Large-scale commercialization of drop-in biofuel technologies requires a deeper understanding of the molecular structure of biocrude oils and their compatibility with fossil crudes in terms of molecular interactions that govern miscibility. For the first time, the compatibility of hydrothermal liquefaction (HTL) derived biocrude obtained from pinewood with straight-run gas oil (SRGO) was comprehensively investigated by theoretical prediction using Hansen double sphere plots and experimental confirmation from miscibility studies to achieve a biofeed compatible for coprocessing at refineries. The Hansen solubility parameters (HSPs) for biocrude, biocrude components (residue and light and heavy distillate fractions), and SRGO were determined by plotting a three-dimensional Hansen solubility sphere plot based on the experimental solubility data obtained on their solubility studies in 38 different solvents. The compatibility of HTL biocrude oil with SRGO was verified from the solubility distance (Ra) and relative energy difference (RED) values obtained from the center of their Hansen spheres and difference in HSPs, respectively, in a Hansen double sphere solubility plot. The experimental data obtained on miscibility studies confirmed that pyridine, cyclohexanone, and a pyridine-cyclohexanone solvent mixture (1:1) occupy a well-defined Hansen space and show fitting to HSPs of the biocrude-SRGO blend, improve the overall compatibility of the blending mixture, and display a maximum miscibility of 72%. To correlate the compatibility with the molecular structure, the compatibility of light, heavy, and residual fractions obtained by fractional distillation of HTL biocrude (pinewood) was also evaluated with SRGO using the Hansen double sphere plot, and a close agreement with differential scanning calorimetry (DSC) results as well as the experimental data on miscibility studies was verified. Furthermore, the comprehensive estimation of the detailed composition and chemical nature of biocrude and light, heavy, and residual fractions by the means of elemental (CHN/O), GC-MS, and GC × GC analysis was also presented. Additionally, the correlation between compatibility and interactions within chemical functionalities of blend components was established by analyzing the contribution of aromatic, aliphatic, and oxygen containing functional groups to the miscibility using quantitative 13C NMR spectroscopy. The present study reports a mixing strategy to assess the compatibility of biocrudes, heavy distillate fractions, asphaltenes, residues, and polymers with existing petroleum infrastructure for the cost-effective biorefinery process to balance economic and environmental considerations.