(Northwest Petroleum Bureau of China Petrochemical Xinxing Petroleum Company, Urumqi, Xinjiang 8300 1 1)

Keyword prototype; Basin analysis; Oil and gas storage; Secondary filling; Tarim River

Tarim basin is a large superimposed basin formed by superimposing Mesozoic and Cenozoic foreland basins on Paleozoic craton basin, which has been recognized by everyone and confirmed by oil and gas exploration practice for more than ten years. It is the only basin in China where large-scale industrial oil fields of marine origin have been discovered, the only basin with Cambrian-Ordovician as industrial oil and gas source, and the only basin with industrial oil and gas fields of marine and continental origin.

After the Shashen 2 well deployed by Northwest Petroleum Bureau of the former Ministry of Geology and Minerals in Shaya Uplift in the north of Tarim Basin encountered industrial oil and gas flow in the Ordovician dolomite of the Lower Paleozoic, it achieved a major breakthrough in oil and gas exploration in Tarim Basin and found marine carbonate reservoirs in the basin, thus achieving a major breakthrough in Tarim Basin from foreland basin to craton basin for the first time. Furthermore, a major breakthrough was made in the non-anticline field, which accelerated the exploration in the craton field in the hinterland of the basin. However, looking back on the history of more than ten years, we have experienced several sea-land transitions, shallow-middle-deep jumps, and cycles between cratons and foreland basins. Only 17 oil and gas field was discovered, mainly medium-sized. By the first half of 2000, the proven oil and gas reserves reached 10× 108 ton oil equivalent. At the end of 1996, Northwest Petroleum Bureau discovered a 100-million-ton oilfield in the Lower Paleozoic carbonate rocks in the middle section of Shaya Uplift. At present, the oil-bearing area is controlled at 700km2, and the estimated proven reserves are 3×108 ~ 5×108t ... Tarim Oilfield Company discovered Kela No.2 gas field in the west of Kuqa foreland basin this year, and submitted the reserves of 200× 108m3. There seems to be a new opportunity for oil and gas exploration in Tarim basin. However, after more than half a century of geological survey and more than ten years of oil and gas exploration, why are there so few large oil and gas fields found in Tarim Basin that match the basin size and resources, and the degree of exploration is so low? Is this result accidental or inevitable? Is this phenomenon mainly caused by mistakes in exploration decision-making or exploration target selection, or is it a reflection of objective geological conditions in the basin? What is the prospect of oil and gas exploration in the basin? Where should the main exploration direction be in the near and medium term? These are all practical problems that need to be solved urgently in front of the explorer. To understand these problems, it is very necessary to understand the prospect of the basin, establish the correct guiding ideology, point out the basis for oil and gas exploration for explorers, and then take a scientific deployment plan under the guidance of more practical theories and combined with the oil and gas geological characteristics of the basin. The correct exploration idea comes from the in-depth understanding of the law of underground reservoir formation and practical theoretical guidance.

1 basin and oil and gas system prototype

Mr. Zhu Xia's contribution to basin analysis is that he reminds everyone of the role of "historical, dynamic and systematic structural analysis" and gives the definition of "prototype" on the basis of structural analysis. The accurate description of this definition provides the most basic unit for the analysis of superimposed basins, and it has been widely used in the national key scientific and technological research in Tarim Basin, and has achieved fruitful results. It is generally believed that the multi-cycle, multi-stage and different-style evolution history of the craton plate in Tarim Basin is realized through the study of two systems, four tectonic stages and multi-plate tectonic stages. Including the later analysis of the petroleum system in Tarim basin, the results should be regarded as based on the "prototype" analysis. The analysis and evolution history of the prototype basin in Tarim basin shows that craton basin and foreland basin have coexisted for a long time in history. According to the sequence of two types of basin prototypes and the characteristics of China plate activity, the ancient plate in Chinese mainland is relatively small, and the sum of the areas of Tarim plate, China plate and Yangtze plate is only 1/3 of that of North America plate. Therefore, the activity is strong, and the accordion stress field controlled by plate structure from Neoproterozoic to late Paleozoic has a strong influence on craton basin, which is a feature of Tarim plate marginal environment. Within the plate, the basement of Tarim Craton is a broken structure, and the change of horizontal stress field increases the possibility of vertical movement of lithosphere. Moreover, under the normal thermal system environment in the early stage of evolution, the deformation of Tarim Craton Basin is mainly manifested in the formation of large uplift structures. Under the influence of crustal rheological factors and their heterogeneity and the control of boundary activity mode, the uplift structure in craton migrated, and the uplift was strong or weak. From the spatial distribution of craton basin and foreland basin, under the continental plate tectonic system, passive marginal basin and foreland basin with narrow distribution range developed along the periphery of craton; However, under the continental intraplate deformation system, the foreland basin advances rapidly from the piedmont on both sides. Especially after the early Yanshan period, the Indian plate collided with the Eurasian plate, and Tarim entered the foreland basin stage. During Triassic-Jurassic period, the combination of foreland flexure and strike-slip pull-apart basin developed in the south of the basin, which reflected that the whole basin was in a compression-torsion environment, indicating that the late Hercynian movement at the end of Early Permian was the most important event in the history of basin structural evolution, and the structural systems developed before and after the basin were completely different. Therefore, the basins formed under the two systems have different styles, inconsistent resource abundance and structure, and the two fields should have different exploration ideas and methods.

