English Study.doc

Lesson One The Essential Conditions of Formation of Oil ReservoirIn order to have a gas or oil deposit in any area, three subsurface conditions must be met. First, there must be a subsurface source rock that generated the gas or oil at some time in the geologic past. Second, there must be a separate subsurface reservoir rock that holds the gas or oil. Third, there must be a trap or a high point on the reservoir rock that concentrates the gas or oil.The rocks that make up the crust of the earth are very old, millions or even billions of years old. During this vast expense of geologic time, sea level has not been constant. Several times in the past, the seas have risen to cover the land with shallow water and then fallen to expose the land. This has happened many times to every continent of the world. While the land is covered with the shallow seas, sediments are deposited in the coastal areas, along the beaches, and in the shallow waters offshore. These sediments are relatively simple materials such as sands along beaches and river channels, muds offshore and in the lagoon behind the beaches, and limes as shell beds and reefs. These ancient sediments piled layer upon layer, form the sedimentary rocks that make up the uppermost part of the crust of the earth.The source of gas and oil I the organic matter which is buried and preserved in the ancient sedimentary rocks. In every environment in which sediments are deposited, along with the mineral grains of sand, mud, and lime, organic mater from dead plants and animals is deposited and then buried deeper and deeper in the earth as it is covered with more sediments. The most common organic-rich sedimentary rock, thought to be the source rock for gas and oil, is black shale. Shale is originally deposited as organic-rich mud on ancient ocean bottoms. Temperature is the major factor in turning the organic matter into oil.Because gas and oil are relatively light in density compared to subsurface waters, they rise by buoyancy through the subsurface rocks after they have generated. Some subsurface rocks, called reservoir rocks, have voids or space (pores) and can hold fluids. A common reservoir rock is sandstone, which is composed of sand grains. The pores are located in the spaces between the individual sand grains. Limestones, however, and brittle rocks and are often have solution pits in them. The fractures and solution pits add to the pore spaces in limestones. Because many pore spaces in the reservoir rock intercommunicate, the gas or oil can flow through the rock (permeability) in and upward direction.A trap is a high point in the reservoir rock where the gas or oil is concentrated as it flows in an upward direction along the reservoir rock layer. The trap either can be a structural trap formed by the deformation of the rock, such as a folded arch (a dome or and anticline) or a break (a fault), or a stratigraphic trap formed when the rock was originally deposited (in a river channel or reef) or when part of the reservoir rock wa removed by erosion (an angular unconformity).How are these subsurface deposits of gas and oil located? During the early days of oil drilling, the causes of the subsurface formation of gas and oil, the knowledge of how it moves through the pores of the rocks and how it accumulates in traps were not understood. Many subsurface traps, however, are leaky and oil and gas seeps on the surface of the ground encouraged the drillers. By the early 1900s, the principles behind occurrence of subsurface oil and gas were becoming known. Oil companies soon realized that by mapping the layers of rocks cropping out on the surface and the location of the ground, the rocks could be projected in the surface and the location of traps could be predicted. Unfortunately, almost all the traps that can be detected from surface seeps of oil or rock outcrops have long since been drilled. There is still a lot of gas and oil to be found, but it is in traps hidden in the subsurface which are more difficult to be found, but it is in traps hidden in the subsurface which are more difficult to detect. Today, seismic techniques are commonly used to find traps. The only way to know for sure, however, if there is commercial gas or oil in a subsurface trap is to drill a well. Unfortunately, even this is not foolproof as many wells have drilled through subsurface gas or oil deposits without detecting them.Lagoon 泻湖Reef 珊瑚礁Brittle 易碎的Arch/ Dome 穹窿Anticline 背斜 syncline背斜Stratigraphic 地层学的Unconformity 不整合 angular unconformity 角度不整合Seep 油苗、渗出Crop 收获 crop out 露头Fool-proof 十分简单明了的Reading Material Introduction to Sedimentology and Petroleum GeologySedimentology is the study of sedimentary rocks and their formation. The subject covers processes which produce sediments, such as weathering and erosion, transport and deposition by water or air, and the changes which take place in sediments after their deposition (diagenesis).Like all natural sciences, Sedimentology ha an important descriptive component. In order to be able to describe sedimentary rocks, or to understand such descriptions, it is necessary to familiarize oneself with quite an extensive nomenclature. There are specialized names for types of sedimentary structures, grain-size, distributions and mineralogical composition of sediments. We also have a genetic nomenclature, which names rock types according to the particular way in which we think the have formed. When we are reasonably confident about their origin we may use the genetic nomenclature.Sedimentology covers the study of both recent (modern) sediments and older sedimentary rocks. By studying how sediments form today in so-called recent environments, we can understand the conditions under which various sedimentological processes take place. From such observations we may be able to recognize older sediments which have been formed in the same way. This is called using the principle of uniformitarianism.Conditions on the surface of the earth have fluctuated widely throughout geological history, however, so that the principle of uniformitarianism cannot be applied without reservations. One important aspect of sedimentological research is attempting to reconstruct changes in environments on the earths surface throughout geological time. This applies particularly to climate, vegetation and the composition of the atmosphere and the oceans.The study of the development of plants and animals throughout geological time (paleontology) is important to Sedimentology, not only for dating beds, but also because organism are an important component of many sedimentary rocks (certain limestones, for example), and organic processes affect the weathering process and the composition of sea-water. Many organisms make very specific demands of their environment, and fossils are consequently a great help in reconstructing the environment in which the sediments are deposited. Paleontology is the study of ecological conditions as we are able to reconstruct them on the basis of remains or traces of plants and animals in rocks. Traces of animals in sediments have proven to be very useful environmental indicators.When we study rocks, we should attempt to give objective descriptions of the composition, structure etc. of the rocks, and on the basis of these try to interpret how they were formed. However, it is impossible to give a completely exhaustive, objective description of a rock. We know from experience that we have a tendency to observe what we are looking for, or what we anticipate finding. Descriptions of sedimentary sequences from the 1940s and 1950s, for example, contain few observations about sedimentary structures which we would consider fairly conspicuous and important today. We observe sedimentary structures because we have learned to recognize them and understand their genetic significance. Our observations are therefore selective.Many sedimentologists use a standard checklist for what they should observe in the field, so that their descriptions are as comparable a possible. Facies analysis should be a creative process and the various models should be kept in mind when making the observations. It is also desirable to quantify observations as far as possible. Field observations can be supplemented by laboratory analyses. These might be surveys, texture analyses (e. g. grain distribution), microscope analyses (perhaps using a scanning electron microscope) or chemical analyses. Pure description of sedimentary rocks are useful because they increase the data base on which we can build our hypothesis against which to test our observations. Data collection can then be focused on observations and measurements which can support or disprove the hypothesis.Although there were a number of scattered investigations earlier, it was not until the end of the 1950s and the 1960s that geologists began to undertake systematic studies of recent environments of sedimentation which a view to finding connections between the external environment and the sediments which accumulate. The environme