Introduction
In 1859, the United States successfully drilled the world's first industrial oil well, marking the onset of the conventional petroleum industry [1]. Since then, the development of hydrocarbon accumulation theory has experienced 4 stages: marine source and anticline accumulation theory (from the mid-19th century to the early 20th century); negative structure exploration and dynamic accumulation theory (from the early 20th century to the mid-20th century); continental source and trap accumulation theory (from the mid-20th century to the early 21st century); and inner-source exploration and in-situ accumulation theory (from the early 21st century to present). Thus, the petroleum geology theory is improving to guide the petroleum exploration. In the past two years, some scholars became interested in the overall evaluation on all types of petroleum reservoirs in super basins, expecting the synchronous exploration and coordinated development of conventional and unconventional reservoirs [2]. More and more studies have shown that many large petroleum basins in the world feature the orderly coexistence of conventional and unconventional reservoirs. Some scholars proposed the concept of “whole petroleum system”, which is a natural system composed of effective source rock and its associated conventional and unconventional petroleum resources, as well as the necessary geological elements and evolution process in the petroleum basin [3⇓-5]. At present, the whole petroleum system has become a hot spot in petroleum exploration and development, which highlights the comprehensive evaluation of accumulation elements, accumulation mechanisms, and accumulation models of all types of reservoirs in the basin [6]. However, the research on the reservoir characteristics and distribution patterns of the whole petroleum system is still in the exploration stage. Therefore, the research on the whole petroleum system of the super basin is meaningful for the evolution of major petroleum exploration and development theories, and also for the definition of exploration and development measures depending upon petroleum types. It is of great significance to guide the exploration and development of all types of petroleum together in super basins.
A super basin refers to a petroleum basin which has two or more sets of source rocks, multiple sets of reservoirs, and the petroleum of over 6.85×108 t of oil equivalent (toe) produced and 6.85×108 toe or more to be produced [7]. The Songliao Basin in China is one of the 25 super basins in the world, with multiple oil-bearing series, various types of petroleum reservoirs, and abundant resources. The total quantity of oil resources in the north of Songliao Basin is 253.69×108 t (including 76.22×108 t conventional oil, 21.85×108 t tight oil, 151.36×108 t shale-type shale oil, and 4.26×108 t interlayer-type shale oil), with 71.20×108 t of reserves proved and 182.49×108 t of reserves to be proved. The northern Songliao Basin is a binary resource system: oil dominated in the shallow-medium strata (Lower Cretaceous Quantou Formation-Upper Cretaceous Mingshui Formation), and gas dominated in the deep strata (Lower Cretaceous Huoshiling Formation-Lower Cretaceous Denglouku Formation). The fourth resource evaluation of PetroChina shows that the northern part of the Songliao Basin has a large quantity of oil resources with a huge exploration potential, and the oil-dominated shallow-medium strata are still an important research target in the Daqing Oilfield for increased reserves and production. However, with the continuous exploration in this region, the remaining resources in the shallow-medium strata are decreasing in quantity, and increasingly difficult to explore. It is urgent to change the exploration strategies and innovate the geological exploration theories for tapping the potential of oil resources in shallow-medium strata as most as possible to support the sustainability of the oilfield. Based on previous studies, from the perspective of the whole petroleum system in super basin, this paper presents a systematic analysis on the formation, distribution and accumulation of hydrocarbons in the shallow-medium strata in northern Songliao Basin, hopefully to get an overall understanding of conventional oil, tight oil and shale oil reservoirs in the whole petroleum system in the shallow-medium strata in northern Songliao Basin. The study results can guide the multi- horizon three-dimensional exploration and coordinated development of conventional and unconventional petroleum resources in the Songliao Basin, and provide references for petroleum exploration and development in other super basins.
