Warm the oil in a stockpot over medium heat. Add the shrimp shells, onions, carrots, and celery and saute for 15 minutes, or until lightly browned. Add the garlic and cook 2 more minutes. Add 1 1/2 quarts of water, the white wine, tomato paste, salt, pepper, and thyme. Bring to a boil, then reduce the heat and simmer for 1 hour. Strain through a sieve, pressing the solids. You should have approximately 1 quart of stock. You can make up the difference with water or wine if you need to.