The role of structural composite oil and gas control is reflected in the following three aspects:
First, the formation and development of different structural systems control different structures, and thus control different Oil and gas combinations. For example, the Sichuan Basin is a multi-system superimposed basin, which determines the formation and evolution of oil generation and favorable reservoir facies belts.
The negative structural composite parts have greater subsidence, which is a good depositional environment and provides conditions for oil generation. For example, the composite part of the Neocathaysian subsidence zone and the east-west subsidence zone.
Forward structural composite parts are often where oil and gas reservoirs or better oil and gas display wells exist, because these parts have larger structural amplitudes and larger trap areas. What's more important is that multiple activities have intensified the development of fractures and changed their properties, which is beneficial to oil and gas migration and storage. For example, in the southern Sichuan gas field, each trap structure is in a composite position of east-west and north-south structures.
Second, the preformed structural system controls the generation of oil and gas, while the migration and accumulation conditions of oil and gas are provided by the structures (or structural systems) formed later. The later structural system not only provides necessary structural traps, but also transforms the original oil and gas combination. The oil and gas in the lower combination can be accumulated in a very new combination.
Thirdly, the structural composite part increases the structural amplitude and creates a large number of fractures in the trap range, which better improves the percolation performance of the reservoir. The multiple composite parts can also change the properties of fractures, increase the development of tensile and tensile-torsion fractures, and make oil and gas migrate more fully and be relatively enriched.
The following takes the Sichuan Basin as an example to conduct a more detailed analysis of the role of structural composites in controlling oil and gas (Le Guangyu et al., 1994). As a large Mesozoic and Cenozoic structural basin, the Sichuan Basin has a relatively regular rhombus outline and is controlled by surrounding structural zones and related fault systems (Figure 9.42, Figure 9.43).
Figure 9.42 A sketch of the structural zoning of the Sichuan Basin and surrounding areas
(According to Le Guangyu, 1994)
1—Zoning boundary line; 2—Basin Scope; 3—Fault; 4—Fold; 5—Division number
According to research by Professor Le Guangyu, the internal structural superposition pattern of the Sichuan Basin is obviously controlled by the Huayingshan Fault Zone and the Renshou-Cangxi Fault Zone. It is divided into three deformation areas: eastern Sichuan, central Sichuan and western Sichuan. The structural forms and superposition patterns of each area are different.
Eastern Sichuan District is located between the Huayingshan Fault Zone and the Qiyaoshan Fault Zone.
Among them, the northwest-trending Dabashan fold and the northeast-trending East Sichuan fold develop in the northeastern Sichuan region. They overlap and interfere with each other in the northeastern Sichuan region, forming both a combination and a composite. Both folds have obvious structural zoning: the Dabashan fold is divided into the northern belt (North Dabashan belt), the middle belt (South Dabashan belt) and the southern belt (Dabashan front belt), and the eastern Sichuan fold is also divided into the eastern belt. Belt, Middle Belt and West Belt. The northern Dabashan fold belt and the eastern Sichuan fold fold belt (part of the NNE trending folds close to the outer edge of the basin) do not directly intersect; the southern Dabashan fold belt and the eastern Sichuan fold belt combine with each other to form a pair of "convergent double arcs"; The Bashan front belt is compounded with the northern section of the western belt of the eastern Sichuan fold to form a cross fold (Fig. 9.44).
Both the eastern and western belts of the East Sichuan Fold maintain a certain trend extension. Only the middle belt is an arc protruding to the northwest, united with the Nan Dabashan arc protruding to the south and west, and the two arcs converge eastward at The Wushan area spreads out in a trumpet shape to the west, and the Wubaochang Basin is sandwiched within it. This is an important type of combined arc system, called a convergent double arc or a figure-eight double arc, which reflects the combined effect of two-way lateral pressure directed to the south-west and north-west.
