Introduction
With the continuous development, the remaining oil and gas resources are gradually decreasing, and the demand for energy is growing. Unconventional oil and gas, as an important replacement resource, have begun to draw high attention around the world. As an important part of unconventional oil and gas resources, tight oil plays a crucial role in the global energy, and has achieved commercial development in many countries. The United States has discovered nearly 20 tight oil basins, as well as multiple pay formations such as Bakken, Barnett, and Woodford. In addition, Canada, Argentina, Ecuador, the United Kingdom and Russia, etc., have also achieved breakthroughs in tight oil exploration [1-2]. In fact, the exploration and development of tight oil in China also has broad prospects. Many basins have developed numerous types of tight oil reservoirs, such as tight sandstone, tight carbonate rock, and tight mixed rock found in basins such as Ordos, Songliao, Sichuan, Bohai Bay, Qaidam, Junggar, Tuha, and Santanghu [3⇓⇓-6]. China has gone through more than ten years of continuous exploration and development of tight oil, from the exploration stage of tight oil concept and technology, industrial testing and trial production stage, to scale discovery and development and production stage [7]. Significant progress has been made in geological theory innovation, geological understanding, and engineering technology. However, with poor physical properties, low abundance, and low natural productivity, the exploration and development of tight oil reservoirs are often not as expected. It is urgent to obtain new knowledge and develop new technologies to promote significant breakthroughs and rapid development in tight oil exploration and development.
Tight oil resources in the Fuyu oil reservoir of the Lower Cretaceous Quantou Formation in the northern part of the Songliao Basin have a great potential. The positive structural belt of the Fuyu oil reservoir is enriched and has produced oil at high yield. Discoveries have been made on a large scale and production capacity has been built. The PetroChina Daqing Oilfield Company first explored the Fuyu oil reservoir from the late 1950s to the early 1960s, and obtained industrial oil and gas flows at local structural highs from the 1970s to the 1980s. By the late 1990s, industrial discoveries had been made in multiple structures in the Fuyu oil reservoir. After continuous exploration, tight oil production broke through 71.26 t/d in Well YH 1 in 2011, which opened a new chapter in tight oil exploration in the Songliao Basin. In the following two years, tight oil wells such as QH 2, PH 1 and PH 2 were drilled successfully, making the scale of tight oil exploration continue to expand. Since 2013, horizontal wells have been constructed on a large scale, and made a huge contribution to the increase of reserves and production in Daqing Oilfield. However, at present, there are problems such as unclear understanding of the relationship between multiple factor configurations during the accumulation period, reservoir controlling factors, and oil and gas enrichment patterns in the Fuyu oil reservoir of the Quantou Formation in the northern Songliao Basin, resulting in unclear research on the tight oil enrichment model of the Fuyu oil reservoir. Taking the tight oil in the Fuyu oil reservoir as a case, and based on the geological understanding obtained in Daqing Oilfield in recent years, this article discusses the controlling factors, and oil and gas enrichment model of tight oil in the Fuyu oil reservoir, with the aim to deeply explore the exploration potential and promote the development of tight oil in the northern part of the Songliao Basin, and provide strong theoretical support for tight oil production.
1. Geological setting
The Songliao Basin is a large continental oil and gas bearing basin deposited from the Mesozoic to the Cenozoic, and with a dual structure of lower faults and upper depressions. It includes six first-order structural units, namely a central depression, a western slope, a northeast uplift, a southeast uplift, a northern subduction, and the southwest uplift (Fig. 1a ), and covers 26×104 km2. The Songliao Basin has gone through stages of thermal uplift, rift, depression, and shrinking uplift during its formation and evolution, and developed formations during faulted rift, faulted depression, depression and reversal. The middle and shallow formations in the northern part of the Songliao Basin include the Cretaceous Quantou Formation, Qingshankou Formation, Yaojia Formation, Nenjiang Formation, Sifangtai Formation, Mingshui Formation, the Paleogene, the Neogene, and the Quaternary (Fig. 1b ). With the source rock of the Qingshankou Formation as the center, there are oil layers such as Heidimiao, Sartu, and Putaohua above the source rock, shale oil of the Gaotaizi Formation, the first and second members of the Qingshankou Formation (Qing 1 and Qing 2 members) inside the source rock, and the Fuyu and Yangdachengzi oil layers below the source rock [8]. The deep layers in the northern part of the Songliao Basin include the basement weathering crust and the Cretaceous Huoshiling Formation, Shahezi Formation, Yingcheng Formation, and Denglouku Formation. Three plays were found around the source rock of the Shahezi Formation: the sandstone reservoir of the Denglouku Formation, the sandy conglomerate reservoir of the Yingcheng Formation, and the volcanic reservoir of the Yingcheng Formation, which are featured by lower source and upper reservoir; the tight sandstone of the Shahezi Formation, which is self-source and self-reservoir; and the bedrock of the Central Paleouplift and the volcanic rock of the Huoshiling Formation which are upper source and lower reservoir.
