Study on Ordovician sedimentary system and characteristics of source-reservoir-cap assemblage in Xichang basin

The Ordovician in Xichang Basin is mainly distributed in the eastern part of Kangdian ancient land, Yuexi, Ganluo and Butuonan, among which Xinjigu is the thickest (thickness 8 19.67m) and Gangluo Pugeluogou is the thickest (thickness 995.20m). But Butuo Tujue Rize Village is thinner (5 15.26m). The lower series is Hongshiya Formation (O 1h) and the lower member of Qiaojia Formation (O 1Q). The Middle Series consists of the upper member of Qiaojia Formation (O2q), Daqing Formation (O2d) and Linxiang Formation (O3l), as shown in Table 5. 14. There are some differences in sedimentary facies, rock assemblage and biological abundance, which is undoubtedly the result of structural constraints in this area.

From April 20th to the beginning of May and September 20th, 2007 1 to 20th, the author carried out profile survey on Butuojue Rize Village and Ganluo Yutian Xinjigu.

Table 5. 14 Division of Ancient Ordovician in Rize Village and Xinji

5.3. Petrological characteristics of1

The types of Ordovician rocks are complete and diverse, including clastic rocks, argillaceous rocks, carbonate rocks, oolitic hematite beds and oolitic chlorite rocks. See table 5. 15 for rock types of each group.

Table 5. 15 Ordovician rock types in Xinjigu and Rizecun sections

Note: The rock composition content (%) is above the diagonal, and the rock thickness (m) is below the diagonal.

Hongshiya Formation is mainly composed of timely sandstone, siltstone and argillaceous rock. The lower member of Qiaojia Formation in Juerize Village, Butuo is mainly composed of granular limestone and a small amount of timely sandstone, while Xinji Valley is mainly composed of timely sandstone, siltstone, argillaceous rock and a small amount of granular limestone. In Rizecun section, the upper member of Qiaojia Formation is a single granular limestone, while in Xinjigu section, the upper member of Qiaojia Formation is a variety of granular limestone, including timely sandstone, feldspathic sandstone, siltstone and granular limestone, dolomite limestone, siliceous limestone and dolomite. Daqing Formation is composed of granulite limestone and dolomite in Rizecun section, mainly granular dolomite in Xinjigu section, followed by siliceous dolomite and residual bioclastic granular dolomite. Linxiang Formation in Rizecun section is composed of microcrystalline limestone and bioclastic microcrystalline limestone. There are residual sand dolomite and granular dolomite in the Xinjigu section.

5.3. 1. 1 clastic rock

(1) Timely sandstone

1) Seasonal sandstone is mainly distributed in Hongshiya Formation, with a small amount in the lower member of Qiaojia Formation. Light gray, grayish white thin-thick layer, with detrital content accounting for 90% ~ 95% of the rock, mainly composed of single crystal, containing a small amount of polycrystalline quartz, undulated extinction, and inclusions and secondary enlarged edges can be seen. The general content of plagioclase and microcline is 1% ~ 2%, and the heavy minerals are zircon and tourmaline. The content of interstitial materials is 5% ~ 10%, mainly chemical cements, mainly siliceous, followed by heterogeneous groups of calcium, iron and clay. Most of them are fine particles, and they can also be fine particles or medium particles, which are sub-circular. Contact cementation of parallel bedding pores. The above characteristics show that it has a high maturity and belongs to the product of high energy environment.

2) Seasonal sandstone with internal debris of phosphorite is dark gray and grayish green, thin to medium thick. The phosphorus internal debris is mainly composed of sand and gravel, and the content of oolite is 15% ~ 30% of the rock, so phosphorus metasomatism feldspar and muscovite can be seen. The cement is mainly siliceous, followed by phosphorus, dolomite and calcium. Coarse particles, fine-medium particles, sub-circular. Four layers can be seen in Rizecun section, and only one layer can be seen in Xinjigu section, which is mostly produced in the lower part of Hongshiya Formation and is the product of coastal and tidal flat environment.

3) Feldspar (cuttings) is timely sandstone, gray, grayish green and grayish brown, thin-thick. The feldspar content is generally 10%, the highest is 15%, and the lowest is 5%. The composition is plagioclase, microcline and striped feldspar. It can be seen that the dissolution edges are sericitization and hydromica, and some are pyritization. The mica content is generally around 2%, and the heavy minerals are mainly zircon. Separation and grinding are medium, fine, medium and sub-circular.