Figure 9.43 Structural outline of the Sichuan Basin and its surrounding areas
(According to Le Guangyu, 1994)
1—Quaternary; 2—Cretaceous (Some areas contain Paleo and Neogene systems); 3-Jurassic; 4-Upper Paleozoic and Triassic; 5-Sinian and Lower Paleozoic; 6-Pre-Sinian metamorphic rock-grade magmatic complex; 7 —Anticline axis trace; 8 — Syncline axis trace; 9 — Structural basin; 10 — Dome; 11 — Reverse fault and overthrust fault; 12 — Fault;
Figure 9.44 Structural zoning and distribution in northeastern Sichuan Overlay relationship sketch
Strata: 1—Cretaceous and Paleo and Neogene; 2—Jurassic; 3—Triassic and Permian; 4—Cambrian—Carboniferous; 5—Previous Cambrian. Dabashan arc structural belt: North zone: 6-anticline axis; 7-syncline axis; 8-fault; middle zone: 9-anticline axis; 10-syncline axis; 11-fault; front zone: 12 —Anticline axis; 13 — Syncline axis; 14 — Fault. Arc-shaped structural belt in eastern Sichuan: East zone: 15-anticline axis; 16-syncline axis; 17-fault; middle zone: 18-anticline axis; 19-syncline axis; 20-fault; west zone: 21- Anticlinal axis; 22—syncline axis; 23—fault. Micangshan east-west structural zone: 24-Micangshan east-west structural zone. Others: 25—Geological Boundary
The western belt of the eastern Sichuan fold (including anticlinal belts such as Huaying Mountain, Tieshan, Tongluo Gorge, and Qili Gorge) is formed by the migration and expansion of folds into the basin. of. At this time, the push from the Daba Mountain was delayed or weakened, so that anticlinal belts such as the Huaying Mountain were not affected by it. The Huanghuangkou anticline belt extending to the north was orthogonal to the South Daba Mountain fold and was restricted by it. Afterwards, the compression and overthrow from the Daba Mountain area intensified again, resulting in wide-spread folds in the frontal zone, which compounded and superimposed with the established NNE-trending folds, resulting in restricted, transverse and migratory interferences. Type.
The Huanghuangkou anticline is not only limited by the South Daba Mountain arc but also the northwest-oriented folds of the Daba Mountain front belt, making them end at its two wings and unable to penetrate. When the northeast-trending folds are weak, the northwest-trending folds may penetrate them to form transverse folds. For example, near Xuanhan County, the northwest-trending Yue'erliang-Doufuliang anticline crosses northeast-trending folds such as Shuangshimiao (Fig. 9.45). The composite superimposed folds in the northern section of Qilixia are a new interference type - migration type (Du Siqing, 1996). The Triangle Mountain nose-shaped anticline that dips to the northwest and the Wuwan Ji-shaped syncline that dips to the south-east are superimposed on different wings of the northeast-trending high and steep anticline, and the axis traces are connected, forcing the early northeast-trending anticline to The axis and hub migrate to form an arc (Fig. 9.46). The northwest-trending folds in the Daba Mountain front belt are mainly surface folds caused by the southward overthrow of the main body of the Daba Mountain. They have basically disappeared as the top surface of the Lower Permian reaches deeper, and only the NNE-trending structure exists alone.
Figure 9.45 Geological map of the Shuangshimiao cross-fold fold
1—Anticline axis of the northeast-trending structural belt; 2—Anticline axis of the northwest-trending structural belt; 3—Northwest-trending structural belt Reverse fault; 4—steep zone; 5—ground (rock) layer boundary: (1) leaf limb boundary layer, (2) to (20) sandstone marker layer; 6—structural number
Southeastern Sichuan The area is bounded by the Changshou-Zunyi Fault that runs north to south, the Gulin-Yanjin Fault that runs east-west, and the Qingshanling Fault that runs northeast, and forms a right-angled triangular block, with its southern boundary extending beyond the scope of the basin. Among them, the mining area in southern Sichuan to the west of the Shilongxia belt and to the north of the Changyuanba belt also forms a smaller right-angled triangle, which is nested within the large triangle. The actual folds are controlled by the boundaries and form three sets of joint arc systems with right-angled triangle boundary fractures as chords (Figure 9.47): ① Chongqing arc-shaped fold system, with north-south right-angled edges as chords, forming an arc bundle protruding westward; ② The Na (Xi) Xu (Yong) fold system is controlled by the east-west right-angled edge, with the east end submerged and the west end slightly curved; ③ The Yong (Chuan) Yi (Bin) double-echelon fold system is controlled by the hypotenuse (Qingshan Ridge fault), the local structure is arranged obliquely along two series axes of NE50° and NE20°, forming a double row of wild goose rows. The three sets of arc systems all protrude towards the center of the triangle and converge towards each endpoint, and the deformation is strong at the edges. This reflects the combined effect of the three-way lateral pressure on the vertical boundary. The fold system progressively expands from the edge of the triangular block to the inside; in addition, along the The hypotenuse also acts as a shear force (right-hand rotation).
