1. Stratigraphic Concept
In geological history, layered rocks formed in a certain era are called strata. Strata are a layer or group of rocks with a certain layer in the crust. Under normal circumstances, the strata formed first occupy the lower part, and then the strata formed last occupy the upper part. The interface between layers can be an obvious layer or sedimentary cross section. It mainly includes sedimentary rocks, volcanic sedimentary rocks and shallow metamorphic rocks which undergo certain metamorphism. In terms of lithology, the stratum includes various sedimentary rocks, volcanic rocks and metamorphic rocks; in terms of age, the stratum has the concept of old, new and time.
Rock is a solid combination of one or several rock forming minerals with certain structural structure. According to its genesis, it can be divided into sedimentary rock, magmatic rock and metamorphic rock. The rocks closely related to oil and gas fields are mainly sedimentary rocks, which are mainly sandstone, limestone and mudstone. Under special conditions, magmatic rocks (such as granite) and metamorphic rocks can also form oil and gas reservoirs.
Magmatic rock is a rock formed by the cold solidification of magma. Magma is a kind of viscous silicate melting material which is rich in volatile components under high temperature and high pressure. When magma moves to the weak zone of the earth's crust (fracture or imbalance), its temperature and pressure will gradually decrease, volatile matter will continuously separate out, magma will gradually differentiate itself, and magma rock will be formed after condensation and crystallization, also known as igneous rock.
The process of magma movement from the deep to the shallow is called intrusion, and the rocks formed by condensation and consolidation at this stage are called intrusive rocks. The process of magma flowing out of the ground or into the sky along the fissures is called magmatic eruption or volcanic activity, and the rocks formed by condensation and consolidation in this stage are called extrusive rocks or volcanic rocks.
4. Metamorphic Rocks
A type of rock formed by metamorphism of sedimentary rock, magmatic rock or pre formed metamorphic rock is called metamorphic rock. Such as slate, phyllite, schist, gneiss, marble, quartzite, etc. Marble is formed by metamorphism of limestone, dolomite and other carbonate rocks; quartzite is formed by metamorphism of quartz sandstone or siliceous rocks; slate and phyllite are formed by metamorphism of clayey and silty protoliths. In the process of metamorphism, no matter the deformation of rocks or the recombination of material components, they are all carried out in the solid state, which is different from magmatism.
5. Sedimentary Rocks
Sedimentary rock is one of the three types of rocks (sedimentary rock, magmatic rock and metamorphic rock) that make up the lithosphere of the earth. It is a kind of rock formed by the transport, deposition and post deposition of the original material components of sedimentary rocks such as weathering products, volcanic materials and organic materials of parent rocks under the condition of the crust surface.
Sedimentary rock is a kind of rock formed by weathering and denudation of any early formed rock (magmatic rock, metamorphic rock and pre formed sedimentary rock), organic matter, volcanic matter and cosmic matter under the condition of normal temperature and atmospheric pressure on the surface, which is transported, deposited and diagenesis in situ or by external force. More than 99% of oil and gas are stored in sedimentary rocks.
6. Formation of Sedimentary Rocks
Any rock formed in the early stage is subject to long-term weathering by the sun, rain, water, glaciers, wind and organisms, and then is moved to low-lying areas or appropriate places for deposition or accumulation by the wind, surface water (mountain torrents, rivers, tidal waves) and organisms, and finally the sediment continues to thicken, continuously experiencing compaction and glue Only by diagenesis such as junction, recrystallization, metasomatism and precipitation of dissolved matter can sedimentary rocks be consolidated. This kind of action is called diagenesis. Diagenesis is the last stage of sedimentary rock formation.
The formation of sedimentary rocks generally undergoes the following processes: the formation of sediments, the weathering of parent rocks, the transportation and deposition of weathering products, and the diagenesis of sediments.
