Accumulation model of quasi-continuous low permeability-dense sandstone large oilfield in Ordos Basin

Formation model of quasi-continuous low-permeability-dense sandstone large oilfield in Ordos Basin Zhao Jingzhou, Bai Yubin, Cao Qing, Dean (School of Earth Science and Engineering, Xi'an Shiyou University, Xi'an 710065, China) The permeability is less than 2x10-3pm2, porosity is about Less than 12%. It is generally believed that tight reservoirs are continuous oil and gas accumulations and are one of the unconventional oil and gas.

The study shows that the middle and lower combinations of the Triassic Yanchang Formation in the Yishan Slope of the Ordos Basin are mainly dense and near-dense sandstone reservoirs, and their accumulation patterns are not typical of conventional lithologic reservoirs, but also atypical continuous types. Unconventional reservoirs, but a type of transition between conventional and unconventional reservoirs or continuous and discontinuous oil and gas accumulation, referred to herein as quasi-continuous reservoirs or hydrocarbon accumulation. It mainly has the following characteristics of accumulation: 1 large-area quasi-continuous distribution of reservoirs, no clear boundary; 2 large-area hydrocarbon generation, high-intensity filling; 3 reservoirs with poor physical properties, strong heterogeneity; 4 traps between conventional Between trap and no trap, it is unconventional trap; 5 oil, water is different, no obvious edge and bottom water; 6 reservoir pressure system is complex, and many negative pressure anomalies; 7 oil and gas migration and accumulation is mainly Non-buoyancy drive, close migration into reservoirs; 8 reservoir formation and distribution are mainly controlled by hydrocarbon sources and reservoirs, with small structural impact; 9 good preservation conditions, small changes in reservoirs; 10 abundant resources, great potential for exploration and development. The proposed model of quasi-continuous tight sandstone reservoirs indicates that such reservoirs have great exploration potential, but traditional geological research and exploration ideas need to be adjusted accordingly.

First: Zhao Jingzhou (1962-), male, oil and gas accumulation geology, unconventional oil and gas geology and exploration.

Fund Project: National Large Oil and Gas Field and CBM Development Project (2011ZX05007-004).

Oil and Gas Geology 1 Concept and division of low-permeability tight reservoirs Currently, the definition of low-permeability reservoirs at home and abroad is confusing, and there is little discussion on tight reservoirs. Reservoirs with a permeability of less than 10X10 3pm2 are referred to as low permeability reservoirs or tight sandstone reservoirs, while reservoirs with permeability greater than 10x10-3pm2 are considered good reservoirs. In the former Soviet Union, the permeability of low-permeability reservoirs was limited to 50x10-3pm2. In China, Li Daopin also used 50x10-3pm2 as the upper limit of permeability of low-permeability layers, and classified low-permeability reservoirs into 6 types of low-permeability layers. (Class I), permeability 50x10-3~10x10-3pm2; ultra-low permeability layer (n-type), permeability 10x10-3~1x10-3pm2; ultra-low permeability layer (m class), permeability 1x10- 3~0.1x10-3pm2; dense layer (Chinese), permeability: 0.1\10-3~0.01\10-3, claw 2; unconventional dense layer and ultra-dense layer (V), the permeability is 0.01x10 -3 ~0.0001x10-3pm2; fracture-void layer (w), the permeability change is large or less than 10x10-3pm2. The 1998 oil and gas industry standard issued by the People's Republic of China, "Evaluation method for oil and gas reservoirs" 0 will also have low permeability. The permeability of the layer is limited to 50x10-3pm2, and the classification criteria for porosity and permeability of oil-bearing clastic reservoirs and gas-bearing clastic reservoirs are determined. Among them, low-permeability oil-bearing clastic reservoirs are divided into low-permeability reservoirs (pore ~1x10-3pm2) and ultra-low permeability reservoirs (pore low-permeability gas-bearing clastic reservoirs are classified into low-permeability reservoirs (porosity 15). % 10-3pm2) Two categories. Zhao Jingzhou et al. According to the study of the Mesozoic reservoirs in the Ordos Basin, the low-permeability sandstone reservoirs are divided into four categories, namely: low permeability layer (Class I), permeability 50x10 -3 ~10x10-3pm2; ultra-low permeability layer U), permeability 10x10-3~1x10-3pm2; ultra-low permeability layer (m class), permeability rate less than 0.1x visible, China's current low permeability reservoir The upper limit (permeability 50x10-3pm2) mostly follows the low permeability reservoir boundary of the former Soviet Union. However, with the advancement of oil extraction technology and the deepening of oil and gas accumulation theory, reservoirs that were originally thought to be low permeability have become conventional reservoirs. In view of this, Hu Wenrui 4 recently proposed a new low-permeability reservoir division standard, which is called a general low-permeability reservoir with a permeability of 1.0x10-3~0.5x10. The reservoir of -3pm2 is called ultra-low permeability reservoir, and the reservoir with permeability below 0.5x10-3pm2 is called ultra-low permeability reservoir.

