Accumulation

As we have previously discussed, planktonic aquatic organisms, Phytoplankton and Zooplankton, are the primary source material for the creation of petroleum. This also means that they are the primary source material for virtually all organic matter in sediments. The latter statement is further supported by examining the total volume of terrestrial organic matter carried by rivers to the sea. This is estimated at 7 x 10¹¹ kg per year, or less than 1% of total organic matter in the oceans. Since phytoplankton makes up the bulk of the planktonic aquatic organisms, it also is responsible for the majority of the organic matter in sediments.

Planktonic aquatic organisms such as phytoplankton, are shallow water creatures; they are restricted to the photic zone in oceans because growth is totally dependent upon photosynthesis. Photosynthesis requires sunlight and therefore phytoplankton are restricted to the upper 50 - 100 m of oceanic water bodies because this is the maximum depth to which sunlight penetrates the water environment. Growth also depends upon mineral nutrients with the greatest supply coming from land. Supplies are not only from inorganic sources such as dissolved minerals from rocks, but also organic sources of mineral nutrients contained in earlier generations of plants and animals being carried into the ocean in erosional waters. Therefore phytoplankton are restricted to zones, continental shelf areas, bordering and overlapping the continents. Such shelf areas experience periodic waterblooms which are voluminous concentrations of phytoplankton. This increase in phytoplankton also causes an increase in the numbers of Zooplankton. Why? Because Phytoplankton are at the bottom of the food chain and Zooplankton feed on them.

There are also deep water organisms that live and thrive below the photic zone therefore not directly dependent upon photosynthesis. They consist of bacteria that thrive on CO₂ dissolved in the water. They convert the CO₂ into organic nutrients. These organisms are often referred to as marine snow because when illuminated by artificial light, they glitter like tiny flakes of snow.

But there are two questions that must be resolved in order to facilitate the hypothesis that microscopic marine life, primarily phytoplankton, are the primary source of all hydrocarbons. The first issue involves an accumulation mechanism; how do the planktonic organisms accumulate in the numbers necessary to produce the vast volumes of oil and gas? Accumulation of living organisms necessitates an oxygen-rich environment. The second question deals with preservation; how are they preserved from destruction from biogenic or chemical agents? Accumulation of dead organisms necessitate an oxygen-deficient environment. An apparent contradiction.

Most living organisms require an oxygen-rich environment; shallow marine phytoplankton certainly do. The massive planktonic accumulation of living organisms observed along the continental shelf areas of the world depend upon the circulation of O₂ rich water. The periodic blooms observed in these areas are the result of the upwelling of cold, stratified, nutrient-rich bottom water into the shallower photic zone.

Upwelling conditions that coincide with spring weather create even more massive volumes of plankton because of the result of an increase in solar activity. The Sun is higher in the sky which generates longer days, deeper penetration into the ocean by sunlight, and higher water temperatures. This massive explosion of phytoplankton creates a red tide which is an annual occurrence throughout world ocean basins.

However, there are some side affects to upwelling and an excessive production of plankton, called hypertrophy. The excessive production of plankton creates a condition where the natural demand for oxygen exceeds supply. The natural demand for oxygen exceeds supply which means there is mass mortality of living organisms including fish and benthonic invertebrates. Everything dies; it suffocates. Another side affect is the creation of stratification within the water. This stratification prohibits mixing of surface and deeper oceanic waters. This means that not only is Oxygen not renewed to the deeper water, but the deeper water becomes colder, denser, and more saline. The surface water is separated from the deeper water by a narrow zone called a thermocline. But what happens to the massive volume of dead plankton that suffocated? They sink to the bottom of the ocean; they fall through the surface water zone, through the thermocline and into an O₂-deficient floor of the ocean.

Remember the two questions we discussed earlier that had to be resolved in order to facilitate the hypothesis that microscopic marine life, primarily phytoplankton, are the primary source of all hydrocarbons. The first involves an accumulation mechanism; how do the planktonic organisms accumulate in the numbers necessary to produce the vast volumes of oil and gas? The answer lies in the hypertrophy condition discussed above; mass mortality occurred because

Click to to view a chart showing areas of the world that are favorable for the accumulation of organic matter. The shaded areas show both onshore and offshore favorable areas.
the numbers of living organisms used up the supply of oxygen - natural demand for oxygen exceeded supply. The second question dealt with accumulation: Living organisms necessitate an oxygen-rich environment, but preservation requires protection from biogenic or chemical agents which means an oxygen-deficient environment. This answer lies with the thermocline. The corpses sink to the bottom of the ocean; they fall through the surface water zone, through the thermocline and into an O₂-deficient floor of the ocean. There is a massive accumulation of organic matter over successive years, centuries, eons, that creates source beds - sedimentary layers containing source material needed to create oil and gas. Ancient oil-source sediments attributable to coastal upwelling are likely to be more phosphatic (i.e., containing phosphorous -- scientists use phosphorous as an indicator of coastal upwelling) than normal source sediments. Examples of such deposits occur in California and Montana.