Fig. 1. Structural units (a) and comprehensive stratigraphic column (b) of the Songliao Basin. |
There are five primary structural units in the northern part of the Songliao Basin: a central depression, a western slope, a northern subduction, a northeast uplift, and a southeast uplift (Fig. 1a ). The study area is mainly located in the central depression. The Fuyu oil reservoir as the target layer in this study is located in the upper part of the fourth and third members of the Quantou Formation of the Lower Cretaceous (Fig. 1b ), with thickness of 66-240 m. It is characterized by the development of a large river-shallow water delta sedimentary system. The reservoir is relatively tight, with porosity of 5% to 12%.
Tight oil in the Fuyu oil reservoir is mainly distributed in the Daqing placanticline, the Sanzhao Sag, and the Qijia-Gulong Sag. It is the most important tight oil production zone in the northern part of the Songliao Basin [9].
2. Geological features of Fuyu oil reservoir
2.1. Structural features
The Songliao Basin has a typical dual structure of lower faults and upper depressions, and has gone through contemporaneous and multi-stage superimpositions, and gradually evolved from an early separate faulted depression to a late large and united depression. Initially developed in the early depression to the late unified lake basin after structural subsidence and reversal, the Fuyu oil reservoir has spread in two sags and one uplift at present, namely the Qijia-Gulong Sag, the Daqing placanticline, and the Sanzhao Sag. On the whole, it is a nose structure surround by sags (Fig. 2 ).
Fig. 2. Structures of Fuyu oil reservoir in northern Songliao Basin. |
Multi-stage tectonic movements happening in the Songliao Basin controlled the sedimentation of the basin. During the sedimentary period of the Quantou Formation, faults were not developed, while strong faulting occurred under regional stress during the sedimentary period of the Qingshankou Formation. A large number of source faults were formed at the T2 interface, providing pathways for large-scale downward migration of oil and gas. During the sedimentary period of the Lower Cretaceous Denglouku Formation to the Upper Cretaceous Nenjiang Formation, the Qijia-Gulong Sag and the Sanzhao Sag had long been the subsidence centers, and they were conducive to the development of source rocks and hydrocarbon generation. In the late stage of sedimentation of the Mingshui Formation, it experienced strong folding deformation and faulting [10]. In the early stage, faults were activated, and new secondary faults developed in stress concentrating areas. At the same time, reversal structures, namely the Daqing Placanticline and the Changchun Ridge anticline, were shaped, which were conducive to the enrichment of oil and gas in the Fuyu oil reservoir and laid the foundation for a structural pattern of large-scale oil and gas enrichment.
2.2. Sedimentary and reservoir features
During the sedimentary period of the Fuyu oil reservoir, the climate was relatively dry, and the Songliao Basin was in a depression period. The tectonic subsidence was stable, the terrain was flat, and the sedimentary range was extremely expanded [11]. During that period, large river systems around the basin were well developed and controlled by multi-directional source systems. The basin experienced sedimentary evolution from river facies to shallow water deltas in lake basins. Early rivers were dominant, and small and shallow lake basins were scattered. Then middle rivers were still dominant, while lake basins became large, and a larger lake basin appeared in the Gulong area. Finally, in the late stage, a unified lake basin was formed, and sediments of shallow water delta facies were developed. Overall, the Fuyu oil reservoir was developed in a large river and shallow water delta system [12]. The reservoir sand bodies are mainly meandering channel sand bodies, distributary channel sand bodies, crevasse fans, and sheet-like sand bodies, distributed as networks and branches (Fig. 3 ).
Fig. 3. Sedimentary facies of Fuyu oil reservoir in northern Songliao Basin (modified from Reference [8]). |
Single Fuyu oil sand bodies are small and poorly continuous, but they are superimposed and merged, which provides good space for oil and gas enrichment.