Figure 9.46 Migrating composite superimposed folds in the northern section of Qilixia
(According to Le Guangyu et al., 1991)
1—Middle Triassic (Leikou Po Formation); 2—Upper Triassic (Xujiahe Formation); 3—Middle and lower part of Lower Jurassic (Artesian Group); 4—Upper part of Lower Jurassic (Artesian Group); 5—Middle Jurassic Lower Shaxi Miao Formation; 6—the lower section of the Middle Jurassic Upper Shaximiao Formation; 7—the upper section of the Middle Jurassic Upper Shaximiao Formation; 8—the anticline of the NNE structural belt; 9—the reverse fault of the NNE structural belt; 10 —Anticline axis of the northwest structural belt; 11 — Syncline axis of the northwest structural belt; 12 — Translational reverse fault of the northwest structural belt; 13 — Rock formation occurrence (normal, inverted); 14 — Rock formation steep zone
Each The migration and expansion of the fold system is uneven, and the east-west folds spread to the inside of the triangular block earlier; then the north-south folds expand from east to west and cross them, forming a row of domes or "dome-basin type" in Luzhou Xuyong. "Composite superimposed folds (see Figure 8.29). The positive structures in both directions are superimposed to form a dome, the negative structures are superimposed to form a basin, and the positive and negative structures are superimposed to form a saddle. When the early folds are asymmetric, their inclined axial surfaces will also be involved in the late folds and curved, so that the sequence of the two stages of folds can be judged (Figure 9.48).
The Central Sichuan Area is a northeast-trending strip sandwiched between two major faults in the basin, and its interior is divided by alternating northwest-trending uplifts and depressions. From north to south there are: North Sichuan Depression, Central Sichuan Uplift, Anyue Depression, Weiyuan-Zigong Uplift and Guanyinchang Depression. The local structure overlaps this northwest-trending uplift and is also controlled by two structural lines: northeast and northwest.
Figure 9.47 Right-angled triangle joint structural pattern in southeastern Sichuan
(According to Le Guangyu, 1989)
1—Main axis of joint structural anticline; 2—Fold bundle Boundary; 3 - right-angled triangle boundary fault; 4 - Gulin arc structural zone
Figure 9.48 Superimposed fold pattern map
(According to Le Guangyu, 1991)
< p>(a) Perspective view; (b) Plane viewS1—early axial plane; S2—late axial plane
The composite folds of the Northern Sichuan Depression range from the north of the Yingshan fault to At the front of Daba Mountain, wide spreading folds and short-axis structures are developed in the northwest direction, and there are also many northeast-trending folds. But it has been greatly weakened than that in eastern Sichuan. A bundle of northwest-trending broad folds is limited to the west flank of the Huanghuangkou anticlinal belt, but extends west across the northeast-trending folds north of Tongjiang and Bazhong (Fuyangba area). South of Bazhong, the structures in these two directions tend to accommodate and restrict each other, such as the bends of the Shujiacao anticline and the Yingshan-Shuanghechang anticline. The so-called "Bazhong-Yilong scroll structure" seems to be the result of two-way folds accommodating and restricting each other.
Short-axis dome-shaped folds develop in the central Sichuan uplift area, but their extension and arrangement still have a certain directionality. The Nanchong Structural Belt is an arc-shaped projecting to the north. Its eastern arc limb includes structures axially oriented to the northwest such as Dacheng and Xiandu River. Its western arc limb is the northeast-trending fold between Pengxi and Jianyang. The Nanchong Arc is influenced by the northeast and northwest A joint arc controlled by two construction lines.
Between Wusheng and Hechuan, the structures in the so-called "Central Sichuan-South Sichuan Transition Zone" are arranged in a series of arcs protruding to the southeast, which can be called Wusheng arcs. They are combined with Nanchong arcs to form a pair of anti-convex double arcs. The eastern ends of the two arcs converge near Guang'an, while the western ends converge less obviously, which is related to the gentle transition between the Central Sichuan Uplift and the Anyue Depression.
The Anyue Depression and the Weiyuan-Zigong Uplift are developed in the central and southern Sichuan region. The Anyue Depression is a broad syncline trending northwest, with only a few northeast-trending local structures superimposed on it. The Weiyuan Dome is the largest trap structure in the basin. In addition to its formation possibly being related to basement uplift, its location at the superposition of two positive structures in the northwest and northeast is also an important factor. The Molinchang structural nose protruding from the southern wing of the Weiyuan dome corresponds to the curvature of the southern section of the Ziliujing anticline and is obviously controlled by the hidden northwest-trending structure.
The Western Sichuan District is the depression belt on the front edge of the Longmen Mountains. The northern section is the Western Sichuan Depression and the southern section is the Chengdu Basin.
Figure 9.49 Cross-section of Zhongba Gas Field
(According to "Sichuan Oil and Gas Field")
The overall structure of the northwest Sichuan Depression is composed of the Cretaceous The large east-trending syncline is called the Zitong syncline. The local structure inside it is axially variable, but it is still controlled by two structural lines: northeast and northwest. On the east side, the northward extension of the Jiulongshan anticline axially oriented to the northeast is restricted by the axial east-west Dalianghui anticline belt at the front edge of the Micang Mountains; on the west side, close to the front edge of the Longmen Mountains, the northeastward oriented short-axis anticline is broken The continuous connection is distributed in a strip, including Hewanchang, Shuangyushi, Haitangpu, Zhongba and other structures. These local structures are greatly influenced by the Longmenshan structure, and generally appear as overlapping axial superpositions of two phases of structures. For example, in the middle bar structure (Figure 9.49), the Jurassic and Triassic are in angular unconformable contact, and the overlying layer and underlying layer both form anticlines, with the upper and lower overlapping axial directions consistent. However, the structure of the underlying layer is often complex with developed faults, while the structure of the overlying layer is generally simpler. In addition, structures such as Laoguanmiao and Fushunchang near Mianyang form arcs protruding to the north. The Mianyang arc should be a combined arc composed of folds in the northeast and northwest directions.
