When heated continental crust cools, its density increases, and it may subside according to the condition of isostasy. If the subsiding basement is covered with sediments a sedimentary basin is formed. In this paper the isostatic subsidence model is applied to the southwestern block of the Los Angeles basin with particular reference to the Wilmington oil field, an oil giant. The subsidence record of the sediments within this oil field agrees with the theory for thermal isostatic subsidence. The predicted gravity field and near surface thermal gradient are also in good agreement with observations. When the predicted thermal history of the sediments is combined with laboratory derived equations for the conversion of kerogens to petroleum we conclude that near ideal conditions for kerogen conversion have existed near the base of the Upper Miocene sediments in this oil field. Geochemical studies confirm that this is the origin of the petroleum. A discussion of the subsidence history of the remainder of the Los Angeles basin is given.