The lithology of the Fuyu oil reservoir is mainly fine sandstone and siltstone, followed by mudstone, which account for 46.07%, 36.54%, and 16.01%, respectively. Lithic-feldspar sandstone and feldspar-lithic sandstone are dominant. Among them, lithics account for 76.8%, which is composed of quartz, feldspar and other debris, and the rest is interstitial material that is 23.2%, mainly composed of carbonate cement and clay mineral matrix. The main types of cementation are contact cementation, pore cementation, and inlay cementation.
The space in the Fuyu oil reservoir is diverse. Primary intergranular pores contribute to the most reservoir space, followed by dissolution pores, and local micro- cracks. Pores or pore-microcracks are dominant types of reservoir space. Micro-pores constitute the main storage space, and nano-throats are channels in the Fuyu oil reservoir [13-14]. The Fuyu oil reservoir is relatively tight, its porosity is 5% to 12% and permeability is (0.01-1.00)×10-3 μm2. It is tight sandstone reservoir with low porosity, and low to ultra-low permeability.
3. Controlling factors for the enrichment of under-source tight oil
3.1. Source-reservoir confirmation controls the distribution of oil reservoirs
Oil and gas generated by mature source rock are the material basis for the formation of various oil reservoirs, and source rock controls the distribution of oil and gas at the macro level. Oil and gas generated by mature source rocks in the Qing 1 Member are the primary source of tight oil in the Fuyu oil reservoir [15⇓-17]. The source rocks are distributed in the central depression, in which lamellar algae are the first natters generating hydrocarbon. The organic matters are mainly Type I, followed by Type II1. The Ro mostly ranges from 0.7% to 1.3%, indicating the source rocks are mature to highly mature. The average value of TOC is 2.97%. The average value of chloroform bitumen "A" is 0.481%. The mature source rock of the Qing 1 Member covers 1.65×104 km2, and its thickness is generally 40-90 m, and hydrocarbon expulsion intensity is (200-480)×104 t/km2. It is mainly distributed in the Qijia-Gulong Sag, the Daqing placanticline, and the Sanzhao Sag [18-19]. With good types of organic matter, high organic matter abundance, high hydrocarbon generation potential, large sedimentary thickness and wide distribution, the source rock of the Qing 1 Member lays a sufficient material foundation for the enrichment of tight oil in the Fuyu oil reservoir.
The thermal evolution of the source rock increases with depth (Fig. 4a ). The Qijia-Gulong Sag and the Sanzhao Sag are the primary centers of hydrocarbon generation and expulsion. As the degree of thermal evolution increases, the source rock continuously generates hydrocarbon, and the (S1+S2)/TOC (i.e. the hydrocarbon generation potential index per unit organic carbon) continuously increases. When the hydrocarbon expulsion threshold is overcome, the source rock begins to expel hydrocarbon, and the hydrocarbon generation potential index begins to decrease. Based on the hydrocarbon generation potential method, the threshold for a large amount of hydrocarbon expulsion from the source rocks of the Qingshankou Formation is determined to be about 1 700 m. In most areas of the Qijia-Gulong Sag and the Sanzhao Sag, the source rocks are below 1 700 m (Fig. 4b ), and a large amount of hydrocarbon has been expelled. In comparison, the source rocks in the Daqing Placanticline are relatively shallow, resulting in less expulsion of hydrocarbon.
Fig. 4. Ro and hydrocarbon generation potential of source rocks in the Qing 1 Member in northern Songliao Basin. |
From the comparison of the distribution of tight oil and source rocks in the Fuyu oil reservoir, it can be found that most of the industrial oil flow wells are drilled in the distribution range of effective source rocks (Fig. 5a ), indicating that effective source rocks have a certain controlling effect on the distribution of tight oil.
Fig. 5. Relationship between source rocks, porosity and petroleum initially-in-place of the Fuyu oil reservoir in northern Songliao Basin. |
Porosity and permeability control the oil and gas enrichment in tight oil reservoirs to a certain extent. Under the same geological conditions, reservoirs with better porosity and permeability are often more conducive to oil and gas enrichment because of the favorable configuration between the abnormal high pressure (charging force) induced by hydrocarbon generation in source rocks and the larger capillary resistance (enrichment resistance) in tight oil reservoirs. Reservoirs with good physical properties usually attract oil and gas to accumulate [20-21]. From the distribution of reserves in the Fuyu oil reservoir, tight oil is mainly distributed in areas with porosity of 5% to 12% (Fig. 5b ). From the relationship between reservoir physical properties and oil content in the study area, it can be seen that porosity, permeability and oil content are positively correlated (Fig. 6 ). In summary, the distribution of oil and gas in the Fuyu reservoir in the northern part of the Songliao Basin is significantly controlled by the physical properties of the reservoir.
