Identification of fluid types and their implications for petroleum exploration in the shale oil reservoir: A case study of the Fengcheng Formation in the Mahu Sag, Junggar Basin, Northwest China

The Fengcheng Formation in the Junggar Basin is a typical shale oil reservoir. Fluid type identification and controlling factors on fluid combination types are significant challenges in shale oil exploration and development. By using two-dimensional (2D) nuclear magnetic resonance (NMR) technology, the accurate boundary values of each fluid type can be determined. Combined with thin sections, scanning electron microscopy (SEM), cathode-luminescence (CL), X-ray diffraction (XRD) and NMR, the effect of lithologies on fluid combination types has been evaluated. The results show that a total of six fluid types can be identified in the Fengcheng Formation, including free oil, adsorbed oil, kerogen, hydroxyls, adsorbed water and free water. Their boundary values on the NMR T1-T2 map can be determined by comparing fluid signals under the original state and water-saturated state after oil washing. In this study, different fluid types eliminated kerogen and hydroxyls can be combined into four fluid combination types defined as Type 1 (free oil, adsorbed oil and adsorbed water), 2 (free oil, adsorbed oil, free water and adsorbed water), 3 (adsorbed oil, free oil and free water) and 4 (movable fluids), existing in the dolomitic shale, mixed shale, felsic shale and sandstone, respectively. Meanwhile, the geometric means of T2 (T2gm) and T2 curves indicate that the pore size can effectively distinguish different lithologies and can be considered as an essential factor influencing the occurrence state of fluids. As the average pore size increases, free water emerges in the mixed shale, and adsorbed water disappears in the felsic shale. Furthermore, after the lithology transforms into sandstone, the fluid type is simple, and only movable fluids exist. Finally, the oil test results imply that the reservoirs with fluid combination Type 4 and Type 3 have relatively high oil production. Therefore, the subsequent orientation for petroleum exploration in shale oil reservoirs targets the sandstone and felsic shale interlayers in the Fengcheng Formation.