5.4. Formation of1anti-S-shaped (similar) structure
Bachu Uplift is an inherited paleouplift, which began in the pre-Sinian period. In Sinian, the high part of its uplift was located in the northeast of the present Mohammad slope, which was a wide and gentle uplift near the east-west direction. During the Silurian-Devonian period, the high part of the uplift moved northward, and the high part of the uplift was located in the present Selbuya-Mazatak fault zone. In the early Hercynian movement, the fault activity on both sides of Bachu uplift was not obvious, and the stratum thickness gradually decreased from the east and west sides to the high part of the uplift. Due to the influence of the early Hercynian movement, the structural strike of Bachu uplift changed in Carboniferous, from near east-west direction in early Paleozoic to northwest-southeast direction, and the southwest was in a gradual transition relationship with Maigaiti slope. Late Hercynian-Indosinian tectonic movement strongly uplifted Bachu Uplift and the northern part of the adjacent Maigaiti Slope, resulting in the absence of Upper Permian-Cretaceous on the uplift. Triassic stratum thickness 108 ~ 550 m is only found in Well Heshen 2 at the southern end of Longdong and Well Badong 2 at the eastern end of Kartak Uplift. According to the outcrop profile of Xiaohaizi area in Bachu, there are a large number of volcanic rocks in the Lower Permian, which thicken southward, indicating that there was a strong tectonic movement in the late Hercynian period, which made volcanic rocks pierce the Carboniferous strata. In the early Himalayan movement (early Paleogene), Bachu uplift was further uplifted and denuded, and the coexistence pattern of Bachu uplift and Maigaiti slope was formed again. At this time, the Maigaiti slope is a slope that is wide and gently inclined to the south, and the Paleogene is superimposed from south to north in turn, and it is pointed out in the northern part of the Maigaiti slope and Bachu uplift.
Fig. 5-9 Oil and gas migration and accumulation pattern diagram of the slope in central and northern Tazhong.
Labyrinth fault is a normal fault from Sinian to Middle Ordovician. From the end of the Late Ordovician, due to the regional compressive stress, the fault was transformed into a compressive thrust fault, accompanied by uplift. There were tensile and compressive activities in the late Hercynian, volcanic eruption in the early Permian, and then this area rose again, lacking Mesozoic deposits.
During the Neogene sedimentary period, Tarim Basin was forced to sink due to the rapid uplift of Kunlun Mountain. Under this background, the relative subsidence of Bachu Uplift led to the expansion of the sedimentary area of Anju An Formation in Neogene. At the same time, the boundary faults on the north and south sides of Bachu Uplift, such as Selibuya Fault (NNW), Gudongshan Fault (NNW), Mazatak Fault (NW), Aqia Fault (NNW) and Minzhen Fault (NW), are active, and are accommodated by Pamir's zigzag clockwise torsional stress field, and finally form an anti-S shape. During the deposition of the Neogene Pliocene Artux Formation, Bachu Uplift sank again, and accepted the deposition of sandstone and mudstone with the thickness of Artux Formation 1253 ~ 1303 m (Figure 5- 10).
Fig. 5- 10 mazatak anti-s structural map (similar)
5.4.2 Oil control function of structural system
5.4.2. 1 Hotan River gas field is located in Mazatak anti-S-shaped (quasi-) fault zone.
In February, 1998, China Oil and Gas Co., Ltd. designed the Niaoshan 1 well on the Niaoshan structure. The industrial gas flow is taken from the lower Ordovician dolomite of 3872 ~ 3885 m, and the daily initial gas production is 4. 1× 108m3. In September, 1998, Ma 4 well in Mazatak fault zone obtained high-yield gas flow in Ordovician limestone, with the depth of 20 17 ~ 228 1m and the daily initial natural gas production of 27.4× 104m3. In June, 1999, Ma 4 gas field was renamed Hotan River gas field, which was proved.
The Mazatage anti-S-shaped (quasi-) fault structure appeared in the late eastern Hugary and was active again in Hercynian. A fault anticline sandwiched by two faults was formed in Himalayan period (Figure 5- 10), and the faults on both sides were active again in Himalayan period. The main fault passes through Miocene, and the secondary detachment fault on the south reaches the sky along Miocene and Pliocene faults.
Natural gas generated from Cambrian-Ordovician source rocks in Himalayan period migrated from deep to middle Ordovician limestone and overlying Carboniferous limestone and glutenite to form oil and gas reservoirs.
Natural gas fields are mainly distributed in the anti-S-shaped (quasi-) arc belt protruding northward, which is a structural belt with moderate in-situ stress, such as Well Ma 4-Well Ma 8 (Figure 5- 10). However, the late Himalayan detachment fault on the south side does not have reservoir-forming conditions because it has not formed traps.