Effective source rocks macroscopically control the distribution of oil reservoirs, and the physical properties of reservoirs control the enrichment of tight oil to a certain extent. The good match between effective source rocks and high-quality reservoirs jointly controls the distribution style of oil reservoirs. According to the superposition maps of source rocks and porosity with reserves, it can be found that most oil reservoirs in the Fuyu oil reservoir are within the distribution range of effective source rocks, and rich in zones with good reservoir properties.
Fig. 6. Physical properties vs. oil distribution in the Fuyu oil reservoir in northern Songliao Basin. |
3.2. Pressure difference between the source and reservoir is the driving force for the enrichment of tight oil
Compared to conventional oil reservoirs, tight oil reservoirs have the characteristics of low porosity and low permeability. Oil and gas need to overcome a greater capillary resistance when flowing. When the pressure caused by hydrocarbon generation reaches the fracturing pressure of the source rock and breaks through the capillary resistance of the tight reservoir, oil and gas start to flow into the tight reservoir along faults. The difference between the pressure caused by hydrocarbon generation and the capillary resistance in reservoir determines the migrating distance and distribution of oil and gas in tight sandstone [22-23].
As source rocks become mature and generate hydrocarbon, overpressure appears. From the distribution of pressure coefficient, it can be seen that the overpressure in the Qing 1 Member gradually decreases from the center of the depression to the surrounding areas until it disappears. The highest pressure coefficient (1.4-1.6) appears in the Qijia-Gulong Sag, then 1.2-1.4 in the Sanzhao Sag (Fig. 7a ). The simulation of pressure evolution history shows that the late stage of sedimentation of the Nenjiang Formation, the late stage of sedimentation of the Mingshui Formation, and the late stage of sedimentation of the Paleogene are three peak periods of hydrocarbon expulsion from the Qingshankou Formation source rocks (Fig. 7b ). Based on fluid inclusion analysis, some scholars believe that the late stage of sedimentation of the Nenjiang Formation and the late stage of sedimentation of the Mingshui Formation are two important periods for crude oil injection into the Fuyu oil reservoir [24-25]. After being discharged from the source rock, crude oil is injected into the reservoir under the drive from hydrocarbon generation. The greater the pressure caused by hydrocarbon generation, the stronger the ability of crude oil breaks through the small pore throats in the reservoir near the source-reservoir interface [26], and the longer the distance of crude oil migrates. As the burial depth of the Yaojia Formation increases, the pressure gradient gradually increases until the pressure gradient at the bottom of the Qingshankou Formation and the fourth member of the Quantou Formation suddenly decreases. It is speculated that the abnormal pressure caused by the third peak hydrocarbon expulsion from the source rock of the Qingshankou Formation exceeded the fracturing pressure of the mudstone, so that oil and gas flew into the reservoir above or below the mudstone, leading to the release of overpressure in the mudstone [27]. When the excessive pressure generated by the mature source rock of the Qingshankou Formation was about 10-20 MPa, the displacement pressure in the Fuyu oil reservoir was generally less than 5 MPa. Under the drive of the source-reservoir pressure difference, oil and gas entered the Fuyu oil reservoir from the source rock along the source faults and accumulated in favorable zones (Fig. 7c ).
Fig. 7. Pressure coefficient distribution in Qing 1 Member and excessive pressure evolution of Qingshankou Formation in northern Songliao Basin (modified from Reference [8]; section position in |
3.3. Good match between faults and sand bodies controls dominant transport channels for oil and gas migration
Oil and gas generated from source rocks mainly migrate to reservoirs through transport systems composed of faults, fractures, sand bodies, etc. Different transport systems have different fluid transport capabilities. Oil and gas usually flow through transport systems with high capabilities. Therefore, the quality of the transport system has a controlling effect on oil and gas migration and enrichment [28-29].
