Introduction
The carrier system refers to the hydrocarbon transport network composed of formation permeable bodies, faults and unconformities in a 3D geological body, which is the “bridge and link” connecting source rocks and traps. In recent ten years, remarkable progresses have been made in the research on the carrier system. Chen et al.[1] analyzed the classification and characteristics of carrier systems; Yang et al.[2] believed that the occurrence and properties of faults, unconformity surfaces, carrier beds and fractures were important factors that influence the hydrocarbon transporting ability and effectiveness; Liu et al.[3] pointed out that the classification of carrier systems should take source rocks into full consideration, and studied the difference in hydrocarbon transporting ability of different transporting media; Lin et al.[4] proposed the methods and procedure for carrier system researches, and held that the reconstruction of paleo-porosity, paleo-pressure and paleo-structure, and reservoir-forming period analysis were the key techniques in the analysis and research of hydrocarbon carrier systems; Wu et al.[5] examined the source, pathway and migration direction of oil and gas by using modern hydrocarbon migration theories and geochemical tracing technologies; and Song et al.[6] characterized the comprehensive transporting ability with the opening indexes of fault and the transporting index of sand bodies.
The above researches mainly focus on the description, depiction and effectiveness evaluation of carrier systems, but few of them are on the modeling of carrier systems. Researches related to the modeling of carrier systems include two aspects: (1) stochastic modeling of sand bodies restrained by 3D seismic attributes[7]; (2) fault surface morphological modeling based on 3D seismic fault surface interpretation data[8]. These two types of modeling adopt their own mesh systems, and aren’t connected. Strictly speaking, there is no technology developed specifically for mesh modeling of carrier systems at home and abroad.
The mesh modeling of 3D carrier system is the extension of 3D geological modeling, and its development is closely related to 3D geological modeling. 3D geological modeling is a comprehensive technology that uses computer technologies to combine tools such as spatial information management, geological interpretation, spatial analysis and prediction, geostatistics, entity content analysis and graph visualization under a virtual 3D environment to do geological analysis[9]. In 1993, Houlding[10], a Canadian scholar, proposed the concept of “3D geological modeling”. Driven by the increasingly growing demands and propelled by the computer, 3D geometric modeling and other relevant disciplines, the 3D geological modeling technology has been developing rapidly, and the petroleum exploration industry is the industry where the 3D geological modeling technology is used the most widely and deeply. Since 2000, some types of 3D geological modeling software (such as Petrel, RMS, GoCAD) have been widely used in the basic geological research of oilfields[11,12,13]. Since 2005, China has made some research achievements in 3D geological modeling, and has launched some application software such as DeepInsight developed by Beijing Grid- world Software Technology Co., Ltd. with the support of Beihang University, GeoView by China University of Geosciences, GSIS by Peking University[13,14,15].
Mesh modeling of carrier system can fundamentally improve the efficiency and computational accuracy of hydrocarbon migration and accumulation simulation. In a 3D geological body, the carriers, especially faults and unconformities, take only a very small volume fraction, and differ widely in parameters from surrounding strata. Thus, only by dividing them separately, can they be assigned relevant parameter values accurately. If the traditional 3D stratum mesh modeling method is used (faults and unconformities are included in the stratum mesh), neither can proper parameters be assigned to faults and unconformities separately, nor can their original natural forms be maintained, thus the transporting direction and ability may be changed.
At present, there are mainly two types of 3D hydrocarbon migration and accumulation simulation technologies: multiphase Darcy flow simulation technology, and invasion percolation simulation technology. Core algorithms of the multiphase Darcy flow method include the finite element method, finite volume method and finite difference method. In comparison, the invasion percolation simulation technology emerged late. In 1983, Wilkinson[16] proposed a new percolation theory; in 2000, Meakin[17] studied the invasion percolation and secondary migration mechanism from the perspective of experiment and numerical simulation, and Carruthers et al.[18] used the improved invasion percolation technology to simulate the migration of fluids; in 2007, Zhou et al.[19] used the percolation model to explore the change rules of hydrocarbon migration pathways; in 2009, Hantschel et al.[20] introduced the invasion percolation technology in detail, and Shi[21] described the technical background, technical methods and application effects of the invasion percolation method and gave improvement suggestions; and in 2013, Guo et al.[22] proposed the 3D-IP model and used it to simulate hydrocarbon migration and accumulation.
Main foreign commercial software of 3D hydrocarbon migration and accumulation simulation includes PetroMod, Temis Suite and Trinity etc., and that developed by Chinese institutions includes BASIMS by the Petroleum Research Institute of Petroleum Exploration & Development and TSM by Sinopec Petroleum Exploration and Production Research Institute. Simulation technologies of such software include 3D three-phase Darcy flow simulation technology and invasion percolation simulation technology[20, 23-27], all of which adopt the single 3D stratum mesh system, without the support of hybrid mesh of fault surfaces, unconformity surfaces and other carrier systems, therefore, it is difficult for them to effectively simulate the hydrocarbon migration and accumulation in faults and unconformities in 3D space.
To solve the problem that the transporting direction and ability may change because the traditional 3D stratum mesh modeling method cannot separately assign parameters to fault surfaces and unconformity surfaces and maintain their original natural forms, a hybrid mesh modeling method of 3D carrier systems and a 3D hydrocarbon tracking technology based on hybrid-dimensional mesh system of carrier systems—a special invasion percolation simulation technology have been proposed in this work. With the western Luliang Uplift in the Junggar Basin as an example, the new technology has been used in depicting the transporting effect of fault surfaces, unconformity surfaces and sand bodies, indicating the hydrocarbon migration pathways and simulating the process of oil accumulation, reservoir adjustment and secondary reservoir formation, to reveal the hydrocarbon distribution rules, hoping to provide basis for decision making in future exploration deployment.
