I. Formation of Oil and Gas The main components of oil and natural gas are hydrocarbons. How on earth did it form? There were many theories in the past, but they can be basically summarized into two kinds, namely, organic origin theory and inorganic origin theory.
1. inorganic genetic theory inorganic genetic theory holds that oil is formed by the chemical reaction of inorganic carbon and hydrogen in the deep crust at high temperature and high pressure. In the laboratory, simple compounds of carbon and hydrogen can be used to synthesize petroleum by inorganic synthesis; In addition, the gas and lava flow from the volcano also contain hydrocarbons; Hydrocarbons also exist in many inorganic substances. Inorganic genetic theory includes acetylene theory, carbide theory, universe theory, magma theory and so on.
Inorganic genesis theory is mainly based on the chemical reaction phenomenon that petroleum can be synthesized under special experimental conditions and the assumption that there are substances in the earth, so it can not be accepted by most scholars. However, before people understand the internal structure of the earth, the existence of inorganic genesis theory is conducive to deepening the understanding of petroleum genesis and promoting the study of petroleum genesis.
2. According to the theory of organic genesis, oil and natural gas are transformed from organic matter in sedimentary rocks under certain conditions. The main evidences are as follows: first, more than 99% of the oil and gas fields found in the world are distributed in sedimentary rocks; Second, oil has the unique optical rotation of living organic matter; Third, there are biomarkers in oil; Fourth, hydrocarbons can be obtained by using bio-fat, protein and carbohydrates in the laboratory; Fifth, the complexity of petroleum composition; Sixth, the process of organic matter transforming into oil and gas has been found in modern marine and lake sediments.
According to the modern scientific theory of the organic origin of oil and gas, the original organic matter was buried under certain environment and conditions, and under certain depth, temperature and other suitable conditions, it experienced biochemical, thermal catalysis, thermal cracking, high temperature metamorphism and other stages, and gradually transformed into oil and natural gas. According to the different oil-forming depths, the theory of organic genesis can be divided into shallow theory and deep theory. The former thinks that oil and gas were formed in the early stage when the sedimentary burial was not deep, while the latter thinks that oil and gas were formed when the organic matter was buried to a certain depth and temperature.
3. Although there are many theories about the origin of oil, the organic origin of oil is generally accepted. A large amount of organic matter is the material basis of oil and gas generation; External factors are the conditions to promote the preservation of organic matter and its transformation to oil and gas. Organic substances that generate oil and gas are the remains of animals and plants in oceans and lakes, among which aquatic plankton (such as fish and algae) and various microorganisms (foraminifera and ostracods) are the main organic substances rich in fat, protein and carbohydrates. Most of the remains of these creatures have either become food for other creatures, or become carbon dioxide, which is free in the atmosphere, and only a few are deposited in low-lying areas of oceans or lakes as tiny sediments. Nevertheless, as long as the universality, fast reproduction speed and long time of the biological world are taken into account, the amount of organic matter on the earth can meet a large number of oil and gas generation.
Organic matter entering the sediment can be preserved under anaerobic conditions. With the continuous strengthening of environmental reduction, organic matter decomposes under certain physical and biochemical effects, completing the process of "deoxidation, hydrogenation and carbon enrichment" and forming dispersed hydrocarbons-oil and natural gas.
4. The rock stratum that can generate oil and natural gas is called source rock or source rock (referred to as source rock). The strata composed of source rocks are source rocks, which are the actual places of oil and gas generation in nature. Fine particles such as mudstone, shale, sandy mudstone, argillaceous siltstone and carbonate rock in sedimentary rocks can form good source rocks. According to different lithology, source rocks can be divided into two categories-argillaceous source rocks and carbonate source rocks. These fine-grained source rocks are deposited in relatively calm water bodies. This environment is also suitable for the mass reproduction of organisms. In addition, after the organic matter settles to the seabed and lake bottom, it is buried by fine-grained rocks, which is beneficial to preservation.
