics. The results indicated that reservoir quality is controlled by lithology, palaeo‐topography and diagenesis. The extent of palaeokarstic reservoir dissolution was shown to be determined by lithologic differences, whilst only gypsiferous dolostones and finely crystalline dolostones emerged in reservoir formation. Palaeo‐topography was also found to regulate reservoir development. The reservoirs are primarily located 20 m below the Ordovician unconformity in palaeo‐topographic hills and palaeo‐topographic slopes, whilst their depth is palaeo‐topography dependent. Lithology and palaeo‐topography also coordinate the distribution of palaeokarst breccia facies. Clast‐supported and matrix‐supported chaotic breccias were found in argillaceous dolostone strata. However, crackle–mosaic breccias occurred in gypsiferous dolostones and crystalline dolostones, located in palaeogeomorphic residual hills and slopes. Reservoirs generally develop in crackle–mosaic breccias of Ma51 and Ma52 sub‐Members, the quality of such reservoirs is dependent on the development and distribution of palaeokarst breccia facies and diagenesis. Dissolution of dolomite and anhydrite nodules was found to result in moldic pores and dissolution‐enlarged pores. Overall, the most damaging effect on reservoir quality resulted from calcite cement infilling moldic pores, enlarged pores and fractures.