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PDF | A great variety of proven hydrocarbon plays and trap styles are In book: Geology of the Netherlands, Chapter: Petroleum geology. Chapter 2 Fundamentals of Petroleum Geology Chapter Outline Introduction Formation of Organic Matter Origin of Petroleum Occurre. Petroleum geology is not a well-defined academic subject. It includes many different aspects of the Earth sciences which are used in petroleum exploration and.
The very lightest materials floated to the top. Relative dating does not tell us how long ago something took place. Sufficient permeability fluid flow capability to be able to deliver the hydrocarbons to extraction wells. This process can take tens of thousands of years to occur. When a non-porous rock face is moved into a position above and opposite a porous rock face. The fluids in a sedimentary rock whether water. Halliburton 49 Basic Petroleum Geology.
The Uranium technique for apatite. Absolute dating did not replace relative dating. This produces either a partial or complete record of the time elapsed from the deposition of the lowermost bed to the deposition of the uppermost bed. In early geologic endeavors. Basic Petroleum Geology and Log Analysis Figure 7 Layered rock sequence illustrating relative age and deposition of strata in horizontal layers Basic Age Dating Principles To establish a relative time scale.
Although they may seem rather obvious to us today. This characteristic is basic to two of the principles used to interpret geologic events from the sedimentary rock record. Stratification is the characteristic layering or bedding of sedimentary rocks. If igneous intrusions or faults cut through strata. If a sequence of sedimentary rock layers are folded or tilted. First is the principle of original horizontality. Second is the principle of superposition which states that each layer of sedimentary rock in a sequence that has not been tectonically disturbed is younger than the layer beneath it and older than the layer above it.
This rule also applies to other surface deposited materials such as lava flows or beds of ash from volcanic events.
Stratigraphy is the study of the origin. By combining faunal succession and stratigraphic sequences. In Others may be interbedded. Halliburton 14 Basic Petroleum Geology. This stratigraphic ordering of fossils eventually became known as the principle of faunal succession and states that fossil faunas and floras in stratigraphic sequence succeed one another in a definite.
Basic Petroleum Geology and Log Analysis assignment of rock sequences. Smith was also the first person to define formations within a rock unit.
Some formations consist of one rock type. While mapping the vertical rock sequences he established a general order of fossils and strata from the oldest at the bottom to the youngest at the top. William Smith. The petroleum industry relies on the application of these principles for exploration and production.
Correlation is the process of relating rocks at one site with those at another site. A formation is a rock unit that is mappable over a laterally extensive area and has the same physical properties and contains the same fossil assemblages.
He learned that he could Figure 8 Correlation of strata. Although it will never be continuous from the beginning of time.
Halliburton 15 Basic Petroleum Geology. You can only know that it accumulated sometime after the formations deposition. Basic Petroleum Geology and Log Analysis Distribution of oil and gas fields based on geologic age It is important to know the geologic age of reservoir rocks because rocks of different ages frequently have different petroleum characteristics and productivity. It is also important to note that the age of the rock does not necessarily coincide with the time of oil accumulation.
Halliburton 16 Basic Petroleum Geology. Clastic Sedimentary Rocks-formed as a result of the weathering or fragmentation of pre-existing rocks and minerals and classified on the basis of their textures. Sedimentary rocks are divided into coarse-grained: Within each textural category. Many limestones also contain dolomite.
Halliburton 17 Basic Petroleum Geology. Chemical or Biochemical Sedimentary Rocks-formed as a result of chemical processes. Primary carbonate deposition results from the precipitation and deposits formed by plants and animals that utilize carbonates in their life processes.
The most abundant mineral chemically or biochemically precipitated in the oceans is calcite. Gypsum and halite are formed by the chemical precipitation during the evaporation of seawater. If a shale is fractured. Halliburton 18 Basic Petroleum Geology. Limestones are sedimentary rocks consisting chiefly of the mineral calcite calcium carbonate.