On the other hand, in the case of dense layers, China has in the past interpreted it as a non-reservoir or ineffective thickness layer of industrial oil gas flow, so many classifications related to low-permeability reservoirs either do not include dense layers or Permeability is very low (mostly considered to be below 0.1x10-3pm2). In fact, the dense layer is generally treated as an unconventional reservoir in foreign countries, especially the United States, and has already been produced from it. The US government has also formulated corresponding tax incentives to encourage the exploration and development of such oil and gas. So far, the dense layer has been widely treated as an unconventional oil and gas reservoir in the world. The reason why the dense layer will have a different understanding is because the dense layer that our scholars have called in the past is not the same as the widely accepted concept of dense layer in the world. In foreign countries, the upper limit of the permeability of the dense layer is generally set to 0.1x10-3pm2, but refers to the formation or in situ permeability of 6-8, and more refers to the tight gas layer. Spencer6 also proposed to in situ the penetration rate at 0. Zhao Jingzhou, et al. The natural gas reservoir of the quasi-continuous low-permeability-dense sandstone reservoir in the Ordos Basin is called near-tight reservoir. The natural gas reservoir with in-situ permeability greater than 1X10-3pm2 is used as a conventional reservoir. . In China, the upper limit of the permeability of the tight layer is 0.1X10-3pm2, which should refer to the absolute permeability of the ground of the reservoir, which is significantly different from the permeability of the formation. Yuan Zhengwen et al. and Xu Huazheng 10 have experimentally studied that the dense layer with a formation permeability of less than 0.1X10-3pm2 is equivalent to a ground permeability of less than 1X 10-3pm2, and the natural gas reservoirs are also divided into three categories, namely: dense layer. The permeability is less than 1X10-3pm2, which is equivalent to the formation permeability of less than 0.1X10-3pm2; the low permeability layer has a permeability of 1X10-3~10X10-3pm2, which is equivalent to the near dense layer of the United States based on the formation permeability (permeability is 0.1). X10-3~1X10-3pm2); high permeability reservoir with permeability greater than 10X10-3pm2, which is equivalent to the United States based on the permeability of the formation. The air permeability of the conventionally tested low permeability reservoir is comparable to its in situ effective gas. The penetration rate is 10 to 1000 times higher. At present, the definition of tight gas reservoirs at home and abroad is generally that the absolute permeability of the ground is less than 1X10-3pm2, but the permeability of 1X10-3 is also considered as tight gas. M. Compared with tight gas, tight oil is up to It has only recently attracted attention, and the discussion of its concept is almost a blank. According to the investigation results of tight oil reservoirs at home and abroad, combined with the understanding of reservoir characteristics and reservoir formation in tight reservoirs in the Ordos Basin, the dense reservoirs are defined as reservoirs dense, only through special measures such as large-scale fracturing. The source rock reservoirs with economic yield have an absolute permeability of less than 2X10-3pm2 and a porosity of less than 12%. Tight reservoirs mainly include tight sandstone reservoirs and tight carbonate reservoirs.

The reason why dense reservoirs are defined as “outsourced reservoirs” is to distinguish them from shale reservoirs formed in source rocks. In addition, the porosity and permeability upper limits of the tight reservoirs given in the paper are mainly based on statistical results of a large number of related reservoir surveys at home and abroad (including the study of the Ordos Basin, see below), and only A rough boundary does not have absolute meaning. The specific boundary depends on the starting point of the definition of the tight reservoir or the factors that are considered (economic factors, development factors or geological factors).

In terms of the relationship between low-permeability oil reservoirs and tight oil layers, the paper distinguishes them. It is proposed that oil-bearing reservoirs can be divided into three categories according to permeability: 1 conventional reservoirs with a permeability greater than 10X10-3pm2; 2 low-permeability reservoirs ( Or near-tight reservoirs, permeability 10X10-3 ~2X10-3pm2; 3 tight reservoirs, permeability less than 2X 2 distribution of tight sandstone reservoirs in the Ordos Basin according to the definition of tight reservoirs, such reservoirs in China The Ordos Basin is the most widely distributed, with the largest number of large oil fields found, and the largest proved reserves and yields, such as Ansai, Jing'an, Wuqi, Zhidan, Yanchang Oilfield and Changfeng's Xifeng Oilfield of the Triassic Yanchang Formation. .

Studies have shown that the tight sandstone reservoirs in the Ordos Basin are mainly distributed in the Triassic Yanchang Formation. According to the characteristics of sedimentary reservoirs and oil-bearing characteristics, the extended group can be divided into three groups of lengths from top to bottom. A total of 10 reservoir groups have three reservoir formations, namely the upper combination (long i length 3) and medium combination ( Length 4+5 - length 7) and lower combination (length 8 - length m) ().