1. Mesh modeling technology of 3D carrier systems
The purpose of the mesh modeling of carrier systems is to establish the mutual relationship between strata (3D body), fault surfaces and unconformity surfaces (2D surface), so that the originally isolated carriers can be connected into a transporting network system, and the relationship between a mesh and the mesh in front and back, left and right, upper and lower levels of it can be found out. To achieve this technical goal, the concepts of 3D stratum mesh and 2D surface mesh were proposed, i.e. hybrid-dimensional mesh system.
Mesh generation technologies of geological modeling mainly include structured mesh and unstructured mesh generation technologies. The unstructured mesh has superior geometric flexibility, and can simulate any complex geometries, but the mesh quality is hard to be ensured and the mesh efficiency is reduced accordingly. Therefore, in recent years, more attention has been paid to the hybrid mesh technology integrating the advantages of structured and unstructured meshes[28,29,30]. In the existing hybrid mesh system of 3D geological bodies, all meshes are 3D mesh bodies. To solve the modeling problem of fault and unconformity surfaces, the unstructured hybrid-dimensional mesh generation method is proposed, and the formed mesh system contains multi-dimensional mesh forms such as the 3D body mesh, 2D surface mesh, line mesh and point mesh.
1.1. Proposal of the unstructured hybrid-dimensional mesh generation method
The stratum mesh cut by a fault consists of stratum body A, stratum body B and fault surface C (Fig. 1a, 1b). There are two types methods: (1) the existence of the fault is neglected in the modeling of the geological body, in other words, the mesh form and volume remain unchanged, but the fault factor is considered in property modeling such as porosity and permeability (Fig. 1c); and (2) the existence of the fault is considered in the modeling of the geological body, and the geological body on the two sides of the fault are divided into two parts for modeling, meanwhile, the two sides of the fault are also handled separately in modeling porosity and permeability (Fig. 1d). The third processing method is proposed in the following section.
Fig. 1.
Fig. 1.
Modeling thought of hybrid-dimensional mesh system.
1.1.1. Advancement of the surface mesh
The existence of fault is considered in the modeling of the geological body, and the geological bodies on the two sides of the fault are divided into two parts for modeling, forming stratum meshes. In addition, the fault surface is taken as the third mesh, i.e. surface mesh (Fig. 1e). At this point, the mesh system isn’t the original single stratum mesh (3D body mesh) any more, and it also contains the surface mesh (2D surface mesh), therefore it is called a hybrid-dimensional mesh system (hybrid mesh system).
1.1.2. Processing method of the unconformity surface
If the unconformity carrier is thick enough, the mesh needs to be subdivided vertically; in this case, the unconformity carrier is regarded as a common geological body, then the conventional method is used for mesh modeling, and the unconformity mesh is a body mesh (stratum mesh), not a surface mesh.
If the unconformity carrier is thin, relative to the total thickness of geological body, there is no need to subdivide the mesh vertically; in this case, the above fault hybrid mesh processing method can be used, and is not repeated here.
1.2. Division of “natural” mesh of 3D geological body
At present, important methods used in 3D geological modeling at home and abroad mainly include triangular mesh division, corner-point mesh division and perpendicular bisection (PEBI) mesh division. To better depict the porosity, permeability, pore-throat radius and other geological parameters of carrier systems such as fault surfaces and unconformity surfaces, a “natural” mesh division method is proposed as follows (Fig. 2).
Fig. 2.
Fig. 2.
“Natural” mesh division process of a 3D geological body (x, y and z are respectively coordinates on x, y and z directions in 3D space).
(1) Data preparation. Framework parameters for establishing the 3D mesh body include the boundary point data in the study area, the stratum structural surface data, and the distribution data of fault surfaces and unconformity surfaces.
(2) Forming 2D PEBI meshes in plane. The PEBI mesh is also known as the perpendicular bisection mesh; in other words, the connecting lines between the central point of the mesh and that of all adjacent meshes pass through the corresponding side boundary of the mesh by perpendicular bisection. A single mesh can be triangular, quadrangular, pentagonal and hexagonal, and its shape is decided by the distribution of data control points, thus a relatively balanced distribution can be established. Therefore, in this study, the PEBI mesh was adopted to build the model. Based on the boundary points and the data points of structural surfaces of target strata, the 2D PEBI mesh division method was adopted to form 2D PEBI meshes in plane.
(3) Building the stratum surface, fault surface and unconformity surface. Based on the stratum structural surface data, the triangular mesh interpolation method was adopted to form a stratum surface; and the same method was used to build the fault surface or unconformity surface.
(4) Forming the stratigraphic columnar PEBI mesh body. With the 2D PEBI mesh as the plane mesh of each stratum and with the stratum thickness as the height of columnar mesh, the 3D PEBI mesh division method was used to form a stratigraphic columnar PEBI mesh body.
(5) Building the 3D “natural” mesh body. On the basis of the stratigraphic columnar PEBI mesh body, with the addition of unconformity surface and fault surface, the intersection points of columnar body between surface and surface, and surface and stratum were obtained, and after searching and sorting again, the 3D “natural” mesh body was finally built.
1.3. Conversion of 3D natural mesh body to geometric mesh system of the carrier system
The 3D natural mesh body contains geometric mesh bodies, surfaces, lines, and points etc.
Surface mesh: it consists of the fault surface and/or unconformity surface. The initial fault surface and/or unconformity surface have no thickness, and only after a certain thickness (given according to actual thickness) is assigned, the surface mesh has volume, and it is similar to a thin plate.
Line mesh: a line segment is formed by intersection of any two surface mesh units. After thickness is given to the surface mesh unit, the line mesh is similar to a fine needle (or pipeline). The line mesh is a hub connecting two surface mesh units and is the only way along which oil and gas must pass from one surface mesh unit to another.
Point mesh: it is an intersection point formed by intersection of any two line mesh units. When the line mesh unit has width, the point mesh unit is similar to a small ball (or four-way conversion interface of pipeline). The point mesh is a hub connecting two line mesh units and is the only way along which oil and gas must pass from one line mesh unit to another.
By converting all the elements, the model of carrier system is built (Fig. 3). The details are as follows.
Fig. 3.
Fig. 3.
