Oil and gas accumulation and evolution

(1) Gui Zhong sag in southern Guizhou is rich in oil, natural gas and asphalt, and many ancient oil reservoirs have been discovered.

According to the preliminary ground geological survey statistics of Qiannan Depression, 586 oil and gas seedlings and bitumen were found in this area, mainly in Cambrian-Silurian strata. Eight ancient reservoirs, such as Majiang ancient reservoir and Kaili residual reservoir, have been discovered in Dushanbi uplift, among which the original reserve of Majiang ancient reservoir exceeds 15× 108t.

After exploration in 1960s and 1970s, oil, gas and water shows of different degrees were found in 12 shallow wells in Ping Huang and Anshun areas. According to drilling, the lower Silurian sandstone section of huzhuang anticline is about 50m thick, which is the main gas-producing horizon. Among them, Well Hu 4 1 (5400m3/d) has the largest gas production, while other wells such as Well Hu 37, 23, 47, 45, 18, 27 and 30 all have gas production in different degrees, ranging from several cubic meters to many. After acidizing and fracturing of the Middle Ordovician Dawan Formation, the total crude oil in Well Hu 47 is 2,300 kg, and oil immersion or a small amount of crude oil is common in other wells. After drilling the Lower Ordovician Tongzi Formation in Well Zhuang L, the water leakage was about 1300m3 for the first time and about 130m3 for the second time. When drilling limestone in the lower part of Lower Cambrian, the core shows gas. In the shallow well drilling of Yeshan syncline and Tang Kai syncline, there are different degrees of oil and gas display, and the display horizons are mostly in the Middle and Lower Silurian, the upper part of the Lower Ordovician and the lower part of the Middle Ordovician. During the test of Kai 1 well, crude oil 100kg, crude oil of Kai 8 well and crude oil of Kai 10 well were obtained, and the oil quality was viscous. Well 60/CK 1, Well 60/CK3, Well 60/CK9, Well Ni 2, Well Ni 4 and Well Ni 7 in Anshun area all contain oil in Permian and Triassic.

Oil sands and natural gas are shown in Yashui (ultra-deep) well 1 in Changshun sag, oil sands and natural gas are shown in 1 ~1970m interval, asphalt is shown in Qianya 2 well, micro gas is shown in Wang Shen well D2 in Wangyou structure, and natural gas is shown in Wang Shen well1.

There are 88 oil and gas seedlings and asphalt in the central Guangxi depression, including 57 asphalt. According to horizon statistics, there are 32 Devonian, 35 Carboniferous, 5 Permian/kloc-0 and 6 Triassic. Among them, the representative underground oil and gas shows are: Well Luo 1 and Well Luo 2, Well Da 2 and Well Da 5 in Dapu structure, Well Yan 2 in Yankou structure, Well Bei 1 and Well Bei 2 in Beishan structure are all gas injected in the Lower Carboniferous; Well Li Miao Structure 1 and Well Larry Structure 1 are middle Devonian gas injection wells. Gas sample analysis: c1:ch 40.21%~ 67.9%, n 232.5% ~ 99.2%; D2: 43.56%~8.88% of methane and 280.548%~94.55% of nitrogen. ; The Lower Carboniferous is dominated by CH4 or N2 gas; The Middle Devonian is N2 gas. Well Yankou Structural Rock 2 is 0.5 ~ 1.5m and 597 ~ 6 12m away from the wellhead, and the horizon is Carboniferous Tang Ying Formation (C1yt). Li Miao structure 1 well gushed out from the wellhead at 986 ~ 1293 m, and oil-bearing asphalt was found, belonging to Middle Devonian Donggangling Formation (D2d). There are 7 shallow wells near the Chehe River in Nandan, of which Devonian gas injection occurred at 197 1 and ZK 1 and 1976 were still bubbling. Gas was injected into the 1987 1 175 and 173m sections constructed in February, which caused fires due to improper use of fire and burned for 1 hour for 30 minutes. Green crude oil was discovered in Triassic in well D907 in Beishan, Heshan.

Longtoushan, Dachang, Nandan, has Middle Devonian paleo-(reef) reservoirs with an area of tens of square kilometers and asphalt reserves of1.1×108t. In Hechi, the accumulated asphalt has reached more than 10,000 tons.

In 2007, when Guangxi Geological Exploration Institute drilled the geothermal well Liure 1, natural gas gushed at 143 ~ 20 1.33 m, and the flame was blue-yellow, with a flame height of 0.3 ~1.2 m. The composition of natural gas was methane. The gas reservoir is located in the east of Hechi-Yizhou-Luzhaiyan west arc fold-fault structural belt, and Jiangmen anticline structure is Xi Rui. The anticline structure is composed of lower Carboniferous Simen Formation (C 1s), Luocheng Formation (C 1l) and the lower part of Upper Carboniferous, and the gas reservoir is obviously controlled by the anticline structure. The gas-producing layer is the Lower Carboniferous Simen Formation (C 1s), and its lithology is bioclastic microcrystalline limestone and bioclastic siliceous rock. There is 30m mudstone mixed with siliceous mudstone in the upper part of the gas reservoir as the cap rock (Figure 2- 15).