1. Geological setting
The Songliao Basin is the largest Mesozoic-Cenozoic continental petroleum basin with a dual-structure (lower fault and upper depression) in Northeast China, covering an area of 26×104 km2. The basin includes six primary structural units: central depression area, western slope area, northeastern uplift area, southeastern uplift area, northern dip area, and southwestern uplift area (Fig. 1a ). During the formation and evolution, the Songliao Basin experienced 4 stages: thermal uplifting and rifting; rifting; depression; and shrinking and uplifting. The basement is Paleozoic and Pre-Paleozoic metamorphic rock, igneous rock and other rocks. Upwards, there are strata of faulting period (Lower Cretaceous Huoshiling Formation, Shahezi Formation and Yingcheng Formation), faulting-depression transition period (Lower Cretaceous Denglouku Formation), depression period (Lower Cretaceous Quantou Formation, and Upper Cretaceous Qingshankou Formation, Yaojia Formation, Nenjiang Formation, Sifangtai Formation and Mingshui Formation), and inversion period (Paleogene and Neogene) (Fig. 1b ). The sedimentary cover is mainly composed of Mesozoic and Cenozoic clastic rocks [8⇓-10].
Fig. 1. (a) Structural unit division and (b) comprehensive stratigraphic column of Songliao Basin. |
The northern part of the Songliao Basin, with an area of 11.95×104 km2, spans five primary structural units including the central depression area, western slope area, northern dip area, northeastern uplift area, and southeastern uplift area [11]. The shallow-medium strata in northern Songliao Basin include the Lower Cretaceous Quantou Formation to the Upper Cretaceous Mingshui Formation, which were developed in the depression period [12]. During the deposition of the third and fourth members of the Quantou Formation (Quan 3 and Quan 4 members), the second and third members of the Qingshankou Formation (Qing 2 and Qing 3 members), and the Yaojia Formation, the water body changed frequently, forming multiple sets of river-delta sedimentary sand bodies, which are the main reservoirs in the shallow-medium strata [13]. The exploration practices demonstrate that the shallow-medium strata in northern Songliao Basin include two sets of main oil source rocks (Qingshankou Formation and Nenjiang Formation) and seven major oil reservoirs (Heidimiao, Saertu, Putaohua, Gaotaizi, Gulong, Fuyu and Yangdachengzi).
2. Formation conditions of the whole petroleum system
2.1. High-quality source rocks provided sufficient petroleum sources
Two sets of source rocks, Qingshankou Formation and Nenjiang Formation, are developed in the Late Cretaceous of the Songliao Basin. The Nenjiang Formation source rocks have a low organic matter maturity to provide a limited oil generation capacity, so the shallow-medium petroleum mainly came from the Qingshankou Formation mature source rocks.
During the deposition of Qingshankou Formation, the first large-scale lake transgression event occurred in the Songliao Basin, forming a set of high-quality source rocks deposited in semi-deep to deep lake. The Qingshankou Formation source rocks are dominated by the dark shales in the first member of Qingshankou Formation (Qing 1 Member) and the middle and lower parts of the second member of Qingshankou Formation (Qing 2 Member).
The main hydrocarbon generating material is the layered algae, and the organic matter type is mainly I-II1. The Ro value ranges from 0.7% to 1.3%, indicative of the mature to high mature stage with a great hydrocarbon generation potential. Specifically, the Qing 1 shale covers an area of 4.2×104 km2, with a thickness of 30-100 m, an average TOC of 2.36%, and an average hydrogen index (HI) of 750 mg/g. The Qing 2 shale covers an area of 2.8×104 km2, with a thickness of 60-230 m [14], an average TOC of 1.8% in the middle and lower parts, and an average HI of 630 mg/g.
The Qingshankou Formation source rocks are thick and wide, with high organic matter abundance and good organic matter type, and they are well preserved. The hydrocarbons generated from the mature source rocks, except the part retained in the formation itself, migrated upward to form the Heidimiao, Saertu, Putaohua and Gaotaizi oil reservoirs, and downward to form the Fuyu, Yangdachengzi and other oil reservoirs, providing sufficient petroleum sources for the shallow-medium strata in northern Songliao Basin.
2.2. Multiple types of reservoirs provided varied storage spaces
A good storage space is one of the key elements for petroleum accumulation. The northern Songliao Basin witnesses an alternation and supervision overlapping of multiple orders of sedimentary cycles, multiple sets of sedimentary facies belts, and multiple types of sand bodies vertically. The reservoir rocks are mainly sandstone, followed by argillaceous rocks, dolomitic rocks, etc.