Conversion process of “natural” mesh to geometric mesh of carrier system.
(1) The geometric mesh body (i.e. stratum entity mesh) is directly converted into a body mesh (also called stratum mesh).
(2) The surfaces from fault surfaces and unconformity surfaces constitute surface meshes (Fig. 4).
Fig. 4.
Fig. 4.
Legend of surface, line and point meshes (different colors represent different meshes).
(3) The common intersection line of two surface meshes constitutes line meshes (Fig. 4).
(4) The common intersection point of two line meshes constitutes a point mesh (Fig. 4).
(5) After a thickness is assigned to the surface mesh, a “plate body mesh” is formed; the corresponding intersection line is changed to a “needle mesh”; and the corresponding intersection point is changed to a “spherical mesh”.
(6) The above four types of meshes are combined and arranged by rule, to form a mesh body of 3D carrier system.
1.4. Comparison of 3D mesh generation technologies
The mesh system of carrier systems is a hybrid-dimensional mesh system (hybrid mesh) consisting of body, surface, line and point meshes. At present, all international 3D basin simulation software and petroleum system simulation software adopts the single stratum mesh system (stratum mesh) consisting of body mesh or single fault surface mesh system rather than the hybrid mesh system. In the stratum mesh, the fault surface mesh and the stratum mesh belong to two types of mesh systems, and there is no connection between them. The comparison of the hybrid mesh system and other systems is as follows.
1.4.1. Stratum mesh system
Advantages: (1) the relationship between mesh units is relatively simple, which is only “up”, “down” and “lateral”; (2) no matter how big the mesh number is, all mesh units are of one type, i.e. the stratum mesh; (3) hydrocarbon migration and accumulation can be only tracked in the stratum mesh, and the tracking algorithm is relatively mature and easy to be achieved; and (4) the 3D dynamic visualization technology is relatively mature, and the dynamic display of simulation results is easy to realize.
Disadvantages: (1) the fault surfaces and/or unconformity surfaces cannot be divided separately and they are included in the stratum mesh, so they cannot be assigned values accurately separately; (2) the relationship between carriers cannot be reflected effectively, thus the migration and accumulation process of oil and gas in the fault surfaces and unconformity surfaces cannot be simulated effectively.
1.4.2. Hybrid mesh system
Compared with the stratum mesh system, the hybrid mesh system includes surface mesh, line mesh and point mesh in addition to stratum mesh.
Advantages: fault and unconformity surfaces can be taken out alone and the connection between carrier systems can be established, therefore fault surfaces and unconformity surfaces can be described effectively, providing key parameters for hydrocarbon migration and accumulation in the fault surfaces and unconformity surfaces.
Disadvantages: (1) the process of establishing this kind of 3D mesh is much more complicated, needing more geological parameters; and (2) the mesh types are multiple and the relationship between each other is complicated, thus it is more difficult to realize the hydrocarbon migration and accumulation simulation and 3D dynamic visualization.
1.5. Geological parameters of the 3D carrier system
1.5.1. Parameters of stratum meshes
The transporting ability of a stratum mesh is closely related to sedimentary facies. The distribution of sand body and physical properties of rock directly influence its transporting ability. Rock physical property parameters include porosity, permeability and pore-throat radius, and they are assigned to each stratum mesh by using the spatial interpolation method.
1.5.2. Parameters of fault surface and unconformity
surface meshes
The transporting ability of a fault surface mesh is related to the geological characteristics of the fault zone. The shale gouge ratio and the rock physical properties in the fault zone directly influence transporting ability of the fault. The parameters of the fault surface mesh include shale gouge ratio, rock porosity, permeability and pore-throat radius, etc. After the parameters of each fault are obtained, they are assigned to the fault surface meshes through human-computer interaction.
The unconformity surface mesh is related to rocks in the weathered zone and their physical properties. The main parameters are rock porosity, permeability and pore-throat radius etc., and they are assigned to each unconformity surface mesh by using the spatial interpolation method.
1.5.3. Parameters of line and point meshes
Parameters of line meshes can automatically inherit the surface mesh parameters, or be assigned to line mesh by human-computer interaction; similarly, parameters of point meshes can automatically inherit the parameters of line mesh, or be assigned to point mesh by human-computer interaction.
2. 3D hydrocarbon migration and accumulation simulation based on the hybrid-dimensional mesh of carrier system
Compared with the 3D three-phase Darcy flow model, the invasion percolation numerical model is simpler with fewer simulation parameters, so it has wider applicability and is widely used now. In this study, on the basis of the original 3D invasion percolation simulation technology[16,17,18,19,20,21,22], the buoyancy flow mode was adopted and the calculation model for the transporting ability of the fault surface mesh was supplemented.
2.1. Oil and gas flow modes
The buoyancy flow mode refers to the upward floating of oil and gas under density difference in the formation pore water, which is generally intermittent flow. Therefore, it is hard to be quantitatively characterized with Darcy’s formula[31]. Buoyancy flow includes free upward floating and restricted upward floating.
2.1.1. Free upward floating
Free upward floating (unobstructed flowing) refers to the situation where oil droplets and air bubbles float upward freely without the restriction of the capillary resistance. Unobstructed flowing mainly occurs in the following cases.
(1) It may occur when the channel diameter of pore medium is greater than the oil droplets and air bubbles; (2) when oil and gas flow from smaller pore-throat to larger pore-throat, the capillary resistance act as a propelling force in this case, and oil and gas flow freely without the restriction of the capillary resistance; (3) oil and gas have flowed previously and the path has been wetted (oil-wetted), or the hydrocarbon saturation in pores has reached the minimum migration saturation.
2.1.2. Restricted upward floating
As rock composition and channel pore diameter change continuously, the migration of oil droplets and air bubbles will not always be unobstructed. Therefore, if they are obstructed when floating upward, they have to wait for subsequent supplement of oil and gas fluids to increase the buoyancy force, and only in this way they can overcome the capillary resistance produced by the deformation of oil and gas fluids and continue to float upward. This is a discontinuous migration process, during which the minimum oil (gas) column height needed for oil and gas to overcome the capillary resistance and continue to migrate is called the critical height.
