Microscopic Reservoir Characteristics of Laminated Shales: A Case Study of Wulalike Formation in the Western Margin of the Ordos Basin

Key words: 
• Ordos Basin /
• Wulalike Formation /
• shale gas /
• laminated shales /
• pore type /
• pore structure

Abstract: [Objective] Microscopic reservoir characteristics of laminated shales in the third member of the Middle Ordovician Wulalike Formation, western margin of the Ordos Basin have been studied to further understand the effectiveness of laminated shale reservoirs.[Methods] This study employs an integrated approach of X-ray diffraction (XRD), scanning electron microscopy (SEM) and mineral mapping, organic geochemical analysis, CO2 and N2 adsorption and mercury injection capillary pressure tests to comparatively analyze differences in mineralogy, organic geochemistry and pore structures between different textures within laminated shales obtained by mechanical separation.[Result] The laminated shales consist of limestone and tuff laminae and silica-rich beds. Limestone and tuff laminae with density of 60~180/m have lower contents of organic carbon than silica-rich beds. Laminated shales contain both pore and fractures. In silica-rich beds, clay minerals-related interparticle and intraparticle pores are abundant with rare organic pores. Calcite and dolomite intraparticle dissolved pores are dominant pore type in limestone laminae. Tuff laminae contain pyrite intercrystalline pores, clay-related interparticle and intraparticle pores, and dolomite intraparticle dissolved pores. Dissolved pores of carbonate minerals in limestone and tuff laminae are related to dissolution of acid fluids generated by organic matter thermal evolution in silica-rich beds. Near-horizontal fractures develop along boundaries between limestone and tuff laminae and silica-rich beds. Density of the near-horizontal fracture is 63~130 /m, and their width is 0.2~4.9 mm. Limestone and tuff laminae have less abundant micro-pores but more abundant macro-pores than silica-rich beds. The volume of macro-pores in limestone laminae is 2.5~4.3 times, and the contribution of macro-pores to total pore volume is 1.9~2.1 times as much as adjacent silica-rich beds. The volume of macro-pores in tuff laminae is 1.5-2.3 times as much as adjacent silica-rich beds. [Conclusion] In laminated shale reservoirs, oil and gas generated in silica-rich beds migrated along pores and fractures, and preferentially enriched in adjacent limestone and tuff laminae. Limestone and tuff laminae contain more abundant macropores and more free gas than silica-rich beds. The Wulalike Formation laminated shale reservoirs have high proportion of free gas, which are related to limestone and tuff laminae.