Relationships among composition, porosity and permeability of Longmaxi Shale reservoir in the Weiyuan Block, Sichuan Basin, China

Shale composition might influence the petrophysical properties of shale reservoirs. The relationships among composition, porosity and permeability of Longmaxi Shale in Weiyuan area, Sichuan Basin, China, were systematically investigated by the integration of geochemical, mineralogical and petrophysical data. The cumulative pore volume of the organic-rich Longmaxi Shale is mainly contributed of pores with diameter of 2 nm-100 nm. The total organic carbon (TOC) content is the main control on porosity, as the percentage of organic porosity is > 60% for the organic-rich shales. The clay content is positively correlated with porosity, partly due to the clay associated interparticle pores and pores developed in organo-clay complex. Carbonate and quartz content dilute the TOC and clay content, which accounts for their negative relationships with porosity. In terms of shale composition, the clay content exerts the strongest influence on permeability. At low clay contents (< 30%), the clay content shows a positive correlation with permeability due to the good pore connectivity caused by pores developed in organo-clay complexes, whereas at high clay content (> 30%), the clay content exhibits a negative impact on permeability because abundant clay minerals block pore throats, reduce the resistance to mechanical compaction and result in low permeability. The influence of the pore network on permeability is evident from a positive correlation between the pore volume percentage of meso-pores (VOLmes) and permeability, which is due to (a) fine and coarse meso-pores including a large fraction of organic pores, which have better connectedness than other interparticle (mostly > 50 nm) and intraparticle (mainly isolated) pores; and (b) large quantities of fine and coarse mesopores increasing the capacity to access other pores, which form the dominated pathways for gas flows. According to the above illustrated relationships, appropriate porosity and permeability indicating fine storage capacity and favorable flow capacity occur in the Longmaxi Shale in Weiyuan Block, which implies good reservoir quality and excellent exploration prospects in this region.