Among them, the middle and lower reservoirs of the Triassic Yanchang Formation (ie, the long 4+5-long m oil formation) are mainly tight sandstone reservoirs, followed by near-tight sandstones or low-permeability sandstone reservoirs; The average porosity of the Chang 3 reservoir is generally greater than 12% to 14%, and the permeability is generally 2x10-3 ~ 50x10-3pm2M, which is dominated by conventional reservoirs.

In the middle and lower combination of the Triassic Yanchang Formation, the Chang 6 reservoir is the most typical tight sandstone reservoir with the largest proven reserves and is the main oil reservoir in the Ordos Basin. According to the test analysis of 13300 porosity samples and 12763 permeability samples in the Chang 6 reservoir in the eastern part of the basin, the porosity is distributed from 3.00% to 22.01%, and the average and median values ​​are 9.16%, of which the porosity is less than 10 % of the samples accounted for 65.8%, less than 12% of the samples accounted for 92.6%, that is, the porosity of the main body was distributed below 12%; the permeability was distributed at 0. 10-3pm2, the average value of less than 1.0X10-3pm2 accounted for 68. The sample accounted for 88.4% (). Because the Chang 6 reservoir in the Ordos Basin is a typical quasi-continuous hydrocarbon accumulation, the upper limit of the porosity and permeability of the tight sandstone reservoir is roughly determined to be 12%, and the long 4+5 and Chang 7 reservoirs are also typical tight sandstones. Reservoir. For example, the Chang 4+5 reservoirs of Kawaguchi, Yanku, Gangu and Subei found in the eastern part of the basin have an average porosity of 8. Petroleum and natural gas geological permeability / (丨gm-Ordos Basin Chang 6 sandstone physical distribution histogram a Porosity; b. Permeability porosity/% average = 1.241 towel value =. MS sample number 丨 2763 length 8 oil range length 9 oil range Ordinal sandstone reservoir distribution of the Triassic Yanchang Formation in the Ordos Basin 3.50xl03pm2; In the Changyi Changwu area, the porosity is 3.20%~10.47%, and the permeability is 0.09x10-3~0.15x103pm2. In the Gaoqiao-Luochuan area, the porosity is distributed on the plane, and the tight oil of the Triassic Yanchang Formation is widely distributed. In the vast area south of the basin south of Yizizhou, Dingbian, the main structure is mainly distributed in the Yishan Slope (). The Chang 6 tight reservoir has the widest distribution and the largest proved reserves. It is the most crude oil production in the Ordos Basin. The main producing layer; followed by the Chang 8 reservoir, mainly distributed in the southwest of the basin.

3 Understanding of the accumulation pattern of large oilfields in the Ordos Basin There is still disagreement on the understanding of the reservoir formation patterns of tight sandstones (formerly known as ultra-low permeability or ultra-low permeability sandstones) in the Ordos Basin. There are three main types of understanding: lithologic reservoir theory; second, multi-factor control theory; and third, continuous oil and gas accumulation theory.

3.1 Lithologic Reservoirs As early as the early 20th century, American MLFuller and FGClappM recognized the central part of the basin when conducting oil geological explorations in the Ordos Basin (what they called the “Shaanxi Basin” or “Shaanbei Basin”). It is a monoclinic structure, but they have concluded that there is unlikely to be a large amount of oil in the entire Ordos Basin, including the Yishan slope in the abdomen of the basin, on the grounds that the basin has too much sandstone in the longitudinal direction and the edge of the basin. The regional folds are too strong and the deterioration may be too deep, while the central part of the basin is mainly monoclinic and the slope is so flat that it does not allow large oil and gas quality oil and gas accumulation. In the 1960s and 1970s, the explorers in the Ordos Basin proposed that the reservoir type should be dominated by lithologic reservoirs based on the simple structure of the abdomen of the basin and the undeveloped folds. It is believed that the formation and distribution of the reservoir are mainly affected by the sedimentary facies. Controls Since the 1980s and early 1980s, they have clearly recognized that lithologic reservoirs are the main type of Triassic reservoirs in the abdomen of the Ordos Basin. At the same time, research on Triassic reservoirs continues to deepen, with the exception of more and more The researchers emphasized the importance of lithologic reservoirs in the formation of the Triassic reservoirs in the Ordos Basin. They also proposed the theory of delta oil control or the theory of delta accumulation, and believed that the Triassic Yanchang Formation delta sedimentary development, the Triassic oil The formation and distribution of the Tibetans play an important role in controlling E3'20'25-38. In addition, the concept of diagenetic traps has been proposed since the mid-1980s, and it is believed that diagenesis plays an important role in controlling the formation of the Triassic Yanchang Formation. 39-40. However, although the importance of lithologic reservoir theory and sedimentation relative to reservoir formation and distribution control has been accepted by many researchers, and is widely used to guide Orr. Exploration and development of oil and gas fields in the Basin, but exploration practice has proved that no matter the Triassic Yanchang Formation or the Upper Paleozoic, not all sand bodies develop oil and gas distribution, and the reservoir type is not completely lithologic oil and gas in the traditional sense. Tibet, indicating that the factors controlling the formation and distribution of large oil and gas fields in the Ordos Basin are not simple.