The colors of source rocks are mainly dark colors such as brown, grayish brown, dark gray and black, followed by gray and grayish green. The color mentioned here is not the inherited color or secondary color of sedimentary rocks, but the primary color that can reflect the sedimentary environment and organic matter abundance at that time. Dark color usually reflects the reducing environment during deposition. This allows a large number of organic matter to be preserved, making iron in a low price state; Red often reflects the oxidation environment, so that organic matter is destroyed by oxidation.
The distribution of source rocks is controlled by lithofacies palaeogeographic conditions. Source rocks appear regularly and are related to some lithofacies zones. For lacustrine facies, deep lacustrine facies and deep lacustrine facies are the main oil-generating facies belts. Fine-grained argillaceous rocks were deposited there. Because the water body is deep, it has favorable oil-generating conditions such as still water deposition, weak water flow, small waves and reducing environment. The reproduction of a large number of lower organisms is the basis for forming a good oil layer. For marine, shallow marine or intertidal low-energy facies zones, low-energy carbonate strata and muddy strata under the tide have good oil-generating conditions. These areas are not deep in water, still in water and sunny, and the rocks are rich in biological fossils and organic matter. Permian and Triassic carbonate strata in Sichuan Basin, China are examples of carbonate oil-bearing strata in shallow sea.
Second, a large number of oil and gas exploration and development practices in reservoirs and caprocks have corrected people's initial misunderstanding that there are oil lakes and oil rivers underground. People gradually know that oil and natural gas are not stored in any oil lake or river underground, but in those rock layers with interconnected pores and cracks, just like water in a sponge.
A rock stratum with certain porosity and permeability, which can store oil and gas and other fluids and flow in it, is called a reservoir. Reservoir has two basic characteristics-porosity and permeability.
Porosity and permeability of reservoir rocks 1) Porosity reservoir rocks are cemented by rock particles and mineral particles of different sizes. There are tiny pores between cemented particles, just like our common building bricks. If a 3 kg brick is soaked in water and weighed again, it may become 3.5 kg, and the increase of 0.5 kg is due to the water soaking into the pores of the brick. Similarly, oil and natural gas are stored in the pores of oil-bearing rocks. In order to measure the total volume of pores in reservoir rocks, the concept of porosity is put forward to express the development degree of pores in rocks.
The ratio of the total volume of pores in reservoir rocks to the total volume of rocks is called porosity. Expressed as a percentage, that is:
(2- 1) where φ is porosity,%; Vp—— total pore volume in rock, m3; Vr—— the total volume of rock, m3.
Porosity of reservoir rocks can be obtained by experimental methods. Large porosity means that the volume between rock particles is large and the space for storing fluid is large; The porosity is small, the volume between rock particles is small, and the place where fluid is stored is small.
If the reservoir is an oil layer, are the pores of the oil layer full of oil? That's not true. Generally speaking, pores contain oil, gas and water. The ratio of oil-bearing volume in reservoir pores to pore volume is called reservoir oil saturation, that is:
In formula (2-2), so-oil saturation,%; VO-volume of crude oil in rock, m3.
The oil saturation of oil layer can be obtained by direct drilling coring and experiment. The higher the oil saturation, the more oil there is in the reservoir. This parameter is also an important data for calculating oilfield reserves. Sw is used to represent water saturation, that is, the ratio of water volume in reservoir pores to pore volume.
2) Permeability Permeability is a measure of the ability of a rock to allow fluid to pass through. Strictly speaking, all rocks in nature have certain permeability under a large enough pressure difference. Usually, when we say permeable rock and impermeable rock, we mean whether the fluid can pass through the rock under the condition of formation pressure. Generally speaking, sandstone, conglomerate, porous limestone, dolomite and other reservoirs are permeable rocks, while mudstone, gypsum and anhydrite are impermeable rocks. Permeability is often used to measure the permeability of rocks in petroleum industry.
