Timothy Bata1*, Ezekiel H Elijah2, John Jitong Shirputda3 and Titus Adinye1
Received: April 11, 2018; Published: April 25, 2018
Corresponding author: Timothy P Bata, Associate Professor of Petroleum Geology, Abubakar Tafawa Balewa University, Bauchi, Nigeria
The widespread occurrence of Cretaceous oil sands can be attributed to various geological processes, most of which can be linked to the warm climatic conditions that prevailed globally at that time. The extreme global warmth witnessed during the Cetaceous caused a rise in global sea level, which resulted in flooding of most continental margins, depositing transgressive sands at shallow depths directly on the Precambrian basement, or much older sedimentary strata. Another important geological factor that contributed to the widespread occurrence of the Cretaceous oil sands was the availability of viable petroleum source rocks that generated oils at the time of, or shortly after, the Cretaceous oil sands were deposited. Oils migrated into the shallow Cretaceous reservoir sands through the plane of unconformity underlying them. The warm climatic conditions witnessed in the Cretaceous also implied that the reservoir had conditions that were favourable for optimum microbial activities.
Important geological factor controlling the widespread occurrence of these Cretaceous oil sands is the prevalence of Cretaceous reservoir sands with good reservoir capabilities. Deposition of these reservoir sands can be linked to the marine transgression that occurred during the Cretaceous period which was caused by the unique greenhouse climatic condition witnessed in the Cretaceous. This extreme global warmth was a direct consequence of the high level of atmospheric CO2 that prevailed globally at that time [1-3]. Variations in global climatic conditions are an important influence on variations in the global sea level. When global temperatures rise, the volume of seawater increases, as polar ice caps melt. Conversely, when global temperatures fall, polar ice caps grow and the volume of seawater decreases, causing the global sea level to fall. An increase in the rate of sea-floor spreading during the Cretaceous continental break-up also resulted in a decrease in the volume of ocean basins as hot rising magma caused the lithosphere to be lifted along the mid-ocean ridges, resulting in the reduction of the volume of the ocean basins. This combination of warm climatic conditions and sea-floor spreading resulted in a significant rise in the global sea level in the Cretaceous, causing flooding of most of the continental margins by the oceans [3,4].
Another important geological factor that contributed to the widespread occurrence of the Cretaceous oil sands was the availability of viable petroleum source rocks that generated oils at the time of, or shortly after, the Cretaceous oil sands were deposited. The Cretaceous period is known to be closely associated with organic-rich sediments (petroleum source rocks) that were deposited during the various widespread Cretaceous anoxic events e.g: [5,6]. Oils generated from these organic-rich sediments (source rocks) could easily migrate into the Cretaceous transgressive sands through the underlying plane of unconformity. There is field evidence showing cases of hydrocarbon migration over hundreds of kilometres through the plane of unconformity underlying the Cretaceous transgressive sands. An example is the heavy oil occurring in the Athabasca oil sand which is believed to have been partly derived from shale source rocks several hundred kilometres away from the reservoir sand [7,8].
Temperature is known to have a significant effect on the activities of microbes that are capable of degrading oils e.g. [3,9,10]. The temperature range in a petroleum reservoir will determine what type of microorganisms can thrive in the reservoir, while the specific temperature within the range will affect the rate of microbial activities. Microorganisms are often classified according to their optimal temperature range. Psychrophilic microorganisms commonly thrive in environments with temperatures ranging between 0 °C and 20 °C. Mesophilic microorganisms commonly thrive in environments with temperatures ranging between 20 °C and 40 °C, while thermophilic microorganisms commonly thrive in environments with temperatures ranging between 40 °C and 80 °C . 80 °C is considered as the sterilization temperature for most microorganisms, beyond which only hyperthermophilic microorganisms can thrive e.g. .
Microorganisms, therefore, have an optimum temperature at which they are most efficient. The further from this temperature a petroleum reservoir strays, the less productive the microorganisms in it will be. An accepted rule of thumb is that the rate of biodegradation at least doubles for every 10 °C increase , (Figure 1). The optimum temperature for microbial activity is between 25 °C and 30 °C e.g. , which is in fact very similar to the estimated average global surface temperature of 28 °C witnessed during the Cretaceous [1,2].
Figure 1: Chart showing variation in the relative rate of microbial activity with temperature. Note the optimum temperature for microbial activity is between 25 °C and 30 °C after which microbial activities decline down to the sterilization temperature at 80 °C.
Figure 2: Effect of variation in temperature with depth on microbial activities. Note that optimum microbial activities occur at a depth of about 300 m.
Measurements of microbial cell densities in aquifer systems carried out by  revealed a significant decrease in microbial cell densities with depth, suggesting relative increase in microbial activities at shallow depths similar to the depth of occurrence of most Cretaceous oil sands. Assuming a normal geothermal gradient of about 25 °C/km for the present day, optimum microbial activity will be expected to occur at a depth of about 300 m, after which microbial activity begins to decline until it gets to the sterilization depth of about 2300 m where most microbial activities cease (Figure 2).
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