Chemical cements include siliceous, calcareous and siderite. The content of clay matrix is generally 5% ~ 10%, some of which can reach 15%, and most of them are hydromica. Contact pore cementation.

Among the timely sandstone cuttings, argillaceous rocks are mostly found in cuttings, with a content of < 5%, and mica content is relatively high, generally between 5% ~ 12%. Feldspar content 1% ~ 5%, poor sorting, generally unequal grain sand structure, medium abrasive grain. Chemical cements are calcareous, phosphous and siliceous, and clay matrix is generally 2% ~ 5%, and some can reach more than 10%. Contact pore cementation.

Feldspar-time sandstone belongs to the product of coastal or tidal flat environment, while lithic-time sandstone is mainly found in Neogene paleoprofile and belongs to the product near the bottom.

(2) Feldspar sandstone

It is only found in the upper member of Qiaojia Formation in Xinjigu section, and it is gray and brownish gray with medium thickness. The content of feldspar in the detritus is 25% ~ 30%. Its composition is mainly plagioclase, followed by microcline and striped feldspar, and its edge is secondary enlarged, carbonated or sericitized, and its edge is irregular. Muscovite is 1%, and heavy mineral is zircon. Most of them are fine particles, or they can be unequal particles, which are divided into edges and circles. Chemical cements are dolomitic and siliceous. Generally, the clay heterobase is 3%, mostly hydromica. Pore contact cementation belongs to coastal environment.

(3) Siltstone

Mainly distributed in Hongshiya Formation and Qiaojia Formation. Gray, grayish green, purple and other colors, thin layer-medium thick layer. The clastic composition is mainly chronological, followed by feldspar and mica, with a content of 5% ~ 10%. Mica has directionality, and the particle size is mostly coarse silt, which can contain a small amount of particles. The interstitial materials are calcareous, dolomite, siliceous and clay matrix (hydromica, directional).

5.3. 1.2 Mud (shale)

Mainly found in Hongshiya Formation. Grayish black, purple, grayish yellow, grayish green and other colors. Thin-medium thick layer. The quality is impure, the mud content is generally high, ranging from 30% to 40%, and the clay mineral is mainly hydromica. It is directional. According to the X-ray diffraction analysis of clay minerals in Xinjigu section 137, the composition is illite 55%, chlorite 12%, quartz 20%, feldspar 13%, and graptolite, brachiopod and other fossils can be produced in argillaceous rocks. It belongs to the product of low-energy environment in shallow sea.

5.3. 1.3 carbonate rock

Except Hongshiya Formation, other horizons are widely distributed. Rock types are extremely rich. However, granular limestone (dolomite) has a single type of granular debris, mainly composed of biological debris and sand debris, followed by spherulites, and oolites are rare. The interstitial materials are generally microcrystalline matrix and bright crystals are rare.

(1) limestone

1) bioclastic limestone (including bioclastic and bioclastic microcrystalline limestone) is mainly found in Qiaojia Formation. Gray-black, dark gray, thick block. There are brachiopods, trilobites, bryozoa, Echinoderms, Ostracoda, Gastropoda, Claudiobranchia, Spongebone Needles, Green Algae, a small amount of kelp and radiolaria. Biochips are silicified, pyritized or dolomitized. Bioclastic limestone content is generally 60% ~ 70%, and terrigenous mixed quartz silt does not exist or does not exist. The content of authigenic mineral pyrite is generally 2% ~ 3%, and the filler and gypsum are recrystallized into fine crystals. Pore basement cementation. Rock may contain about 10% of particles or sand. Selective dolomitization is more common. When the content of bioclastic is less than 50%, it is bioclastic or bioclastic microcrystalline limestone.

The organisms are mainly normal salinity organisms, and the forming environment energy is not high, which belongs to shallow sea environment.

2) Sandy limestone was found in Qiaojia Formation and Daqing Formation in Rize Village. Gray-black, gray thin layer block. The particle size of sand debris is uneven, ranging from 1.2 mm to 0.05 mm, generally 0. 1 ~ 0.6 mm. At the same time, different contents of brachiopod, trilobite and sea lily stem biological debris are also seen. Occasionally see thin-skinned ostriches. The authigenic mineral pyrite is 2%, and the interstitial material is microcrystalline matrix. Local recrystallization is powder-fine crystal. Selective dolomitization, basement cementation.