Zhu believes that a structural unit is a structural form and a sedimentary entity; It is this prototype that can be distinguished and compared according to the mechanism of geodynamics, rather than their combination-basin; In some cases, these prototypes or structural units should be regarded as a process response system and its subsystems. The concept of this basic unit and system is the brilliant crystallization of Mr. Wang's application of activity theory and historical view to analyze the evolution characteristics of the basin in different stages. Its essence is "T environment -S action -M response", which makes the prototype of China basin globally comparable, thus highlighting the particularity of China basin in the global tectonic environment. This not only fills the blank of A.W.Bally( 1975) basin classification, but also lays a foundation for the study of oil and gas system in China basin, and clears up the train of thought.

Since 1972 Dow put forward the concept of oil system at AAPG annual meeting, the term has undergone several evolutions abroad, and the research in this field has become a new hot spot in western literature. Since it was introduced into China, it has been widely paid attention to in oil and gas exploration practice, and many people have studied the oil and gas system between the basin and the reservoir-forming assemblage. But it is not a simple summary of the research results of generation, storage, cover, transportation, circle and protection, but a new idea and method to analyze these petroleum geological conditions at a higher level. It emphasizes the holistic view, comprehensive view, dynamic view and hierarchical view, and traces the genetic chain conditions and action process of oil and gas from source rock to trap through the analysis of each element from static to dynamic. Therefore, the establishment of relationship and the recovery of action process are the soul of oil and gas system research. It is generally believed that the petroleum system is a set of (? ) mature source rocks and related reservoir rocks and caprocks. 1980 Mr. Wang pointed out that the basin should be considered as a whole in petroleum geological work, and its panorama should be investigated first. But a basin, especially a great basin, always contains several different tectonic parts produced by different geodynamic mechanisms, which we call the "prototype" of the basin. Prior to this, Mr. Wang summarized the basic conditions for the formation of oil and gas reservoirs as four "M", four "S" and three "T". Four s are actions, and four m are the results of actions. Therefore, we think that the analysis of petroleum system is consistent with the prototype analysis of the basin. If this mechanical equivalence is not too rigid, I think Mr. Zhu Xia suggested the following in the preface of On the Structure of Oil-bearing Basin in China published by Petroleum Industry Press (1986 Edition):

(1) We should learn international things, and new theories and methods should be combined with China's national conditions, especially geological theories, because geology is very regional and has its own characteristics;

(2) Historical, dynamic and systematic structural analysis is particularly important for exploring oil and gas reservoir types with complex migration process and source-reservoir relationship.

Tarim basin is a large superimposed basin composed of Paleozoic craton basin and Mesozoic foreland basin. Its evolution has experienced the transformation from ocean-continental plate tectonic system to continental intraplate deformation system, forming two types of oil and gas resources: craton and foreland basin.

2 Structure and distribution of oil and gas resources in Tarim Basin

Resource structure is a big problem related to the decision-making of oil and gas exploration deployment, exploration technical ideas and exploration technical methods. There are many sets of source rocks in Tarim basin. The development and distribution of these source rocks and their influence on oil and gas reservoirs are controlled by basin prototype and its superimposed evolution. There are many achievements in its evolution and resource structure. Generally speaking, it has the following characteristics.