2.2. Driving force and resistance of hydrocarbon migration
2.2.1. Driving force
In the buoyancy flow mode, the driving force of hydrocarbon migration is the buoyancy force, and its calculation formula is:
2.2.2. Resistance
In the buoyancy flow mode, the resistance of hydrocarbon migration is the capillary force, and its calculation formula is:
2.2.3. Transporting ability of the fault surface mesh
The transporting ability of the fault surface mesh can be determined by the shale gouge ratio (SGR)[32]. The conversion formula of the transporting ability is as follows:
where Pmig is 0-1, 0 represents “close” and 1 represents “open”; SGR is the proportion of accumulative thickness of shale within the range of fault displacement in the formation thickness, which is 0-1, and the bigger the value is, the better the closure will be and the poorer the connectivity will be[32]; SGRclose and SGRopen are different in different areas. Taking the Shahejie Formation in the Bohai Bay Basin as an example, its SGRclose is 0.95 and SGRopen is 0.18.
2.3. Tracking principles of hydrocarbon migration pathways
No matter it is a stratum mesh, surface mesh, line mesh or point mesh, the sea level elevation of the central point of mesh unit is taken as the reference point in the pathway tracking process. For two adjacent mesh units, the mesh unit with higher sea-level elevation is called high mesh for short and that with lower sea-level elevation is called low mesh for short.
2.3.1. General principle
(1) Under the condition of floating upward freely: for two adjacent mesh units, oil and gas flow from low mesh unit to high mesh unit. The flow process is not restricted by mesh type.
(2) Under the condition of floating upward restrictively: after oil and gas encounter resistance and stop flowing, if the subsequent supplement of hydrocarbon fluid increases the buoyancy force to a level at which oil and gas can overcome the resistance, they will break through from the mesh unit with the minimum resistance and flow forward. The flow process is not restricted by mesh type, and the flow direction does not depend on the height of the mesh, but only on the resistance value.
2.3.2. Priority principle under equal resistance
According to the basic rule of the invasion percolation model[20,21], during migration, oil and gas only move forward along the direction of the minimum resistance, and will stop only when the resistance is greater than the buoyancy, and when the subsequent oil and gas make the height of oil and gas column meet the requirement that the buoyancy overcomes the resistance, they will continue to move forward or change moving direction. When the resistances in multiple directions in the front are equal and less than the buoyancy force (the probability of this is small), the priority principle needs to be determined, which can be determined by software randomly or given by experience according to different situations. In this study, the principle of first entering the surface mesh was taken.
2.4. Calculation of flowable oil and gas quantity in migration pathways
After oil and gas encounter a barrier and stop in the course of flowing forward, the flowable oil and gas quantity in the migration pathways needs to be calculated, i.e. the supplementary quantity of oil and gas fluids, and the new oil and gas column height and its buoyancy after the supplement need to be calculated, so as to determine whether oil and gas fluids can break through the resistance and continue to migrate forward. Apparently, in the buoyancy flow mode, the calculation of flowable oil and gas quantity is a key technology for tracking the hydrocarbon migration pathways.
Fig. 5.
Fig. 5.
Flow chart of tracing algorithm.
Fig. 6.
Fig. 6.
Schematic diagram of tracing process and paths (“flowable” refers to the mesh containing flowable hydrocarbons; and “residual oil” refers to the mesh containing residual hydrocarbons).
From the starting point of the source tracing, the tracking is performed according to two paths, “downwards” and “leftwards”: (1) the first path passes through the “bridge-shaped” accumulation mesh area, and a special treatment is needed in the tracing process; (2) “bifurcation” and “merging” situations appear on the second path, and problems of “multiple sources” and “split flow” need to be solved in the tracing process.
After source tracing, all “flowable” meshes (meshes containing flowable oil and gas) are sorted from larger to smaller according to the overflow elevation, and the recorded mesh number is output. Then, the porosity and hydrocarbon saturation of all flowable meshes are searched by mesh number, and the volume of all flowable oil and gas is calculated.
2.5. Calculation of hydrocarbon accumulation quantity and residual quantity
2.5.1. Determination of hydrocarbon accumulation area
In the above source tracing process, the hydrocarbon accumulation area (i.e. accumulation mesh unit group, see Fig. 6) is passed. By searching, the mesh group where the oil and gas saturation reaches that of the conventional oil and gas reservoir (no lower than the minimum saturation of hydrocarbon migration) and hydrocarbons surrounded by trap is determined as the hydrocarbon accumulation area.
2.5.2. Calculation of hydrocarbon accumulation quantity
After the tracking is completed, the hydrocarbon saturation in all meshes of the whole 3D geological body is in a relatively stable status (unchanged temporarily). According to the porosity and hydrocarbon saturation at this moment, the hydrocarbon volume in the accumulation mesh unit groups are calculated one by one, and the entire hydrocarbon accumulation volume is obtained by summing them up. The calculation formula is as follows:
2.5.3. Calculation of residual hydrocarbon quantity in migration pathways
After the tracking is completed, the mesh units with hydrocarbon saturation no more than the minimum saturation of hydrocarbon migration in all meshes of the whole 3D geological body are called the residual hydrocarbon mesh unit group. Like the calculation method of the accumulation quantity, according to the porosity and hydrocarbon saturation at this moment, the hydrocarbon volume in the residual hydrocarbon mesh unit groups are calculated one by one, and the entire hydrocarbon residual volume is obtained by adding them up.