When the sand content is less than 50%, it is sand or limestone containing sand. In general, rocks contain about 10% normal salinity cuttings, and dolomitization is common and thin.

3) Globular limestone is only found in the lower member of Qiaojia Formation in Rizecun section. Dark gray, gray-black thick layer, uniform particle size, 0. 1 ~ 0.2 mm, mostly about 0. 1~0.2mm, oval, dark color, rich in organic matter, internal structure, content of about 60%. At the same time, there are 2% ~ 10% brachiopods, ostracods, sea lilies and so on. Mortar matrix can be recrystallized into fine powder crystals with dolomite phenomenon. Basement cementation. The rocks can contain 5% ~ 10% terrigenous quartz silt, which belongs to shallow sea environment.

4) Mud (microcrystalline) limestone is mainly developed in Linxiang Formation in Rizecun section, followed by the lower member of Qiaojia Formation. Dark gray, gray-black, extremely thin-extremely thin, with microcrystalline structure. The rock can contain ≤5% brachiopods, sponges, trilobites, ostracods and a few complete brachiopods fossils. Pyrite is 1% ~ 3% and is rich in organic matter. Locally, it can be recrystallized into powder crystal or dolomite, with horizontal stripes and biological disturbance structure and developed cracks. It belongs to the product of still water and low energy environment.

5) Silty limestone is only found in the lower member of Qiaojia Formation in Rizecun section. Dark gray thin layer, containing 2% pyrite, with developed cracks, width 1 ~ 0.5 mm, filled or semi-filled.

(2) Dolomite

The rock is mainly developed in the upper member of Qiaojia Formation, Daqing Formation and Linxiang Formation in Xinjigu section.

1) residual sand dolomite, developed in Linxiang Formation. Dark gray very thin layer-medium thick layer, mainly thin layer. The content of residual sand debris is 60% ~ 75%. Dolomite is recrystallized into fine crystals, which have silicification and can contain 2% ~ 3% organic matter. The autogenous time is generally around 2%, and mud can be seen. Most of the matrix is recrystallized into fine powder crystals, and a few are medium crystals. Rock has residual stripes and biological disturbance structure. When the sand content is less than 50%, it is fine-grained dolomite with residual sand powder. Its general content is 30%, and the middle of intergranular cracks is filled with organic matter.

It is speculated that the rocks should be sandy limestone and sandy microcrystalline limestone, and the residual sandy dolomite formed by freshwater dolomitization belongs to the sandy beach on the edge of the platform.

2) Residual spherulite dolomite (including residual spherulite dolomite) is small in quantity and only 3.66m thick. Dark gray thin layer-medium thick layer, containing 70% spherulites, with particle size of 0. 1mm, recrystallized into dark powdery crystals, containing organic matter. Siliceous pyrite 1%, spherulite silicification. The matrix is powdery dolomite. Basement cementation. When the content of residual spherulites is 10%, it is powdery dolomite with residual spherulites, which has biological disturbance structure.

3) Residual bioclastic fine-mesophase dolomite, dark gray, medium thick layer, with residual bioclastic content of about 30%, including brachiopods, lily bulbs, trilobites and green algae. The matrix is recrystallized into fine-medium crystals. Karst caves and dissolved holes are well developed.

4) Granular dolomite, dark gray, gray, gray-black, thin-thick. The particles are powder-coarse particles, mostly fine-medium particles, with banded structure and semi-self-shape-self-shape. There are 1% ~ 5% residual biological debris in rocks, such as brachiopods, trilobites, ostracods, lily stems and algae debris. Crystal caves are developed, which may have siliceous bands or nodules, as well as secondary calcite veins or plaques.

Those particles with fine particles usually have a striped or banded structure. Visible bottom scour. Generally speaking, Linxiang Formation is darker in color and finer in particles than Daqing Formation. However, Daqing Formation is mostly medium-coarse grained and light in color. In Linxiang Formation, rocks are pseudobreccia due to extremely developed fractures.

5.3.2 Sedimentary facies characteristics of profile

According to the measured profile, the Ordovician sedimentary facies are divided as follows:

5.3.2. 1 outer shelf facies

This facies is developed in Linxiang Formation in Rizecun section (Table 5. 16).