3.2 Multi-factorial control theory In the 1980s, scholars began to notice that the formation and distribution of the Triassic reservoirs in the Ordos Basin are not only controlled by the sand body sedimentary environment, but also by the source rocks and their reservoirs. The configuration relationship controls M. But until the end of the 20th century, especially since the 21st century, the complexity of the oil control factors of the Triassic reservoirs in the Ordos Basin began to be recognized by more and more researchers. Zhang Wenzhao emphasized that the hydrocarbon generation center, sedimentary facies and local structures are the main controlling factors for the formation and enrichment of Triassic reservoirs. Yang Hua et al. 3'42 pointed out that the large-area distribution of lacustrine source rocks and the development of large-scale compound delta reservoirs are the main factors for the formation and enrichment of large delta reservoirs in northern Shaanxi, emphasizing the growth of Chang 7 high-quality oil source rocks. Low-permeability oil and gas accumulation and concentration play a leading role.

Wu Fuli et al. pointed out that the formation of Triassic reservoirs in northern Shaanxi is controlled by multiple factors such as oil source, reservoir, caprock and structure, and further points out that reservoirs of different horizons are in conditions of accumulation or There is a compensation relationship in the distribution, which is controlled by multi-factor compensation. The Triassic reservoir type shows regular sequence variation in the longitudinal direction. From the bottom to the top, it is lithologic reservoir, tectonic-lithologic composite reservoir and structure. Reservoir.

It can be seen that although the oil control factors obtained by different researchers are not exactly the same, the common point is that the formation and distribution of the Triassic Yanchang Formation reservoirs in the Yishan Slope of the Ordos Basin are not controlled by a single factor, but by hydrocarbons. Multiple factors such as source and reservoir control. Under the guidance of multi-factor control theory, oil and gas exploration in the Ordos Basin has made new progress.

However, whether it is the traditional lithologic reservoir theory or the multi-factor storage theory, the proposed reservoir model cannot fully explain the unique properties of the tight sandstone large oil and gas fields in the Ordos Basin. Its main specialities are: continuous or quasi-continuous distribution of large areas of oil and gas reservoirs, no clear reservoir boundaries; complex oil and water distribution, oil and water heterogeneity, generally no clear bottom water, no obvious oil and water boundaries .

3.3 Continuous oil and gas accumulation theory Because traditional hydrocarbon accumulation theory is difficult to explain the unique reservoir-forming characteristics of low-permeability tight sandstone reservoirs in the Ordos Basin, Zou Cai et al. introduced the theory of continuous hydrocarbon accumulation in 60-51 in recent years, and considered the tight oil in the Ordos Basin. And tight gas is a continuous oil and gas accumulation.

Zhao Jingzhou, and so on. The formation model of quasi-continuous low-permeability-dense sandstone large oilfields in the Perdos Basin was proposed by the US Geological Survey. It is believed that tight sandstone reservoirs, basin center gas, shale gas, coalbed methane and natural gas hydrates belong to this type. . They point out that continuous aggregation refers to the accumulation of oil and gas with a large spatial distribution and no clear boundary, and it exists more or less independent of the water column. M. They believe that the difference between continuous aggregation and conventional aggregation is that conventional oil and gas accumulation is due to The buoyancy of oil or gas in water causes accumulation in local structures or stratigraphic traps, resulting in discontinuously distributed oil and gas fields or reservoirs 67; while continuous oil and gas accumulation has two common characteristics, one is that there is widespread oil or The gas volume is filled with a huge volume of rock body, and the second is independent of the buoyancy of oil or gas in the water.

The geological characteristics of continuous oil and gas accumulation are generally: the rock is distributed under saturated water, the lack of obvious traps and caps, the general filling of oil or gas, the wide distribution range, low matrix permeability, abnormal pressure (High or low) and closely related to the source rock. Its production characteristics are usually: large in-situ oil and gas, low recovery, lack of dry wells in the true sense, dependence on fracture permeability, and desserts with good production characteristics in the gathering. 62'57. National Geological Survey of the United States Schenk69, head of the oil and gas resources evaluation team, proposed that there are 16 geological standards for determining continuous natural gas accumulation: regional distribution; diffuse boundaries; pre-existing “oil and gas fields” combined into individual regional aggregates; no obvious traps and Cover layer; no clear oil-water or gas-water interface; hydrocarbon emplacement is not caused by hydrodynamics; usually has abnormal formation pressure; huge resource, low recovery; geologically controlled "dessert"; free production Little water (except coalbed methane); formation water is located in the upward direction of oil and gas; there are few real dry wells; reservoirs are adjacent to source rocks; final production of oil or gas wells is lower than conventional gas reservoirs; reservoir matrix infiltration The rate is very low; the reservoir generally develops natural cracks. At the AAPGHedberg conference in 2005, SchenkM extended these feature advancements to continuous oil and gas agglomeration (not just continuous gas agglomeration), arguing that a continuous oil or gas agglomeration may have all or some of its characteristics.