During the large-scale generation and expulsion of hydrocarbons from source rocks, active faults can serve as channels for vertical oil and gas transport. The top of the Fuyu oil reservoir has developed multiple stages of faults. Among them, in the extensional stress field dominated by subsidence in the late stage of sedimentation of the Qing 1 Member, the most developed extensional faults are those generated by the extensional activity of some deep and large faults in the basement. The existence of these faults can not only vertically connect the source rocks of the Qingshankou Formation with underlying reservoirs, but also provide lateral migration channels for local source rocks and reservoirs [27]. There are two types of source faults developed in the Fuyu oil reservoir. Type I faults upwards to the Qingshankou Formation and downwards to the Fuyu oil reservoir connect the source rock of the Qingshankou Formation and the Fuyu oil reservoir. This type of faults let oil and gas unidirectionally flow downwards and only into the Fuyu oil reservoir. Type II faults extend upwards to the Gaotaizi reservoirs and above, and downwards to the Fuyu oil reservoir. The faults connect the source rocks with many reservoirs and let oil and gas flow in both directions (Fig. 8 ). Both Type I and Type II faults control the reservoirs, but Type I faults that match well with the source overpressure are more conducive to oil enrichment in the Fuyu oil reservoir.
Fig. 8. Fault types and hydrocarbon transport in northern Songliao Basin (see the section position in |
The dense fault zone is a series of fault polygons in similar or identical directions. They are connected and concentrated in obvious strips to some extent [30]. Influenced by multiple tectonic movements, the northern part of the Songliao Basin has developed many faults. Many T2 dense fault zones in the source rocks of the Qingshankou Formation penetrate the Fuyu oil reservoir. During the enrichment process of tight oil in the Fuyu oil reservoir, these dense faults were not only high-speed channels letting oil and gas backflow, but also blocked oil and gas that migrated vertically along the faults from escaping. Exploration and development practices have shown that the production and abundance of oil layers sandwiched by dense fault zones are relatively high. Oil abundance in the dense fault zones is poor, and the success rate of exploration wells is 25%. The wide and gentle zones between these dense fault zones are favorable zones for oil and gas enrichment, and the success rate of exploration wells is as high as 58%.
Shallow-river-controlled delta fronts and delta plain sand bodies were developed in the Fuyu oil reservoir. They are mostly shaped in lens or thin layers with poor continuity, and superimposed vertically and merged laterally. The numerous strip-like sand bodies in the third and fourth Members of the Quantou Formation provide effective channels for lateral migration of oil and gas.
Faults and sand bodies together make up a tight oil transport system in the Fuyu oil reservoir. Under the pressure difference between the source and the reservoir, oil and gas generated by the source rock can migrate downward along the faults or laterally through the sand bodies to favorable zones. The transport system determines the migrating direction and spatial distribution of oil and gas.
3.4. High paleogeomorphology and local positive structures are conducive to tight oil accumulation
Based on the exploration practice of tight oil in the Fuyu oil reservoir, it is believed that the influence of structures on tight oil in the Fuyu oil reservoir is mainly reflected in two aspects: The Paleogeomorphology during the sedimentary period of the Fuyu oil reservoir controlled the distribution of sand bodies, thereby controlling the pattern of oil and gas enrichment. The positive structures and fault-horst zones induced by the tectonic activities in the basin were favorable for oil and gas migration and accumulation.
The Paleogeomorphology during the sedimentary period of the Fuyu oil reservoir controlled the distribution of sand bodies in the Fuyu oil reservoir, thereby controlling the enrichment of oil and gas. The influence of Paleogeomorphology on the sedimentation of the Fuyu oil reservoir is summarized as valley-controlled source and slope-controlled sand bodies. In other words, valleys controlled river flow, slopes controlled the distribution of sand bodies. Valleys on the basin edge are crisscrossed, and sags are clear. Sediments migrated along the valleys, the saddles of low uplifts, or the slope and transition zones between faults, and finally settled in the sag centers. From the source area to the sedimentary area, river sand moved a long distance, and became continuously distributed. The sand bodies evenly distributed provide favorable space for the enrichment of oil and gas in the Fuyu oil reservoir below the source rock.