The Chengdu Basin is a Quaternary depression superimposed on the Western Sichuan Depression (Figure 9.50). It complements the Longquanshan fold and fault zone, and both trend in the north-northeast (about NE30° ); the Western Sichuan Depression complements the Longmenshan thrust belt, and both trends are NE (NE45°~50°). Two stages of fold and fault zones, two stages of depression, and two structural trends form an oblique compound superposition relationship. The boundary between the Western Sichuan Depression and the Central Sichuan Uplift is not the Longquanshan fault zone but the hidden Renshou-Cangxi fault zone. The Longmenshan structure continued to be active during the Himalayan period, and together with the Longquanshan structure formed an offset on both sides of the Chengdu Basin. The axes of the Sanhechang, Gaojiashan, and Wuzhongshan anticlines and related faults on the west side all dip to the north and west. , the axial planes and associated reverse faults of the Xiongpo, Sudock, and Yanjing anticlines on the east side all dip to the south-east, consistent with the hedging pattern.
Figure 9.50 Structural outline of the Chengdu Basin
(According to Sichuan Bureau of Geology and Mineral Resources)
1—Unconformity line between the Quaternary system and the underlying rock strata; 2— Quaternary isopach (m); 3—hidden fault; 4—peripheral anticline; 5—thrust fault; 6—Feilai peak
On the south side of the Chengdu Basin, north-south folds are developed. Such as Zhougong Mountain, Zonggang Mountain and other structures. They are the northern part of the Sichuan-Yunnan structural belt that extends into the basin. Their amplitude has been greatly reduced and soon subsided, but there are still north-south structures in the hinterland, such as the Daxing structure. The south extension of the northeast-trending structure often inserts into the north-south trending structure, or changes its orientation to accommodate it. Both structures are cut and disturbed by the late northwest-trending thrust fault at the edge of the basin.
Figure 9.51 Concentric fold pattern map
(According to Le Guangyu, 1994)
The sedimentary cover of the Sichuan Basin has undergone complex structural changes in the Mesozoic and Cenozoic. Most of the fold systems are concentric folds generated by longitudinal bending folding mechanisms, but there may also be some upper thin folds and puncture structures generated by the transverse bending mechanism supported by fault blocks, magma or rock masses. A typical concentric fold section can be divided into three zones: upper, middle and lower (Figure 9.51). The upper zone is a rupture zone, which is a buckling top with developed tension cracks; the middle zone is a concentric zone, where the rock layers are parallel and curved with consistent shapes; the lower zone is a compression zone, with the upper part still having concentric characteristics but with developed thrust faults, and the lower part being a weak layer. Slip and squeeze in, the bottom is the slip surface.
The pectinate fold area in eastern Sichuan has obvious concentric fold characteristics and vertical zoning. However, because there are more than one detachment layer and detachment surface, the shape is more complex. From bottom to top, the bottom surface of the sedimentary caprock, that is, the bottom surface of the Sinian system, is the first detachment surface, and its sliding front is a shovel-type reverse fault that cuts into the lower part of the sedimentary caprock. As the sedimentary rocks slide along the ground from southeast to northwest, the surface layer lags behind relatively, resulting in relative shearing between the surface layer and the bottom layer. Most of the high and steep anticlines in eastern Sichuan have steep southeast wings, gentle northwest wings, and a west-tilt axis. The faults that cut deeper, as the front of the bottom slip surface, all thrust to the northwest; the faults that cut the Permian and Triassic systems and disappeared in the Silurian system are two groups of ***-yoke, but they thrust to the southeast. The thrust group is more developed and has a larger fault throw. This feature is consistent with the tilt of the axis and the relative shearing of the top and bottom of the caprock. When the front-edge thrust fault develops, the folds of the entire sedimentary layer in its hanging wall become more coordinated, and the scope of the concentric zones expands (such as the Huayingshan anticline and the Dachigan anticline). The second detachment surface is the top surface of the Sinian Dengying Formation, on which the weak layer of the lower Cambrian system detached slightly.