SPE, AAPG, WPC, and SPEEM define "contiuous-type deposits" as: oil and gas accumulations that are spread over a wide area and are not significantly affected by hydrodynamic forces. The four types of continuous mineral deposits listed by the four organizations include “basin center” gas, shale gas and gas hydrate, as well as natural bitumen and oil shale. The so-called continuous and unconventional oil and gas are conceptually Basically the same, this is not exactly the same as the US Geological Survey.

4 Proposed formation model of quasi-continuous large oilfields and its characteristics 4.1 Quasi-continuous large oilfield accumulation model The study on the formation and distribution of oil reservoirs in the Ordos Basin shows that the Triassic Yanchang Formation is dense and partially compact (or low). Infiltration) Sandstone reservoirs are not conventionally considered lithologic reservoirs in the conventional sense, but also atypical continuous unconventional reservoirs, but between conventional reservoirs and unconventional reservoirs or discontinuous and continuous reservoirs. The type of transition between the two is called “quasi-continuous reservoir” or “quasi-continuous oil and gas accumulation. The so-called quasi-continuous reservoir refers to the accumulation of oil and gas by unconventional traps and the large-scale quasi-continuity of oil and gas reservoirs. Distribution and tight hydrocarbon accumulation without clear hydrocarbon reservoir boundaries. The unconventional traps referred to here are those that have "constrained" and "constant continuous unconventional oil and gas reservoirs" in conventional oil and gas reservoirs. A transitional type trap between traps, or a transitional type trap between a tangible conventional trap and an invisible unconventional trap. Common for unconventional hydrocarbon accumulation ring Closed is mainly an unconventional lithologic trap and dynamic trap. The so-called unconventional lithologic trap refers to a large area consisting of many small and medium lithologic traps that are adjacent to each other in the lateral direction and vertically overlap each other. Distributed trap groups. Compared with conventional lithologic traps, the main features of unconventional lithologic traps are numerous and adjacent to each other, lacking clear boundaries; while conventional lithologic traps are often isolated and distributed, Relatively clear, in addition to unconventional lithologic traps, oil and gas migration and accumulation dynamics also play an important role in controlling the formation and distribution of continuous tight reservoirs. The determined range of hydrocarbon migration is the range of dynamic traps. "Dynamic trap" is a new type of oil and gas geological closure type of tight sandstone quasi-continuous reservoir model proposed by Li Mingcheng and Li Jian (2010) M for the formation of low-permeability tight reservoirs. Closed is the most important type of hydrocarbon accumulation in the low-permeability and tight reservoir of oil and gas, and it is also a three-dimensional space in the low-permeability tight reservoir that can retain oil and gas accumulation. Quasi-continuous reservoirs are oil and gas accumulations similar to those of continuous reservoirs formed under unconventional traps such as unconventional lithologic traps and dynamic traps. Typical continuous unconventional reservoirs (coal methane) Unlike shale gas, quasi-continuous reservoirs are source-external accumulation and near-source accumulation, and oil and gas are quasi-continuously distributed, and traps are between tangible and intangible; while continuous reservoirs are sourced, Self-generated and self-storing, oil and gas are continuously distributed, no obvious traps.

In fact, continuous unconventional reservoirs and discontinuous conventional reservoirs represent two endmember types in the reservoir formation sequence in a complex geological environment, and there should be different transition types between the two. Quasi-continuous tight sandstone reservoirs in the Ordos Basin represent a transitional type between unconventional continuous reservoirs and discontinuous conventional reservoirs.

4.2 Formation characteristics of quasi-continuous large oilfields in the Ordos Basin The formation and distribution of quasi-continuous large oilfields in the Ordos Basin mainly have the following characteristics.

1) Large-area quasi-continuous distribution of oil reservoirs. There are no clear boundaries. Conventional oil and gas reservoirs generally have discontinuous distribution, and the distribution area is small, mostly in the range of several to several tens of square kilometers, but the majority is not hundreds of square kilometers. The tight sandstone reservoirs of the Triassic Yanchang Formation in the Ordos Basin are widely distributed. As early as the 1960s and 1970s, the older generation of petroleum geologists in China had discovered the phenomenon of the extension group “seeing oil in the well and not flowing in the well”. After decades of exploration, it has been found that the tight sandstone reservoirs of the Triassic Yanchang Formation have the characteristics of large and low, ie, large distribution area, low abundance, low permeability and low yield; the reservoir area is as high as tens of Thousands of square kilometers, mostly in the hundreds of square kilometers; moreover, there are no clear boundaries in tight sandstone reservoirs. The boundaries currently defined are mostly artificial boundaries, including exploration and development work boundaries or economic boundaries. With the expansion of the development scope, most of the oil-bearing area will be further expanded; or as the oil layer transformation technology advances or the oil price rises, some wells that were originally thought to be low-yield or even only show may become economic production wells. In fact, Erdos The phenomenon of "seeking oil in the wells of the Triassic Yanchang Formation in the basin" is a reflection of the reservoir-forming characteristics of quasi-continuous reservoirs; while the "wells in the wells" is mainly due to the fact that such reservoirs are commonly used as conventional reservoirs in the past. Therefore, the reasons for the exploration using conventional oil testing and mining techniques (such as "test without pressure").

