Oil is buoyant when it is in it’s complete form. Over time, the more volatile components will gas off or separate, or man made chemicals will break down the structure of the mass of oil. Then the mass becomes an oil/chemical mass, parts of which float as a “sheen” on the surface, enter the atmosphere as molecules, or sink to the behthic zone as globules that form from the denser components of the oil.
Initially, the unchanged oil mass will move rapidly, depending on the winds, tides and movement of the open ocean. later, the components will be pushed, separated and moved as the shallower, warmer water is reached. Warmer temperatures accelerate the venting of volatile components and the wave action helps the formation of tarry globules.
As the more buoyant oil/chemical mass reaches narrow formations, such as channels and estuaries, the narrowness magnifies the force of the water, making it more possible for the chemical/oil mass to move more forcefully, farther and more effectively. The mass might move back and forth with the tides and flows, but always, some is left behind to settle to the bottom soil, where it incorporates to forever alter the structure and content of the soil.
Some works its way through the riparian structure to wind up in shorelines and in the water tables where higher up trees, grasses and shrubs may be affected.
The life forms which depend on the inland underwater bottom soil for habitat and food can be either poisoned, suffocated or suffer illness and injury. There are effects on metabolism, reproduction and the ability of juveniles, larvae, seeds, spores, polyps and eggs to survive to maturity. The animals which drink the water and look for fish and other freshwater foods may either be poisoned, physically impaired by contact with globules of tarry oil, sickened or forced to migrate into already populated areas.
Birds and animals which depend on everything from water to nesting, burrowing and hunting grounds can be affected by both microscopic and visible components of the chemical/oil mass.
Aquatic species will definitely be affected by the changing composition of the water.
How far inland are we talking about? It could be miles, considering the composition of the oil/chemical mass, water temperature, land elevation, the volatility of the weather and the force of tides in the region. Hurricanes will push columns of water deep inland, taking the oil/chemical mass with them. Tsunamis from Caribbean and other earthquakes, even if small, will serve to transport the oil/chemical mass through the brackish (mixed saline and freshwater) zones and into pure freshwater habitats and biomes.
Where estuaries and channels are involved, the force will be greater and the movement during unusual events can push the mass even further inland, where the oil and chemicals might have time to settle. As the oil and chemicals settle, this will permanently alter atmospheric soil, water tables and freshwater shoreline and benthic zone soil.
When oil spills, aggravated by chemical dispersants, occur close to low lying land structures, the damage goes on for an extensive period of time and the healing may never happen until the oil and chemicals are broken down or weathered over thousands of years by the atmosphere.
When oil spills occur close to rocky and steep seashores, the violent movement of the water can toss and spray the oil, incorporating it onto and into the riparian, intertidal, benthic and deeper zones until nature breaks the oil down.
Where there are convoluted coastlines with coves, rocky structures, bays and estuaries, the force of crashing waves from storms and hurricanes as well as earthquake tsunamis can bring the oil far deeper into bays, channels and coves, and even deeper inland to marshes and beaches that are normally protected by high and complex barriers.
The full effects of oil spills are never known until great amounts of time pass. By then, the oil is incorporated into atmospheric and underwater soil and shorelines, or is an ever moving tarry mass that comes and goes with the tides.