Heat oil in a large soup pot. Add carrot, celery and onion. Cook until vegetables have begun to soften and onion turns translucent, 3 to 4 minutes. Stir in butternut squash, thyme, chicken broth, salt and pepper. Bring to a boil, reduce heat and simmer until squash is fork-tender, about 30 minutes. Use an immersion blender to purée soup. Alternatively, let the soup cool slightly and carefully purée in batches in an upright blender.