Since the 1980s, due to the adoption of advanced unconventional oil layer reconstruction techniques and other measures, the Ordos Basin has emerged from the exploration and development dilemma of “seeking oil in wells and not flowing wells”, and oil exploration and development has made important breakthroughs.

2) Large-scale hydrocarbon generation, high-intensity filling The Mesozoic source rocks in the Ordos Basin are mainly distributed in the Chang 4+5-Chang 9 section of the Triassic Yanchang Formation, mainly semi-deep lake-deep lake facies. Among them, the Chang 7-segment source rock has the largest distribution range and high organic matter abundance, which has been confirmed as the main source rock of the Mesozoic reservoir in the Ordos Basin. The main source rock ("Zhangjiatan black shale") is a high-quality source rock with black shale and oil shale. The sedimentary area of ​​the deep lake area is 3.104km2, plus the area of ​​the surrounding shallow lake area. 5. 104km2, with a total area of ​​approximately 8.5x(). Its thickness is 510m in the eastern part of the Qingjian River; it is as thick as 30m in the Zhidan area; the thickest is more than 120m in the center of the Fuxian Lake basin, and the thickness of the single layer can reach more than 60m. Zhao Jingzhou, and so on. Quasi-continuous low-permeability-dense sandstone large oilfield accumulation model in the Erdos Basin U\fi and other geographical names The Triassic tight oil distribution in the Ordos Basin is related to the Chang 7 source rock. The Chang 7 source rock controls the Ordos Basin. The distribution of most of the reservoirs in the Mesozoic, especially the reservoirs in the strata above the long s. In addition, the Chang 9 Lijia shale is another important source rock in the Ordos Basin. The organic matter abundance of the source rock is also high, and the total organic carbon content is 1.19%~8.64%, the average value is 5.03%; the chloroform asphalt “A” content is distributed in 0.4724%~1.2997%, with an average of 0. 8603%M. However, the distribution of the source rock is relatively limited, mainly distributed in the area of ​​Zhidan-Ganquan-Fuxian County, with a thickness of up to 20. The oil-source comparison shows that the Chang 9 source rock mainly controls the length 9 and The distribution of long t. reservoirs may also have some control over long s reservoirs. 67. Due to large distribution area, high abundance of organic matter, good type and moderate maturity, and a large amount of hydrocarbon generation in major source rocks such as Chang 7 It has produced a widespread distribution of overpressure phenomena, providing sufficient power for the outward discharge of oil and gas and filling of adjacent tight reservoirs, so that the high-quality source rocks of the Chang 7 segment and the like exhibit large area hydrocarbon generation and high strength in the basin. The characteristics of the filling form the general oil-bearing appearance of tight reservoirs such as long 6 and long s.

3) Poor physical properties of reservoirs, heterogeneous strong Middle and lower combined reservoirs of the Triassic Yanchang Formation in the Ordos Basin are the least physical layer of the Mesozoic oil layer, with the main body being tight sandstone reservoirs; followed by near dense (low Infiltration), the average porosity is generally less than 12%, and the average permeability is less than 2x103pm2. Moreover, because the reservoir is mainly superimposed and composited, the reservoir heterogeneity is strong, and the lithology and physical properties change in the lateral direction. Large, single sand bodies have poor continuity in the lateral direction. The reason why such a poor reservoir can form a large-area quasi-continuously distributed reservoir is mainly due to its proximity to the equally widely distributed Chang 7 main source rock, which has superior hydrocarbon supply conditions. On the other hand, the dense and uneven oil and natural gas geology of the reservoir makes it difficult for oil and gas to be transported over long distances in dense reservoirs such as Chang 6 and Chang 8 to form a concentrated distribution, but it is characterized by large area and low abundance. The characteristics of the degree distribution. On the contrary, in the Chang 2 Member and the Jurassic Yan'an Formation away from the source rock, due to its good physical properties and close to the conventional reservoir, the oil and gas is relatively easy to carry out lateral migration and concentration, and the hydrocarbon supply conditions are poor. Thus, a tectonic-lithologic composite reservoir or even a tectonic reservoir with relatively isolated and dispersed distribution, relatively small scale, and certain control by structural traps is formed.