The third detachment surface (main detachment surface) is the top surface of the Ordovician hard layer dominated by carbonates, and the Silurian weak layer is the main detachment layer; as shown in the sections of the Datianchi structure and Huangnitang structure, the Ordovician The top surface of the system is only slightly upward arched, while the strata above the Silurian system form large concentric folds. The fourth detachment surface is the top surface of the Lower Triassic Jialingjiang Formation, on which the gypsum-salt-rich Leikoupo Formation detached, further increasing the fold amplitude of the overlying layer. Local detachment sometimes also occurs in the anticline wings or synclines. The thickening and convex curvature of the rock strata point to the depth. As shown in the Zhangjiachang section (Figure 9.52), the thrust fault cuts the Carboniferous, Permian and Lower Triassic. System, the rock layers in between form synclines and increase in thickness, while the strata above the Leikoupo Formation are only slightly deflected; a similar phenomenon is also seen in the syncline zone between the Dachigan anticline and the Fangdoushan anticline.
In addition to controlling the deformation of high and steep anticlines, detachment also plays an important role in controlling the formation of some low and gentle structures. For example, the formation of the Yingshan structure in central Sichuan is that under the action of horizontal pressure, the top of the Jialingjiang Formation and the gypsum-salt layer of the Leikoupo Formation undergo plastic compression, causing local intrusion and thickening, resulting in arching and fracture of the overlying brittle layer, while the underlying brittle layer The subterranean stratigraphy remains gentle and has no structural display (Fig. 9.53).
Faults associated with folds mainly occur in the hard layers of the Permian and Triassic systems, and pinch out upwards and downwards in weak layers such as the Jialingjiang Formation gypsum salt layer and Silurian shale; usually Two groups of yoke thrust faults with opposite tendencies are formed. The thrust is associated with the two wings of the syncline and is combined into a fault depression; the backthrust is associated with the two wings of the anticline and is combined into a fault arch. The occurrence of this type of first-yoke thrust fault has increased the fold amplitude of the Permian and Triassic rock formations.
The Mesozoic and Cenozoic fold movements in the Sichuan Basin and its surrounding areas have a certain migration and expansion effect, and each movement expands and contracts toward the basin in turn. The Indosinian folds are mainly distributed around the Upper Yangtze platform; the Yanshan folds extensively affect the platform, but are mainly limited to the periphery of the basin; the fold movement did not enter the basin until the Himalayan period. However, this migration and expansion is asymmetric, and the center of migration is not the central Sichuan block but the Longmenshan front depression, especially the Chengdu Basin in its southwest section. This may be related to the extremely deep depression and thick sedimentation in the Western Sichuan Depression.
Figure 9.52 Zhangjiachang structural cross-section
(According to "Sichuan Oil and Gas Field")
Figure 9.53 Time section of line 89-D29 in Yingshan area
p>
(According to the Central Sichuan Mining Area)
Reflective layer layers: I—top boundary of Ziliujing Group; I1—bottom boundary of Da’anzhai Formation; II0—top boundary of Xiangxi Group; II1—Xiangsi Ding boundary; III—top boundary of Leikoupo Formation; IV—upper part of Jialingjiang Formation; VI—top boundary of Yangxin Series; VII—bottom boundary of Yangxin Series; VIII—top boundary of Middle Ordovician
Mesozoic and Cenozoic sediments The shrinkage of the basin is also centered on the Chengdu Basin. In the Late Triassic, from the Kuahongdong Period, Xiaotangzi Period to the Xujiahe Period, the sedimentary basin gradually expanded; from the Jurassic to the Paleo and Neogene periods, the basin gradually shrank from its maximum extent, and finally became localized. In this corner of western Sichuan, this is consistent with the progressive enhancement of tectonic movements in the basin and the migration and expansion of folds to the western Sichuan depression.
Each structural zone in the Sichuan Basin is characterized by progressive deformation and migration and expansion.
Professor Le Guangyu believes that there are four main structural lines in the Sichuan Basin and its surrounding areas, namely the Northeastern Formation (including the North-Northeast, Northeastern and North-Eastern structural lines), the Northwestern Formation, East-West Formation and North-South Formation. The first two formations have the greatest influence on basin structure. Each group system evolves progressively and migrates and expands gradually. It is impossible to determine the relative sequence of the formation of relevant structures based on the direction of the structural lines alone, let alone a representation of a certain movement.
