In a large saucepan, heat the olive oil until shimmering. Add the shallot and cook over moderate heat until just beginning to brown. Add the garlic and cook until fragrant. Add the lobster shells and cook, stirring, until beginning to brown in spots, about 8 minutes. Add the wine and cook over moderately high heat until nearly evaporated, about 15 minutes. Add the demiglace and cook over moderate heat until slightly reduced, about 5 minutes. Strain the sauce into a medium saucepan, pressing hard on the shells to extract as much sauce as possible. Add the port, season with salt and pepper and simmer over moderately low heat for 5 minutes. Cover and keep warm.