Table 5. 16 Division of Paleo-Ordovician Sedimentary Facies in Rize Village and Xinji

The rock assemblage is grayish black, dark gray, very thin-very thin microcrystalline limestone, containing bioclastic and bioclastic microcrystalline limestone, mixed with shale and siltstone, and containing flint nodules and argillaceous bands. Biological assemblage includes brachiopods, trilobites, sponges, gastropods, ostracods, radiolarians, echinoderms, calcium balls, etc. Salinity is normal, and complete brachiopod fossils can be seen buried in situ. Horizontal stripes are common in sedimentary structures, and biological disturbance and bottom scouring structure can be seen. These rocks are rich in organic matter and pyrite. This feature shows that it belongs to the environment of still water and low energy outer shelf.

5.3.2.2 Inland shed (shallow sea)

We should meet in Linxiang Group, Daqing Group and Qiaojia Group. There are differences in rock types and biological abundance between north and south.

Xinji Paleogene: The rock assemblage of Linxiang Formation and Daqing Formation is gray-black, gray medium-thick residual bioclastic fine-mesomorphic dolomite, residual sand dolomite, dolomite mixed with shale and limestone. Karst caves are developed by dissolution.

The remains are brachiopods, trilobites, mollusks, sea lilies and green algae. Sedimentary structures, horizontal stripes and biological disturbance structures. The rock assemblage of Qiaojia Formation is gray-black, dark gray, medium-thick and fine grained bioclastic limestone. Silty silty limestone, dolomitized bioclastic limestone, dolomitic limestone, siliceous dolomite, timely sandstone with granular dolomite, and flint nodules and bands.

Biological assemblage includes brachiopods, trilobites, lily stems, spongy spicules, ostracods, krabbe animals, bryozoa and green algae.

Rizecun Formation: It is characterized by grayish black and purplish red bioclastic microcrystal limestone, bioclastic limestone, fine-medium grained sand limestone, fine-medium grained sand limestone, siltstone, calcareous shale and unequal grained sandstone. Oolitic hematite layer near the bottom. Limestone is locally dolomitized. There are many kinds of organisms, including trilobites, brachiopods, echinoderms, foraminifera, ostracods, bryozoa and green algae. Horizontal bedding is very common in sedimentary structures.

Shoal facies (beach facies and bioclastic beach facies) on the margin of 5.3.2.3 platform.

1) Beach facies: developed in Linxiang Formation and Daqing Formation in Xinji ancient section. It is characterized by gray and dark gray thin-medium-thick residual sandy dolomite containing flint nodules and bands. A small amount of residual stem fossils of sea lily can be seen. The sedimentary structures are residual laminae and biological disturbance structures.

2) Bioclastic beach facies: found in the lower member of Qiaojia Formation in Rizecun profile. Gray-black thick layer of fine-grained bioclastic limestone containing spherulite bioclastic limestone. Selective dolomitization. The biological combination is mainly brachiopod; Trilobites, Ostracoda, Echinoderm, bryozoa, Gastropoda, Clarambranchia, Coral, etc. Brachiopod fragments were found on the 84th floor, most of which were densely distributed along bedding and bulge.

5.3.2.4 finite platform facies

It is found in Daqing Formation in Rizecun section. It is characterized by dark gray medium-thick layered-massive, powdery-microcrystalline sand or spherulite dolomite, dolomite, bioclastic or beaded microcrystalline limestone and bioclastic microcrystalline limestone. Dissolved pores are well developed, and there are dolomite patches in the rocks. Biological debris includes trilobites, brachiopods, ostracods, echinoderms, spongy spicules, bryozoa, mollusks and green algae.

5.3.2.5 tidal flat subfacies

1) Qingshui tidal flat subfacies (intertidal zone-supratidal zone) can be found in Linxiang Formation in Neogene paleoprofile. It is characterized by dark gray medium-thick silty dolomite and residual spherulite dolomite. There are bird's-eye, layered structure and flaky cracks in the sedimentary structure, and there is degumming phenomenon. No residual biological fragments were found.

2) Muddy tidal flat subfacies were found in Hongshiya Formation in Rizecun section. It is characterized by gray, grayish green and purplish red thin to medium thick layers of fine-grained chronological sandstone, feldspar chronological sandstone, coarse siltstone and silty hydromica claystone. Sedimentary structures include sand bedding, lenticular bedding, vein bedding, wave bedding, horizontal bedding and biological disturbance structure.