Experiments show that when fluid passes through the core, if the pressure difference between the two ends of the core is not too large, the volume of fluid passing through the core per unit time is directly proportional to the pressure difference between the two ends of the core and the cross-sectional area of the core, but inversely proportional to the viscosity of the fluid and the length of the core, that is:
In formula (2-3), k is the absolute permeability of rock, μ m2; Q—— liquid flow, cubic centimeter per second; A—— the cross-sectional area of the core, cm2；; L—— core length, cm; Δ P —— pressure difference at both ends of the core,105 Pa; ; μ —— liquid viscosity, MPa·s
Equation (2-3) is called Darcy's law of linear seepage, which is obtained under the assumption that only one liquid flows in the pores of rocks and this liquid does not have any physical or chemical reaction with rocks. When the flow of fluid conforms to Darcy's linear seepage law, the obtained k value is the absolute permeability of rock. But in the actual oil layer, the fluid seepage situation is much more complicated. Two-phase (oil-gas, oil-water, gas-water) or even three-phase (oil-gas-water) fluids often coexist in the stratum. Therefore, when there are many fluids in the reservoir, the concept of absolute permeability must be revised. If the core is saturated with 25% bound water and 75% crude oil, the permeability to oil will be lower than that measured when it is saturated with 100% crude oil. When the saturation of a phase decreases, the permeability of that phase also decreases. When there are multiphase fluids, the permeability of rock to each fluid is called the effective permeability or phase permeability of the phase. The symbols Ko, Kg and Kw respectively represent the effective permeability of oil, gas and water.
Effective permeability is not only related to rock properties, but also to the properties and quantity ratio of fluids in rocks. In practical application, the concept of relative permeability is often adopted, which is defined as the ratio of effective permeability to absolute permeability. Under the condition of specific oil (gas, water) saturation, the relative permeability of oil, gas and water can be calculated by the following methods, namely:
(2-4)
(2-5)
In formula (2-6), Kro—— is the relative permeability of oil; Krg- relative permeability of gas; Krw- relative permeability of water.
Generally, the effective permeability of rock to each phase is always less than the absolute permeability of rock. The sum of effective permeability of each phase is always lower than absolute permeability, or the sum of relative permeability of each phase is less than 1.0.
Attached Figure 2- 1 1 shows the curve of relative permeability of oil phase and water phase with water saturation when oil-water two-phase seepage occurs in the reservoir. The relative permeability curve can be determined by core experiments, and can also be calculated by relevant empirical formulas according to the wettability, lithology and some basic parameters of reservoir rocks.
Fig. 2- 1 1 oil-water two-phase relative permeability curve 2. Types and basic characteristics of reservoirs At present, most oil and gas reserves in the world are concentrated in sedimentary reservoirs, among which clastic reservoirs and carbonate reservoirs are the most important. Only a small amount of oil and gas is stored in magmatic rocks and metamorphic rocks. Petroleum geology divides reservoirs into three types according to rock types: clastic reservoirs, carbonate reservoirs and other rock reservoirs.
1) Clastic rock reservoir Clastic rock reservoir is one of the important reservoirs in major oil and gas-bearing areas in the world. For example, the main oilfields in the West Siberian Basin of the former Soviet Union, Burgen Oilfield in Kuwait, Bolivar Lakeside Oilfield in Venezuela, Putewan Oilfield in the United States and Daqing Oilfield in China are all clastic reservoirs.
The rock types of clastic reservoirs are conglomerate, glutenite, coarse sandstone, medium sandstone, fine sandstone and siltstone. At present, the clastic reservoirs found in China are mainly medium-fine sandstone. The pore types of clastic reservoirs are mainly primary intergranular pores (Figure 2- 12), and the porosity is generally 5% ~ 40%. In addition, there are secondary dissolved pores, intergranular pores produced by cement recrystallization, cleavage cracks, bedding cracks and mineral interlayer cracks. Its oil storage performance is not only controlled by sedimentary environment, rock composition and structure, but also by the changes of underground temperature, pressure and pore water composition in the long diagenetic history, mainly including compaction, dissolution and cementation.