Cambrian-Ordovician and Carboniferous-Permian source rocks developed in Paleozoic craton basin, with Cambrian-Ordovician as the main source rock. Lower Paleozoic source rocks are mainly distributed in the eastern part of the basin, and Bachu uplift is also an important distribution area near Hotan River. They are mainly controlled by the passive marginal basin of the craton, and the middle of the basin is an intra-craton depression basin, which is quite different from the uplift and depression. It has experienced a long evolution process and is a set of highly mature source rocks. Early Hercynian mature, there was a period of vigorous oil generation. The resource structure is dominated by oil, and the basin structure has changed to a relatively flat terrain. In passive and active terrigenous basins and craton depression basins with shallow water bodies, Shaya uplift in the north of the basin has three paleo-uplifts, Shaxi, Akkule and Korla, and there is also a uplift belt composed of paleo-uplifts in the middle of the basin, and oil and gas migrate to paleo-uplifts around mature oil-generating areas. During this period, oil-based reservoirs were formed, but the erosion at the top of the ancient uplift destroyed the ancient reservoirs and formed asphalt. By the late Hercynian, the tectonic movement of the basin was strong, and the Paleozoic erosion of Shaya uplift was serious. The oil and gas system of craton basin with Cambrian-Ordovician as source rock was destroyed again in uplift, and the slope was relatively well preserved. It is speculated that there is a high abnormal magmatic activity thermal system, the evolution degree of Cambrian-Ordovician source rocks becomes higher, and the gas-oil ratio increases significantly. By Himalayan period, the oil and gas generation capacity was greatly reduced. The calculation results show that the resources provided by the Lower Paleozoic source rock sheet are about 1.20× 1.08t, of which the oil and gas resources preserved in the early stage are 37× 1.08t, which is still about 1/5 of the total resources in the Paleocene basin, which fully shows the importance of early resources. The more likely residual form is a secondary oil and gas reservoir that has undergone secondary transformation, degradation or differentiation in the later stage. Condensate is a kind of light oil with high maturity, which accounts for a high proportion in the resources generated in the later period (Gu Yi et al., 1997).

The main source rocks in Mesozoic-Cenozoic foreland basin are Triassic-Jurassic, and the resources formed with Carboniferous-Permian in Himalayan period are 40.5× 108t and 25.8× 108t, respectively, which are mainly distributed in 56% of the northeastern depression, 2 1% of the central uplift and 26544 of the southwestern depression. The resources formed by three sets of source rocks account for 4/5 of the total resources in the basin, and the resource structure is mainly natural gas.

3 Characteristics of oil and gas reservoirs and secondary oil generation in Tarim Basin

As mentioned above, the evolution of source rocks in Tarim basin is complex, and the evolution history of the basin determines that the formation of oil and gas reservoirs is also multi-stage. The analysis of fluid inclusions confirms that there are at least three stages of oil and gas migration and accumulation in Tarim Basin. Previous exploration areas were concentrated in uplift areas and piedmont depression areas with strong tectonic activity, and most oil and gas reservoirs were formed in the late and recent period, taking Kela 2 gas field as an example. Kela 2 gas reservoir is a massive dry gas bottom water gas reservoir with high methane content and low non-hydrocarbon gas content. It is a typical high-quality natural gas, and the PVT property of natural gas is abnormally high. The study of fluid inclusions shows that Kela2 gas field is the result of one-time accumulation since 5.0Ma[ Kangcun Formation (Nk)], that is, the gas reservoir formed later than the trap formation time [since the deposition of Kuqa Formation (N2k)], and the gas source came from Triassic-Jurassic in Kuqa Depression. According to the source rock potential index as high as 50.5t/m2, the oil and gas system in Kuqa foreland basin is an ultra-high pressure gas-bearing system in which crassus structural belt and source rocks are superimposed.

However, in recent years, the oil and gas reservoirs discovered in the upper craton of Tazhong, Tabei and Bachu uplift have the characteristics of early accumulation or secondary charging. Taking Tahe Oilfield as an example, from the whole Akkule uplift, many oil and gas reservoirs have been formed in this area under the influence of lithology, reservoir physical properties, fault separation or local structural control. There are two sets of clastic reservoirs of Triassic and Carboniferous and carbonate reservoirs of Ordovician in the vertical direction. The oil-source correlation shows that the source rocks are all Cambrian-Lower Ordovician. Trap type Triassic is mainly low-amplitude structural type, Carboniferous is superimposed with lithologic factors on the basis of large structural background, and Ordovician karst reservoir is covered by mudstone of Carboniferous Bachu Formation, forming unconformity karst trap. In recent years, the accumulated proven oil and gas geological reserves have exceeded 1× 108t.

