Basin Evolution and Reservoir Properties

Introduction

Igneous and metamorphic rocks form the basement of sedimentary basins. With time, compression and heating associated with subduction basin subsidence change the chemical and mechanical properties of these sediments and fluids. In the broad field of geology the term basin can be used to describe or represent a number of features. The different types of basin include topographic or drainage basin, sedimentary basin and structural basin. In some cases a single basin can have the aspects of all the above mentioned basins. Basins are shaped like bowls and have sides that are higher than the bottom. Basins can be oval or circular in shape similar to the bathtubs in bathrooms. Basins are formed due to forces that act above the ground like erosion or forces acting below the ground such as earthquakes. Basins are created over thousands of years but can be created overnight as well. In response to a range of processes that influence the elevation of the Earth's surface, basins take on a variety of shapes and sizes. Some basins are filled with sediment deposited wholly on land, whereas others are filled with sediment deposited below sea level in marine habitats. This assignment consists of information about reservoir properties, basin evolution, energy and fluid migration. In this assignment The reservoir rock of the Geysers Field located in Northern California is chosen as the main reservoir. 502 MWe of installed generating capacity is available at the Geysers Field in Northern California. 95 wells provide a combined steam withdrawal rate of 8 to 5 million pounds per hour. It is presently being constructed 4 major generating plants, which will increase the installed capacity to 908 MWe by 1979. The reservoir rock is fracturing graywacke, a pretty competent rock with moderate permeability and porosity between the interstitial spaces. Sea level datum indicates that the reservoir contains dry steam with an initial pressure of around 514 psia. The static pressure gradient is the difference between the saturated steam pressure and the total depth of all wells dug to this point in time.

The reservoir rock of the Geysers Field

Reservoir properties

The reservoir rock of the Geysers Field located in Northern California mainly consists of fractured graywacke which is a very competent rock with very low interstitial porosity and permeability. Graywacke is a variety of sandstone which is generally characterized by its dark colour, hardness and poorly sorted grains of quartz, feldspar and other small rocks of lithic fragments arranged in a clay and muddy matrix. Graywacke is a type of sandstone that is widespread in the Earth’s rocky crust and shares about 20-25% of all other sandstones. Graywacke is further categorized in two groups depending upon the quantities quartz, feldspar and other rock fragments. The three types of Graywacke are:-

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Lithic graywacke- It contains 95% of quartz and more rock fragments as compared to feldspar.

Feldspathic graywacke- It contains 95% of quartz and more feldspar as compared to other rock fragments.

Quartz graywacke- It contains >95% of quartz.

Lithic graywacke is a sandstone of low level of maturity due to the presence of large amounts of matrix such as illite, clay minerals and metastable fragments of rock. This rock is dark gray to dark green in colour and is characterized by poor sorting due to the presence of a high number of dark rock fragments and clay-chlorite matrix. Lithic graywacke is formed by synsedimentary muddy or clayey detritus which is then converted into a dense mixture of chlorite, sericite and quartz during the diagenetic process. Feldspathic graywacke contains considerable amounts of feldspar and other rock fragments in addition to quartz. The proportion of Feldspar varies in wide limits and is always greater than the percentage of rock fragments. In certain variations of feldspathic graywackes feldspar is found in more quantities than quartz. Matrix of feldspathic graywacke is similar to that of lithic graywacke. Clay minerals that are mainly present are of kaolinite group and they originate from the chemical weathering of kaolinization of feldspar. Feldspathic graywacke is a common type of sandstone whereas lithic graywacke and quartz graywacke are rare types of sandstone. At a confining pressure of 20 MPa graywacke shows a very low permeability value of ~4.89x10-7 mD (Braz et al. 2018). Mineral assemblages of zeolite are found in graywackes that contain reactive volcanic lithic materials in addition to quartz and albite. Trenches are formed where oceanic lithospheric slabs descend into the mantle. The trench sediments mainly consist of mainly fine grained graywacke turbidites. Turbidity currents mainly enter the trenches at submarine canyons and flow along trench axes. Sediments can be transported through the trenches for upto 3000km. Graywackes are used for various purposes such as in construction industries and in architecture. They are used as decorative aggregates and are used in flooring and interior decoration. They are also used to decorate gardens and are used as paving stones as well. Graywackes are also used to build houses and roads. They prove to be an effective material in the construction industry due to its properties. Graywackes are also used to manufacture cement and mortar. Due to its properties graywackes are also used to build sea walls and reservoirs.