Research on the evolution history of basin structures indicates that the tectonic movements during deposition periods of the Qingshankou and Mingshui formations had a significant impact on the enrichment of oil and gas in the Fuyu oil reservoir. During the Qingshankou period, strong faulting occurred and a large number of faults were produced at the T2 interface. These faults effectively matched with the overpressure of the Qing 1 Member, and oil and gas generated by the source rock were injected along the faults towards the lower Fuyu oil reservoir under the drive of overpressure [31]. In the late sedimentation stage of the Mingshui Formation, due to strong compression and uplift, a series of reverse anticlines were formed. The faults, nose shaped structures, and reverse anticlines well matched with peak hydrocarbon expulsion, and provided superior structural conditions for large-scale oil and gas migration and enrichment. Previous exploration of tight oil in the Fuyu oil reservoir was mostly concentrated in the Daqing Placanticline, the Sanzhao Sag, the Longhupao area, etc., and industrial oil flows were obtained in anticline zones, nose shaped structural zones, and some uplifts inside local sags (Fig. 9 ). Exploration practice has confirmed that positive structures have a controlling effect on the enrichment of tight oil in the Fuyu oil reservoir. With the continuous deepening of research on tight oil in the Fuyu oil reservoir, the exploration target has gradually shifted to the Qijia-Gulong sag that’s less explored. The dense fault zones in this area are almost fault terraces. Wide and gentle zones between the dense fault zones are favorable for oil and gas enrichment. With lateral connection of source with reservoir, and good enrichment conditions, it is believed that breakthrough to tight oil exploration can be made in the fault horsts in the syncline area.
Fig. 9. Overlay of the paleo-structure during the end of deposition period of Mingshui Formation in the Fuyu oil reservoir and productivity columns of exploration wells in northern Songliao Basin (the height of the column reprensents well productivity). |
4. Enrichment model and differential enrichment law
4.1. Tight oil enrichment model
The tight oil in the Fuyu oil reservoir in the northern part of the Songliao Basin is tight oil from the play featured by upper source and lower reservoir. Currently, large tight oil reserves have been discovered in the Longhupao-Da'an terrace, the Daqing Placanticline, the Sanzhao Sag, and the Changchun Ridge anticline belt, and conventional oil reservoirs on a certain scale have been found in some structural highs such as in the Qijia-Gulong Sag, the northern Daqing Placanticline, and the Sanzhao Sag [8] (Fig. 10 ).
Fig. 10. Distribution of petroleum initially-in-place in the Fuyu oil reservoir in northern Songliao Basin (modified from Reference [8]). |
Tight oil enrichment in the Fuyu oil reservoir is controlled by multiple factors such as oil source, structure, fault, sand body, overpressure, and reservoir. Oil and gas from the overlying Qing 1 source rock is the main source of tight oil and gas in the Fuyu oil reservoir. The source- reservoir pressure difference is the main driving force, and the three-dimensional transport system composed of faults and sand bodies is an important pathway for oil and gas migration. Positive structures and fault horsts are favorable places for oil and gas enrichment. Based on factors such as source-reservoir relationship, enrichment dynamics, and transport system, three below-source tight oil enrichment models were established (Fig. 11 ).
Fig. 11. Below-source tight oil enrichment models in northern Songliao Basin (see the section position in |
Source-reservoir connection and vertical or lateral hydrocarbon migration model. The source rock supplies hydrocarbons directly and vertically, and the sand body is adjacent to the overlying source rock of the Qing 1 Member. The source rock is in close contact with the reservoir over a large area, without the need for fault communication. The oil generated by the source rock breaks through the source-reservoir interface under the drive of overpressure, and the reservoir can directly capture oil and gas expulsed from the source rock in favorable areas. The source rock supplies hydrocarbons directly and laterally, and the sand body is close to the source rock. Under the faulting effect, the sand body comes into direct contact with the Qing 1 source rock. Under the drive of overpressure, oil and gas migrate laterally into the reservoir, and then accumulate there under the jointing effect of source-reservoir pressure difference and buoyancy.
Source-reservoir separation and fault transport model. The reservoir does not directly contact with the source rock, and oil and gas migrate through faults into the reservoir. Forced by overpressure, oil and gas generated by the source rock overcomes the blocking force of the faults, the capillary force in the reservoir, formation pressure difference, and the buoyancy of the oil itself, and flow downwards through the source faults into the target sand body.