3. Application example
The western Luliang Uplift in the Junggar Basin is taken as the example. The study area is located on the west side of the Luliang Uplift of the Junggar Basin (Fig. 7a), including Xiayan uplift, northeast part of Dabasong uplift and west part of Sangequan uplift. It is next to the western Well Pen-1 sag and the Sannan sag on the southeast, and it is connected with the Mahu sag and the Yingxi sag on the northwest, with an area of 3 502 km2 (Fig. 7b). The target strata are Jurassic system and Cretaceous system, and the source rock is the lower Wuerhe Formation of the Permian System (Table 1). The proved reserves in the area are 2.086 4×108 t, including 0.779 7×108 t in Cretaceous system, which is mainly distributed around Well LU9 at the northeast corner of the study area; 0.767 9×108 t in Jurassic Toutunhe Formation, which is mainly distributed around Well SN4, Well SN21 and Well LU9; 0.479 9×108 t in Jurassic Xishanyao Formation, which is mainly distributed around Well SN7 and Well LU9; and 0.058 9×108 t in Jurassic Sangonghe Formation, which is mainly distributed around Well XY11.
Fig. 7.
Fig. 7.
Location of the study area (a) and division of structural units (b).
Table 1 Strata and lithologic characteristics of the study area.
| Strata | Lithology and rock stratum | Tectonic movement period | ||
|---|---|---|---|---|
| System | Series | Formation | ||
| K | K1 | Hutubi (K1h) | Interbedded sandstone, sandy conglomerate and mudstone, and mudstone is local seal | Middle stage of late Yanshanian |
| Qingshuihe (K1q) | ||||
| J | J3 | Absent | Early stage of middle Yanshanian | |
| J2 | Toutunhe (J2t) | Interbedded sandstone, sandy conglomerate and mudstone with coal interlayers; mudstone is local seal | Early stage of early Yanshanian | |
| Xishanyao (J2x) | ||||
| J1 | Sangonghe (J1s) | |||
| Badaowan (J1b) | ||||
| T | T3 | Baijiantan (T3b) | Interbedded sandstone, sandy conglomerate and mudstone | Indosinian |
| T2 | Karamay (T2k) | |||
| T1 | Baikouquan (T1b) | |||
| P | P3 | Upper Wuerhe (P3w) | Late Hercynian | |
| P2 | Lower Wuerhe (P2w) | Interbedded sandstone, sandy conglomerate and mudstone, mainly mud shale (source rock) | ||
3.1. Geological characteristics
3.1.1. Source rock
There develops lower Wuerhe Formation source rock in the middle Permian series, which is mainly distributed in the Western Well Pen-1 sag on the southern side and the Mahu sag on the northwest side of the study area (Fig. 7b). In the Western Well Pen-1 sag, the effective source rock has a thickness of 100-250 m, TOC of 2%-4%, Ro of 1.2%-2.2%, and total hydrocarbon-generating intensity of (50-400)×104 t/km2. In the Mahu sag, the effective source rock has a thickness of 150-275 m, TOC of 1.5%-2.5%, Ro of 1.2%-2.0%, and total hydrocarbon-generating intensity of (50-400)×104 t/km2. The two sags have a combined hydrocarbon supply area of about 3 000 km2, an average hydrocarbon-generating intensity of about 150×104 t/km2, and hydrocarbon supply quantity of 45×108 t. Apparently, the hydrocarbon source is sufficient.
3.1.2. Reservoir characteristics
The Jurassic and Cretaceous reservoir rocks in the study area include sandstone and sandy conglomerate, with pores dominated by primary intergranular pores. The lower Cretaceous series belongs to high-porosity medium-permeability reservoir, the middle Jurassic Toutunhe Formation belongs to medium-porosity medium-permeability reservoir, the Xishanyao Formation belongs to low-porosity low-permeability reservoir and the lower Jurassic series belongs to medium-low porosity low-permeability reservoir (Table 2).
Table 2 Reservoir characteristics.
| Strata | Reservoir lithology | Pore type | Porosity/% | Permeability/10-3 μm2 | Evaluation result | |
|---|---|---|---|---|---|---|
| K1 | Sandstone and sandy conglomerate | Intergranular pore | 26 | 300 | High-porosity medium-permeability | |
| J2 | J2t | Sandstone and sandy conglomerate | Intergranular pore | 18 | 57 | Medium-porosity medium-permeability |
| J2x | Sandstone and sandy conglomerate | Intergranular pore | 14 | 2.4 | Low-porosity low-permeability | |
| J1 | J1s | Sandstone | Intergranular pore | 16 | 3.6 | Medium-low porosity low-permeability |
| J1b | Sandstone and sandy conglomerate | Intergranular pore and intragranular pore | 12 | 1.4 | Low-porosity low-permeability | |
3.1.3. Seals
The lower Cretaceous series and the middle-lower Jurassic series are made up of interbedded sandstone, sandy conglomerate and mudstone, among which mudstone layers can act as local seals (Table 1).
3.1.4. Trap types
Main trap types include fault-block, fault-nose (such as Well Lu 9), lithologic-stratigraphic (such as Well Lu 15) and fault-lithology traps (such as Well SN21).
3.2. Carrier system
3.2.1. Faults
The study area mainly has developed 94 faults in five periods, including 18 faults developing in the Hercynian period, 23 faults in the early Yanshanian period, 23 faults in the middle Yanshanian period, 13 faults in the late Yanshanian period, and 17 faults in the Himalayan period. The faults cutting the lower Cretaceous series and the Jurassic system are mainly the Yanshanian faults (Fig. 8). These faults played different roles during different geological periods. Sometimes they acted as channels, and sometimes as barriers.
Fig. 8.
Fig. 8.
Plane projection of Yanshanian fault surfaces (different colors only represent different faults).
3.2.2. Carrier beds of sandstone and sandy conglomerate
In the Jurassic system and Cretaceous system, are underwater distributary channel, interdistributary bay, sheet sand and shore-shallow lake deposits (Fig. 9). The sandstone and conglomerate in underwater distributary channels are the most important carrier beds.
Fig. 9.
Fig. 9.
Distribution of sedimentary facies of main reservoirs.
3.2.3. Unconformities
The unconformity between Jurassic system and Cretaceous system is the key unconformity that controls hydrocarbon migration. On the northeast side, the slope was relatively large and the erosion time was relatively long, so the unconformity had better transporting ability.