Figure 2- 12 Schematic Diagram of Particle and Pore Distribution in Clastic Rock Reservoir
Sandstone is the main body of clastic reservoir, which refers to sedimentary rocks with certain morphology, lithology and distribution characteristics, mainly sandstone. Sandstones related to oil and gas mainly include alluvial fan sandstone, delta sandstone, coastal sandstone, river sandstone, turbidite sandstone and lake sandstone.
Among oil-bearing sandstones, sandstone bodies with good permeability, high oil saturation and industrial oil flow are called oil sands. It is the smallest oil-bearing unit in the reservoir, and it is also a relatively independent unit to control the oil-water movement in waterflooding oilfield. Oil sand body is one of the most remarkable characteristics of continental clastic reservoir, so it is necessary to study the nature, shape and distribution of oil sand body when compiling oilfield development plan, analyzing development performance and adjusting development. Oil sands often appear in two forms: one is lenticular oil sands distributed discontinuously in a single layer; The other is that the sand bodies are connected with each other to form a composite oil sand body, which is called conjoined. The connected body can be composed of several or even a dozen sand bodies, forming a unified oil-water movement system. The main oil and gas reserves are distributed in this connector, which is also the main development object.
2) Carbonate reservoir The reservoir space per unit volume of carbonate reservoir is small, but its thickness is large. The connected porosity of carbonate reservoirs dominated by limestone and dolomite is generally 1% ~ 3%, and individual reservoirs can reach 10%.
Carbonate reservoirs are usually deposited in shallow sea facies. The lithology is relatively stable, with wide distribution area and large thickness. For example, the thickness of sinian dolomite in Sichuan basin is 500 ~1200m; The thickness of Proterozoic dolomite in Renqiu Oilfield is 2 140m. Therefore, although the storage space per unit volume is small, the storage space in the whole reservoir is still large due to its large thickness.
In carbonate reservoir, the distribution of fractures and caves is non-uniform, and it has the characteristics of stratigraphic and directional (Figure 2- 13). Cracks and caves can be seen everywhere in carbonate reservoir rocks, with various types and sizes. The permeability of large holes and cracks is extremely high and the output is high; The permeability of pores, cracks and surrounding rocks is extremely low, and the output is also low.
Figure 2- 13 Fractured Reservoir
3) Reservoirs other than clastic rocks and carbonate rocks, such as magmatic rocks, metamorphic rocks and clay rocks, are classified as other types of reservoirs. Although there are many types of rocks in this kind of reservoirs, the amount of oil and gas stored in them accounts for only a small part of the world's total oil and gas reserves, and its significance is far less than that of clastic rocks and carbonate rocks. A certain amount of oil and gas has been obtained in this kind of reservoir at home and abroad. This expands the field of studying oil and gas reservoirs. So far, China has obtained industrial oil gas flow in volcanic rocks, crystalline rocks and clay rocks, and has a certain production capacity.
3. In any area of caprock, only source rocks and reservoirs are not enough to form oil and gas reservoirs. In order to prevent the oil and gas generated in the source rock from escaping into the reservoir, an impermeable cap rock is needed. Sealing refers to the protective layer above the reservoir, which is used to seal the reservoir and prevent oil and gas from escaping upward. The quality of caprock directly affects the accumulation and preservation of oil and gas in reservoir.
In nature, any caprock is only relatively isolated from gaseous and liquid hydrocarbons. Oil and gas accumulation under formation conditions has different natural energy, which can drive oil and gas to escape to the surrounding area. Therefore, it is necessary to have a good seal to prevent hydrocarbons from escaping and make them accumulate to form oil and gas reservoirs.
The sealing effect of caprock is due to its dense lithology, no cracks, poor permeability and high displacement pressure. Displacement pressure refers to the lowest pressure required for wetting phase fluid in rock samples to be displaced by non-wetting phase fluid. Because most sedimentary rocks are wetted by water phase, the water in them must be driven away before oil and gas enter them. If the driving force of oil and gas migration does not reach the displacement pressure required to enter the caprock, oil will be blocked under the caprock. The displacement pressure of rock is directly related to the size of pore throat, and the smaller the pore throat, the greater its value.
