Most of Louisiana has been underwater during the past 200 million years or so. It wasn't until Tertiary (about 65 million years ago) time that north Louisiana began to develop as dry land. South Louisiana did not begin to develop until late Tertiary, about Pliocene (5 million years ago) time. But most of south Louisiana didn't begin to form as dry land until Pleistocene time which was about 1.7 million years ago.

So the development of Louisiana is a relatively recent phenomena when considered in a geologic time context. Yet, Louisiana has many prolific oil and gas reservoirs that have produced and are currently producing large volumes of hydrocarbons. How can this be? If Louisiana were mostly underwater during its development, how can there be so much hydrocarbons?

This apparent contradiction is not a contradiction at all. You will learn in Chapter 3, Oil Formation, that microscopic marine life (Phytoplankton and Zooplankton) are the primary source material for virtually all organic matter in sediments. And it is the organic matter in the sediments that is responsible for the creation of petroleum. Therefore, the extended periods of submergence of the area of the earth that we now call Louisiana is the reason why Louisiana has the vast reserves of hydrocarbons.

Here is a short movie showing Louisiana's Physiographic Development.