GEOL3200
- The rocks and sediments that fill in the Gulf of Mexico, especially for Miss.-Louisiana-Texas, can be broken into three main types of rock units, based on the dominant rock type (relates to how the dominant rock type formed). a) What are these three rock types (in order from oldest to youngest) and provide a brief description of how they formed. b) The first (oldest) unit is important in the petroleum industry due to the structures it forms in the subsurface. Explain how these structures formed and why they influence hydrocarbon exploration. (Hint: it’s not just one feature that results from this unit—do other structures or geologic features form as a result that may influence hydrocarbon accumulation?).
- a) Unlike other parts of the united states of America, Louisiana; Texas has sedimentary rocks, igneous and metamorphic rocks.
Sedimentary rocks
They are the oldest types of rocks. They form when erosion occurs on preexisting rocks. The minerals and sediments disintegrates are sweep away towards the ocean base and accumulates overtime in layers (Class notes, 2020). Weathering of existing rocks is due to wind, water, or ice. After a long time, the suspensions compact to form sedimentary rocks. Sedimentation is the major process in formation of sedimentary rocks like sandstone and siltstone in Texas.
Igneous rocks
They are formed when a volcanic eruption occurs and the magma flows from mountain crest or valve. When the molten rock reaches the surface of the earth it cools and solidified to form lava (Class notes, 2020). Over a long period, the lava crystallizes because of the impact of pressure and temperature to form igneous rocks. The rocks divide into volcanic and plutonic rock depending on their position on the geographical surface.
Metamorphic rock
The rocks are a hybrid of igneous and sedimentary rocks. They form when there is an impact of pressure, heat, and chemicals on the existing igneous and sedimentary rocks (Class notes, 2020). The sediments flow in form of suspension on the ocean bed where they accumulate to form a rock stratum. Over time, the layers exert pressure on the ones below leading to the formation of metamorphic rocks such as Precambrian in Texas.
b). The major structures of sedimentary rocks that define hydrocarbons exploration are permeability and porosity. Porosity means the voids in the rocks full of water but has high oil or gas content (Class notes, 2020). It is expressed as a rock percentage while permeability is the capability of rocks to allow fluids to pass through. Consequently, sedimentary rocks like limestone are highly permeable to enable miners to access the hydrocarbon content in the form of petroleum a few miles from the earth’s surface.
- Describe the difference between relative and absolute dating of geologic units. Provide techniques for each (don’t just name them, you need to describe them or you’ll only get partial credit). Some questions to help: what are some fundamental concepts used in relative dating? What is the common technique used for absolute dating? What are some other techniques that can be used for absolute dating?
Relative Dating
Relative dating is a scientific procedure to identify the phase difference of rocks without having any knowledge of their pre-existing age of the rock. It uses principles to know the period of existence of rocks in years. These principles include superstition in which old rocks appear at the bottom while the youngest are at the top of sedimentary rocks. The principle of original horizontality explains that unbuckled sedimentary rocks often settle horizontally (Bjorlykke, 2010). Other vital principles in relative dating rope in lateral continuity, cross-cutting relationships, and enclosure. Relative dating justifies the estimated maturity of a rock using the time principle. For instance, the geologist can note the age of rocks in the Paleozoic period. Relative dating essentially depends on recent erosion processes, geographical landscape, and effects of faults scarps.
Absolute Dating
Absolute dating, on the other hand, estimates the time of life of rocks using the arithmetical range of years. It is due to the chemical and physical features of the rock strata. Absolute dating operates differently from relative dating in that it avails a geometric series. Relative, however, puts events in their order of existence in a systematic manner. The main method of absolute dating is radioactive decay in which young and old rock elements such as uranium are used to establish a rock era (Bjorlykke, 2010). Absolute dating has radiometric, radiocarbon, and potassium-argon techniques. Radiocarbon dating is based on radioactive decay where cosmic radiation gets into the earth’s surface, releases carbon -14 which assimilates by plants and animals until their time of death. The geologist thus calculates the age of the organic matter using carbon-14 techniques.