Source-reservoir separation and faults-sandbodies match model. The reservoir does not have direct contact with the source rock. After oil and gas enter the Fuyu reservoir through the faults, it is difficult to preserve them in the sand bodies near the faults, so they need to migrate laterally a certain distance before enrichment in favorable areas. By the driving force of overpressure, oil and gas generated by the source rock transport downwards along the faults into the Fuyu reservoir, and then accumulate in structural or lithological traps at structural highs after lateral migration by the driving force of buoyancy.
4.2. Differential enrichment laws and exploration directions
The source rock in the Qing 1 Member in the Daqing placanticline is shallow and less mature, and its hydrocarbon generation ability is poor. The oil in the Fuyu reservoir mainly came from the mature source rocks in the Qijia-Gulong Sag and the Qing 1 Member in the Sanzhao Sag. The Qijia-Gulong Sag, the Sanzhao Sag and the Daqing Placanticline constitute a structural pattern with two sags sandwiching an uplift. Therefore, oil generated by the source rocks of the Qing 1 enters the two sags, and then through effective faults and sand bodies, some oil accumulates in the structural highs of the Daqing Placanticline. Four types of sand bodies were developed in the Daqing Placanticline: meandering rivers, braided rivers, crevasse fans, and sheet-like sand. These sand bodies are overlapped with each other, and the overall connectivity is good. The porosity is mainly 8% to 15%. The fault density is 0.93 faults/km2. Type I faults, totally 2 155 pieces, are dominant, and account for 90.9%. The number of Type II faults is 217. At present, the proven and probable petroleum initially-in-place are mainly distributed in the southern positive structural belt and nose shaped structures (i.e. the Xingxibi, Gaoxibi, Puxibi, Xinzhaobi nose shaped structures) in the Daqing Placanticline.
In the Sanzhao Sag, the mature source rocks of the Qing 1 Member cover 3 550 km2, the mudstone is 40-80 m thick, and the Ro value is 0.8%-1.1%. The source rock has good quality and can generate up to 124×108 t of hydrocarbons. It is the main source of tight oil and gas for the Fuyu oil reservoir in the Sanzhao Sag. The pressure coefficient of the source rock in the Qing 1 Member ranges from 1.1 to 1.4, providing an injection power for oil and gas to flow back into the Fuyu oil reservoir. The fault density is 0.81 faults/km2, and both Type I and Type II faults were developed. There are 3 585 Type I faults, accounting for 82.1%, and 784 Type II faults. Four types of sand bodies were developed in the Sanzhao Sag: meandering channels, distributary channels, underwater distributary channels, and crevasse fans. The sand bodies are influenced by the sediments from north, northeast and east, and far away from the sources. The slopes are small, the sand bodies are thin and poorly continuous, and the porosity is mainly 7% to 15%. These thin sand bodies are overlapped vertically to form favorable reservoirs. At present, the proven and probable petroleum initially-in-place are mainly distributed in the positive structural zone and surrounding nose shaped structures (i.e. the Yushulinbi, Shengpingbi, Songfangtunbi, Mofangtunbi, and Toutaibi nose shaped structures).
The mature source rocks of the Qing 1 Member in the Qijia-Gulong Sag cover 4 510 km2, the mudstone is 60-100 m thick and the Ro value ranges from 1.2% to 1.7%. The source rocks have good quality and high maturity, and better hydrocarbon generation conditions than those in other areas. They can generate hydrocarbon of 266×108 t. Oil and gas in the Fuyu reservoir mainly come from the overlying source rocks. The pressure coefficient of the Qing 1 Member is 1.2-1.6, and the injection power is large. The conditions for oil and gas backflow and enrichment are superior. The fault density is 0.42 faults/km2, including 971 Type I faults accounting for 91.8%, and 87 Type II faults. Western and near sediments and steep terrains make it easy to form short-axis deltas where the sand bodies are thick and continuous, and the porosity is 7% to 12%. It’s found that the wide and gentle zone between the fault zones is favorable for oil and gas enrichment. The horst zone serves as a channel for oil and gas migration, and the lateral connection between the horst zone and the source rock is conducive to oil and gas enrichment. The Fuyu oil reservoir in the Qijia-Gulong Sag was less explored. Wells drilled into the upper Sartu, Putaohua, and Gaotaizi formations found good oil show, indicating a high remaining resource potential. Breakthrough can be made in the fault-horst zone in the syncline area.