California	Montana

Question: Why are California oil fields (illustrated here) an example of phosphatic hydrocarbons?
Answer: Coastal area and therefore source of Cretaceous upwelling. What does this say about the California coastline over time? Has been a coastline area of an oceanic body for a long time.

Question: Why is Phosphoria formation in Montana an example of phosphatic hydrocarbons? It is land-locked; no ocean??
Answer: It was a favorable area for accumulation of organic matter in North America during the Cretaceous.

Click to view what the area looked like during the Cretaceous ... notice it was a sea!!

Again, refer to the chart that shows the areas of the world that have or have had favorable conditions for the accumulation of petroleum-rich sediments. Notice the many onshore areas that are favorable. What does this tell you about that area's past?

But not all petroleum is the result of planktonic processes in the ocean. Massive planktonic accumulation of living organisms can also occur in isolated, semi-isolated saline or brackish water basins. There are two types of basins: A positive water balance basin is one where surface water flows out as saline water flows in. An example is the Black Sea and Lake Maracaibo. The other type is a basin with a negative water balance. In this instance, all water flows into the basin where it is quickly evaporated in a hot, arid climate. The dense hypersaline water (high concentration of O₂, low concentration of nutrients) sinks to bottom of the basin and flows out as deep currents. Some examples of this condition are the Mediterranean Sea, Red Sea, and Caspian Sea.

There are some present-day deposits where we find shale (sedimentary rock) that are high in organic matter; some of these shales are so exceedingly rich that they secrete hydrocarbons when heated - oil shale. These source beds were created when large freshwater lakes spread across Colorado, Utah, and Wyoming during the Eocene Epoch. These freshwater lakes accumulated massive amounts of organic matter which later formed the Green River Formation .

Other source beds that contain shale that has high concentrations of organic matter occur throughout the United States. Click "here" to view a map showing the complete U.S. Some hydrocarbons occur at or near the surface and have survived in this manner for a long period of time. They are composed of heavy, low volatility, high viscosity, dark colored hydrocarbons and may be an example of a reservoir bed in which much of crude has flowed out and left a heavy tar behind. Such deposits are called tar sands. An example is the Athabascan tar sands found in Alberta, Canada in which about 50,000 bbl oil are extracted each day.

This figure shows the areas of the world that have or had favorable conditions for the accumulation of petroleum-rich sediments.

Question: Why are there no Phosphatic hydrocarbons in Louisiana? Looking at the this figure, we see that the conditions favorable to the accumulation of organic material (continental shelf/slope, oceanic upwelling, proximity of continental land mass) were present. Yet a map of the US does not show this type of deposit in Louisiana. And indeed, Louisiana petroleum does not have much phosphorous in it, not enough to classify it as phosphatic.

Answer: The reasons are simple. As we have seen, upwelling brings the deep ocean, organic rich waters into the euphotic zone where planktonic organisms feed, reproduce, and die creating large amounts of organic material deposited on the continental shelf. However, there existed a barrier, a reef structure at the edge of the continental shelf where it met the continental slope. This reef which existed during Cretaceous time, prohibited the upwelling current from reaching the euphotic zone which precluded the accumulation of the needed type of OM and thus prevented this type of hydrocarbon, rich in phosphorous, from being deposited. But even though the phosphatic hydrocarbons did not develop, the area is rich in hydrocarbons because planktonic organisms lived, flourished, and accumulated on the inland side of the reef which were later transformed into oil and gas. The occurrence of hydrocarbons in Louisiana will be discussed in more detail in a later chapter.

Sources:

"Geology of Oil," Steven Cooperman, Ph.D.

"Understanding Petroleum Exploration and Production," National Energy Foundation, Student Activity Guide

"The Upstream: A Guide to Petroleum Exploration and Production," Exxon Corporation Informational Brochure

NORTH, F. K., 1985, Petroleum Geology: Allen & Unwin, Inc., Winchester, MA.