- Describe the requirements for organic matter to be preserved so that if buried and subjected to increased temperatures for a given amount of time it turns into hydrocarbon? Some helpful questions: what are the conditions for organic matter to be preserved? what type of organic matter produces gas/oil vs. coal? does grain size influence the preservation of organic matter and why? high or low energy deposition environments?…
Conditions for organic matter to be preserved
The preservation of hydrocarbon requirements comprises oxic, environmental conditions rich in hydrogen sulfides, carbon compounds, water contents, low temperatures, and small size of humus.
Type of organic matter that produces gas and coal
Coal and oil are constituents of fossil fuels. The category of organic matter that produces oil in aquatic-based known as kerogen. The oil or gas forms when the organic matter subjects to high pressure from heavy rocks and temperature to form humus thus petroleum oil (Pommer et al., 2015). The process is continuous and occurs in millions of years. Lignite, bituminous, and peat are just but a few oils due to organic matter decomposition.
Features of organic matter and how they affect its preservation
The size of the grain affects the preservation of organic in that small grain size of humus equals the minimal amount of hydrogen sulfides and carbon compounds. Arguably, the large size of grain relates to a high level of preservation. It therefore certifies that mineral content increases with the surface area of humus (Pommer et al., 2015). High decomposition energy means a decrease in water but a high oxygen to produce nutrients for humus preservation.
- What are the two main types of hydrocarbon traps? Provide two examples for each. What is important about the relative age of hydrocarbon/migration and trap formation between the two main types of hydrocarbon traps?
Hydrocarbon traps
They include structural and stratigraphic traps. The underground traps exist because of hydrogen accretion. The structural traps are due to a bend or a fracture on a rock and have an infinite size because of different forms of distortion. They comprise anticline, fault traps, and salt domes (Bjorlykke, 2010). The stratigraphic traps occur as an effect of the accumulation of sedimentary rocks on the reservoir which is either porous or semi-porous. They consist of primary and secondary stratigraphic traps.
Importance of relative carbon age of hydrocarbon and trap formation
The carbon age of hydrocarbon reflects on the hydrocarbon trap formation. Structural and stratigraphic traps occur after millions of years of relative carbon sediment deposition in the reservoir. It is due to a split, a bend, or ground salt on preexisting rocks to mean that the higher the carbon age the higher the chances of hydrocarbon traps formation.
- Briefly explain how CO 2 in the atmosphere varies on a seasonal basis, and over the past
50 years. What is the most likely reason for this?
The level of carbon for oxide fluctuates in a specific pattern over the past fifty years as shown in the graph below. The level, however, is seasonal because plants undergo photosynthesis and respiration during spring and summer with the help of carbon dioxide absorption. The graph thus shows a peak between April and June. The graph slopes towards October during fall and winter when plants release more carbon dioxide. Other factors that affect carbon dioxide rise and fall include vegetation and the magnitude of the variations (Class notes, 2020). In forested areas, more carbon accumulates than in the plateaus. The main reason for this change is the variation of slow mixing between the Northern and southern hemisphere as well as the different ratio between land and atmosphere.
- How does the trend of CO2 concentration in the past 50-60 years compare to the past 800,000 years of Earth’s history? Where do we get current and past information on atmospheric composition, thus inferences to how climate has changed over Earth’s history?
The level of carbon dioxide concentration in the atmosphere in the past 800,000 years is relatively lower than the 50-60 years ago. It is because of the increased combustion of fossil fuels like charcoal, petroleum, and coal (Class notes, 2020). Unlike a hundred thousands of years ago, industrialization breeds accumulation of carbon dioxide from car exhausts, combustion of charcoal, and diesel ignition. The carbon molecules settle in the atmosphere for decades thus the rise in the graph. Industrial effluents and mining activities far lead to carbon dioxide accumulation. Conclusively, we get current and past information about atmospheric conditions from the National Geographic Society database.
References
Bjorlykke, K. (2010). Petroleum geoscience: From sedimentary environments to rock physics. Springer Science & Business Media.
Class lecture notes (2020). Exploration techniques /methods.
Pommer, M., & Milliken, K. (2015). Pore types and pore-size distributions across thermal maturity, Eagle Ford Formation, southern Texas. AAPG Bulletin, 99(9), 1713-1744.