In a word, oil shows are mainly concentrated in Kaili area of Qiannan Depression, southern Guiyang and Liucheng and Luzhai areas in the northeast of Gui Zhong Depression, and asphalt and natural gas are widely distributed in this area. Vertically, oil, gas and asphalt in Qiannan Depression are distributed in different degrees from Sinian to Triassic, mainly in Cambrian-Silurian, liquid hydrocarbons are mainly distributed in Ordovician and Silurian in the east of the depression and Permian-Triassic in the north, and oil, gas and asphalt in Gui Zhong Depression are mainly distributed in Devonian and Carboniferous, followed by Permian and Triassic.

According to the previous analysis, the centralized distribution of oil and gas asphalt in this area is mainly controlled by four factors: first, it has good hydrocarbon source conditions; Second, it is distributed in favorable reservoir facies zones; Third, it has favorable paleostructural conditions; Fourth, faults are developed and hydrothermal activities are frequent. The first three factors are favorable conditions for oil and gas generation and accumulation, and the fourth factor is one of the main reasons for oil and gas damage.

The above situation shows that there were extensive oil and gas generation and large-scale oil and gas migration and accumulation in central Guizhou. However, due to the destruction of late subsidence, structural uplift and extrusion, many oil and gas and asphalt display points and ancient reservoirs still exist, and preserved primary or secondary reservoirs may still remain in areas with good preservation conditions. Therefore, a relatively good oil and gas preservation area (unit) should be the direction selection area for oil and gas exploration in this area.

(2) Accumulation conditions and evolution process of Majiang ancient oil reservoir and Kaili residual oil and gas reservoir.

Majiang ancient oil reservoir is the Lower Paleozoic ancient oil reservoir in Majiang-Duyun area, southern Guizhou. According to preliminary calculation, its original oil reserves reach 15.08× 108t, which is one of the Caledonian super-large reservoirs. "Kaili residual oil and gas reservoir" is also a Paleozoic oil and gas reservoir, and it is the earliest area where live oil seedlings and a large number of oil and gas seedlings were discovered in Guizhou. From 1950s to 1990s, 54 shallow wells and 2 deep wells were drilled in this area. Most wells have found oil and gas, and some wells still produce a certain amount of oil and gas, which has attracted worldwide attention (Figure 2- 16).

Figure 2- Stratigraphic Histogram of Well Liure 15

(According to Guangxi Geological Exploration Institute, 2007)

1. Accumulation conditions and evolution process of Majiang ancient oil reservoir

Majiang ancient oil reservoir was formed in the late East Caledonian-Devonian period, and it was cast into the present situation after Hercynian-Indosinian buried thermal evolution and post-Yanshan uplift and erosion transformation.

1) Accumulation conditions of Majiang ancient oil reservoir

A. The main source rocks are muddy source rocks of the Lower Cambrian.

The main source bed of Majiang ancient reservoir is black mudstone of Lower Cambrian basin facies-shelf facies. The peak of oil generation is from the end of Silurian to the beginning of Devonian. Lower CAMBRIAN dark mudstone thickness 100 ~ 600 meters.

B. Development of porous sandstone reservoir in the third member of Wengxiang Formation and karst reservoir in Honghuayuan Formation.

The main reservoirs of Majiang ancient reservoir are sandstone reservoir of the third member of Wengxiang Formation (S 1w3) and carbonate reservoir of Honghuayuan Formation (O 1h). The reservoir characteristics are shown in Table 2-8.

Table 2-8 Reservoir Characteristics of Majiang Ancient Reservoir

Fig. 2- 16 Structural Geological Diagram of Majiang Ancient Reservoir.

(According to China Petrochemical, 2006)

The reservoir space of sandstone in the third member of Wengxiang Formation (S 1w3) is mainly primary intergranular pores, including primary intergranular pores after compaction and partial cementation, followed by secondary fracture pores. The main source rock of Majiang ancient reservoir is dark argillaceous rock of Lower Cambrian, and the peak of oil generation is at the end of Donghuli. At this time, an ancient uplift has been formed in Majiang area. The sandstone reservoir of the third member of Wengxiang Formation (S 1w3) and the mudstone caprock of the fourth member of Wengsi Formation (S 1w4) of Silurian system have not been deposited for a long time, and the pores in the sandstone reservoir have not experienced complex diagenesis, which makes the primary intergranular pores become pores in the reservoir-forming period. This arrangement of source rocks, reservoirs and caprocks in time and space is an important factor for Majiang ancient reservoir to become a large-scale ancient reservoir.