3.3. Simulation meshes and key parameters
Key simulation parameters of the study area: 2 884 plane simulation meshes with an area of 3 502 km2, and 11 strata with 4 interpolated sub-layers, 15 simulation layers in total and 59 faults (Fig. 10a-10c). The total number of formed meshes was 54 406, including 45 972 body meshes, 7 884 surface meshes, 549 line meshes and 1 point mesh.
Fig. 10.
Fig. 10.
Simulation scope and meshes.
Key parameters included porosity, pore-throat radius and hydrocarbon-generating intensity, etc. In addition to assigning attribute (parameter) to stratum body meshes, attribute value was also assigned for each fault surface mesh and unconformity surface mesh separately (Fig. 11a, 11b). The optimal effect of the carrier system modeling can be achieved only by conducting meticulous individual assignment for each fault surface and unconformity surface mesh, thus improving the reliability and accuracy of simulation results of 3D hydrocarbon migration.
Fig. 11.
Fig. 11.
Parameters of simulation meshes.
3.4. Simulation results and guidance to exploration
3.4.1. Transporting effect of faults and fault surface meshes
Fig. 12 shows the information of 59 faults and the fault surface meshes through which oil passes. In Fig. 12a, red meshes on the fault surface have residual oil saturation, i.e. the fault surface meshes through which oil passed in the migration process; and red strips in Fig. 12b are the same as those in Fig. 12a, both of which refer to the fault surface meshes through which oil passed in the migration process; and the light blue lines in Fig. 12b refer to the oil migration pathways. According to these figures, the fault surfaces have played a crucial role in oil and gas transport, allowing both vertical transport (vertical migration) and lateral transport of oil and gas (including migration along the lateral direction of faults and that across faults).
Fig. 12.
Fig. 12.
Distribution of residual oil on fault surfaces (a) and migration pathways (b).
3.4.2. Paleo-structure restoration and paleo-reservoir simulation
According to study on the hydrocarbon generation history of Permian source rock and the structural evolution history[33], it is believed that the end of early Cretaceous and the present are the critical moments for reservoir forming. The “back- stripping method” was used to restore the structure during the late depositional stage of early Cretaceous Qingshuihe Formation, in other words, the top surface of the Qingshuihe Formation was defined as a marker bed, and the stratum above the marker bed was back stripped, so that the marker bed was on a horizontal plane with the height at sea level of 0, and the strata below the marker bed were moved to make their relative positions to the marker bed remain unchanged. Generally, it is difficult to tell the reservoir physical properties and the openness of faults during the formation process of paleo-reservoir. In the example, reservoir physical properties were obtained by uniformly multiplying the present values by a coefficient; and for the openness faults, the initial state was set as open. In the simulation process, if the simulation result was found obviously deviated from the present reservoir distribution, the relevant fault was adjusted as closed.
The simulation result of the end of early Cretaceous (critical moment) reveals that the Jurassic system was relatively gentle then, and for oil sources from the Western Well Pen-1 sag on the south side and the Mahu sag on the southwest side, the lateral migration distances were short, and most of the oil mainly accumulated in the Jurassic system in the southwest and the southeast, thus forming a paleo-reservoir (Fig. 13).
Fig. 13.
Fig. 13.
Simulation results of oil saturation at the end of early Cretaceous and migration pathways.
3.4.3. Simulation results of paleo-reservoir adjustment and subsequent oil accumulation
After further burial and subsequent tectonic movements, the Jurassic system became steeper in slope, shifted in the dip direction towards the southeast, and changed in trap form and amplitude, leading to the adjustment of paleo-reservoir. The simulation result of the present reveals that the paleo-reservoir evolved into be the present reservoirs in Well SN21 and Well SN4 after adjustment, with total proved reserves of over 6 000×104 t (Figs. 14a and 15).
3.4.4. Simulation results of migration pathways and oil accumulation at the present
In addition to paleo-reservoir adjustment and subsequent accumulation, the simulation of the present reveals that there are three groups of main migration pathways upward (towards the north) (Figs. 14b and 15).
The first group of pathways (counting from east to west) passes through Well SN7 and finally arrives near Well LU9. On these pathways, the lithologic reservoir of Well SN7 (with proved geological reserves of 2 081×104 t) and the structural-stratigraphic reservoir of Well LU9 (with proved geological reserves of 10 496×104 t) have been discovered.
The second group of pathways passes through Well XY15 and Well XY11, and finally arrives near Well MD4. On these pathways, XY11 reservoir (with proved geological reserves of 589×104 t) has been proved, Well MD4 has tapped commercial oil flow, and there is exploration discovery near Well XY15.
The third group of pathways passes through Well XY12 and arrives near Well YB1 and finally migrates outside the study area on the northwest side. Oil reservoirs have not been discovered on these pathways.
3.4.5. Analysis on consistency of the tracing of wax content in crude oil and the reservoir distribution with simulation results
Figs. 14b and 15 show the direction of hydrocarbon migration, i.e. from south to north. The analysis on the wax content in crude oil confirms this conclusion. Table 3 reveals that the wax content increases from the Western Well Pen-1 sag on the south side to the central Xiayan uplift and to the northern Sangequan uplift in the study area, and the change trend is obvious, showing that the direction of hydrocarbon migration is from south to north.
Table 3 Wax content in crude oil at different structural positions.
| Structural position | Wax content in crude oil/% | |||
|---|---|---|---|---|
| Content (test well) | Minimum | Maximum | Average | |
| Sangequan uplift | 11.44(LU12), 10.39(LU 122), 7.90(LU125), 8.77(LU151), 9.67(LU156) | 7.90 | 11.44 | 9.63 |
| Xiayan uplift | 6.81(XY10), 7.74(XY11), 7.46(XY13), 6.29(XY21), 6.69(XY23), 6.63(XY24) | 6.29 | 7.74 | 6.94 |
| Western Well Pen-1 sag | 0.75(M16), 2.47(M17), 0.72(M171), 1.28(QS1) | 0.72 | 2.47 | 1.31 |
By the end of 2017, oil reservoir in LU9 well area in the study area has been proved, with reserves of 10 496×104 t in the lower Cretaceous series, middle Jurassic Toutunhe Formation and Xishanyao Formation. The Well SN7, Well SN21 and Well SN4 discovered have reserves of 2 081.00×104 t, 2 622.80×104 t and 3 697.65×104 t respectively in the Jurassic system. Fig. 14a shows that there are mainly four oil accumulation areas, i.e. LU9, SN7, SN21 and SN4 well areas. This shows that the main accumulation areas from the simulation are basically the same as the oil reservoirs discovered by exploration. In addition, oil reservoirs are discovered in MD4 and XY11 well areas too, but the simulation result shows that oil and gas only pass through these areas but don’t accumulate in large scale, which may be related to the insufficiency of Parameter analysis and research.