The Heidimiao, Saertu and Putaohua oil reservoirs are conventional sandstone reservoirs, which are dominated by delta plain, delta front and shore-shallow lake facies, and composed of fine sandstone and siltstone. The reservoir properties are good, with the porosity of 11%-28% (avg. 15.6%).
The Gaotaizi oil reservoir mainly contains tight oil and interlayer shale oil, as well as conventional oil locally. It is dominated by delta front sand body, and composed of fine sandstone, siltstone, argillaceous siltstone, and mudstone. The reservoir is relatively tight, with the porosity of 4.8%-14.0% (avg. 9.2%) [15].
The Gulong oil reservoir mainly contains shale type shale oil. It is dominated by lacustrine to deep lacustrine sand bodies, and composed of shale, mudstone, siltstone, mesolithic limestone, and dolomite. Bedding fractures are well developed; shale and siltstone interlayers are important carriers of shale oil [16]. The shale reservoir is tight, with the porosity of 3%-8% (avg. 5.1%).
The Fuyu and Yangdachengzi oil reservoirs contain tight oil mainly, and conventional oil at some high positions. The reservoir bodies are mainly distributary channels, crevasse splays and sheet sands of large river-shallow water delta sedimentary system, and lithologically composed of medium sandstone, fine sandstone, siltstone and mudstone. The physical properties are poor, with the porosity of 2.1%-14.6% (avg. 10.8%) [17].
2.3. Alternating presence of small faults near the source and large faults far from the source was beneficial for the preservation of various petroleum resources
The scale and quantity of vertical faults have a controlling effect on petroleum accumulation. The alternating faults of different scales in the shallow-medium strata in northern Songliao Basin are conducive to the preservation of various petroleum resources. The history of fault development shows that the faults are well developed in the shallow-medium strata in northern Songliao Basin, especially in the T2 reflection layer where there are a large number of faults up to 20 000, with a high density (up to 2.7 faults/km2), and small throw and extension (1.3 km averagely). The faults cutting the T2 reflection layer were mainly formed in the tectonic movement at the end of the deposition period of Qing 1 Member, and partially break up to the T1 reflection layer, providing good pathways for the upward migration of petroleum from the Qingshankou Formation source rocks to the reservoirs (Fig. 2 ).
Fig. 2. Shallow-medium seismic section of northern Songliao Basin (see the section position in |
In the shallow-medium strata of northern Songliao Basin, small faults near the source and large faults far from the source are alternatively developed around the Qingshankou Formation source rocks. The large faults far from the source and sand bodies together form the upward and downward petroleum transport system. For the Fuyu and Yangdachengzi oil reservoirs below the source, under the pressure induced by hydrocarbon generation of the source rocks, together with the fault system, the petroleum migrated downward along the faults. For the Heidimiao, Sartu, and Putaohua oil reservoirs above the source, the petroleum mainly migrated upward through the vertical faults under the action of buoyancy, and accumulated in reservoirs at effective traps. The small faults near the source that did not break through the Qingshankou Formation inside the source were conducive to the preservation of shale oil. Such faults did not extend out of the source, allowing the petroleum to store inside the source. Moreover, the existence of these faults is supportive to the shale oil reservoir fracturing.
3. Petroleum distribution in the whole petroleum system
3.1. Correlation and differences between various reservoirs
3.1.1. Orderliness and differences of sedimentary systems
The shallow-medium strata of the Songliao Basin, formed in the depression period, are far from the provenance and mainly composed of fine-grained sediments of river, delta, and lake facies. On the plane, complete facies, including rivers, delta plains, delta front, shore shallow lakes, semi-deep lakes, and deep lakes, are orderly distributed from the edge of the basin to the center of the depression. Conventional oil, tight oil and shale oil reservoirs coexist orderly.
Taking the Qingshankou Formation as an example, the sedimentary system shows delta plain, delta front, semi-deep lake and deep lake from the edge of the basin to the center of the depression. The facies belts are different in types of petroleum. The delta plain sand bodies mainly contain structural reservoirs and structural-lithologic reservoirs. The delta front sand bodies, which become tighter towards the center of the depression, are full of tight oil. The delta front sheet-like sand bodies, sandwiched in lacustrine shales, contain interlayer shale oil. The center of the depression is dominated by thick shales of semi-deep to deep lake facies, which contain shale type shale oil. Obviously, the correlation between the spatial distributions of petroleum within the whole petroleum system is to some extent controlled by the orderly distribution of sedimentary systems (Fig. 3 ).