Biological fossils are scarce, only brachiopods are found, but trace fossils are rich, which belong to the combination of needle marks, ancient algae marks, sand bracelet marks, wall marks and halo marks.

3) Subtidal microfacies are developed in the lower part of Hongshiya Formation in Rizecun section. It is characterized by gray, grayish green, purplish red, thin-medium-thick, fine-medium-grained phosphorus-containing internal debris timely sandstone, quartzite sandstone mixed with silty hydromica clay rock. Nearly symmetrical wave marks are mainly seen in sedimentary structures, brachiopods are the main biological fossils, and Cruz marks are the trace fossils.

5.3.2.6 coast subfacies

This facies developed in Hongshiya Formation and Qiaojia Formation in Xinjigu section. It can be subdivided into two microfacies: foreshore and nearshore.

1) Foreshore microfacies: It is characterized by light gray, grayish white, medium thick-fine-medium grained chronosandstone, dolomitic chronosandstone and chronosandstone, and contains phosphorus-containing internal debris. Its composition, structure and maturity are very high. The sedimentary structures are parallel bedding, nearly symmetrical wave marks and monoclinic bedding. Biofossils include brachiopods, ancient algae traces, single traces and vertical worm tubes.

According to the grain size analysis, it has the characteristics of beach, including two stages, jumping population and suspended population, with steep slope and steep demarcation point, and the standard deviation is 0.87.

2) Nearshore microfacies: It is characterized by dark gray, grayish green and purplish red fine-grained feldspar timely sandstone, timely sandstone and coarse siltstone mixed with thin to medium-thick layers of silty hydromica claystone. The lower part near the shore is poorly sorted, mostly with unequal grains and rich in mica sheets. There are parallel bedding, nearly symmetrical wave marks, wavy bedding and sand bedding in sedimentary structures. The biological combination is brachiopod and trilobite fragments.

Trace fossils include single cup trace, single trace and flat tube trace. Trace fossils mostly reflect that organisms are foraging horizontally or moving in one direction, indicating that waves and tides are relatively weak and seawater salinity and temperature are relatively stable. The results of particle size analysis show that it has the characteristics of coastal sand, which is two-stage, jumping population and steep slope suspended population. The cut-off point is abrupt with a standard deviation of 0.68.

5.3.3 Vertical and horizontal changes of sedimentation

In the early Ordovician, Xichang basin was a terrigenous clastic rock deposit in coastal tidal flat environment, distributed in the east of Puge Dacao River in western Vietnam and the Kangdian ancient land in the west. There is little difference between the south and the north. In the north, Ganluobobo Township is the thickest (324m thick) and Tianping is the thickest (309.8m thick), and it gradually decreases to the west, and the new Jigu is 234.3m, and the crotch ditch179 m. It is a set of clastic rocks and argillaceous rocks deposited on the coast without barrier. The southern part is mainly composed of tidal flat clastic rocks and argillaceous rocks. However, in the late Early Ordovician, it was a shallow-sea carbonate deposit. Luowugou deposit is the thickest (468.8m), and it becomes thinner to the east or southeast. The thickness of Rize Village is 365,438+065,438+0.6m, forming a thinning pattern opposite to that in the north. At the same time, it is seen in Dacaohe that the Baiguowan Formation of Upper Triassic directly overlaps with Hongshiya Formation, and its exposed signs are obvious, such as dry cracks and diamond-shaped wave marks, which are the product of tidal flat environment.

The middle and late Ordovician is mainly marine carbonate deposits, which are distributed in Yuexi and eastern Puge, and the western part is still Kangdian ancient land. At this time, the difference between north and south is obvious. The sediments of Minzi Luomu (555.8 meters thick) and Xinjigu (485.4 meters thick) in the north are the thickest, and become thinner to the east and west. The thickness of crotch groove is only 1.9 1.3m, which is dominated by a set of dolomite. From west to east, the sedimentary facies is tidal flat-platform edge sand beach-shallow sea. Linluogou area in the northern part of Xinjigu is a set of coastal deposits of limestone and clastic rocks.

In the south, Louwugou deposit is the thickest, with a thickness of 526.4m, and becomes thinner eastward. The thickness of Rize Village is 203.7m, while the iron foot is only 147.9m, which is mainly composed of limestone deposits. Luowugou is dolomitic limestone and dolomite. From west to east, the sedimentary facies is tidal flat-open platform-shallow sea.