The carbonate reservoir space of Honghuayuan Formation (O 1h) is mainly secondary dissolved pores and structural fractures. The carbonate reservoir of Honghuayuan Formation in Majiang ancient reservoir was exposed or close to the surface after Duyun movement and uplift at the end of Ordovician, and its upper part was leached and dissolved by atmospheric precipitation, forming secondary dissolution pores and structural fractures generated during uplift, which is the main reservoir space. In the reservoir of Honghuayuan Formation (O 1h) in Majiang ancient reservoir, asphalt mainly exists in the secondary dissolution pores and fractures at the top of this formation (10 ~ 25m).

C. The argillaceous rocks in the fourth member of Wengxiang Formation are good regional caprocks.

The existence of caprock is also an important condition for the formation of ancient oil reservoirs. Before the formation of Majiang ancient reservoir, the argillaceous rocks in the 4th member of Silurian Wengxiang Formation covered the sandstone reservoir in the 3rd member of Wengxiang Formation (S 1w3) in a large area, with the thickness of 168 ~ 5 12m, forming the direct and regional caprocks of the reservoir. Because the argillaceous rocks in the fourth member of Wengxiang Formation (S 1w4) are relatively dense, and mudstone accounts for a large proportion, it is a set of homogeneous caprocks with good sealing ability. A good caprock of Majiang ancient oil reservoir was formed.

D. The trap types of ancient oil reservoirs are structural-lithologic compound traps dominated by structural traps.

The peak of oil generation of the main source rocks in Majiang ancient reservoir is in the late Caledonian. At this time, the paleostructure is characterized by "two ridges and two depressions": Wuling sag in the north, Qiannan sag in the south, Qianzhong uplift in the west and Xuefeng uplift in the east. Majiang ancient oil reservoir is located in a favorable position on the ancient uplift on the south side of the saddle of "two ridges and two depressions" and is a favorable structure for oil and gas traps. The trap types of ancient oil reservoirs are structural-lithologic compound traps dominated by structural traps.

E. it perfectly combines raw materials, storage and covering.

The vertical combination of source and reservoir in Majiang ancient reservoir has the characteristics of ancient source and new reservoir, that is, the spacing between source and reservoir is 2200～2600m m. Silurian was deposited immediately after Duyun movement lifted Danghuayuan Formation (O 1h) reservoir and leached and dissolved. At this time, the sandstone reservoir of the third member of Wengxiang Formation (S 1w3) entered the peak of oil generation shortly after deposition. Because the diagenetic change of sandstone in the third member of Wengxiang Formation (S 1w3) is relatively simple, today, in the upper part of Honghuayuan Formation (O 1h), asphalt is mainly filled in secondary dissolution pores and fractures. The caprock is a direct shielding caprock formed by argillaceous rocks in the fourth member of Wengxiang Formation (S 1w4) covering sandstone reservoirs in the third member of Wengxiang Formation (S 1w3).

F. Preservation conditions of ancient oil reservoirs

Majiang ancient oil reservoir is located in the western edge of the Caledonian Xuefeng uplift fold belt. Influenced by Duyun movement in the middle and late Late Ordovician, Majiang area formed a broad and gentle ancient uplift. After the uplift and denudation in the Late Ordovician-Longmaxi period, the core stratum of Honghuayuan Formation (O 1h) was exposed to the surface, and the deposited carbonate rocks rose to the surface after shallow burial, cemented recrystallization or local dolomitization, and were leached and dissolved by atmospheric fresh water-mixed water, forming a large number of dissolved pores and holes. From Dazhong Dam period of Silurian to Guangxi movement at the end of Silurian, Majiang ancient uplift was further developed and formed on the basis of the prototype formed during Duyun movement, and gradually accumulated oil and gas.

At the end of Silurian, when the ancient oil reservoir was formed, the argillaceous rocks in the fourth member of the third member of the urn (S 1w4), the main reservoir, formed a good caprock with a thickness of 260 (Danzhai Yanzhai) ~ 455 m (Majiang grindstone). Generally speaking, the argillaceous rocks in the fourth member of Wengxiang Formation (S 1w4) constitute a unified regional cap rock of the ancient reservoir, which is a very important condition for the formation and protection of the ancient reservoir.

2) Evolution process of Majiang ancient oil reservoir

A. regional thermal evolution and metamorphism

After the Guangxi movement, Majiang ancient oil reservoir was located in the eastern margin of Qiannan (Late Paleozoic) depression and began to experience long-term continuous burial. With the long-term continuous thermal action, it is inevitable that the crude oil in ancient reservoirs will evolve in the direction of cracking and polycondensation. Before Eogene, when the regional fold uplifted due to Yanshan movement, the ancient oil reservoir had been buried for nearly 300 million years, with the maximum burial depth of 4000 ~ 5000m·m and the maximum burial temperature of1/kloc-0 ~ 225℃. The preservation state of hydrocarbons has entered the stage of oil-gas cracking and asphalt polycondensation, and a large amount of asphalt existing in ancient reservoirs today reflects the above understanding.