Reservoir-forming model of 5.4.2.2 gas reservoir
Asphalt is widely distributed in Carboniferous and Ordovician reservoirs, and has the characteristics of multi-stage and multi-source mixing, which indicates that there is a destruction process of Ordovician ancient reservoirs in gas area, and a small amount of mixed source condensate oil of Carboniferous and Cambrian is obtained in some sections of gas field. According to the structural development history, the reservoir-forming model of this gas field is established.
Formation and destruction of ancient oil reservoirs (Late Caledonian-Early Hercynian): During Sinian-Ordovician, under the influence of extensional movement, a set of extensional normal faults developed in Hetianhe gas field, forming a fault shielding structure near east-west, and at the same time forming a structure under Cambrian salt, which provided a directional area for downward oil and gas migration and accumulation. In the late Early Paleozoic, the early extensional faults were transformed into compressional faults, and the oil and gas in the CAMBRIAN trap migrated upward through the faults and accumulated in the Ordovician carbonate reservoirs. From Silurian to Devonian, the Ordovician suffered weathering and denudation, on the one hand, a good Ordovician carbonate weathering crust reservoir was formed; On the other hand, the leaching and biodegradation of atmospheric fresh water destroyed the Ordovician ancient reservoir, forming a large number of asphalt and heavy crude oil (the Ma 4 well area in the east of the gas field was less affected by tectonic movement, and the Cambrian ancient reservoir was well preserved).
Formation and dispersion of natural gas (late Hercynian): By Carboniferous, large-scale transgression occurred in Tarim Basin, and the Carboniferous system with a thickness of 800 ~ 1000 m was deposited. By the late Hercynian (that is, the end of Early Permian), large-scale volcanic activity in Tarim Basin promoted the rapid maturity of Cambrian and Carboniferous source rocks. The source rocks of the lower CAMBRIAN in Hetianhe gas field and the northern area entered the dry gas stage, and the crude oil in the early ancient reservoirs of the lower CAMBRIAN was cracked into dry gas, while the source rocks of the CAMBRIAN were in the stage of condensation and moisture. At the same time, the volcanic activity in the late Hercynian period also led to the fault activity on both sides of Hetianhe gas field, which cut through the Permian and made natural gas migrate vertically.
Natural gas accumulation and accumulation (Himalayan period): Indosinian-Yanshan period, Bachu area was in a uplift state, and the geothermal gradient became smaller due to weathering and erosion. During the Himalayan period, due to the sharp rise of Kunlun Mountain, the Tarim Basin declined relatively and accepted thicker Tertiary deposits. In the late Himalayan period, due to the influence of regional compressive stress, the faults on both sides of Hetianhe gas field were violently active, resulting in deep faults that broke the basement, forming today's fault barrier structure, forming better structural traps in Carboniferous and Ordovician, providing favorable places for oil and gas accumulation. The crude oil cracking dry gas and kerogen cracking dry gas in the Cambrian ancient reservoir migrated upward through faults on both sides of the gas field, forming secondary gas reservoirs in Hetianhe gas field.
Carbonate and clastic reservoirs in 5.4.2.3
(1) carbonate reservoir
Carbonate reservoirs in Hetianhe gas field are concentrated in Ordovician buried hill and Carboniferous bioclastic limestone section, with natural gas reserves of 454.64× 108m3, accounting for 73.5% of the proven geological reserves of the gas field. Reservoir physical properties are affected by sedimentary facies belt and diagenesis such as later dissolution and fracture.
1) Ordovician buried hill (o)
Ordovician sedimentary facies is mainly carbonate platform facies, and lithology is mainly micrite limestone, silty limestone, biolimestone, sandy limestone, oolitic limestone, gray dolomite and dolomite. The reservoir type is fracture-cave type, which is a typical dual-medium reservoir. The effective reservoir space is mainly intergranular pores, intragranular dissolved pores, intergranular pores, dissolved pores, karst caves and fractures, and the throat is mainly slender fractures and reticular micro-fractures.
Matrix porosity is low and reservoir heterogeneity is strong. Porosity distribution of core analysis ranges from 0.089% to 27.45%, with an average value of 65438 0.95%. The porosity formed by dissolved pores is relatively high. The porosity of cores from Well Ma 5, Well Ma 40 1 and Well Ma 4 174 was analyzed by self-absorption method, and the porosity ranged from 0.11%-16.57%, with an average of 6.85%. The distribution range of permeability is 0.005× 10-3μm2-3 μ m2.
Unfilled dissolution holes are developed in the weathering crust at the top of Ordovician buried hill, forming an effective reservoir space. Karst caves vary in size, and most of them are filled with karst collapse rocks. The fillings are supported by surrounding rocks, and the dissolution is strong, forming a large number of dissolution holes, which plays an important role in natural gas storage.
2) Carboniferous bioclastic limestone section.