4) The formation of conventional oil and gas reservoirs between conventional traps and trapless traps is inseparable from traps, and the trap limits are clear. The trap of the Mesozoic tight sandstone reservoir in the Ordos Basin is not a trap form in the traditional sense, but a type of transition between a conventional trap and a trap, or between a tangible and an invisible trap. A special unconventional trap. Its main form is the unconventional lithologic trap, which is represented by the fact that many small and medium lithologic traps or desserts are superimposed in the longitudinal direction and compounded on the plane without a clear boundary. This is because the Triassic Yanchang Formation is a set of river-delta facies deposits, which has undergone multiple evolutions of sedimentary cycles. As a result, the channel sediments in the various layers of the Yanchang Formation are often superimposed in the longitudinal direction, often on the plane. The multi-period compounding forms a superimposed composite sand body configuration with large-area contiguous distribution. Its prominent features are strong reservoir heterogeneity, and lithology and physical properties vary greatly in the lateral direction. In addition, the faults and fold structures in the basin are not developed. Therefore, the traps of tight reservoirs such as Chang 6 and Chang 8 in the Triassic Yanchang Formation are not isolated and distributed as in conventional tectonic reservoirs or lithologic reservoirs, but by numerous medium and small rocks. The sexual traps or desserts are superposed on each other in the longitudinal direction and are contiguous in the transverse direction, thereby forming a large-area lithologic trap group or a dessert group () that is adjacent to each other.

In addition to unconventional lithologic traps, dynamic traps may also be a special type of trap that controls the formation of tight reservoirs in the Ordos Basin. The study of pressure evolution history shows that the main source rocks of the Triassic Yanchang Formation have formed a general overpressure in geological history. This is the main driving force for the tight hydrocarbon migration in the Ordos Basin (see below), which plays an important role in controlling the formation and distribution of tight reservoirs in the Mesozoic.

Conventional oil and gas reservoirs generally have distinct oil, gas and water, and have clear side or bottom water. However, a large number of oil test and test results show that the Chang 6 and other tight sandstone reservoirs discovered in the Triassic Yanchang Formation in the Ordos Basin are basically borderless and bottom water, and oil and water are stored together, and there is basically no pure oil layer. The pure water layer is also less, and the oil and water are the same layer. This is distinct from typical conventional lithologic reservoirs. The reason is that the oil and water in the tight reservoir of Chang 6 are the same as the reservoir, and the lack of edge and bottom water is mainly due to the dense reservoir, the small pore throat, the large change of lithology in the lateral direction, and the smooth formation of the oil. It is difficult for water to form a good differentiation in it, thereby forming a phenomenon of oil, water storage and lack of free water. The reservoirs in the reservoir period have been relatively dense, which may be an important cause of the lack of free water in the reservoir. In addition, due to oil, water heterogeneity and lack of free water, the tight sandstone reservoirs such as Chang 6 are not only lacking in edge and bottom water, but also there is no phenomenon of updip formation water or regional oil and water inversion.

The reasons for the complex oil and water distribution in the tight sandstone reservoirs in the Ordos Basin are related to the tightness and heterogeneity of the reservoirs, and on the other hand, the oil and gas charging and transport methods are also closely related. Because the oil and gas charging is mainly a wide vertical filling method, and it is difficult to carry out long-distance migration after entering the reservoir, but mainly for the near migration and accumulation, it will inevitably cause gas, oil, water heterogeneity and distribution complexity. There are not many but more common water production phenomena.

6) The reservoir pressure system is complex and has many negative pressure anomalies. The tight sandstone reservoirs of the Triassic Yanchang Formation in the Ordos Basin are complex in present. The same reservoir generally does not have a uniform pressure system, reflecting the poor internal connectivity of the reservoir. This is closely related to the dense reservoir and strong heterogeneity. Moreover, today's formation pressure is generally characterized by negative pressure. For example, in the Xingzichuan oilfield in Ansai, the reservoir pressure coefficient of the present extension group is less than 0.8, mainly distributed in the range of 0.4~0.8, which is characterized by abnormal low pressure.

The study shows that the negative pressure phenomenon in the Ordos Basin is mainly due to the strong tectonic uplift and erosion in the later stage of the basin, and also related to the reservoir heterogeneity and poor connectivity. Due to the poor connectivity of the reservoir, it is difficult for the fluid to communicate and balance within the reservoir, making it difficult to form a normal pressure system.