7. Geological Age
In the development of the earth, a set of layered rocks formed in a certain era are the strata of that era. Under normal circumstances, the later formed strata are always above the first formed strata, which is the stratigraphic sequence law.
According to certain principles, all strata in the crust are divided into five parts: Archean, Proterozoic, Paleozoic, Mesozoic and Cenozoic. Each boundary can be divided into several systems, each system can be divided into several systems, and the system can also be subdivided.
The above stratigraphic units, such as the universe, boundary, system, series and stage, are divided according to the evolution stage of organisms. They are applicable units in international and regional scope, usually called time stratigraphic units. Each stratigraphic unit has a corresponding time unit for the formation of this stratum. According to the stratigraphic sequence and the fossils contained in it, the whole history of the earth is divided into many natural stages or periods, which are called geological ages. When they are arranged from the old to the new, they are compiled into geological year representatives.
Basic Types of Construction
The scale of geological structure, which is thousands of kilometers, can be identified only by comprehensive analysis of geological and geophysical data and interpretation of remote sensing data, such as the lithospheric plate structure; the small ones, which are measured in millimeters or even micrometers, can be observed only by means of optical microscope or electronic microscope, such as deformation of mineral grains, lattice dislocation, etc. The basic types of geological structure are: horizontal structure, inclined structure, fold structure and fault structure.
1. Horizontal Structure
The original rock stratum is generally horizontal, and it is called horizontal structure if it remains horizontal or nearly horizontal without fold change under the influence of vertical crustal movement. As is common in the red beds of the tertiary system. In the horizontal structure, the new strata are always above the old strata.
2. Inclined Structure
Inclined structure refers to the angle between the strata and the horizontal plane after the structural movement of the strata. Inclined strata are often a wing of a fold, a wall of a fault, or caused by uneven lifting movement.
3. Fold Structure
The bending of rock under lateral compressive stress is called fold. Fold only refers to the single bending of rock stratum, while the continuous bending of rock stratum is called fold. There are two basic types of folds: anticline and syncline. Anticline is the core of the rock relatively old, the two wings of the relatively new fold. Syncline is a relatively new rock layer in the core and an old fold on both wings.
4、 Fault Structure
When the stress exceeds a certain strength, the rock will break, even dislocation along the fracture surface, which will destroy the continuity and integrity of the rock stratum. It is called fracture structure. There are two types of fault structures: joint and fault according to whether the rock on both sides of the fault has obvious sliding.
Joint refers to the fracture structure without significant displacement after rock fracture; fault refers to the fracture structure with large displacement along the fracture surface of rock stratum or rock mass. The elements of faults include: fault plane, fault line, fault plate and fault distance. According to the relationship between the relative movement of the two fault walls, the fault types can be divided into normal fault, reverse fault, horizontal thrust fault, hub fault, etc.
Primary Structure and Secondary Structure
According to the time sequence of structural formation, geological structures can be divided into primary structure and secondary structure, and secondary structure is the main object of structural geology.
A structure formed during diagenesis. The primary structure of igneous rock is mainly produced by the force of magma flow and condensation contraction, and the primary structure of igneous rock is mainly produced by the force of magma flow and condensation contraction. It can be divided into primary flow structure and primary fracture structure. The primary flow structure is the directional structure formed by solid objects in the flow of liquid magma, such as the flow surface formed by the directional arrangement of lamellar, plate-like minerals, flat segregation and xenoliths in the early stage of magma, and the streamline formed by the directional arrangement of columnar and needle like minerals in the early stage of magma; the primary fracture structure has columnar joints and the occurrence of flow surface or streamline according to joints and flow surface The vertical joint, horizontal joint, oblique joint and layer joint are divided in the shape relation.
The primary structure of sedimentary rock is mainly produced by the action of surface force. It includes bedding, bedding structure, intrabedding structure and cross bedding structure.
① Bedding structure refers to the layered structure in sedimentary rocks, and its stratification is revealed by the changes of sediment composition, grain size and color. Structural bedding is the basic reference plane for studying secondary structure and an important primary structure.