The bioclastic limestone section is distributed stably in the gas field, and its thickness is generally about 40m. It is mainly composed of bioclastic beach and gravel beach microfacies of open platform subfacies, and there is a small amount of limestone dolomite flat microfacies deposited at the top of the platform subfacies. The lithology is brownish gray and dark gray micrite limestone, silty limestone is mixed with bioclastic limestone and granular limestone, and a small amount of dolomite or gray dolomite is often mixed at the top.
The effective reservoir space is mainly intergranular pores, intragranular dissolved pores and dissolved pores. Open fractures and structural micro-fractures are the main seepage channels, and the reservoir type is fracture-pore type.
Physical property analysis shows that the average porosity is 3.55% and the maximum porosity is 19. 1 1%. In the concentrated development section of solution holes in Well Ma 3, the computer scanning area ratio is 2 1.6%. The average analytical permeability is 2.23× 10-3 μ m2, and the maximum value is 128× 10-3 μ m2. The porosity calculated by logging is generally 3% ~ 5%, and some intervals reach 8%, and the permeability is (10 ~ 30) × 10-3μ m2.
Carboniferous bioclastic limestone section is well developed, with the density of 10 ~ 14/m. The dissolution in bioclastic limestone section is different from the weathering crust with big cracks and big holes in Ordovician limestone, and the dissolved holes are pinholes or sponges, which mainly belong to sulfuric acid dissolution during burial.
Effective fractures include structural fractures and diagenetic fractures. The density of structural fractures is 2.0 ~ 4.85/m, and it is unfilled-semi-filled, which is the main seepage channel. Diagenetic fractures are mainly vertical fractures developed in bioclastic limestone zone and do not penetrate this layer. The fracture is short and wide, with a length of 10 ~ 50 cm and a width of1~ 5 mm. The fracture surface is half filled with authigenic calcite.
The acidizing effect of bioclastic limestone section is good. Generally, hundreds to thousands of cubic meters of natural gas can be obtained by testing before acidification, and high yield can be obtained after acidification. Typical is the test of well Ma 31414 ~1424m interval, with the daily gas production of 454m3 before acidification and11.46x104m3 after acidification (7.94mm choke
(2) Clastic rock reservoir
Carboniferous sandstone-mudstone profile (C2)
Carboniferous sandstone and mudstone are 269~354.5m thick, sandstone section is 99 ~ 14 1m thick, covered layer is 27.9% ~ 57% thick, maximum single layer thickness is 29.5m, and average thickness is 10~20m ~ 20m. The strata in this section are basically stable in lateral distribution and good in connectivity, and are deposited in Binhai-Binhai subfacies, and the rock types are mainly fine-medium grained lithic sandstone. The porosity of core analysis is 1.99% ~ 2 1.08%, with an average value of12.25%. The permeability is (0.024 ~ 774) × 10-3 μ m, with an average of 3.12×10-3 μ m. The reservoir type is fracture-pore, and the effective reservoir space is mainly intergranular dissolved pores, matrix micropores and intergranular pores, followed by structural fractures and caves.
The reservoir matrix in glutenite section has low porosity and strong heterogeneity, but fractures are developed, which greatly improves the reservoir performance. Ma 40 1 well was tested at 2 126 ~ 2 165.3 m, with a 7.94mm choke and a daily gas production of 16.8372× 104m3. The calculated absolute non-negative flow rate is1/.
5.4.2.4 caprock
Carboniferous regional caprock in this area is one of the key factors for the good preservation of Hotan River gas field.
Carboniferous in Hetianhe gas field consists of three sets of regional caprocks, namely upper mudstone section, middle mudstone section and lower mudstone section, and some banded mudstone caprocks in Carboniferous sandstone mudstone section. They form a good regional reservoir-cap combination with the underlying Carboniferous standard limestone section and Ordovician buried hill, and a good banded reservoir-cap combination with the Carboniferous sandstone and mudstone section tidal flat subfacies sand.
Hetianhe gas field structure was formed in the early and middle Himalayan period. The violent activity of the fault destroyed the effectiveness of the trap of the Cambrian ancient oil reservoir, and made natural gas migrate vertically along the fault into the overlying Ordovician and Carboniferous traps, forming the current gas reservoir pattern. The late Himalayan tectonic movement did not destroy the main structure.
5.4.3 Oil and gas distribution prediction
5.4.3. 1 mazatak anti-s structure
Oil and gas are mainly concentrated in the arc bend of the fault structural belt, which indicates that the structural stress in this section is moderate, which is beneficial to oil and gas accumulation and enrichment, and points out the direction for oil and gas exploration in the future.
Acha-Mutu impact fault zone in northern Bachu uplift, 5.4.3.2.
It also has anti-S-shaped characteristics, so we should pay attention to finding favorable traps for oil and gas field exploration and discovery.
Seven NW-trending fault structural belts in Bachu Uplift, 5.4.3.3.
At present, oil and gas fields have been discovered in two fault structural zones, such as Selibu area and Mazatak fault zone, and the remaining five fault structural zones should be further studied and drilled to find oil and gas fields.