Basic Petroleum Geology and Log Analysis There are five types of sedimentary rocks that are important in the production of hydrocarbons: Sandstones Sandstones are clastic sedimentary rocks composed of mainly sand size particles or grains set in a matrix of silt or clay and more or less firmly united by a cementing material commonly silica.
Limestones are the most important and widely distributed of the carbonate rocks. Shales Shale is a type of detrital sedimentary rock formed by the consolidation of fine-grained material including clay. Evaporites Evaporites do not form reservoirs like limestone and sandstone. Dolomite is a common rock forming mineral with the formula CaMg CO3 2.
The formation of salt structures can produce several different types of traps. The sand particles usually consist of quartz. On evaporation the general sequence of precipitation is: Shales are typically porous and contain hydrocarbons but generally exhibit no permeability. Carbonates are sediments formed by a mineral compound characterized by a fundamental anionic structure of CO Calcite and aragonite CaCO3. One type is created by the folding and faulting associated with the lateral and upward movement of salt through overlying sediments.
Salt overhangs create another type of trapping mechanism. Evaporites make excellent cap rocks because they are impermeable and. Hydrocarbons are generated when large volumes of microscopic plant and animal material are deposited in marine.
Evidence that petroleum does migrate is suggested by the very common occurrence of active seeps where oil and gas come to the surface either directly from the source rock or from reservoir rocks. In either case. This allows the organic material to accumulate rather than be destroyed by aerobic organisms such as bacteria. Organically rich. The amount of petroleum generated is a function of the thickness of the accumulated sediments and organic material.
Over time. These and other properties will be discussed in detail in the sections to follow. During burial the sediments protect the organic material by creating an anoxic oxygen depleted environment. Migration of Hydrocarbons Primary migration is the process by which petroleum moves from source beds to reservoir rocks.
This process can take tens of thousands of years to occur. Secondary migration is the concentration and accumulation of oil and gas in reservoir rock. Halliburton 19 Basic Petroleum Geology. Some of the rock and fluid properties include porosity. The numerous varieties of petroleum are due to the way carbon and hydrogen can combine and form different sized molecules.
A thick black asphalt and yellow light crude are examples of two varieties of petroleum with different molecular weights. At room temperature and pressure. Impurities like. A hydrocarbon molecule is a chain of one or more carbon atoms with hydrogen atoms chemically bonded to them.
The molecular structure of hydrocarbons can vary from simple straight chains to more complex branched chains or closed-ring structures.
Some petroleum contains hydrocarbon molecules with up to sixty or seventy carbon atoms. Temperature affects the chemical structure of hydrocarbons and can break heavier longchain molecules into smaller and lighter molecules.
Halliburton 20 Basic Petroleum Geology. It is both simple and complex and is composed almost entirely of carbon and hydrogen. For a more detailed explanation of the chemical properties of hydrocarbons. Most of this organic matter is in a form known as kerogen. It owes its insolubility to its large molecular size and heat is required to break it down. As kerogen thermally matures and increases in carbon content. Petroleum is the organic substance recovered from wells and found in natural seepages.
It is. The organic content of a rock that is extractable with organic solvents is known as bitumen. Bitumen becomes petroleum at some point during migration. Different types of kerogen can be identified. Halliburton 21 Basic Petroleum Geology.
It normally forms a small proportion of the total organic carbon in a rock. A source rock must contain significant amounts of kerogen. Bitumen forms largely as a result of the breaking of chemical bonds in kerogen as temperature rises. Important chemical differences often exist between source rock extracts bitumen and crude oils petroleum.
Kerogen is that part of the organic matter in a rock that is insoluble in common organic solvents. Figure 10 Maturation of kerogen is a function of increased burial and temperature and is accompanied by chemical changes. We have seen that petroleum is derived mainly from lipid-rich organic material buried in sediments.