Geysers Field located in Northern California

Energy

The Geysers is an important geothermal field located in the Mayacamas mountains California which is 115 km north of San Francisco. This basin is the world's largest geothermal field which consists of 22 power plants and an installed capacity of 1517 MW. This area is the first geothermal field where the geothermal power plant was put into commercial action in the USA. Currently 18 power plants are operative in this region. The geothermal field is spread over 117 square kilometres in the counties of Sonoma and Mendocino. In the entire region more than 350 steam wells have been drilled to tap the natural steam. Some of these wells are as deep as 3 kilometres. The steam that rises from the underground is brought over land through pipes and then the steam is supplied to interconnected power plants. The steam is used to turn conventional turbines which in turn generates green electricity. These geysers account for 20% of the green power that is generated in California. The steam that is required for the power plants is harnessed from a graywacke sandstone reservoir that is located at the top of a mixture of low permeability rocks where the mixture is heterogeneous in nature. The steam reservoir requires heat that comes from a large molten rock chamber that extends for 7 kilometres beneath the ground. This chamber has a diameter of about 14 kilometre. In the late 1980s it was found out that the steam reserves have gone through systematic depletion and the reservoirs were also not recharging quickly to meet the increased steam demand for the power plants. This caused many power plants to shut down in that region. In order to cope up with the increased demand of steam in the power plants the geothermal reservoirs are now charged with treated and recycled wastewater from cities and sewage treatment plants. The power plants use steam from the geothermal reservoirs to produce electricity. There are three different technologies that are used to convert hydrothermal fluids into electricity. These three technologies are dry steam, flash steam and binary cycle (Feng et al. 2018). The type of conversion that is used entirely depends on the fluid and its temperature. The hydrothermal fluids that are primarily used by dry steam power plants are generally steam. The steam reaches the turbines through pipes which are used to turn the turbines to produce electricity. In dry steam plants fossil fuels are not used to produce steam (Lutz et al. 2018). These plants mainly emit excess steam and few gases. Dry steam power plants are the first type of geothermal power plants that were ever constructed. Flash Steam power plants are the most widely used geothermal power plants today. In these power plants hydrothermal fluids of temperatures greater than 360°F are pumped under high pressure into tanks which are at relatively low pressure causing the fluids to vapourize quickly. That vapour is then used to turn the turbines which in turn drives the generator to produce electricity. If any geothermal liquid is left in the tank it is flashed in a secondary tank to produce more electricity.

Location of the field

Basin evolution

In the broad field of geology the term basin can be used to describe or represent a number of features. The different types of basin include topographic or drainage basin, sedimentary basin and structural basin. In some cases a single basin can have the aspects of all the above mentioned basins. Basins are shaped like bowls and have sides that are higher than the bottom. Basins can be oval or circular in shape similar to the bathtubs in bathrooms. Basins are formed due to forces that act above the ground like erosion or forces acting below the ground such as earthquakes. Basins are created over thousands of years but can be created overnight as well. A river drainage basin can be defined as an area that is drained by a river and its tributaries. A river drainage basin is made up of different watersheds. Watersheds are small versions of a river basin. Every stream or tributary of the river has its own watershed which in turn drains into a larger stream or a wetland (Hu et al. 2019). The streams, ponds, wetlands and lakes are part of the river basin. Every river is an integral part of the network of watersheds that forms the entire drainage basin of a river system. The water that is collected in the drainage basins flows downhill towards the bigger rivers. For example the Pease River in northern Texas is a part of the Arkansas Red-White watershed. It is one of the tributaries of the red river. The Red river in turn is a major tributary of the Mississippi river which flows into the Gulf of Mexico. The Amazon basin is the largest river drainage basin in the world and drains an area of more than 7 million square kilometers. Structural basins are formed due to tectonic activities. Tectonic activity can be defined as the movement of large pieces of Earth crust called tectonic plates. A structural basin is an area where the rocks tilt or dip towards the centre of the structure. Tectonic activities are responsible for certain phenomena such as earthquakes and volcanoes. Other natural processes such as weathering and erosion also help to form structural basins (Qu et al. 2018). For the formation of structural basins, rocks and other materials present on the floor of the basin are forced downwards and the materials present on the side of the basin are pushed upwards. Lake basins are examples of structural basins. Sedimentary basins are the key sources of petroleum and other fossil fuels. Sedimentary basins are also a type of structural basin but are not shaped like typical basins. Sedimentary basins are filled with layers of rock and organic material that have been deposited over millions of years. Millions of years ago tiny sea creatures called diatoms lived and died in the ocean basins. Under right conditions the pressure from the sediment fills turns the remains of these diatoms into petroleum.