Fig. 3. Reservoir section of Qingshankou Formation in Yingtai-Gulong area, northern Songliao Basin (k2qn1—first member of Upper Cretaceous Qingshankou Formation, k2qn2—second member of Upper Cretaceous Qingshankou Formation, k2qn3—third member of Upper Cretaceous Qingshankou Formation; see the section position in |
3.1.2. Orderliness and differences of lithology and physical properties
The reservoir lithology and physical properties are different depending upon petroleum types, but present an orderly variation for conventional oil, tight oil and shale oil (Fig. 4 ). For conventional oil, the reservoir properties are good, and buoyancy plays a dominant role in petroleum migration and accumulation. For tight oil, the reservoir is relatively tight, with low porosity and permeability; buoyancy is no longer dominant, but the overpressure generated by hydrocarbon generation of source rock is the main charging power. For shale oil, which is often deeply endowed in the center of the depression, the reservoir is tight, with low porosity and low permeability, and the petroleum can hardly overcome the resistance but accumulate in situ.
Fig. 4. Scale of storage space by reservoirs in shallow- medium strata in northern Songliao Basin. |
In northern Songliao Basin, the conventional oil reservoirs are dominated by medium sandstone, fine sandstone and siltstone, with good physical properties (porosity greater than 12%, permeability greater than 1×10−3 μm2, and pore throat diameter greater than 1 μm). The tight oil reservoirs are mainly composed of fine sandstone, siltstone and argillaceous siltstone, with the porosity of 5%-12%, the permeability of (0.01-1.00)×10−3 μm2, and the pore throat diameter of 0.05-1.00 μm. The shale oil reservoirs mainly consist of shale, thin siltstone and dolomite, with poor physical properties (porosity less than 8%, permeability less than 0.5×10−3 μm2, and pore throat diameter less than 50 nm). Clearly, conventional oil reservoirs, tight oil reservoirs and shale oil reservoirs exhibit large differences and obvious orderly changes in porosity, permeability and pore throat diameter.
3.2. Orderly distribution of petroleum resources
3.2.1. Longitudinal distribution
The whole petroleum system in the shallow-medium strata in northern Songliao Basin has seven sets of main oil reservoirs, namely Heidimiao, Saertu, Putaohua, Gaotaizi, Gulong, Fuyu and Yangdachengzi, indicating an orderly coexistence of conventional oil, tight oil and shale oil. Vertically, radiating from the Qingshankou Formation source rocks, there are conventional oil reservoirs, shale oil reservoirs and tight oil reservoirs, from top to bottom. According to the source-reservoir relationship, the whole petroleum system can be further divided into conventional oil reservoirs above the source (Heidimiao, Saertu, and Putaohua), interlayer shale oil reservoir (Gaotaizi) and shale type shale oil reservoir (Gulong) inside the source, and tight oil reservoirs below the source (Fuyu and Yangdachengzi) (Fig. 5 ).
Fig. 5. Longitudinal distribution of conventional and unconventional petroleum in shallow-medium strata in northern Songliao Basin. k2n2—second member of Upper Cretaceous Nenjiang Formation, k2n3—third member of Upper Cretaceous Nenjiang Formation, k2n4—fourth member of Upper Cretaceous Nenjiang Formation, k2y1—first member of Upper Cretaceous Yaojia Formation, k2y2—second member of Upper Cretaceous Yaojia Formation, k2q1-3—1st-3nd members of Lower Cretaceous Quantou Formation, k2q4—fourth member of Lower Cretaceous Quantou Formation. |
The Heidimiao, Saertu and Putaohua oil reservoirs above the source are overlain by the Qingshankou Formation source rocks. The oil-source correlation suggests that the petroleum in the Heidimiao oil reservoir came from two sets of source rocks (Qingshankou Formation and Nenjiang Formation), and the petroleum in the Saertu and Putaohua oil reservoirs came from the Qingshankou Formation source rocks. Generally, the reservoirs are relatively shallow, and have good physical properties. They are communicated via faults with the source rocks below to form a conventional petroleum system. The faults match the sand bodies to control the distribution of petroleum, and the reservoirs are mostly structural, structural-lithologic, and lithologic reservoirs.