API gravity is a standard adopted by the American Petroleum Institute for expressing the specific weight of oils. The lower the specific gravity. Halliburton 22 Basic Petroleum Geology. API gravities of 20 to 40 degrees sp. These oils have frequently suffered chemical alteration as a result of microbial attack biodegradation and other effects. Heavy oils are those with API gravities of less than 20 sp. It can vary in type and amount of hydrocarbons as well as which impurities it may contain.
Basic Petroleum Geology and Log Analysis Crude Oil Crude oil is a mixture of many hydrocarbons that are liquid at surface temperatures and pressures. Oils of API gravity greater than 40 degrees sp. Not only are heavy oils less valuable commercially. Crude oil may be classified chemically e. The difference between the two is contingent upon conditions of origin. Thermogenic gas is a natural gas resulting from the thermal alteration of kerogen due to an increase in overburden pressure and temperature.
It generally contains appreciable amounts of sulphur. Natural Gas There are two basic types of natural gas. Mississippi and Amazon rivers. It is produced by the partial maturation of kerogen or by the degradation of mature crude oil.
Halliburton 23 Basic Petroleum Geology. A dry gas contains less than 0. Asphalt is particularly suitable for making high-quality gasoline and roofing and paving materials. The major hydocarbon gases are: The terms sweet and sour gas are used in the field to designate gases that are low or high. Because of these factors. Basic Petroleum Geology and Log Analysis Asphalt Asphalt is a dark colored solid to semi-solid form of petroleum at surface temperatures and pressures that consists of heavy hydrocarbons and bitumens.
Natural gas. Biogenic gas is a natural gas formed solely as a result of bacterial activity in the early stages of diagenesis. It can occur naturally or as a residue in the refining of some petroleums. Basic Petroleum Geology and Log Analysis Figure 11 Thermal maturity indicators Condensates Condensates are hydrocarbons transitional between gas and crude oil gaseous in the subsurface but condensing to liquid at surface temperatures and pressures.
Halliburton 24 Basic Petroleum Geology. The temperatures required to produce crude oil occur between 5. Temperatures above 5. Geologic conditions such as volcanism and tectonics folding and faulting can change or effect the temperature gradient.
There are. Temperatures below Graph 1 Temperature vs. Hydrocarbons evolve from an immature stage to oil generation. Basic Petroleum Geology and Log Analysis Pressure Gradient Most pressure that effects rocks is due to the weight of overlying rocks and is called overburden pressure. Overburden pressure is a function of depth and increases one pound per square inch for each foot of depth.
At feet. Diagenetic changes such as dolomitization. Hydrocarbons migrate upward through porous and permeable rock formations until they either reach the surface as seepage or become trapped below the surface by a non-permeable cap rock which allows them to accumulate in place in the reservoir. Sandstone reservoirs have a depositional porosity and permeability controlled by grain size. Diagenetic changes may include precipitation of clay minerals in the pore space. A reservoir is a subsurface volume of porous and permeable rock that has both storage capacity and the ability to allow fluids to flow through it.
Porosity and permeability are influenced by the depositional pore-geometries of the reservoir sediments and the post-depositional diagenetic changes that take place. Sandstone reservoirs are generally created by the accumulation of large amounts of clastic sediments which is characteristic of depositional environments such as river channels. Porosity types of carbonate reservoirs include vuggy pores larger than grains.
Halliburton 25 Basic Petroleum Geology. Basic Petroleum Geology and Log Analysis Abundance and Production of Sedimentary Formations The approximate abundance and production for the three types of sedimentary formations significant to petroleum production are as follows: The hydrocarbons would generally be better separated as a result of lower internal reservoir pressures.
Deep reservoir— Typically created by severe faulting. The hydrocarbons would be less separated with more gas in solution and oil of reduced viscosity because of higher temperatures.
There is often a reduction in porosity and permeability due to increased compaction. Shallow reservoir— Created by the folding of relatively thick. Basic Petroleum Geology and Log Analysis Physical Characteristics of a Reservoir Physical characteristics of a reservoir include original deposition and subsequent changes.