Evolution of the basin

Fluid migration

Sedimentary basins that hold the fossil fuels for the production of electricity also consist of carbon di-oxide reserves. There are certain key factors that define and regulate the storage and use of these carbon di-oxide reserves. Hydrocarbon production suresults in the depletion of resources which in turn helps to increase the CO2 storage by de-pressurising reservoirs and providing reusable boreholes and other infrastructures. This enhanced storage method may be beneficial in some cases but may be temporary in some situations as well. In certain circumstances where offshore gas and oil infrastructures have to be decommissioned after production it reduces the availability. While searching for underground CO2 storages potential areas of overlap with underground water should also be taken into account. It should be checked whether the overlap is geographical in nature. Whereas in some cases of overlap it is seen that impermeable layers separate the two natural resources. The successful storage of CO2 from natural gas and oil reserves depends on the hydraulic integrity of the geological formations that bound it and the wellbores that penetrate it (Zhao et al. 2017). The potential leakage chances comes from shear failure of caprock, out of zone hydraulic fracturing and poor sealing of the cases using cements in enlarged and unstable boreholes. The factors or the parameters that control these are the upper and lower bound pressures, temperatures experienced by the reservoirs, orientation and mechanical properties of the fault, mechanical properties of the rock and reservoir depth and shape. In order to prevent geomechanical leakages certain factors should be kept in mind. To prevent any kind of geomechanical leakages it is important to identify the safe upper limits of injection and preferred injection well locations. It is also important to check the previous records of reservoir temperatures and pressure and the drilling programs that are designed to mitigate the yielding of rocks in new wells. The wellbore integrity indicators of the new wells should also be assessed. To reduce the emission of CO2 into the atmosphere and increase the storage of carbon di-oxide in geological formations it is important to use certain technologies. Deep saline aquifers have the largest potential of CO2 storage in terms of volume, duration and minimum impact on the environment. There are chances of conflict between natural gas storage sites and potential CO2 storage sites but the conflict does not extend to commercial scale as the natural gas storage sites have much less capacity when compared to CO2 storage sites. CO2 can also be stored in unmineable coal seams where the CO2 displaces methane on the coal surfaces due to preferential adsorption. Subsurface storage of waste fluids and the water produced from mining operations also affect the storage of CO2 by competing for pore space, mixing by pressure perturbation and leakage. The depth, pressure and temperatures of the CO2 storage containers are in the range of 380m to 2500m, 2to 28MPa and 20 to 100°C.

Seismicity at the Geysers

Conclusion

From the above assignment it has been concluded that In the broad field of geology the term basin can be used to describe or represent a number of features. The different types of basin include topographic or drainage basin, sedimentary basin and structural basin. In some cases a single basin can have the aspects of all the above mentioned basins. Basins are shaped like bowls and have sides that are higher than the bottom. Basins can be oval or circular in shape similar to the bathtubs in bathrooms. Basins are formed due to forces that act above the ground like erosion or forces acting below the ground such as earthquakes. Basins are created over thousands of years but can be created overnight as well. Basins and sedimentary materials cover the majority of the Earth's surface, which is covered in sedimentary basins. As a result of understanding sedimentary basin evolution and the causes for their existence in certain regions and time periods, fundamental insights into a wide range of Earth processes can be gained. The most detailed record of Earth's lithosphere's history is the imprint of geologic events on sedimentary basin materials. In response to a range of processes that influence the elevation of the Earth's surface, basins take on a variety of shapes and sizes. Some basins are filled with sediment deposited wholly on land, whereas others are filled with sediment deposited below sea level in marine habitats.

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Reference

Braz, C., Seton, M., Flament, N. and Müller, R.D., 2018. Geodynamic reconstruction of an accreted Cretaceous back-arc basin in the Northern Andes. Journal of Geodynamics, 121, pp.115-132.

Feng, J., Dai, J., Li, X. and Luo, P., 2018. Soft collision and polyphasic tectonic evolution of Wuxia foreland thrust belt: Evidence from geochemistry and geophysics at the northwestern margin of the Junggar Basin. Journal of Geodynamics, 118, pp.32-48.

Hu, P., Yang, F., Tian, L., Wu, K. and Wang, W., 2019. Stress field modelling of the Late Oligocene tectonic inversion in the Liaodong Bay Subbasin, Bohai Bay Basin (northern China): Implications for geodynamics and petroleum accumulation. Journal of Geodynamics, 126, pp.32-45.

Lutz, R., Franke, D., Berglar, K., Heyde, I., Schreckenberger, B., Klitzke, P. and Geissler, W.H., 2018. Evidence for mantle exhumation since the early evolution of the slow-spreading Gakkel Ridge, Arctic Ocean. Journal of Geodynamics, 118, pp.154-165.

Qu, W., Lu, Z., Zhang, Q., Wang, Q., Hao, M., Zhu, W. and Qu, F., 2018. Crustal deformation and strain fields of the Weihe Basin and surrounding area of central China based on GPS observations and kinematic models. Journal of Geodynamics, 120, pp.1-10.

Zhao, J., Deng, G., Xu, Q., Shao, X., Zhang, X., Chen, X. and Ma, Z., 2018. Basement structure and properties of the southern Junggar Basin. Journal of Geodynamics, 121, pp.26-35.

Power Technology, 2021. The Geysers Geothermal Field, California, United States of America. [online] Power-technology.com. Available at: [Accessed 4 August 2021].

ResearchGate, 2021. [online] Available at: [Accessed 4 August 2021].

ResearchGate, 2021. [online] Available at: [Accessed 4 August 2021].

ResearchGate, 2021. [online] Available at: [Accessed 4 August 2021].


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