Engler’s theory or organic theory:- this theory was proposed by Engler in 1900. He proposed that petroleum has been formed by the decay and decomposition of dead animals (preferably the sea animals) under high temperature and pressure. The sea animals, which died due to various reasons, were deposited at the bottom of the sea. Gradually, these were covered with mud, silt and sand. Under high temperature and pressure, these animals were dcomposed into petroleum. The gaseous hydrocarbons, that were produced, constituted the natural gas. The theory is supported by the following facts.
a) The presence of brine with petroleum
b) The presence of optically active compounds
c) The presence of nitrogen and sulphur compounds
d) The presence of fossils in the petroleum area.
The theory was further supported by the fact that when the destructive distillation of fish oil and other animal fats under high temperature and pressure was carried out, a petroleum-like liquid containing N, S and optically active compounds was obtained. 
The theory fails to account for a) the presence of chlorophyll in the petroleum b) the presence of coal deposits found near the oil fields and c) the presence of resins in the oil.