Depth The physical characteristics of a reservoir are greatly affected by the depth at which they occur. Halliburton 26 Basic Petroleum Geology. Halliburton 27 Basic Petroleum Geology. The greater the area and thickness of the reservoir. Basic Petroleum Geology and Log Analysis Area and Thickness The total area of a reservoir and its thickness are of considerable importance in determining if a reservoir is a commercial one or not.
Effective Porosity vs. It is usually a function of the amount of space between rock-forming grains. Basic Petroleum Geology and Log Analysis Porosity Porosity is the ratio of void space in a rock to the total volume of rock. Such porosity results from groundwater dissolution. Total porosity is all void space in a rock and matrix whether effective or noneffective.
Total Porosity— Effective porosity is the interconnected pore volume available to free fluids. Primary Porosity— Amount of pore space present in the sediment at the time of deposition. When used in calculations. Halliburton 28 Basic Petroleum Geology. Realistic Porosity— Porosity can approach. In a sandstone. All rock types are affected by fracturing and a rocks composition will determine how brittle the rock is and how much fracturing will occur.
Hydraulic fracturing is a method of stimulating production by inducing fractures and fissures in the formation by injecting fluids into the reservoir rock at pressures which exceed the strength of the rock.
The two basic types of fractures Figure 18 Fractures in rock material include natural tectonically related fractures and hydraulically induced fractures. Halliburton 30 Basic Petroleum Geology. Basic Petroleum Geology and Log Analysis is typically much lower due to cementation and compaction.
This may be achieved if the carbonate is highly fractured along with vuggy porosity.
In a carbonate. Porosity in a well sorted rock is generally high. Other important controlling factors include grain packing. Sorting is a process by which the agents of transportation. If the grains are well rounded and of similar size. Porosity can be reduced considerably because smaller or irregularly shaped grains can be inserted in between the larger grains. Halliburton 31 Basic Petroleum Geology. Rhombohedral packing yields approximately The effects of packing on porosity can be illustrated by considering the change in porosity that takes place when even-size spheres are rearranged from open packing cubic packing to tightest or closed packing rhombohedral packing.
Halliburton 32 Basic Petroleum Geology. Figure 22 Grain packing and its effect on porosity Packing strongly affects the bulk density of the rocks as well as their porosity and permeability. Cubic packing can yield a porosity of The weight Figure 23I Sedimentation process: Layer A is compacted of the by layer B overlying sediments squeezes the particles together into the tightest arrangement possible.
Minerals in solution crystallize out of solution to coat grains and may eventually fill the pore spaces completely. Compaction affects porosity and permeability by reducing the amount of interconnected pore space. The load pressure also squeezes out the water that occupies the pore spaces between the particles.
Porosity and permeability can be reduced significantly due to cementation. Halliburton 33 Basic Petroleum Geology. Compaction is dependent not only on overburden pressure but also on the different types of clastic materials present in the formation. Common cementing agents include calcite CaCO3.
The process of lithification the conversion of unconsolidated deposits into solid rock is completed by cementation. Halliburton 34 Basic Petroleum Geology. The more pressure exerted on a fluid. Permeability is a measure of the ease with which a formation permits a fluid to flow through it. Permeability is measured in darcies. This is generally the main path of the flowing fluids into the borehole. Basic Petroleum Geology and Log Analysis Permeability Recovery of hydrocarbons from the reservoir is an important process in petroleum engineering and estimating permeability can aid in determining how much hydrocarbons can be produced from a reservoir.
Vertical permeability is measured across the bedding planes and is usually less than horizontal permeability.
The more viscous the fluid. Permeability is usually measured parallel to the bedding planes of the reservoir rock and is commonly referred to as horizontal permeability.
Few rocks have a permeability of 1 darcy. They are commonly found in carbonate rocks or other rock types with a brittle fabric and also in clastic rocks with a high content of soluble material. Halliburton 35 Basic Petroleum Geology. As seen in Figure High vertical permeabilities are generally the result of fractures and of solution along the fractures that cut across the bedding planes.