Fig. 14.
Fig. 14.
Simulation results of oil migration and accumulation at the present.
3.4.6. Guidance to exploration
(1) Traps around the paleo-reservoir are favorable areas for oil and gas distribution, for example, the proved reservoirs in Well SN21 and Well SN4 (Fig. 13). Traps around the paleo-reservoir on the southwest side of the study area, especially on the direction of hydrocarbon migration (on the side of the Mahu sag in the study area) have potential conditions to form reservoirs (Figs. 14b and 15).
(2) Areas covered by the first group of pathways in the east, the second group of pathways in the central part and the third group of pathways in the west revealed by the simulation result of the present time are favorable areas for the distribution of oil and gas reservoirs (Figs. 14b and 15). The area covered by the first group of pathways has been explored sufficiently, with proved oil reserves of over one hundred million tons, so it is not the focus of recent exploration; there have been exploration discoveries in the area covered by the second group of pathways, so it is the important area of recent exploration; and oil and gas reservoirs have not been discovered in the area covered by the third group of pathways, and traps on the direction of the pathways (including outside of the study area) are the potential areas of future exploration.
Fig. 15.
Fig. 15.
Migration pathways and oil accumulation simulation results of the present time.
4. Conclusions
A natural mesh division method of 3D geological body has been proposed to divide the fault and unconformity carriers separately, so as to solve the problem in mesh modeling of fault surfaces and unconformity surfaces, and this is the hybrid-dimensional mesh modeling technology of the carrier system, which provides an important mesh system for hydrocarbon migration and accumulation modeling.
A 3D hydrocarbon tracking technology based on the hybrid- dimensional mesh system of carrier systems is proposed - special invasion percolation simulation technology, i.e. the invasion percolation simulation technology considering SGR, which lays a foundation for modeling hydrocarbon migration and accumulation in faults.
The tracing algorithm has been proposed to solve the problems in the bridge-shaped accumulation area encountered in the hydrocarbon migration process and the multiple-source and multiple-path tracing and tracking, which has formed a 3D hydrocarbon migration and accumulation simulation technology based on the hybrid mesh system of carrier systems, providing important technical guarantee for dynamic hydrocarbon simulation.
This new technology can be used to simulate the paleo-reservoir formation, paleo-reservoir adjustment and subsequent accumulation, migration pathways and oil accumulation of the present, and to predict the distribution of paleo-reservoirs and present oil reservoirs (adjusted reservoirs, secondary reservoirs and new reservoirs). These simulation results are of great significance to the study on hydrocarbon accumulation.
The application example reveals that areas around paleo-reservoirs and covered by migration pathways are favorable areas for the distribution of oil and gas reservoirs, which provides an important decision-making basis for future exploration deployment.
Nomenclature
Ck—hydrocarbon accumulation quantity of the kth accumulation area (accumulation mesh unit group), m3;
F—buoyancy, N;
g—gravitational acceleration, 9.8 m/s2;
m—number of accumulation areas;
n—number of mesh units in the kth accumulation area;
pc—capillary pressure, MPa;
Pmig—transporting coefficient of fault (0-1), i.e. probability of connectivity of fault;
QA—total hydrocarbon accumulation quantity of all accumulation areas, m3;
r1, r2—rock pore-throat radius of the current position (mesh unit) and the mesh unit to be flowed into respectively, μm;
s—hydrocarbon saturation of the ith mesh unit in the kth accumulation area, f;
SGR—shear gouge ratio, dimensionless;
SGRclose, SGRopen—SGR values corresponding to the closure and opening of fault respectively;
v—volume of the ith mesh unit in the kth accumulation area, m3;
V—volume of continuous oil (or natural gas), m3;
ρhc—density of underground oil (or natural gas), kg/m3;
ρw—density of formation water, kg/m3;
θ—wetting angle, (°);
σ—interfacial tension, N/m;
ϕ—porosity of the ith mesh unit in the kth accumulation area, f.
Reference
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Through analysis of classification and researches characteristics of pathway system,the study status of it was particularly discussed.The pathway system research was been classified into two types of statics and dynamics.Then pathway systems were been divided into four sorts of reservoir,fault,unconformity surface and composite ones.The five kinds of characteristics of pathway system of independence,dynamic,two-sidedness,complexity and synthesis were pointed out.From two sides of the static and dynamic,research techniques of pathway systems were detailedly introduced.Such as,sequence stratigraphy,sedimentology,diagenesis,basin simulation,dynamic research of oil and gas accumulation,earthly chemistry,experiment simulation of earthly physics,and geological modeling.On the base of discussion above,the problems and the development directions of pathway system researches were indicated.
