Pore Characteristics and Factors Influencing the Oil Content in the Chang 7 Shale of the Yanchang Formation in the Dingbian–Yanchang Area of the Ordos Basin, NW China

ABSTRACT Organic‐rich shales in continental basins in China have strong reservoir heterogeneity, and their development of pore size is controlled by a variety of factors with direct influence on shale oil sweet spots evaluation. Taking as an example the continental shale of the Chang 7 member in the Ordos Basin Yanchang Formation, through experiments such as X‐ray diffraction (XRD), field emission scanning electron microscopy (FE‐SEM), low‐temperature nitrogen adsorption (LPNP) and high‐pressure mercury compression (MICP), the pore distribution characteristics, the causes and the controlling factors of Chang 7 shale were studied, for which the correlation between pore size and oil content was also discussed. The results reveal that the Chang 7 shale is associated with three main lithofacies types: massive layered felsic shale facies (MLFSF), laminated felsic shale facies and lamellar clay shale facies (LCSF). The types of pores include organic matter pores, dissolution pores, intergranular pores, intercrystalline pores and microcracks, with pore sizes ranging from several 100 nm to more than 10 μm. The pore morphologies were mainly ink‐bottle (H2), parallel‐plate (H3) and slit‐like pores (H4), while pore size distributions were dominated by unimodal type, and pore sizes of 10–100 nm contributed most to pore volume. Pore development is increased by quartz, feldspar, chlorite films and R o . In contrast, clay minerals with high total organic carbon (TOC) content have an inhibiting effect on pore development. The S 1 is controlled mostly by the mineral content, average pore size, pore volume, specific surface area, TOC, HI and R o . The average pore size and mineral content, particularly for clay minerals and pyrites with TOC and HI, are the major factors in increasing the S 1 , and a higher T max and R o will reduce the S 1 content. The selected OSI, S 1 and TOC contents are the major parameters for sweet spot evaluation. The shale in the Chang 7 member is classified into five types: enriched resources, medium enriched resources, dispersed resources, low‐efficiency resources and ineffective resources, in which the LFSF lithofacies is mostly the enriched resource type, which is the target for the next step of exploration.