The crude oil of the third member of Wengxiang Formation (S 1w3) and Honghuayuan Formation (O 1h) in Majiang ancient reservoir has been unrecognizable after the geological and geochemical processes in Hercynian-Himalayan period. The liquid hydrocarbon formed in the reservoir is metamorphic asphalt which is highly condensed under the action of temperature and time.

The thermal evolution direction of petroleum is that cracking and polycondensation occur simultaneously. The pyrolysis product-natural gas (dry gas) is generally difficult to preserve, especially in the exposed area with high exposure, which has already escaped. The solubility of asphalt in chloroform of the third member of Wengxiang Formation (S 1w3) is n×11%~ n×10-2%, R max is about 2.0% ~ 2.5%, and the atomic ratio of H/C is about 0.7. The asphalt evolution degree of Honghuayuan Formation (O 1h) is higher than that of Wengxiang Formation (S 1w3). Its solubility in chloroform is n×10-1%~ n×10-3%, R max is more than 2.5%, generally less than 4.0%, and H/C atomic ratio is generally 0.4 ~ 0.7. The results show that with the increase of thermal evolution, the molecular weight of solid asphalt increases, the degree of molecular polymerization increases and the directionality of molecular arrangement increases. Therefore, the content of carbon element is getting higher and higher (the atomic ratio of H/C is getting smaller and smaller), the solubility in organic solvents is getting weaker and weaker (chloroform is soluble), and the reflection ability of incident light with a certain wavelength is getting stronger and stronger (r max/%), which confirms the Red Garden Group. This is consistent with the increase of burial depth and temperature.

B. the destruction of ancient oil reservoirs by yanshan movement

Before the Yanshan movement, the oil stored in ancient oil reservoirs experienced long-term burial and thermal metamorphism in the late Paleozoic and Mesozoic, which was a process in which liquid hydrocarbons were transformed into gas (natural gas) and solid (asphalt).

Yanshan Movement is a widespread and intense event in folding movement, followed by a huge uplift. On the basis of stratigraphic folds and faults, Majiang ancient oil reservoir was gradually cast into the present situation after about 130Ma disintegration. Most of the reservoirs in the third member of Wengxiang Formation (S 1w3) and Honghuayuan Formation (O 1h) have been eroded or exposed to the ground. The distribution area of the third member of Wengxiang Formation (S 1w3) has been reduced from 2450km2 to 876km2, of which only 3.53x108t of asphalt is left. Therefore, the destruction after Yanshan period is the complete destruction and transformation of ancient oil reservoirs.

To sum up, Majiang ancient oil reservoir has unique reservoir-forming conditions, and there is no doubt that there was a large-scale oil and gas generation, migration and accumulation process in the early Paleozoic. By analyzing the main characteristics of Majiang ancient oil reservoir in the process of formation and evolution, it will provide useful reference for us to understand and prospect the oil and gas prospect in Guizhou and its adjacent areas and further explore the oil and gas of Lower Paleozoic.

2. The accumulation and evolution process of the remaining oil and gas reservoirs in Kaili.

The residual oil and gas reservoirs in Kaili were formed in two stages, which were similar to Majiang ancient reservoir in the early stage and have now evolved into asphalt. The late stage was formed at the end of Triassic, and there are still reservoir residues.

The source rocks of Majiang ancient oil reservoir were formed in the Early Cambrian. Ordovician and Silurian reservoirs, Silurian caprocks and "ancient uplift" traps together constitute the Duyun movement and Guangxi movement in the late Caledonian, and they are well configured with each other. From Silurian to devonian period, the source rocks entered the peak of oil generation, and oil and gas migrated to trapped oil and gas reservoirs through erosion surfaces and faults. The key moment of reservoir formation is at the end of Silurian, so it belongs to the Lower Cambrian petroleum system as a source rock. Under the cover of thick sediments of Upper Paleozoic and Mesozoic, and the burial history of nearly 3× 102Ma, the reservoir crude oil evolved into asphalt, wet carbon asphalt and dry gas. Later, after the strengthening, folding and transformation of Yanshan-Himalayan tectonic movement, oil and gas were redistributed. Due to strong uplift and erosion, only 800km2 of ancient oil reservoirs are left today.