Common caprock rocks are shale, mudstone, salt rock, gypsum and anhydrous gypsum. Shale and mudstone caprocks often coexist with clastic reservoirs; Salt rock and gypsum caprock are mainly developed in carbonate rock profile. In areas with weak structural changes, cracks are not developed, and dense marl and limestone can also be used as caprocks.
Three, trap trap refers to the place that can prevent oil and gas from continuing to migrate, and store and block oil and gas to make it accumulate. Traps are composed of reservoirs, caprocks and shielding layers. The basic function of traps is to accumulate oil and gas. On the premise of sufficient oil sources, the existence of traps is a necessary condition for the formation of oil and gas reservoirs. Therefore, it is very important to study the formation and types of traps and their relationship with oil and gas accumulation.
According to the geological factors that control the formation of traps, traps can be divided into three categories: structural traps, stratigraphic traps and lithologic traps.
1. The tectonic movement of the structural trap deforms or displaces the stratum, that is, folds or fractures. When the conditions are satisfied, these folds and faults can form structural traps, such as anticline traps and fault traps (Figure 2- 14 and Figure 2- 15).
Figure 2- 15 Fault Trap
Figure 2- 14 anticline trap
2. The contact relationship between upper and lower strata in stratigraphic cycle is called integration. It reflects the relatively stable subsidence of the crust, and the crust is constantly accepting deposits.
If the crust rises, the old strata will surface and suffer from weathering and erosion, resulting in sedimentary discontinuity. In the future, if it descends and continues to accept sedimentation, it will form an unconformity stratigraphic trap in which the new strata are in discontinuous contact with the underlying old strata. There, the continuously deposited rocks are partially eroded and then covered by impermeable rock caps. The angular contact between the old and new strata is called angular unconformity, which reflects that folding movement occurred in the crust before the new strata were deposited. In the angular unconformity, the unconformity upper new stratum covers the edge of fold erosion or the lower inclined layer, forming traps. If there is sedimentary discontinuity between the old and new strata, but they are still in parallel contact, it is called parallel unconformity, also known as pseudo-conformity. Parallel unconformity reflects the balanced rise or fall of the crust, so the occurrence of new and old strata is basically the same (Figure 2- 16).
Figure 2- 16 unconformity diagram
3. Lithologic Trap In a sedimentary basin, due to the difference of sedimentary conditions, the reservoir changes laterally and is blocked by impermeable rocks, thus forming a lithologic trap. Such as sandstone pinchout and sandstone lens (Figure 2- 17). This change is caused by the abnormal distribution of sand and clay when strata are deposited, such as sand dams in river deltas.
Figure 2- 17 Schematic Diagram of Lithologic Trap
These are three basic trap types, many of which are compound traps formed by the combination of folds, faults and porosity changes.
Four. Oil and gas migration and accumulation 1. Oil and gas migration oil and gas are dispersed after the source layer is formed, and under the action of various external forces, they migrate to nearby traps and gather together to form a unified whole with the traps, forming oil and gas reservoirs. It can be seen that oil and gas migration is an indispensable stage of oil and gas reservoir formation. Any movement of oil and gas in the stratum is called oil and gas migration. The migration of oil and gas generated in source beds to reservoirs is called primary migration. All migration of oil and gas after entering the reservoir is called secondary migration, including migration of oil and gas inside the reservoir and migration of oil and gas along fault planes and fractures (Figure 2- 18).
Figure 2- 18 Schematic Diagram of Oil and Gas Migration
Although oil and gas are flowable fluids, there must be power to make oil and gas flow along various channels. The main sources of power are compressive force, tectonic movement force, hydrodynamic force, buoyancy and capillary pressure. They play different roles in two stages of oil and gas migration. Among them, compressive force plays a leading role in the primary migration of oil and gas, and other forces play a major role in the secondary migration of oil and gas.