7) Oil and gas migration and accumulation is non-buoyant driving, and the study of close-range migration and accumulation shows that the reservoirs in the middle and lower reservoirs of the Triassic Yanchang Formation in the Ordos Basin have been densified during the period of massive oil generation. Because the strata in the period of accumulation have been relatively flat, and the reservoir is dense, there is little free water, so buoyancy and hydrodynamics are weak, and it is difficult to become an effective power for oil and gas migration in the reservoir, contributing little to oil and gas migration. In addition, due to the strong heterogeneity of the reservoir, the lithologic properties in the lateral direction vary greatly, so there is no transport condition for long-distance lateral migration of oil and gas. It can be seen that oil and gas in the tight sandstone reservoir of the Triassic Yanchang Formation lacks sufficient transport mobility and lacks good migration channels, which makes it difficult for large-scale long s long 0 long 7 long 9 long r long 7 long. 8 Chang i, Chang 9 Ordos Basin Triassic Yanchang Formation Stratigraphic Fluid Pressure and Depth Relationship M Study shows that the main factors controlling the formation and distribution of tight sandstone large oilfields in the Ordos Basin are hydrocarbon source and reservoir conditions, followed by caprock control. The role of hydrocarbon source conditions is the most important factor controlling the formation and distribution of tight sandstone oilfields in the Ordos Basin. It can be said that where the effective source rock is distributed, where the tight sandstone reservoir may extend. Exploration and development practices show that the Mesozoic reservoirs are mainly distributed in and around the effective source rocks of the Triassic Yanchang Formation, especially the Chang 7 main source rock distribution area, which is mainly limited to the south of Hengshan Yiyan Pool, especially On the edge of the south of the basin south of a sub-continent, a large area (). In the longitudinal direction, the Yanchang Formation reservoir is mainly distributed on the upper and lower adjacent layers of the main source rock. It is because of the development of the widely distributed long-segment high-quality source rock that the formation fluid pressure/MPa formation fluid pressure/MPa length: length 2 long 3 long W long 6 long 7 c. Qingshen Sajing d. 33 well formation fluid pressure / MPa formation fluid pressure / MPa length, length 2 long, long 2 long 3 long 1 long 1 2 Zhao Chenglin, Hu Yumei, Chen Bizhen, and so on. SY/T6285-1997, Oil and Gas Reservoir Evaluation Method (People's Republic of China Oil and Gas Industry Standard) S. Beijing: Petroleum Industry Press, 998 3 Zhao Jingzhou, Wu Shaobo, Wu Fuli. On the Classification and Evaluation Criteria of Low Permeability Reservoirs: Taking the Ordos Basin as an Example J. Lithologic Reservoirs, 2007, 19(3): 28-31. 4 Hu Wenrui. Introduction to low-permeability oil and gas fields M. Beijing: Petroleum Industry Press, 2009: 246. 9 Yuan Zhengwen, Zhu Jiawei, Wang Shenglang, et al. Natural gas reservoir characteristics and classification of Shahejie Formation in Dongpu Depression J. Natural Gas Industry, 1990, 10(3): 6 Xu Huazheng. Study on the characteristics of tight sandstone gas reservoirs in Dongpu Depression J. Petroleum Science 113 Yang Hua, Fu Jinhua, Yu Jian. Enrichment laws and exploration techniques of large delta reservoirs in northern Shaanxi Province J. Petroleum Journal, 2003, 24 114 Fu Jinhua, Luo Anxiang, Yu Jian, et al. The geological characteristics of the formation of Xifeng Oilfield and exploration Wang Changyong, Zheng Rongcai, Li Zhongquan, et al. Long 8 oil 116 Wang Guicheng, Wang Yujun, Jiyu Oilfield, Ordos Basin.鄂尔多斯盆地英旺油田长8储层非均质性研究J.西安石油大学学报（自然科学版），2010，25（5）：16-19，24. 117郭德运，郭艳琴，李文厚，等。富县探区上三叠统延长组长8油藏富集因素J.西北大学学报（自然科学版），2010，40（1）：93-97. 118段毅，于文修，刘显阳，等。鄂尔多斯盆地长9油层组石油运聚规律研究J.地质学报，2009，83 20杨俊杰，李克勤，宋国初，等。中国石油地质志（卷十二）M.北京：石油工业出版社，992.赵靖舟，等。鄂尔多斯盆地准连续型低渗透-致密砂岩大油田成藏模式21朱明恭，谢广成。延长一永坪油田群//张文昭。中国大油田勘探实践。北京：石油工业出版社，2002：314 22孙肇才，谢秋元。叠合盆地的发展特征及其含油气性――以鄂尔多斯盆地为例J.石油实验地质，1980，23孙国凡，谢秋元，刘景平，等。鄂尔多斯盆地的演化叠加与含油气性――中国大陆板块内部一个大型盆地原型分析24张抗。鄂尔多斯断块构造和资源。西安：陕西科学技术25李克勤。最佳成油配置及低渗油层找油J.石油勘探与开26李克勤。陕甘宁地区上三叠统隐蔽低渗油藏的找油方向27朱国华，王文炯。论陕北安塞延长组三角洲的油气富集条件J.石油与天然气地质，1987，8（4）：440-447. formationinAnsai，NorthShaanxi张松扬。大牛地气田致密砂岩储层测井评价J.石油物29梅志超，彭荣华，杨华，等。陕北上三叠统延长组含油砂体的沉积环境J1.石油与天然气地质，1988，9 30王锡福，宋国初。陕甘宁盆地油气藏形成条件及分布特征//中国石油学会石油地质委员会。中国油气藏研究。

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