For example, taking the Qiyaoshan anticline and part of the northeastward folds to the east as the outer belt (eastern belt) of the eastern Sichuan fold, there is no problem that it was formed during the Yanshan movement based on the angular unconformity relationship. The middle zone (including Fangdoushan, Dachiqian, Yunanchang, Nanmenchang, Datianchi, Wentangjing and other structures) and the South Dabashan structural belt form an "eight-shaped double arc" (Le Guangyu, 1975), which belongs to the basin. First period folds. The Micang Mountain Belt is spanned by the Nandaba Mountain Fold. It was certainly formed earlier, but it is not within the basin. They are closely adjacent to the Yanshan folds on the basin edge, and their structures continue. It is speculated that they took shape during the Yanshan period. The inner belt (the western belt, including Qili Gorge, Tongluo Gorge, Tieshan, Huaying Mountain, Huanghuangkou and other structures) maintains its north-northeast axis, is restricted by the South Daba Mountain Arc, formed slightly later, and is classified as the second stage of folds . The arc system controlled by the right-angled triangle boundary in southeastern Sichuan also belongs to the second stage, and there are still rows of superimposed folds formed by deformation and expansion within it. The northeast-trending folds in the Zigong-Weiyuan area in southern Sichuan are coordinated with the southern extension of the Huayingshan fold, and should also be classified as the second phase. The NW-trending surface folds in the front belt of Dabashan are either restricted by NNE-trending folds or compositely superimposed on NNE-trending folds. The western belt of the eastern Sichuan fold that night should be classified as the third stage. The anti-convex double-arc dome-shaped folds in the central Sichuan uplift are formed by the combined lateral pressure of Huaying Mountain to the west and Daba Mountain to the south (Le Guangyu et al., 1994), and are also classified as the third phase. In the western Sichuan region, some structures in the Longmen Mountain front were formed in the Indosinian period and were superimposed by the Himalayan fold axis; in addition, relatively speaking, the northeast-trending Jiulongshan, Nanyangchang and other structures were relatively early and may belong to In the second phase, the Mianyang Arc belongs to the third phase; structures such as the Chengdu Basin and Longquan Mountain may have formed the latest and can be divided into the fourth phase, but the north-south structures on its south side were inserted by the northeast-trending structures relatively early.
Based on the combined distribution of main structural traces and their joint relationships, the tectonic stress field of the basin under the combined action of the peripheral dynamic system can be generally restored. Generally speaking, as long as no significant rotational deformation occurs, structural lines can reflect stress traces (Le Guangyu et al., 1996); tensile structural lines are perpendicular to the minimum principal stress (principal tensile stress or minimum principal compressive stress) traces and parallel to on the maximum principal stress trace; the compressive structural line is perpendicular to the maximum principal stress (principal compressive stress) trace and parallel to the minimum principal stress trace.
Due to the dynamic changes of the tectonic dynamic system, the Mesozoic and Cenozoic tectonic stress fields in the basin are also unstable fields that change dynamically over time. According to the results of the overall combined inversion of regional structures, only the stress distribution in the structural finalization stage is obtained, and its main stress trace network is shown in Figure 9.54. This joint tectonic stress field has both a unified stress distribution law and obvious zoning characteristics. Each zone is basically bounded by faults and is generally consistent with the structural deformation zone.
The joint and composite superposition of structures in the Sichuan Basin has obvious control over structural traps and oil and gas accumulation.
The gas reservoirs in the Sichuan Basin are mainly of the fractured anticlinal trap type. The direct factor controlling their formation and enrichment is the existing structural traps; paleostructures, especially paleohighs, control the early stage of oil and gas. transport. The existing local structures of various shapes in the basin are all products of the tectonic movement of the Mesozoic belt. They occurred under the combined action of complex dynamics at the periphery and are closely related to the progressive expansion and superimposed interference of the structures of each series. The ancient structures The same is true for the formation of.
Tectonic association has obvious control over the Indosinian paleo-uplift.
The Indosinian movement produced a pattern of one uplift and two depressions in the basin, namely the Luzhou-Kaijiang ancient uplift and the depressions on both sides, which generally trend northeast. This is a weakening manifestation of the Indosinian folds around the Upper Yangtze platform on the rigid block (Le Guangyu et al., 1994). The paleo-uplift is divided into two sections by the saddle. The southern section of the Luzhou paleo-uplift is located just within the right-angled triangle block in southeastern Sichuan. According to simulation experiments, the triangular thin plate is unstable and buckled due to the combined action of three-way lateral pressure. Due to different forces and buckling degrees, different deflection distributions and buckling forms can be obtained. When the plate is thin and elastic, it can form a single dome bulge; if it is a multi-layer elastic-plastic plate, it can form a three-way complex deflection joint form controlled by three boundaries. The Luzhou paleo-uplift is not a simple "broken uplift", but a kind of arch. Its complete overlap with the later triangular arc system is not accidental. They are the products of the joint tectonic action at different stages and under different conditions (Le Guangyu et al., 1994). The Kaijiang ancient uplift in the northern section is located at the boundary between the Dabashan fault and the Qiyaoshan fault, which is the opening of the later "eight-shaped double arc". It is also an arch produced under the combined action of two-way lateral pressure. . These two ancient uplifts were important factors in controlling early oil and gas migration in eastern Sichuan. The late moderate local structures overlapping them controlled re-migration and enriched them into important gas fields.