The Qingshankou Formation inside the source includes three members. The Qing 1 Member develops very thick dark mudstone as a result of massive lake transgression, which is a good source rock in the basin. During the deposition of the Qing 2 and Qing 3 members, the lake basin shrank gradually, with a number of delta complexes developed at the edge of the basin, which extended into the source rock area, forming near-source accumulation. Controlled by the sedimentary facies belts, the Qingshankou Formation exhibits a coexistence of multiple types of petroleum from bottom to top: shale type shale oil in Qing 1 and lower Qing 2; interlayer shale oil and tight oil in upper Qing 2 and Qing 3.
The Fuyu and Yangdachengzi oil reservoirs are communicated via widely distributed faults in vertical direction with the Qing 1 source rock below to form a petroleum system. The reservoir is deeply buried, with poor physical properties, and mainly contains tight oil. Conventional reservoirs are developed at high positions locally near the source.
3.2.2. Planar distribution
On the plane, conventional oil, tight oil and shale oil coexist orderly but enrich differently. Furthermore, conventional sandstone oil reservoir, tight oil reservoir, interlayer shale oil reservoir and shale type shale oil reservoir coexist in an orderly manner from the edge of the basin to the center of the depression.
During the deposition of the Heidimiao, Sartu, and Putaohua oil reservoirs above the source, the sedimentary systems in the north and west corresponded to different control coverage. The Putaohua oil reservoir has the widest distribution of sand bodies, which are universally observed across northern Songliao Basin, with larger thickness in the west and north than in the east and south. The hydrocarbon accumulation is significantly controlled by sedimentary facies. Taking the Putaohua oil reservoir as an example, distributary channels, natural levee, and crevasse-splay are developed in delta plain, with the sand- to-formation ratio higher than 50%, and the dominance of structural oil reservoirs; underwater distributary channels and mouth bars are developed in the delta inner front, with the sand-to-formation ratio of 20%-50%, and the dominance of structural-lithologic oil reservoirs; thin sheet sand is distributed continuously in the delta outer front, with the sand-to-formation ratio less than 20%, and the dominance of lithological oil reservoirs. The structural oil reservoirs, structural-lithologic oil reservoirs, and lithologic oil reservoirs are arranged in a semicircular pattern from the north to south of the basin (Fig. 6 ).
Fig. 6. Above-source petroleum distribution in shallow- medium strata in northern Songliao Basin. |
During the deposition of the Qingshankou Formation inside the source, under the control of sedimentary systems in the north and west, from the edge to the center of the lake basin, the facies transited from delta plain, inner front and outer front to shore shallow lake, semi-deep lake and deep lake, corresponding to the semicircular distribution of sand bodies. The distribution of petroleum is controlled by both sedimentary facies and reservoir properties. In the delta plain, the reservoir porosity is greater than 12%, and conventional oil reservoirs are dominant. In the delta inner front, tight oil reservoirs are dominant. In the delta outer front and shore shallow lake, interlayer shale oil reservoirs are dominant. In the semi-deep lake and deep lake, shale type shale oil reservoirs are dominant. From the north to the south of the basin, conventional oil, tight oil, interlayer shale oil and shale type shale oil reservoirs are developed in a semicircular pattern (Fig. 7 ).
Fig. 7. In-source petroleum distribution in shallow- medium strata in northern Songliao Basin. |
During the deposition of the Fuyu and Yangdachengzi oil reservoirs under the source, controlled by six major sedimentary systems around the basin, sand bodies appeared across the basin. The distribution of petroleum was mainly controlled by reservoir physical properties. When the porosity is greater than 12%, conventional oil reservoirs are dominant, mainly at structural highs near the source, such as the Chaoyanggou anticline, the nosing structure in the eastern part of Sanzhao depression, the northern part of the placanticline, and the northern part of Qijia. When the porosity is less than 12%, tight oil reservoirs are dominant, mainly in Sanzhao depression, the southern part of the placanticline, the southern Qijia-Gulong depression, and the Longhupao-Da'an terrace (Fig. 8 ).