The petroleum system of Kaili residual oil and gas reservoir is relatively complex, and there are two source rocks: Lower Cambrian, Ordovician and Silurian. Although its main reservoirs (O 1h, S 1w3) and caprocks (S mudstone) are similar to Majiang area, the trap-"paleouplift" also formed the last stage of Jurassic, and the relationship between source, reservoir, cap and trap configuration is quite good. However, because the Upper Paleozoic in Kaili area is thinner than that in Majiang area, the source rocks of Lower Cambrian have reached the peak of oil generation in Devonian and Carboniferous, and oil and gas have entered the reservoir, and the key moment of reservoir formation is Devonian-Carboniferous. Therefore, similar to Majiang area, Kaili area has the following petroleum systems as source rocks. However, after a long burial history from Late Paleozoic to Mesozoic, its crude oil has evolved into carbonaceous asphalt and dry gas, which may be the source of dry gas obtained by some drilling in the 1960s. The Ordovician and Silurian source rocks only entered the peak of oil generation in the late Triassic, and the key moment of reservoir formation was at the end of Triassic. Therefore, it belongs to another petroleum system with Ordovician and Silurian as source rocks. That is, there are two petroleum systems in Kaili area. The former has only carbon asphalt and dry gas in oil and gas evolution; The oil and gas in the latter petroleum system evolved into moisture through burial history in the middle and late Mesozoic, and these oil and gas can still be seen in outcrops and underground. This is the most obvious difference from Majiang ancient reservoir. The results of oil-source correlation and the fact that there are both carbonaceous asphalt and light crude oil in the Tang Kai S 1w3 sandstone and the Kaililuo surface also confirm this conclusion. This reservoir was also folded, reformed, uplifted and eroded by Yanshan-Himalayan movement, forming today's residual reservoir landscape.

(3) The oil and gas show of well/kloc-0 in central Guangxi, and the formation and evolution of ancient oil reservoirs.

Distribution characteristics of oil and gas asphalt in 1. Guizhong 1 well.

The 709-meter-long asphalt interval was drilled in the middle of Guangxi 1 well, which confirmed that there was a large-scale oil and gas generation-migration and accumulation process in the exploration area in the middle of Guangxi. Asphalt and liquid hydrocarbons have also been found in the outcrops of Devonian dolomite and reefs nearby, especially in the drilling process at the top of Tang Ying Formation of Middle Devonian in Guizhong 1 Well 3752-3753m, indicating that there is still the possibility of oil and gas accumulation in this area, which has the potential for further exploration.

Well Guizhong 1 encountered three kinds of oil and gas displays: poor gas layer, oil trace sandstone and solid asphalt, which revealed that the oil and gas in southern Guizhou was well preserved and had the geological conditions to form large and medium-sized oil and gas fields, which boosted the confidence of realizing the oil and gas exploration breakthrough in the southern marine new area.

1) poor gas layer

The gas logging anomaly of Carboniferous Huanglong Formation (C2H) in 1207 ~ 1209 m dark gray limestone is obvious, and the total hydrocarbon content is as high as 4.72 1%. The composition analysis is gas reservoir characteristics, and the drilling cuttings have no fluorescence display, and the quantitative fluorescence is 3.5 ~ 3.7, which is comprehensively interpreted as poor gas reservoir.

2) Oil trace sandstone

The gas logging anomaly of light gray fine sandstone in Tang Ying Formation of Middle Devonian at 3752 ~ 3753 m is obvious, with the highest total hydrocarbon content of 2. 179%, complete components, obvious oil marks in cuttings, light oil smell, light yellow fluorescence, yellow dripping, and quantitative fluorescence of 4.6, which is comprehensively interpreted as poor oil layer and comprehensively named as light gray oil mark fine sandstone.

3) asphalt display

A large amount of asphalt was found by observing rock slices during drilling in the well of Guizhong/KLOC-0. Microscopic observation of cuttings shows that asphalt is mainly distributed in Guilin Formation of Upper Devonian and Sipai Formation of Lower Devonian, with four layers of asphalt concentrated display sections, and the cumulative formation thickness is 709 meters, accounting for 1 4% of the total exposed formation thickness of1well in central Guangxi.

Among them, Guilin Formation has two layers of asphalt, of which the interval is 2585 ~ 2797 m, and the thickness is 212m; The interval thickness of 2886 ~ 3 146m is 260m, and the cumulative thickness of display interval is 472m. The lithology of the display section is mainly biolimestone.

Four rows of asphalt show two layers, with an interval of 4345 ~ 4460 m and a thickness of115m; The layer thickness of 4605-4727m 122m, and the cumulative thickness of the display section is 237m m. The lithology of the display section is mainly dolomite.

2. The comparative study of oil and gas geochemistry shows that the ancient oil reservoir/reservoir asphalt in the central Guangxi depression originated from the argillaceous rocks in the middle and lower Devonian.

The molecular parameters of argillaceous rocks in Luofu Formation of Middle Devonian are all within the range of soluble hydrocarbons in reservoir solid asphalt (Figure 2- 17), which indicates that the soluble hydrocarbons in reservoirs in Central Guangxi Depression may come from Devonian source rocks.

