It has been frequently reported that in unconventional shale formations, fractured well productivity can be dramatically reduced by severe proppant embedment due to a reduction in fracture aperture and conductivity. However, the mechanisms of this decline are poorly understood. In the absence of this understanding, very few models take proppant embedment into account when predicting production decline. In this study, we present both new experimental techniques and analytical analysis to explore proppant embedment mechanisms and to quantify stress dependent elastic and plastic deformation, as well as time dependent creep deformation. Two independent experimental setups have been employed for this purpose. To determine elastic and plastic deformation a constant displacement test is conducted while monitoring load in a simple and novel experimental setup. Creep deformation under constant load while monitoring the displacement is combined with these measurements to extract parameters for elastic, plastic and creep deformation from these two experiments. Results show that plastic and creep deformation dominate proppant embedment in shale (over 75%), while elastic deformation is usually small (less than 15%). 1. INTRODUCTION In the petroleum industry, hydraulic fracturing has been used to produce hydrocarbons from unconventional shale reservoirs. These fractures have been created by a pressurized fluid with proppants. Proppants have been used to maintain conductive paths for hydrocarbons to flow. Once the pumping of the fracturing fluid is stopped, created fractures begin to close. Fracture closure is a function of many different phenomena, including proppant fines generation and migration, proppant crush resistance, proppant embedment into the fracture surface, reorientation of proppants during stress variation and proppant flow back (Economides and Nolte, 1989; Sato and Ichikawa, 1998; Reinicke et al., 2006, 2010; Terracina et al., 2010; Alramahi and Sundberg, 2012). Also, closure stress, proppant size, concentration, and distribution, formation hardness, surface roughness (Volk et al., 1981), water saturation, dynamic fluid leak-off, cyclic loading conditions (Lacy et al., 1998), fluid viscosity (Lacy et al., 1997), shale mineralogy (Alramahi and Sundberg, 2012), fracturing fluid effect (Corapcioglu et al., 2014), elastic, creep deformation (Guo and Liu, 2012), and pumping strategy (Huang et al., 2019) are other factors that affect proppant embedment.