Figure 9.54 Schematic diagram of the tectonic stress field of the Sichuan Basin
(According to Le Guangyu, 1994)
1—Minimum principal stress trace; 2—Maximum principal stress Trace; 3—Main discontinuous interface; 4—Peripheral side pressure direction; 5—Quaternary coverage area
Combined composite superposition has obvious control over the local structure.
Folds do not always constitute traps. The structural superposition controlled by structural lines in different directions is an important reason for the formation of structural traps. The combined and composite superposition of structures often produces structural traps of specific shapes, especially joint arc traps with gently curved main axes, or several structural traps arranged into joint arcs. For example, the Datianchi Structural Belt is located at the bend of the combined arc produced by the Daba Mountain in the northern section of the Eastern Sichuan Fold, and has been confirmed to be the most important gas field in northeastern Sichuan; the Artesian Well structure is superimposed on the northwest uplift, and is subject to northwest boundary conditions , the combined action of the pressure pointing to the northwest and the restraining reaction force pointing to the northeast creates a local joint arc trap in the composite background. This arc-shaped trap is conducive to the development of fractures and the formation of important gas fields.
Compound superposition results in a variety of structural trap forms. The two-way positive structures are superimposed to form a dome, the two-way negative structures are superimposed to form the "ten" shape of the basin, and the positive and negative structures are superimposed to form a saddle; the superposition effect controls the formation, trap area and amplitude of traps. Compound traps often take the form of cross, tic-shaped, T-shaped and other biaxial or multi-axial shapes. For example, many gas fields in southern Sichuan have this complex shape. There are also cases of three-dimensional structural superposition. For example, the surface structure of the Shaguoping gas field is originally the broken nose at the western end of the Wentangjing anticlinal belt, which dips south to west and west. The upturned end is diagonally spanned by northwest folds and is thrust. The fault is closed, and the dip end is connected obliquely with a nose-shaped structure dipping from south to west. The latter forms a high point in the hinterland and becomes an important gas field in eastern Sichuan.
Latent structures may have many causes, and structural superposition is one of the important causes. In addition to migration and expansion in the horizontal direction, structural deformation also develops progressively in the vertical direction, causing disharmony between the upper and lower structures. Taking eastern Sichuan and southern Sichuan as examples, under the action of lateral pressure, the main hard carbonate layers such as the Lower Permian and Ordovician tend to fold first and control the main section shape; then the weak Silurian shale layers detach and promote The overlying Lower Permian folds are complicated, and the shallow rock formations are less affected. Therefore, some structural groups are not shown on the surface, but exist in the underground rock formations.
For example, in the Liziba area, located in the center of the right-angled triangle block in southeastern Sichuan, the ground structure is extremely simple, but there are multiple hidden high points in the sun roof structure in the hinterland with different axes, showing the inward migration of three-way folds. superimposed effects. Another example is the Guangfuping structure. The surface axis is slightly curved, but the sun roof structure in the subsurface forms obvious "cross" folds, showing that the composite superposition effect is enhanced at certain levels. The northeast-trending buried fault nose in eastern Sichuan is generally restricted by a high and steep north-northeast trending anticline. When it is a late-stage oblique composite structure, there are also fault-derived hidden structures on the wings of the anticline zone.
Early structural traps and overlapping composite structural traps are important places for oil and gas accumulation in the Sichuan Basin.
Most of the local structures in the basin were formed by the Himalayan movement, but there are local structures of the Indosinian period on the front edge of the Longmen Mountains, and there may be structures of the Yanshan period in eastern Sichuan. Early structures are more favorable for the remigration and accumulation of oil and gas. According to the law of tectonic movement, early structural traps are mainly distributed in the edge zones of basins, such as the Longmenshan front. Indosinian structures overlap with Himalayan structures. Sometimes the trap locations in the upper and lower layers are consistent, but they are not always consistent. There may be early traps lurking below the unconformity surface, which is worth noting. This overlapping composite of different structural layers is caused by two phases of movement, and is different from the composite superposition caused by the deformation and advancement of the same movement in the Himalayan period. Accordingly, ancient structural traps, ancient and modern composite structural traps and progressive composite structural traps should be divided. They may have different effects on the migration and accumulation of oil and gas.
Tectonic superposition changes fault properties and controls crack development patterns.