The carbon isotope composition range of reservoir solid asphalt is also very close to the argillaceous rocks of Luofu Formation. The carbon isotope composition of kerogen in Luofu Formation of Middle Devonian is -24 ‰ ~-27.4 ‰, and kerogen in Tang Ding Formation of Lower Devonian is -26.8 ‰ ~-27.8 ‰. The carbon isotopic composition of the solid pitch of Devonian reservoir in Central Guangxi 1 Well is between-23 ‰ and-27.5 ‰ (Figure 2- 18), and the carbon isotopic composition of pitch in Carboniferous samples is mostly in this range. Generally speaking, these results are more partial than the molecular carbon isotope composition range of n-alkanes in the soluble hydrocarbons of reservoirs in the central Guangxi depression, and may have direct genetic relations. Therefore, the soluble hydrocarbons in the main reservoirs in the central Guangxi depression may come from the muddy source rocks of the Middle and Lower Devonian.

Fig. 2- 17 Comparison of some molecular parameters between the reservoir in the middle of Guangxi 1 well, the ancient reservoir in Dachang and the argillaceous rocks in Luofu Formation of Middle Devonian.

Fig. 2- 18 Changes in Carbon Isotopic Composition of Devonian-Carboniferous Solid Pitch and Kerogen in Central Guangxi 1 Well

Fig. 2- 19 Carbon Isotopic Composition of Solid Pitch and Kerogen in Well1Well Reservoir and Dachang Ancient Reservoir.

The carbon isotopic composition of solid asphalt in Dachang ancient reservoir in Nandan varies from-25.9 ‰ to 28.1‰ (Zhao et al., 2006a, 2007), which is also very consistent with kerogen of middle and lower Devonian source rocks. Therefore, carbon isotope evidence supports that the solid asphalt in Devonian reservoirs in the central Guangxi depression comes from middle and lower Devonian mudstone.

However, the carbon isotope composition of solid asphalt generated by Dachang ancient oil reservoir and metal vein is-18 ‰ ~- 19 ‰ (Figure 2- 19). Unless such a heavy isotopic composition comes from higher plants (coal type), the reasonable explanation can only be solid asphalt, which has not only experienced the high temperature of the formation, but also experienced mineralization.

About the origin of solid asphalt in reservoir, predecessors generally think that it is pyrobitumen formed by thermal cracking. The results of this analysis also support this view: ① the reflectivity of asphalt is high, ranging from 2% to 4.5%; (2) The isotopic composition of solid asphalt is close to kerogen, even slightly deviated, which indicates that once oil and gas are filled/migrated, they undergo a significant thermal cracking process; (3) There is no evidence of supergene alteration such as biodegradation in soluble hydrocarbons, so the possibility of biodegradation to form solid asphalt is very small; ④ Other processes of forming solid asphalt, such as reservoir differentiation and water washing. Although limited research data cannot be ruled out at present, their contribution should be small.

3. The solid bitumen of reservoir is pyrobitumen formed by thermal cracking of migration/accumulation oil and gas, and some ancient reservoirs may be related to magmatic/ore-forming hydrothermal alteration, mainly due to the increase of reservoir paleotemperature.

There are obvious differences in some molecular parameters between free hydrocarbon (chloroform asphalt "A") and adsorbed/encapsulated hydrocarbon (asphalt C or mineral-bound organic matter) (Figure 2-20).

Compared with the hydrocarbon adsorbed/encapsulated in the reservoir, the free hydrocarbon in the reservoir has higher Pr/nC 17 and Ph/nC 18, and the low-carbon n-alkanes in the free hydrocarbon in the reservoir show certain parity advantages. According to the law of hydrocarbon generation and evolution of kerogen, these characteristics are the manifestations of low maturity evolution stage. However, the geological evolution, thermal history evaluation and the determination of solid asphalt reflectivity in this area all show that it is in a high-over-mature evolution stage (Figure 2-2 1), so it is unlikely that the free hydrocarbons in the reservoir come from kerogen. The free hydrocarbon in the reservoir may be the result of thermal evolution of carbonate minerals combined with organic matter in the over-mature stage, which is directly related to the adsorption/encapsulation of hydrocarbons.

Another possibility is biodegradation, and slight biodegradation will increase the ratio of branched alkanes to straight alkanes. Microscopic observation shows that there are no signs of biodegradation except that the reservoir of Rongxian Formation may be connected with surface water and damaged to some extent in geological history. Lower Devonian reservoirs are in geothermal conditions above 100℃ after the main hydrocarbon generation period, and there is little possibility of biodegradation. Biodegradation may lead to the priority loss of n-alkanes with low carbon number, while the content of n-alkanes with low carbon number in free hydrocarbons is very high. Therefore, biodegradation is unlikely.

