Shale reservoirs in the Doushantuo Formation in western Hubei:Porosity and its controlling factors

The pore structures of shale reservoirs are considered as the basis for the evaluation of shale gas resources. In the light of organic carbon content, asphalt reflectance, X-ray diffraction analysis, field emission scanning electron microscopy (FE-SEM) analysis and low temperatures nitrogen absorption method, the present paper gives a detailed discussion of the compositions, pore structures and controlling factors of the shale reservoirs in the Doushantuo Formation in western Hubei. The Doushantuo Formation is mainly made up of siliceous and calcareous shales. The total organic carbon contents vary from 3.29% to 6%, indicating the I-type kerogen and high maturation-overmaturation stages of the source rocks. The organic pores in the shales from the Doushantuo Formation are less developed, and the inorganic pores have provided the main storage spaces, including intergranular pores, interlayer pores and partial solution openings within the brittle minerals and clay minerals, as well as a small amount of microfractures. On the whole, the pore diameters range between 1.1 nm and 284 nm, and micropores, mesopores and macropores have the volumes of 0.005, 0.023 and 0.006 ml/g, respectively, with an average of 0.034 ml/g. There is no significant correlation between the total organic carbon contents and pore volumes in the shales from the Doushantuo Formation, indicating that the organic pores contribute little to the pore volumes. On the contrary, the contents of siliceous and clay minerals are positively correlated with the pore volumes, indicating that the mineral compositions are the main controlling factors of pore development in the Doushantuo Formation. The shales in the Doushantuo Formation in western Hubei may be considered potentially prospective for shale gas because of good hydrocarbon generation conditions and shale gas storage spaces.