2. Oil and gas accumulation The process of oil and gas gathering in traps and forming oil and gas reservoirs is called oil and gas accumulation. It is the result of organic cooperation of oil and gas generation, migration, reservoir and trap structure. Adequate oil and gas sources are the material basis for the formation of rich oil and gas reservoirs in the basin. Good reservoir is the basic condition for oil and gas migration and accumulation. However, to form oil and gas reservoirs, there must be a transport layer and a good sealing layer, that is, a good combination of source, reservoir and cap rocks. That is, the oil and gas generated in the source rock can migrate to the reservoir in time, and the quality and thickness of the cap rock can ensure that the oil and gas migrated to the reservoir structure will not escape.
Verb (abbreviation of verb) Oil and gas reservoir type 1. The concept of oil and gas reservoir refers to the basic accumulation of oil and gas in a single trap with the same pressure system. Only oil is accumulated in the trap, which is called oil reservoir; Only natural gas is gathered, which is called gas reservoir; Oil and free gas accumulated at the same time are called oil and gas reservoirs (Figure 2- 19).
Figure 2- 19 Schematic Diagram of Oil and Gas Reservoir
Under the current technical and economic conditions, the oil and gas reservoirs with exploitation value are industrial oil and gas reservoirs. Western countries call it a commercial oil and gas reservoir. But this concept changes with the needs of the country and different technical conditions. When the country is in urgent need of oil and gas, oil and gas reservoirs with no industrial value should also be exploited, and the economic value is in a subordinate position at this time.
2. Types of oil and gas reservoirs According to relevant data, there are tens of thousands of oil and gas reservoirs discovered in the world with various types. In order to guide the exploration and development of oil and gas resources more effectively, it is necessary to scientifically classify the discovered oil and gas reservoirs. At present, there are many oil and gas reservoir classification methods adopted at home and abroad, mainly including four kinds.
(1) can be divided into high-yield oil and gas reservoirs, medium-high-yield oil and gas reservoirs, low-yield oil and gas reservoirs and non-industrial oil and gas reservoirs according to daily output.
(2) According to the reservoir shape, it can be divided into layered reservoirs (such as anticline reservoirs), massive reservoirs (such as buried hill reservoirs) and irregular reservoirs. The oil and gas distribution of irregular oil and gas reservoirs has no certain shape, such as fault oil and gas reservoirs, stratigraphic oil and gas reservoirs and lithologic oil and gas reservoirs.
(3) According to hydrocarbon composition, it can be divided into oil reservoir, oil and gas reservoir, gas reservoir and condensed gas reservoir. Hydrocarbons in traps exist only in liquid form and are called oil reservoirs. There are both liquid oil and free natural gas in the trap, which is called oil and gas reservoir. Traps where only natural gas exists are called gas reservoirs; Under the formation conditions of high temperature and high pressure, hydrocarbons exist in gaseous state, and with the decrease of temperature and pressure during exploitation, they will become condensate oil after reaching the ground. This kind of gas reservoir is called condensate gas reservoir.
(4) According to the origin of traps, they can be divided into structural reservoirs, stratigraphic reservoirs and lithologic reservoirs. Oil and gas are accumulated in traps formed by deformation or displacement of strata due to tectonic movement, which is called structural oil and gas reservoirs; Oil and gas are accumulated in traps formed by stratigraphic overlap or unconformity coverage, which is called stratigraphic oil and gas reservoir; Oil and gas are accumulated in traps formed by lateral changes in reservoir lithology due to changes in sedimentary conditions, which is called lithologic oil and gas reservoirs.
In order to facilitate exploration and development, the classification of oil and gas reservoirs should follow two basic principles: first, the classification should be scientific, that is, the classification should reflect the genetic types and formation conditions of traps in order to seek regularity; Second, the classification should be practical and can guide oil and gas exploration and development more effectively.