Generally speaking, the Ordovician in Xichang Basin is a transgressive sequence from Hongshiya Formation to Linxiang Formation, from terrigenous clastic rock deposition to carbonate rock deposition. In the meantime, high-energy beaches or biological beaches appear on the local horizon. However, the southern part is deeper than the northern part, and the outer shelf sedimentary facies (Linxiang Formation) appears, which is a favorable oil (gas) generating horizon.

5.3.4 Combination analysis of source, reservoir and cap rocks

The Ordovician oil-generating conditions in Xichang basin are poor. According to the previous oil generation analysis data, the organic carbon in sandy mudstone of Hongshiya Formation in Rizecun is only 0.08%. The organic carbon content in the dark gray carbonate rocks of the upper member of Qiaojia Formation and Daqing Formation is 0.05% and 0.08% respectively. The Linxiang Formation is mainly a set of gray-black limestone. According to microscopic observation, the organic matter content can reach 5% and pyrite content is 2%, which is a favorable source rock. Although the oil-generating conditions of Ordovician are not ideal, the reservoir performance is quite good. Linxiang Formation and Daqing Formation are the most ideal in Xinji ancient profile, which together with the overlying Silurian source rocks and caprocks form a relatively complete source-reservoir-cap assemblage. Among residual sand dolomite and silty dolomite, karst caves are extremely developed. Its diameter is mostly 0.5 ~1cm; Up to 5 cm; Filled or semi-filled. According to the analysis of porosity and permeability, the porosity of 237 layers of silty dolomite in Linxiang Formation is 1.7%. The porosity of 222 layers of silty-mesomorphic dolomite is 3.44%; The permeability is 1.06× 10-6μm2. The analysis of medium-coarse dolomite samples in Area 7 of Daqing Formation shows that the porosity is 1.05% ~ 5.55%, with an average of 3.38%. The permeability is (5.74×10-7 ~ 82×10-3) μ m2, with an average of 3.79× 10-5μm2. At the same time, in the thin section observation, the intergranular dissolved pores are 1% ~ 3%, with a diameter of about 0.2mm and uneven distribution. Intergranular cracks are also well developed and full of organic matter.

In Rizecun section, the reservoirs are Daqing Formation and Qiaojia Formation, and the lithology is mainly granular limestone, micro-coarse grained limestone with particles, dolomitization and karst cave development. Thin section observation shows that intergranular dissolved pores are 2% ~ 3%, and cracks are developed. At the same time, there is biological beach facies, which is a good oil and gas reservoir and can form a source-reservoir combination with the overlying Linxiang Formation oil and gas reservoir.

Generally speaking, the Ordovician in Xichang Basin is not ideal for oil generation, so it cannot form a complete source-reservoir-cap assemblage, but Daqing Formation, Linxiang Formation and overlying Silurian Formation form an ideal source-reservoir-cap assemblage. Regionally, between Xinjigu and Luowugou, the pores and caves of granular dolomite and dolomitic limestone are developed, and the thickness is relatively large, nearly 449 ~ 486 m. Previously, dry asphalt was seen in the caves of Daqing Formation in Kupaigou, which is actually a good regional reservoir. Qiaojia Formation, Daqing Formation and Linxiang Formation form a source-reservoir combination. Hongshiya Formation (caprock) can form a reservoir-cap combination with the underlying Upper Cambrian Erdaoshui Formation (reservoir).

conclusion and suggestion

1) There is little difference between the north and the south of the Early Ordovician basin, which is mainly composed of terrigenous clastic rocks deposited on the shore or tidal flat. Only in the Rizecun area in the south of the basin, the late Ordovician was shallow-sea carbonate deposition. In the middle and late Ordovician basin, the north-south differentiation is obvious, and the water body in the south is deeper than that in the northern Neogene area, and deep-water carbonate deposits appear. Dolomitization is not strong, so it constitutes a source-reservoir combination. The water body in the north is shallow, and there are high-energy platform marginal shoals. At the same time, freshwater dolomitization is strong and accompanied by pore development, which is a good reservoir. Form a complete source-reservoir-cap assemblage with the overlying strata.

2) If the Lower Paleozoic in Xichang Basin is the exploration target layer, the study of Ordovician source rocks and reservoirs should be strengthened in the future.

3) Geophysical exploration in Zhaojue Basin and Mishi Basin can confirm whether there are Ordovician deposits in this area.