② The layer structure mainly refers to ripple mark, mud crack, rain mark, impression, insect mark, etc., which are developed on the top or bottom of sedimentary strata. In the field, it can be used to identify the top and bottom of strata to judge the normal and inversion of stratigraphic sequence.
③ Intraformational structure mainly refers to the primary structure limited to a certain rock stratum, such as grain sequence layer, cross layer, intraformational fold and intraformational fault, among which, besides intraformational fault, the top or bottom surface of the stratum can also be identified according to their internal structural characteristics.
④ The cross layer structure mainly refers to the anticline or fault developed in the multi-layer strata, which is called synsedimentary anticline or synsedimentary fault. Syndepositional anticline is a kind of anticline developed gradually in the local uplifting part of the basin which is regional crust depression and receiving sedimentation continuously. Therefore, it has the original arching bending state. The thickness of the axial stratum is smaller than that of the wing, and the grain size of the axial clastic sediment is larger than that of the same layer of the wing.
Synsedimentary fault, also known as growth fault, refers to the fault which is active at the same time with sedimentation, generally developed at the edge of sedimentary basin and has the nature of normal fault. The basin is located in the descending wall of fault, whose stratum thickness is obviously larger than the ascending wall of fault, and the fault distance increases with the depth, that is, the older the stratum age is, the larger the fault distance is. In addition, the rhythmic layer is also a kind of primary cross layer structure, which is composed of multiple grain sequence layers.
After the formation of rocks, the tectonic deformation caused by tectonic movement is the main research object of structural geology. Secondary structure includes fold, joint, fault, cleavage, lineation, etc.
According to the deformation properties, it can be divided into:
① Continuous deformation structure. The structure in which the continuity of rock stratum is not damaged, and the deformation property belongs to plastic deformation, such as fold, etc.
② Discontinuous deformation structure. The structure where the rock layer is cut and loses its original continuity belongs to fracture deformation, such as joint and fault.
According to geometric elements, it can be divided into:
① Planar structure. The structures represented by geometric planes, such as fold axis plane, joint plane, fault plane, cleavage plane, etc. Generally, the plane structure with systematicness is called foliation.
② Linear structure. Structures represented by lines of geometric significance, such as fold junction, fault scratch, directional arrangement of non equiaxed minerals, intersection lines of two structural planes, etc. The surface and linear structures can also be divided into two types: Abstract (such as the intersection of fold axis, hinge and two structural surfaces) and divisional. The former is only of geometric significance, not of concrete existence, while the latter is of concrete surface and line structure.
According to the distribution characteristics of plane and linear structures in geological body, it can be divided into:
① Permeable structure refers to the plane structure and linear structure, such as cleavage, gneissic texture, schistosity and various small-scale lineations, which are uniform, continuous and arranged in a certain format in the geological body on a certain scale;
② Non permeable structure refers to the surface structure and linear structure that are non-uniform, discontinuous and occur in a separate way in geological body, such as joint surface, fault surface, large-scale rod structure, window lattice structure, etc. The permeability and non permeability of geological structures are relative to a certain scale of observed structures, and the permeability structures on a larger scale can also become non permeability structures on a smaller scale; generally speaking, the permeability structures are aimed at two scales of small structure and microstructure.
The mechanical movement of deformation and displacement of the lithospheric geological body is caused by the internal dynamics of the earth. Tectonic movement is caused by the internal force of the earth, which is the mechanical action of the displacement and deformation of the crust and even the lithosphere, as well as the accretion and extinction of the ocean floor, and the associated seismic activity, magmatism and metamorphism. The tectonic movement produces various geological structures such as folds and faults, which cause the changes of sea and land contours, the uplift and depression of the crust, the formation of mountains and trenches, etc. Tectonic movement plays an important role in the process of crustal evolution.