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Starting from analysis of passage system unit,we conclude that fault,unconformity,carrier bed,occurrenceand character of fracture are the important factors of the ability and effectiveness of transporting oil and gas.Passage systems are the system channels connecting"source"and"reservoir".In the accumulation system frameof different types of oil and gas basins,12 types of oil and gas passage system are pointed out.Study indicates thattransportation of oil and gas is of inconsistency and heterogeneity,and the key to passage system effectiveness isII Journal of Southwest Petroleum University(Science Technology Edition) Jun.2011space-time distribution and evolution of different passage systems.The development characteristics of passage systemsin different locations of fault basin,cratonic basin and foreland basin are studied and we find out that passagesystem determines the direction of oil and gas migration,accumulation period and location.Based on the study,thepassage system types and the controlling factors effectiveness evaluation of prototype basin and find out evaluationparameters of the space-time effectiveness.Taking the amount of migration,accumulation efficiency and the scaleof oil and gas fields as evaluation criteria,the efficiency of passage system is evaluated.Study shows that formedat rift-depression period of fault basin,the platform margin facies fault and platform facies karst of cratonic basin,the foreslope and thrust belt of foreland basin are the most effective passage systems.This study provides a usefulreference for studied on accumulation effectiveness and exploration planning of oil and gas basins.
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Mechanism and quantitative evaluation method of the petroleum migration system
A classification scheme for the petroleum migration system was proposed through the analysis and summary of the migration systems of the main petroliferous basins in China in order to reduce the multiple solutions and make more objective and accurate evaluation possible. The migration elements were classified into three types. The first type was the necessary elements,which included transportation space,power and fluid.The second type was a variety of elements that described the significant features of the migration system,including direction and capability. The third type was the elements that complemented the migration system,such as the scale and hierarchy as well as the formation and function period of the migration system. Based on the migration mechanism,a new nomenclature of the system was proposed: the scale and hierarchy of the migration system+ the period of formation and function + fluid transport direction + the capability of transporting fluid + migration power + migration space. Moreover,according to the contribution of elements on oil and gas migration and accumulation,this article systematically summarized the quantitative evaluation criterion and the assignment principle of the merits of each migration element. The restoration methods and steps of ancient migration system were determined. The recovery of paleo-pores and paleo-structure was the key to migration system recovery during the accumulation period. The method paved the way for the quantitative evaluation of migration,and made it possible to predict potential oil and gas reservoirs better and to improve the exploration success ratio.
Characteristics of the hydrocarbon pathway system and transport model of the Chepaizi and its surrounding areas in the Junggar Basin of China
Based on the analysis of the reservoir distribution characteristics of Chepaizi and its surrounding areas,which combines the basic geologic information and test data,modern hydrocarbon migration theory and geochemical tracer techniques are adopted to investigate the oil-source,migration paths and direction of hydrocarbon in this study area.The features and model of the hydrocarbon migration system in each formation are certified.The burial depth of the strata above Carboniferous is shallower in the northwest and deeper in the southeast,and there is a hydrocarbon supply from the deep sag in the southeast.The results indicate that the light oil in the Neogene Shawan formation issourced from coal source rock of the Jurassic Badaowan formation,which may be mixed with some oils from lacustrine source rocks.Oil that accumulated in the Cretaceous Qinshuihe formation and Jurassic Badaowan formation are sourced from the source rock of Permian Wuerhe formation.The average thickness ratio of sand layer over total strata in the Shawan,Qinshuihe and Badaowan formations are0.50,0.25 and 0.40,respectively.Their sand bodies can be considered to be the preferred paths for hydrocarbon migration in the lateral direction.The wells with oil shows within unconformities at the bottom of Cretaceous and Jurassic are located in the area with porosity higher than 10%,indicating that the unconformities are suitable paths for hydrocarbon migration in the lateral direction.The geochemical tracer parameters show that the hydrocarbon migration direction points to the sand bodies with higher thickness ratio and unconformities with higher porosity.The deep and large thrust faults formed in the Carboniferous-Jurassic connect the hydrocarbon generation center with the accumulation areas.The normal faults subsequently developed exhibit a satisfactory adjustment and reforming effect to the hydrocarbon reservoirs.Hydrocarbon migrates from deep to shallow and from sag to uplift and subsequently accumulates in the higher positions of structures.The hydrocarbon pathway system becomes more complex in the shallow layers than in the deep layers.
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The traps in the ultra-denudation zones at the northwestern margin of Junggar Basin are characterized by wide distribution,shallow buried depth,long trap-source distance,large differences in trap oil-bearing property and etc.Trap oil-bearing property is mainly controlled by the migrating capability of fault-sand transport system,hydrocarbon migration distance,poor reservoir-cap physical properties and other factors.Among them,the fault-sand transport performance can be expressed by fault open-closure index and sand transport index.The hydrocarbon migration distance can be expressed by trap-source plane distance,and trap reserving performance can be characterized by the difference in reservoir porosity between reservoir and cap rock.Whether the trap contains oil or not depends on whether the fault-sand transport index exceeds critical value.The trap oil-bearing degree has a favorable positive correlation with fault-sand transport index and trap reservoir-cap porosity difference,but shows a negative correlation with trapsource plane distance.Through analyzing the relationship between trap oil-bearing property and single factors,multiple factors are combined to establish the multi-parameter fitting relation of trap oil-bearing property in ultra-denudation zones.The application results show that the predicted value of such fitting relation is highly consistent with the true value,so that this relation can be used for quantitative assessment on the trap oil-bearing property in ultra-denudation zones at the northwestern margin of Junggar Basin.
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Three- dimensional hydrocarbon migration and accumulation modeling based on finite volume method
Past, present, and future of basin and petroleum system modeling
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Hybrid mesh generation algorithm for viscous computations of complex aerodynamics configurations
A new surface hybrid mesh generation method based on subdomain decomposition
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A new hybrid mesh generation method was proposed based on subdomain decomposition.First,the structured background meshes were generated by mapping method and the locations of small features such as hole and keyway were determined.Then,the meshes which covered by small features were eliminated from background meshes,and unstructured triangular meshes were generated in this region.Finally,the meshes of target domain were obtained by combining the background meshes and the unstructured triangular meshes.It is capable of preserving the advantages of mapping's high efficiency and high quality of the meshes,high computational accuracy of quadrangle meshes and the powerful geometric adaptive capacity of triangle meshes.Numerical experiments show that the proposed method may generate high quality hybrid meshes for complex surfaces automatically,the quality of resulting meshes and the algorithm efficiency are superior to traditional methods such as advancing front method and paving.
Quantitative fault seal prediction