To determine the permeability of a formation. To be permeable. The reason why horizontal permeability is generally higher than vertical permeability lies largely in the arrangement and packing of the rock grains during deposition and subsequent compaction. Halliburton 36 Basic Petroleum Geology. As a result. These types of formations will exhibit very little permeability. Some limestones may contain very little porosity.
It is because of this water that we are able to measure the resistivity of a formation in logging. MRIL is quickly becoming a highdemand service. Water As hydrocarbons and water accumulate in a reservoir.
Some formations may only contain water. This transition zone is a relatively thin zone above the oil accumulation. The oil accumulation may be of primary importance because it contains crude oil and possibly saturated gas. It is important to note that all reservoirs may not contain natural gas. These measurements depend upon petrophysical characteristics of the formation. Figure 26 Below the lighter fluids is a gas to oil transition zone. Natural Gas 2. Halliburton 37 Basic Petroleum Geology.
With time-honored logging tools such as the induction. We elaborate on different types of petroleum traps such as structural, salt related, and stratigraphic traps. We then discuss various types of reservoir rocks such as clastic sandstone and shale and carbonate rocks. We conclude with geology, geophysics, and petrophysics, in connection with reservoir geometry, volume, and assessment of reserves.
In this section, we discuss how geology combined with geophysical techniques defines the geometry of a petroleum reservoir and how petrophysics is utilized to quantify the reservoir quality and petroleum reserves. The process starts with photosynthesis in which plants convert water and carbon dioxide to complex sugars glucose using the energy of the sun. Glucose is the starting material for the synthesis of more complex organic compounds either in plants or the animals that eat them.
Generally, most of the organic matter produced by photosynthesis is eventually returned to the atmosphere as carbon dioxide. Only about one CO2, molecule in every million taken up by photosynthesis is converted to Chapter 2 Fundamentals of Petroleum Geology 17 hydrocarbons. This recycling of CO2 is achieved by plant and animal respiration and through oxidation and bacterial decay when organisms die.
However, the recycling of carbon as CO2 is not totally efficient in that a very small amount about 0. Sediments, laden with dead plant and animal lake, or sea organisms are heavier than water, and naturally deposit in the lower areas or basins under the sea. These basins are originated by tectonic action and sea level changes. When the sea level rises relative to the base of a depositional basin , the sediments are buried deeper. As ocean basins gradually fill with layers of sediments, the weight of the newer layers increases the pressure on the layers below.
This weight, or pressure at depth, along with heat, converts the organic material to oil and gas. The primary source of the organic matter that is ultimately transformed into oil and gas are the remains of phytoplankton; microscopic floating plants such as diatoms.
The best environment for the accumulation of this organic matter is in quiet waters such as a swamp, lake, or deep ocean basin. Here, the organic matter can lie buried without being disturbed. However, to ensure its preservation and to prevent rapid decay, the water conditions need to be stagnant and reducing oxygen deficient or anaerobic thus eliminating the possibility of aerobic bacterial decay or scavenging by fish, etc.
Along with the organics, muddy sediment also accumulates. Source rock starts life as an organic-rich mud, subsequently to be converted to a claystone, shale, or marl. They come in many combinations and types, from the petroleum products used in cars and other internal combustion engines to natural gas used for heating and cooking.
The ancient societies in Egypt, China, and India made limited use of petroleum mainly as fuel for lamps, medicine, and as caulking for boats and canoes. The modern petroleum age began a century and a half ago. Advances in technology have steadily improved our ability to find and extract oil and gas and to convert them to efficient fuels, lubricants, and other useful consumer products. Petroleum is composed of compressed hydrocarbons and was formed millions of years ago in a process that began when aquatic plant and animal remains were covered by layers of sediments particles of rock and mineral.