As mentioned above, the isotopic composition of solid asphalt is close to or slightly different from kerogen, which indicates that the filled/migrated oil and gas has undergone a significant thermal cracking process.

Figure 2-20 Comparison of molecular parameters of free hydrocarbons and adsorbed/inclusion hydrocarbons in reservoir samples

●-measurement data, ○-literature data

Figure 2-2 1 Guizhong 1 Well Solid Asphalt Reflectance Distribution Map

4. The study of soluble hydrocarbon in reservoir shows that the well in central Guangxi 1 may have undergone two stages of filling.

The first stage is the main hydrocarbon generation period before Indosinian, and the reservoir solid asphalt is the cracking product of this oil and gas reservoir. The second phase is represented by soluble hydrocarbons adjacent to the oil and gas display layer. The free hydrocarbons and adsorbed/encapsulated hydrocarbons (Figure 2-22) adjacent to the oil and gas display layer (375 1 ~ 3752m) are significantly different from other samples in alkane distribution, biomarker composition and carbon isotope composition, as follows:

A. The distribution of free hydrocarbons and normal alkanes in adsorbed/encapsulated hydrocarbons is unimodal, and the content of alkanes below C2 1 is low, which is obviously different from that of free hydrocarbons and adsorbed/encapsulated hydrocarbons below C2 1 in most reservoir samples (Figure 2-23);

B. The relative content of pentacyclic terpanes and tricyclic terpanes in free hydrocarbons and adsorbed/encapsulated hydrocarbons is very high, which is obviously different from other reservoir samples (Figure 2-24 and Figure 2-25);

C. Free hydrocarbons and adsorbed/encapsulated hydrocarbons do not have the advantages of C27 regular steranes, but show C29 > C27 ≥ C28 (Figure 2-26).

The ratio of C23 tricyclic terpane /C30 hopane of reservoir-soluble hydrocarbons shows a very good positive correlation with the ratio of tricyclic terpane/pentacyclic terpane and C2 1/C29 sterane. The first two parameters may be related to the type, maturity and hydrocarbon migration of parent organic matter, while C2 1/C29 sterane may be related to the type and maturity of parent organic matter. The increase of maturity and hydrocarbon migration may lead to the increase of the above parameters. The three ratios of GZ55 sample (near the oil and gas display layer) are all low (Figure 2-27), indicating that it is unlikely that hydrocarbons will undergo obvious thermal alteration in the later period.

Based on this, it can be inferred that hydrocarbons in most reservoir samples are filled earlier, so the adsorbed/encapsulated hydrocarbons are obviously different from free hydrocarbons in many geochemical parameters due to the protection of minerals. The sample GZ55 (adjacent to the oil and gas display layer) may have been filled with oil and gas in the later stage after the early oil and gas filling, but the oil and gas filled in the later stage has no obvious thermal alteration, so the geochemical characteristics are basically similar. The isotopic characteristics of solid asphalt can also support this inference. As shown in Figure 2- 18, there is no obvious difference between this sample and the adjacent samples at the top and bottom of the profile, and the hydrocarbons filled in the later period may not contribute to the solid asphalt. The reflectivity of solid asphalt in this sample is as high as 4.4%, and its distribution range is small, which is obviously the product of thermal metamorphism caused by early oil and gas filling.

The carbon isotope distribution of solid asphalt in Guilin Formation and Sipai Formation gradually decreases from deep to shallow, so it is easier to form reservoirs at the same time (Figure 2- 18).

Therefore, the process of reservoir formation in the central Guangxi depression can be divided into two stages. The first stage is the hydrocarbon generation and accumulation process of corresponding reservoir asphalt before Indosinian period (over-mature stage), which may be as follows: (1) The Lower Devonian began to enter the peak period of oil generation in the early Carboniferous; In the middle and late Carboniferous, the source rocks successively entered the high maturity stage and reached the peak of gas generation. From Late Carboniferous to Early Permian, it entered the over-mature stage, and the generated oil and gas reservoirs began to crack. At this time, the organic matter combined with carbonate minerals may begin to generate a large number of hydrocarbons. The peak of cracking is from the middle and late Permian to the early Triassic, and the hydrocarbon generation process of minerals combined with organic matter has also been completed, and the formed oil and gas reservoirs are basically completely cracked to form reservoir asphalt.

Figure 2-22 Geochemical Characteristics of Devonian-Carboniferous Reservoirs in Central Guangxi 1 Well

Figure 2-23 Chromatograms of Free Hydrocarbons and Adsorption/Inclusion Hydrocarbons in Devonian Reservoir Samples

Mass chromatograms of free hydrocarbon and adsorbed/encapsulated hydrocarbon m/z 19 1 of some Devonian reservoir samples in Figure 2-24.