Fig. 8. Below-source petroleum distribution in shallow- medium strata in northern Songliao Basin. |
4. Petroleum accumulation and distribution models in the whole petroleum system
Based on the source-reservoir relationship, sedimentary system, reservoir lithology, and migration/accumulation dynamics, three models of petroleum accumulation are recognized in the whole petroleum system in the shallow-medium strata in northern Songliao Basin. The models include: conventional oil accumulation by buoyancy-driven charging above the source, shale oil accumulation by retention inside the source, and tight oil accumulation by source-reservoir pressure difference- driven charging below the source [18] (Table 1 ).
Table 1. Characteristics of conventional oil, tight oil and shale oil reservoirs in the whole petroleum system in the shallow- medium strata, northern Songliao Basin |
| Reservoir type | Structural units | Sedimentary facies | Lithology | Accumulation model | Accumulation mechanism | Migration characteristics | Driving force | Resistance | Spatial location relative to the source | Fluid features |
|---|---|---|---|---|---|---|---|---|---|---|
| Conventional oil | Structural high, uplift between depressions, nosing structure | Delta plain, delta front, shore-shallow lake, semi- deep lake | Fine sandstone, siltstone, etc. | Source and reservoir separated, accumulation by buoyancy-driven charging | Closure | Secondary migration | Buoyancy | Gravity | Far from the source | Differential oil and gas |
| Tight oil | Marginal slope area of depression, nosing structure, uplift area in depression | River, delta plain, delta front, shore- shallow lake | Fine sandstone, siltstone, argillaceous siltstone, etc. | Source and reservoir in close proximity, accumulation by source- reservoir pressure difference-driven charging | Stayed | Primary migration or short- distance secondary migration | Source- reservoir pressure difference, buoyancy | Capillary resistance | Near the source | No unified oil-water contact, continuous distribution in large area |
| Shale oil | Depressed area | Delta front, semi-deep lake, deep lake | Shale, sandstone interlayer, dolomite interlayer, etc. | Source and reservoir integrated, accumulation by retention | Retention, stayed | No migration or Micro- migration | Pressure induced by hydrocarbon generation | Capillary resistance, viscous force, friction force | Inside the source |
4.1. Conventional oil accumulation by buoyancy-driven charging above the source
Conventional oil reservoirs highlight the separation of source and reservoir, petroleum accumulation far away from the source, and dominance of sandstone and carbonatite. They have good physical properties, and were charged with oil and gas mainly by the buoyancy. Under the control of traps, the reservoirs were formed and distributed in structural units within the basin. They are characterized by multi-layer three-dimensional hydrocarbon accumulation at structural high, stable traps, high porosity, and high internal pressure. In the shallow-medium strata in northern Songliao Basin, conventional oil reservoirs are mainly Heidimiao, Saertu and Putaohua, which constitute an assemblage with the Qingshankou Formation source rocks below. The structural reservoirs, structural-lithologic reservoirs, and lithologic reservoirs are dominant. The structural reservoirs are mainly located in the Daqing placanticline and Longhupao structure. The structural-lithologic reservoirs are mainly found in the nosing structure zone around the Qijia-Gulong depression and the nose-like dip zone within the Sanzhao depression. The lithologic reservoirs are mainly discovered in Qijia-Gulong depression and Sanzhao depression (Fig. 9 ).
Fig. 9. Three-dimensional superposition of reserves in Heidimiao, Saertu and Putaohua reservoirs above the source in shallow-medium strata in northern Songliao Basin. |
Conventional sandstone reservoirs have large pore throats, small throat capillary resistance, and poor self- sealing capacity. Buoyancy is the main driving force for petroleum migration. Faults and sand bodies together serve as the petroleum migration pathways. The petroleum generated from mature source rocks of Qingshankou Formation migrated upward to the Heidimiao, Sartu and Putaohua reservoirs through the dominant pathways, and accumulated in effective traps at structural highs, forming a multi-layer three-dimensional petroleum pool. In the western slope in northern Songliao Basin, for example, there are mainly structural reservoirs, structural-lithologic reservoirs, lithologic-structural reservoirs, lenticular bodies, and lithologic reservoirs. Faults, unconformity surfaces and continuous sand bodies are the main pathways for petroleum migration. Structures, sand bodies, and faults together control the enrichment of petroleum in the slope zone. The configurations between micro-structure and sand bodies, and between small faults and sand bodies, control the formation of petroleum reservoirs [19]. The petroleum generated by the mature source rocks of Qingshankou Formation in the Qijia-Gulong depression migrated upward along the faults at the lower slope under the buoyancy, and then migrated westward along the faults, unconformity surfaces and sand bodies; they finally accumulated in the effective traps along the dominant pathways (Fig. 10 ).