The exploration of tight oil in the Fuyu oil reservoir is mostly concentrated in the Daqing Placanticline and the Sanzhao Sag, but the Qijia-Gulong Sag is less explored although it has good tight oil enrichment conditions and exploration potential. (1) Superior hydrocarbon supply: The overlying Qing 1 Member has a large area of source rock with high maturity and hydrocarbon generation ability. (2) Sufficient charging power: Driven by the pressure caused by hydrocarbon generation and 1.2-1.6 pressure coefficient, oil and gas migrate downward and accumulate. (3) Good transport conditions: Type I faults in the Qijia-Gulong Sag are more conducive to oil and gas migration (Fig. 12 ). In comparison, both Type I and Type II faults are developed in the Sanzhao Sag. (4) Good reservoir conditions: The sand bodies are near sources, superimposed and merged, the porosity is 7% to 12%, and almost all sand bodies bear oil. (5) Strong oil fluidity: The oil is light oil, the density is lower than 0.82 g/cm3 and the viscosity is lower than 20 mPa·s. The oil properties are better than those in the Daqing Placanticline and the Sanzhao Sag (Fig. 13 ). It is believed that the Qijia-Gulong Sag is an important target for future exploration and increasing tight oil reserves in Daqing Oilfield.
Fig. 12. Fault distribution in the Fuyu oil reservoir in northern Songliao Basin. |
Fig. 13. Relationship of crude oil density, viscosity, and asphaltene and gum content in Fuyu oil reservoir. |
5. Conclusions
The tight oil in the Fuyu oil reservoir is characterized by upper generation and lower storage. The upper mature Qing 1 source rocks provide sufficient oil and gas. The positive tectonics and nose structures are good place for oil and gas accumulation. Although the sand bodies are poorly continuous, these sand bodies of various facies are superimposed and merged into a large scale, providing good conditions for oil and gas enrichment. The reservoir is mainly fine sandstone and siltstone, and the rock types are mainly lithic-feldspar sandstone and feldspar-lithic sandstone. The reservoir is relatively tight with porosity of 5% to 12%, permeability of (0.01-1.00)×10-3 μm2, micro-pores as space and nano-throats as flow channels.
The controlling factors for the enrichment of tight oil in the Fuyu oil reservoir were analyzed by studying the source-reservoir matching relationship, transport system, enrichment dynamics and structural background. Effective source rocks macroscopically control the distribution of oil and gas. Reservoir physical properties control the degree of oil and gas enrichment. The matching relationship between effective source rocks and high-quality reservoirs jointly controls the distribution style of oil and gas. The advantageous channels composed of faults and sand bodies control the migration and enrichment of oil and gas. The source-reservoir pressure difference is the primary driving force for the enrichment of oil and gas. The positive structures caused by basin structural activities are favorable place for oil and gas migration and accumulation. Breakthrough can be made to tight oil exploration in the fault horst zone in the syncline area.
Based on factors such as source-reservoir relationship, enrichment dynamics, and transport system, three enrichment models of tight oil in the Fuyu oil reservoir have been established: (1) Source-reservoir connection and vertical or lateral hydrocarbon migration model. Source and reservoir are adjacent, overpressure is the driving force, oil and gas vertically backflow or lateral migrate. (2) Source-reservoir separation and fault transport model. Source and reservoir are separate, overpressure is the driving force, and oil and gas migrate downwards through faults to reservoirs. (3) Source-reservoir separation and good faults and sand bodies matching model. Source and reservoir are separate, overpressure is the driving force, faults and sand bodies are transport channels, and oil and gas laterally migrate into reservoir and accumulate.
The differences in oil source, fault distribution, driving force, and physical properties of sand bodies and reservoir resulted in the differential enrichment of tight oil in the Fuyu oil reservoir. Previous tight oil exploration in the Fuyu oil reservoir more focused on the Daqing Placanticline and the Sanzhao Sag, but less on the Qijia-Gulong Sag. This study found that the Fuyu oil reservoir in the Qijia-Gulong Sag has superior hydrocarbon supply, sufficient driving power, good transport channels, continuously distributed sand bodies, strong oil fluidity and favorable enrichment conditions, so suggests that the Qijia-Gulong Sag is an important target for future exploration of tight oil in the Fuyu oil reservoir.
Nomenclature
HI—hydrogen index, mg/g;
Ro—reflectivity of vitrinite, %;
S1—free hydrocarbon content, mg/g;
S2—pyrolysis hydrocarbon content, mg/g;
TOC—total organic carbon content, %.