As bacteria and chemicals broke down the organic plants and animal material, increasing layers of sediment settled on top. Heat and pressure transformed the layers of sediment into sandstone, limestone and other types of sedimentary rock, and transformed the organic matter into petroleum. Tiny pores in the rock allow the petroleum to seep in. These reservoir rocks hold the oil like a sponge, confined by other, low permeability layers that form traps. For a rigorous definition and more on petroleum systems see, for example, Magoon and Dow We also want to briefly discuss different geologic time periods, how different geologic structures are formed and how such structures have evolved over millions of years.
A typical outcrop, such as the one depicted in Fig. Subsequent chapters show how geophysical data can help geologists with their analysis in building more reliable models.
The evolution of formation of geologic structures is accomplished through careful analysis of natural processes, modeling, and various FIGURE 2.
Chapter 2 Fundamentals of Petroleum Geology 19 hypotheses. For example, Fig. At the top of the figure, going back several million years, the sediments characterized by ABCD were deposited under the sea bed.
Then the second model from the top , the uplift of the beds above the sea level, caused by tectonic forces, expose them to erosion. Note the erosion has stripped away sediment package D and part of C, creating an irregular collection of hills and valleys model 3 from above.
Finally, at the bottom model, we note creation of a new package of sediments marked as E created from subsidence below the sea that is deposited on top of C. The irregularity of C package is preserved as an unconformity. Later on, we will see e. From Levin Here, we can see the flat layers of rock that at first glance look like continuous layering of sediment. For example, much older formations e. Source Rock Burial depth and temperature Reservoir Rock Migration pathways Seal Rock Trap Three-dimensional 3D seismic surveys enable the geologist and geophysicist to investigate many of these key elements—identifying likely migration paths, inferring the relative timing of trap formation and charge and measuring the geometry and size of closed structures.
In some cases, reservoir quality and even the presence of fluid hydrocarbons may be estimated. Rock physics is a key component of analyzing the reservoir. Much of this is related to the source rock, seals and the capacity of the reservoir to contain hydrocarbons. Porosity is a key ingredient and will determine the supply of petroleum that is contained in the rocks. Seismic velocity can be related to porosity. The empirical Wyllie Time Average equation Wyllie et al.
Dt is the observed interval transit time inverse velocity. Dtma is the matrix solid interval transit time. Dtf is the fluid interval transit time.
As a general rule, velocity decreases will accompany porosity increases, as related by another empirical relationship by Raymer et al. Dt and Dtma are as above. RHG is an arbitrary constant 0. Sufficient permeability fluid flow capability to be able to deliver the hydrocarbons to extraction wells.
Sufficient hydrocarbon saturation volumes of hydrocarbons relative to other fluids to be an economic resource. Since oil is lighter than water and gas is lighter than both, when a hydrocarbon reservoir is found, it is stratified with gas on top, oil in between, and water on the bottom, if all three phases are present. Sedimentary rocks fall into one of four basic groups.
These are sandstones, shales, carbonate rocks, and evaporites. These rocks are generated by two principal processes: Erosion, transport, and deposition of sediments, as well as 2.
Chemical solution and precipitation. The erosion process is one in which solid particles resulting from land weathering are transported and usually deposited in water environments as sediments. The solid materials result from complete weathering of igneous rocks. Sediments accumulate as fragmented material and result in sedimentary deposits having a clastic texture.
As the sedimentary material is transported, abrasion processes round the grains. Porosity results from 24 Geophysics for Petroleum Engineers the space between the grain particles that is not filled with cement or clay. Permeability, which is the property that permits fluid to flow through the pores, is controlled by the amount of cement, the degree of compaction, and the magnitude and variation of grain sizes. Sedimentary Geochemistry. Sandstones and Sandstone Reservoirs.
Carbonate Sediments. Shales, Silica Deposits and Evaporites. Heat Transport in Sedimentary Basins. Introduction to Geomechanics: Stress and Strain in Sedimentary Basins. Source Rocks and Petroleum Geochemistry. Petroleum Migration.