Professor Le Guangyu et al. (1994) studied that the main faults in and around the Sichuan Basin were under tension in the late Paleozoic, showing normal fault activity, and turned to compression in the Mesozoic. If they are transformed into thrust fault activity, they should all belong to transformational composite superimposed faults or surface composite superimposed faults. Taking the Huayingshan fault as an example, the section tilts eastward. In the central Sichuan area to the west, the Silurian system is rarely preserved, and the Devonian and Carboniferous systems are missing. However, in the eastern Sichuan area to the east, the remaining Silurian system is thicker and contains Carboniferous. System distribution, Permian basalt or diabase are distributed along the Huayingshan fault, all of which reflect the extensional activity of the late Paleozoic fault rising in the west and falling in the east; in the Indosinian period, the uplift and depression on both sides reversed, and the east side arched and uplifted , and then formed a significant thrust fault zone during the Yanshan-Himalayan period. In the extension stage, platforms and troughs are formed, controlling the distribution of reef shoals, dolomitization and paleokarst; in the compression stage, deformation zones and local structures are controlled, each exerting different control effects on oil and gas reservoirs.
Tectonic superposition also directly controls the regional fracture distribution pattern and the development of local structural fractures. For example, in southeastern Sichuan, under the control of triangular boundaries, the fold amplitude and fracture development gradually decrease inward, forming a ring of triangular zones. Most high-yield gas fields are distributed in the central ring zone with moderate structural strength. The superposition effect also increases the fracture system on the local structure and improves the reservoir conditions. From the perspective of structural assemblage superposition, in addition to the old gas areas in southeastern Sichuan and central and southern Sichuan, the eastern Sichuan (especially northeastern Sichuan) and western Sichuan areas have good exploration prospects (Le Guangyu et al., 1986).
Wang Tonghe (1986) studied that the Bohai Bay Basin is an extensional fault basin superimposed on the basement of the North China Platform and developed from the Cenozoic. The crust of this area was in a relatively stable development stage until the end of the Permian since it did not become the North China Platform in the Cryptozoic Eon. The Indosinian movement in the early Mesozoic Era began to have an impact on this area, but the Triassic sedimentation and structural pattern still inherited the "north-south differentiation" characteristics of the Late Paleozoic Era. The Yanshan Movement in the late Mesozoic caused strong folds and faults in this area in the northwest-southeast compression stress field (Figure 9.55). Moreover, due to the obvious differences, the stratigraphic preservation and burial conditions that play a role in the formation of buried hill oil and gas reservoirs are also different, which lays the reservoir conditions for the formation of ancient buried hill oil and gas reservoirs.
The end of the Mesozoic to the beginning of the Paleogene was the main period of stress field changes in the Bohai Bay, and block faulting generally occurred on the background of large-scale uplift. Under the action of northwest-southeast extension, the Paleogene basin structure tracked and accommodated the northeast-northeast fault, accompanied by the northwest-northwest fault, and the northeast-northeast basin structure was cut or closed by the latter. . *** A series of grabens, horsts or stepped fault blocks were simultaneously confined and developed, controlling Paleogene sedimentation. The action of the tensile stress field causes differential activity of fault blocks, which increases the contact area between the Paleogene oil generation and the ancient buried hill reservoir, the height of the ancient buried hill, or the uplift amplitude. Due to the different structural positions of each structural unit in the basin, the magnitude and direction of stress and the heterogeneity of the medium, the type and development degree of ancient buried hills are not only related to the rock strata, but also depend on the faults that control the differential activity of the fault blocks. and changes in appearance and activity periods.
Figure 9.55 Mesozoic structural outline of the Bohai Bay Basin
1—Back axis; 2—Syncline; 3—Reverse fault; 4—Regional compressive stress direction
< p>Figure 9.56 Paleogene graben structural map of the Bohai Bay Basin(According to Wang Tonghe, 1986)
1—Basin; 2—Uplift area; 3—Normal fault; 4 —Regional tensile stress field
Since the Neogene, the structure and distribution range of the sedimentary area in the Bohai Bay Basin have undergone significant changes compared with the Paleogene. Young unconsolidated sediments cover Paleogene barrier structures showing large sedimentary plain landforms.
Tectonic activity shows NE-NNE dextral shear stress field activity, causing strong torsional deformation in the Paleo and Neogene systems of the Linqing Depression; many reverse faults appear in the Lower Liaohe Depression. The Beijing-Qingdao line is superimposed with a secondary NE-SW tensile stress field (Figure 9.56, Figure 9.57), which changes the Paleogene subsidence zone from northeast to northwest, which is manifested in the entire basin moving from north to south to Beijing- The Qingdao line dips downward, that is, the sediments of the Lower Liaohe Depression migrate from north to south, while the depocenters of the Jizhong, Huanghua, and Jiyang depressions each migrated from south to north, forming a northwest-beaded Neogene subsidence zone from Beijing to Qingdao. .
Figure 9.57 Distribution map of rift basins in the Bohai Bay Basin and its surrounding areas since the Neogene
(According to Wang Tonghe, 1986)
1—Fault basins ;2—Sea area; 3—Uplift area; 4—Dextral shear stress field; 5—Depositional center migration direction