*-C29Ts； G-γ paraffin

Figure 2-25 Mass Chromatogram of Free Hydrocarbon and Adsorption/Inclusion Hydrocarbon m/z2 17 of Devonian Reservoir Samples

Fig. 2-26 Regular Distribution of Steranes in Soluble Hydrocarbons in Well Guizhong 1 Reservoir and Dachang Ancient Reservoir

+is a free hydrocarbon; ● Used for adsorbing/encapsulating hydrocarbons; The red circle is the soluble hydrocarbon in the reservoir (375 1 ~ 3752 m) adjacent to the oil display layer.

Fig. 2-27 Regular Distribution of Steranes in Soluble Hydrocarbons in Guizhong 1 Well Reservoir and Dachang Ancient Reservoir

Is a free hydrocarbon; ● Used for adsorbing/encapsulating hydrocarbons; ▲ It is the reservoir sample GZ55 (375 1 ~ 3752m) adjacent to the oil display layer.

The second stage may be oil and gas filling in the late stage of high maturity, represented by the upper oil display layer of Tang Ying Formation, or it may come from the source rocks that are still in the transitional stage of high-over maturity in this area. The oil and gas formed in the post-Indosinian high maturity stage migrated to the reservoir, which may occur after the Yanshan movement, and the strata lifted to avoid the cracking process caused by thermal action above 180℃. It should be pointed out that the solid asphalt in this layer should be the result of filling before Indosinian period, which is characterized by high reflectivity and similar isotopic composition to the upper and lower layers.

5. Evolution process of oil and gas in central Guangxi depression.

The evolution process of oil and gas in the central Guangxi depression can be divided into three stages: one is the thermal cracking of Indosinian oil and gas reservoirs; Second, the thermal metamorphism in the higher stage of magmatic/ore-forming hydrothermal superposition in the late Yanshan period (such as Nandan Dachang); Third, the Himalayan tectonic activity and the large-scale uplift of strata destroyed the Upper Devonian oil and gas reservoirs.

The evolution process of oil and gas in the central Guangxi depression can also be divided into the transformation and destruction of oil and gas reservoirs by thermal action and tectonic movement: ① During Hercynian-Indosinian period, oil and gas reservoirs experienced significant thermal cracking and evolved into solid asphalt and methane gas. Soluble hydrocarbons in the reservoirs are more likely to be the manifestation of hydrocarbon generation of reservoir organic matter, and have also entered the end of the evolution stage, so the accumulation process in this period is of little significance to "oil". In addition to the thermal effect caused by formation subsidence, the isotopic characteristics of solid asphalt in ancient reservoirs show that it also experienced thermal alteration at higher temperature caused by magmatic activity in the late Yanshan period, which led to obvious deviation in the isotopic characteristics of gas adsorbed by asphalt and reservoir (Figure 2- 19). This process may mainly occur in Dachang metal metallogenic area in Nandan, and the time is early Cretaceous (90 ~ 100 Ma). The carbon isotope composition of solid asphalt in some reservoirs in the middle of Guangxi 1 well is slightly heavier than that of kerogen in Devonian source rock in the middle of Guangxi depression, and the asphalt reflectivity changes irregularly with depth, indicating that the weak influence of magmatic activity cannot be ruled out. ② For gas, kerogen cracking gas and petroleum cracking gas may be formed. Due to the limitation of natural gas research data in this area, it is still difficult to determine its type. However, according to the gas analysis results of Yang 1 well in Nanpanjiang Depression, its nitrogen content is between 54% and 74%, which may reflect the characteristics of gas generation and late accumulation in high evolution stage. The amount of hydrocarbon gas generated is very small, and the main accumulation is nitrogen gas formed by the cracking of NH4+ clay salt in clay minerals at higher temperature. Therefore, we should strengthen the geochemical analysis of related natural gas display and further evaluate the transformation and preservation of natural gas in the central Guangxi depression. Oil shows that the upper reservoir of Tang Ying Formation is in a high mature stage, and whether there is corresponding natural gas accumulation is also a problem worthy of consideration. Therefore, for the central Guangxi depression, the natural gas accumulated in Yanshan-Himalayan period should be the next exploration direction, and the favorable accumulation and preservation area should be the goal of the strategic selection area. The strata in the west of Gui Zhong sag are relatively intact (the target layer is buried deeply), and faults and magmatic activities are relatively undeveloped, which may be more conducive to the accumulation and preservation of natural gas. ③ TOC of Rongxian Formation of Upper Devonian is the lowest in Devonian, while TOC of Guilin Formation of Upper Devonian is obviously lower than that of Sipai Formation of Lower Devonian (Figure 2-22). Microscopic observation shows that the development of asphalt is not as good as that of Sipai Formation, which may reflect that the upper Devonian reservoir was affected by the reconstruction or destruction of Himalayan tectonic uplift.