According to the analysis of reservoir asphalt and fluid inclusions, as well as the calculation data of saturation pressure or dew point pressure, oil and gas accumulation in Tahe oil region has experienced four stages: early Hercynian destruction, coexistence of late Hercynian destruction and reconstruction, early Yanshan-Himalayan reservoir formation and distribution, and late Himalayan light oil and gas filling. The plane distribution of fluid properties of Ordovician reservoirs in Tahe Oilfield is quite different, especially in areas 3, 4 and 6. Only Tahe Area 4 and Area 6 have heavy oil, and its density is much higher. Only Ordovician reservoir in Tahe No.3 in this area has condensate gas cap, and light oil, conventional crude oil and heavy oil are distributed downwards. Reservoir formation occurred in the late Hercynian period, but mainly in the Himalayan period. The high concentration of 25- norhopane series compounds in crude oil proves that there is oxidative degradation after reservoir formation in the late Hercynian period (Sha 6 1 well area), and the northern crude oil is less affected by oil and gas filling in the later period. Vertically, the three reservoirs have different properties and formation ages. The reservoir-forming period reflected by Tahe No.3 Carboniferous crude oil is mainly Indosinian-Late Yanshanian.

Through the analysis of the actual data of oil and gas reservoirs found in the basin, it is not difficult to find that the formation of oil and gas reservoirs in Tarim Basin has the following general characteristics:

(1) Integral sealing condition is the primary condition for oil and gas accumulation. Every reservoir-forming period in Tarim geological history is based on the formation of the whole closed environment, and every reservoir-forming period is destroyed by the opening of the whole closed environment. Moreover, the development horizon or development time of regional caprocks related to overall closure determines the effective accumulation period of oil and gas reservoirs.

(2) Himalayan period is an important period for oil and gas accumulation. The formation and distribution of oil and gas in the present basin are obviously controlled by the structural framework of the basin. There are three relatively independent structural units and three combinations of effective reservoir-forming conditions, which determine the size of today's oil and gas prospects and different exploration targets. While deepening the study of late reservoir-forming law, we should pay attention to the exploration of craton ancient oil reservoirs.

(3) Multi-stage long-distance migration and accumulation of oil and gas. The coexistence of marine and continental oil and gas and the existence of various types of oil and gas reservoirs lead to the complexity of oil and gas distribution. However, the relationship between several large oil fields and oil sources clearly reflects that the closer the big trap is to the oil and gas source, the easier it is to form large oil and gas accumulation.

(4) Multi-stage reservoir formation is mainly related to the process of generating oil and gas with different properties from various oil sources at different stages. The accumulation of multi-stage oil and gas reservoirs forms multiple oil-bearing systems, and oil and gas exist in multiphase state, thus forming the dynamic accumulation process of oil and gas reservoirs.

(5) In a word, the above characteristics obviously show the compound oil and gas accumulation law.

4. Guiding ideology and deployment principles of oil and gas exploration in Tarim Basin.

Judging from the geological characteristics and oil and gas achievements of the above basins, the guiding ideology of oil and gas exploration in Tarim Basin should be:

(1) Exploration based on multi-purpose layer; Occupy the ancient uplift and slope of craton basin, close to the main source rocks; Looking for large primary oil and gas fields (reservoirs) for carbonate rocks of Lower Paleozoic with large traps as the main target; Look for secondary small and medium-sized oil fields (reservoirs) by taking into account the low-amplitude trap groups of Upper Paleozoic and Mesozoic Cenozoic.

(2) Strengthen the anatomy of the foreland slope and thrust fold structural belt in the foreland basin, take large traps as the main attack direction and Mesozoic and Cenozoic high porosity and permeability clastic rock reservoirs as the target, and look for large natural gas fields.

Under the guidance of the above guiding ideology of exploration and deployment, the exploration and deployment of oilfield companies should adhere to the following principles:

(1) Pay equal attention to oil and gas, give priority to oil, support and develop regional economy, and actively promote natural gas exploration of "West-East Gas Transmission".

(2) Pay equal attention to anticline oil and gas reservoirs and non-anticline oil and gas reservoirs, and pay attention to the effectiveness and pertinence of method and technology series.

(3) Both marine craton basin and continental foreland basin should be considered, each with its own emphasis.

(4) Insist on exploration procedures, intensify exploration and speed up exploration; At the same time, we should fully consider the conditions of exploration materials and financial support, and adhere to the principle of doing what we can.

(5) Adhere to the principle of sustainable development, while actively exploring new fields and cultivating exploration reserve bases, continuously carry out selection and evaluation based on the whole basin.

(6) Rely on scientific and technological progress and innovation, actively introduce, digest and absorb new exploration methods and theories at home and abroad, constantly update exploration ideas, and further improve and innovate methods and technologies in combination with the actual characteristics of oil and gas exploration objects.

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