The tectonic movement is mainly manifested as the mechanical movement of the crust, but it is not limited to the movement of the crust, usually also involves the lithosphere. In general, the slow tectonic movement is not easy to be detected. Under special circumstances, the tectonic movement is violent and rapid, which is manifested as an earthquake, which may also cause landslides, tsunamis and so on. In these circumstances, people can detect the tectonic movement.
According to the direction of crustal movement, it can be divided into vertical movement ("Ascending Movement", "Land Making Movement" - along the Earth Radius Direction) and horizontal movement ("orogeny" - along the earth tangent direction)
1. Horizontal movement: refers to the force that the earth's crust plays a major role in the horizontal direction, that is, the movement of the earth's crust and rock strata under the action of the force that is tangent to the ground (including the compression and tension of the earth's crust), which makes the adjacent blocks be squeezed, separated or pulled apart, or sheared, staggered or even rotated. Horizontal movement mainly makes the strata of the earth's crust bend and break, forming huge fold mountains and fault structures. Therefore, horizontal movement is also called orogeny.
2. Vertical movement: the vertical movement of the earth's crust refers to the upward or downward movement of the crustal block along the direction of the earth's radius. Vertical movement is often manifested as a large-scale uplift or depression, resulting in land and sea changes and relief. As the earth's crust rises and the sea recedes, part of the sea floor is called land; when the earth's crust falls and the sea water invades, the original land becomes sea. Therefore, the vertical movement is also called the land making movement.
Horizontal motion and vertical motion are the basis of analyzing the formation of terrain, but it should be pointed out that these two kinds of motion are often accompanied, and the results of motion can not be arbitrarily separated and distinguished, in fact, the two are interrelated and affect each other.
Under the strong compression of the crustal movement, the rock strata will undergo plastic deformation, resulting in a series of wavy bends, which are called folds. In principle, folding is a very simple process, but there are many kinds of folds due to different reasons.
Syncline is a concave fold, the later it is formed, the closer it is to the center. Syncline is a typical downward curved fold, pointing upward. Its top is also eroded, so we can only see the edge it forms.
Anticline and syncline are similar, but reverse. Anticline is a convex fold. The earlier it is formed, the closer it is to the center, and the side is often syncline. However, in reality, there will be fracture and erosion to separate the two. Anticlines often produce many excellent gas traps, which are suitable for oil exploration.
3. Monoclinal fold
Monoclinal fold refers to the bending of all layers in the same direction. There are many reasons for its formation. The most common one is the differential compaction of the stratum structure.
4. Sharp Edge Fold
This kind of fold is often called V-shaped fold by oil companies. As the name implies, it often presents V-shaped fold. It is often formed due to local compressive stress, but some very specific conditions are needed for forming. There are four development stages in total: curve forming core, parallel fold, two wings extending / middle sharpening, sharp edge fold tightening.
5. Recumbent fold
A recumbent fold is an inverted fold, an overturned or completely reversed fold, with an axial plane of a certain angle, and the strata on one side are overturned. The axis of recumbent fold is basically horizontal.
6. Isoclinal Fold
The angle of the two wings of the isoclinal fold is between 0 ° and 10 °, and the two wings are basically parallel.
In geological structure, domes are formed by symmetrical anticlines crossing each other and penetrating to the top of each other. It is due to the horizontal stress of refolding and the impact / puncture action of the atmosphere (the vertical displacement of the deeper and lighter part moving towards the surface).
To some extent, the basin is a fold opposite to the dome, because the structural deformation of the strata lying on the ground previously leads to large-scale structural reconstruction of the strata. A structural basin is a geological depression.
Not all folds can be easily distinguished, and not all folds can be classified (because the field of vision is not good enough, or the geology is strongly eroded, or other functions hide the folds). Geological fold is very complex, if you work in the field of geology, you may not be so lucky to meet several types of geological fold completely in line with the above. But don't be discouraged. It is because of the difficulty of geology that it has become a science we know and love.