Analysis of Geochemistry - Coursework Example

? Geochemistry – Answer ALL questions PART 1. What are the important processes that affect carbonate rocks during diagenesis ? Diagenetic processes range between purely physical ones, bio-chemical and organic processes and physiochemical processes. Carbonate rocks are affected to some degree by all of these processes during diagenesis. The formation of carbonate rocks is affected by dissolution and is important in creating secondary porosity. Similarly compaction and cementation are critical to carbonate rock formation especially for the loss of porosity. Before cementation and compaction, other processes such as bioturbation and burrowing also affect carbonate rocks. Mechanical infiltration also plays a vital part in the downward movement of clay sized particles. Carbonate rocks such as limestone are affected by inversion, recrystallisation and grain growth as well. Typical processes that affect carbonate rocks are (sequentially) biochemical degradation, polycondensation and insolubilisation. 2. Why is the smectite to illite transition important in the process of petroleum formation? The smectite to illite transition is an important diagenesis reaction especially for shales with progressive burial. This transformation has the potential to flush hydrocarbons from various shales, accelerating the generation of hydrocarbons, producing high pore fluid pressures and for provision of various cementation agents to different sandstones. 3. How would you expect the alkane/alkene ratio to change as a hydrocarbon mixture matures over geological time, and why? The alkane/alkene ratio would tend to decrease with time as a hydrocarbon mixture matures over geological time. This would take place as the biodegradation process would tend to remove alkanes so that the hydrocarbon mixture moves towards aromatics. 4. During early burial, what are the major differences between subaerially and subaquatically polycondensed materials? Early burial has a major impact on the way in which deposited materials decompose to later. Typically subaerially polycondensed materials tend to be more porous and produce less biodecomposition content when compared to subaquatically polycondensed materials. Moreover, subaquatic polycondensed materials tend to have a higher water content (up to 60%) and tend to form finer sediments when compared to subaerially deposited polycondensed materials. 5. What is a geochemical fossil? A geochemical fossil is the remains of any organic object that has been transformed by diagenesis into a fossilised state. The resulting fossilised state may not have been able to preserve the original structure of the fossilised object. 6. Explain how crude oils can be grouped based on their sulfur content. Crude oils are classified using sulphur content such that low sulphur content crude oils are known as “sweet” while high sulphur content crude oils are known as “sour”. Typically low sulphur content crude oils are preferred because sulphur tends to display acidic behaviour. PART 2. 1. Draw the van Krevelen diagram and explain how it illustrates several important facts concerning the formation of petroleum. A van Krevelen diagram is generally plotted to evaluate the origins and maturity levels of kerogen and petroleum content. The van Krevelen diagram plots the ratio of hydrogen to carbon against the ratio for oxygen to carbon. Figure 1 - van Krevelen diagram sourced from (Monash University, 2008) Looking at the diagram above, it becomes obvious that the van Krevelen diagram differentiates between various hydrocarbon products depending on certain regions on the plot. The formation of liquid petroleum can be seen as resting between oxygenated products and gas products which signifies petroleum formation. Additionally the van Krevelen diagram illustrates the evolution paths of various kerogens through the various paths shown on the plot. The plot above shows four possible paths (I, II, III and IV) that denote increasing burial. Based on this the particular mix of petroleum obtainable is arrived at. 2. What is the above equation? The equation above relates the mass flow rate of crude oil from a cavity to another cavity along with variations in other parameters including viscosity. 3. Define all of the parameters contained within the equation. (10 marks) is the mass flow rate is the effective oil permeability is the viscosity of the crude oil is the density of the crude oil is the exposition area through which flow is allowed is the porosity 4. Explain why this equation is important in understanding the formation and migration of crude oil. As crude oil is formed, it tends to migrate but this migration depends on critical factors such as the permeability of the crude oil and the porosity resulting from its formation. Oil can only migrate if enough porosity has been introduced around the organic mass for transportation and if the permeability is suitable enough for movement. This equation tends to deal with both permeability and porosity at the same time and so delineates the migration and formation of crude oil. 5. Discuss the general reaction pathway which the organic inventory follows as original organic residue is transformed into crude oil, paying special attention to inorganic processes that may participate. When organic inventory is deposited, the mixture is generally unstable and out of equilibrium. Diagenesis occurs first and tends to provide equilibrium to the organic inventory through shallow burial. The pressures and temperatures at this stage are small so the transformation of sediments occurs in mild conditions. Biodecomposition tends to affect early diagenesis through the action of microbial elements. The sulphates are thus reduced to free oxygen which is consequently consumed by aerobic organisms. As a result of these activities, the organic inventory and the newly formed sediments tend to approach equilibrium and tend to form a new polycondensed structure known as kerogen. Typically kerogens with the highest carbon and hydrogen content have the best potential to produce hydrocarbons. As more deposition occurs, the pressures and temperatures increase significantly due to the weight of overlying structures. This leads to the expulsion of water since the new mixture is also out of equilibrium. The permeability and the porosity tend to decrease significantly too during this stage. The kerogen tends to produce liquid petroleum through diagenesis as well as through this catagenesis process though the production of liquid petroleum is low through the latter process. 6. Explain how you would go about constructing a computer model which coupled diagenesis with oil formation. The computer model to couple diagenesis to oil formation would have to account for both organic and inorganic processes through the use of numerical methods. The various step wise transformations from diagenesis would have to be simulated numerically based on the organic inventory stock to see what results would be achieved. Read More