Sauté onion, green pepper, celery, and garlic in shortening until tender. Stir in tomato paste, water, and salt and pepper; bring to a boil. Reduce heat; cover and simmer 2 hours.
Return mixture to a boil; stir in oysters and shrimp. Cook, uncovered, 5 minutes or until shrimp are tender and oyster edges curl. Stir in rice.
Spoon jambalaya into individual bowls; serve warm.