Fig. 10. Petroleum accumulation model of shallow-medium strata in western slope of northern Songliao Basin (see the section position in |
4.2. Shale oil accumulation by retention inside the source
Shale oil reservoirs emphasize the integration of source and reservoir, and in-situ accumulation. The reservoir lithology is generally shale, siltstone, or carbonatite interlayer. The reservoirs have poor physical properties, with low porosity and low permeability. Microfractures and micro- to nano-scale pore throats are the main storage space of shale oil. Shale oil mainly occurs in free or adsorbed state [20-21]. Usually, shale oil is accumulated in-situ in the shale, and partially in siltstone, carbonatite and other interlayers within the reservoir under the pressure induced by hydrocarbon generation. It is continuously distributed in a large scale.
Exploration practices show that shale oil reservoirs, as well as certain tight oil and conventional oil reservoirs, are developed in the Qingshankou Formation inside the source. The in-source shale oil mainly appears in Qing 1 and Qing 2, with shale type shale oil in dominance, and a certain amount of interlayer shale oil. For the in-source petroleum of Qingshankou Formation, conventional oil reservoirs, tight oil reservoirs, and shale oil reservoirs are distributed in a circular pattern, from the edge to the center of the basin. They lie at the edge of the basin, on the slope, and in the center of the basin, respectively (Fig. 11 ).
Fig. 11. Petroleum distribution and reserves superimposition of Qingshankou Formation inside the source in shallow- medium strata in northern Songliao Basin. |
The mature source rocks of Qingshankou Formation began to generate liquid hydrocarbons intensively since the oil window. Once the hydrocarbons reached the saturation at which they were adsorbed by the source rock itself, they began to accumulate in shale pores and microfractures. Shale has a small grain size, a nano-scale pore structure, large BET surface area, and strong molecular adsorption. The presence of inorganic pores (water wet) and organic pores (oil wet) made the shale have the property of mixed wettability in most cases. As the degree of thermal evolution increased, hydrocarbons were generated continuously inside the shale, the oil wet ratio increased, and the capillary resistance decreased. According to the measured data of shales in the Qijia-Gulong area, the pressure caused by hydrocarbon generation of mature source rocks is about 13 MPa, the capillary resistance is 13-25 MPa (Fig. 12 ), the viscous force is about 0.4 MPa, and the friction is about 0.04 MPa[22]. After the hydrocarbon generation induced a pressure to a level of breakthrough pressure, oil and gas were expulsed and migrated into the reservoirs, forming the conventional oil reservoirs and tight oil reservoirs. With the petroleum expulsion, the pressure inside the source rock decreased, and the remaining petroleum was retained and stored in situ in the source rock, forming a large area of shale type shale oil in continuous distribution [23]. The shale oil reservoirs of Qingshankou Formation are vertically developed with multiple sets of siltstone and interlayers rich in calcite, ankerite, shell and other minerals [24]. As the source rock further generated hydrocarbons, the abnormal pressure in the formation rose. Due to pressure induced by hydrocarbon generation, oil migrated by overcoming the resistance in the shale and accumulated in sandstone, dolomite and other interlayers in shales, forming the interlayer shale oil accumulation.
Fig. 12. Relationship between pressure induced by hydrocarbon generation and resistance in shale oil reservoirs in northern Songliao Basin. |
4.3. Tight oil accumulation by source-reservoir pressure difference charging below the source
Tight oil reservoirs feature source and reservoir in close proximity and accumulation near the source. These reservoirs are tight, and composed of fine sandstone, silty fine sandstone, siltstone, and tight carbonatite, with micro-to nano-scale storage space. The physical properties are poor. The hydrocarbons were mainly charged intensively via fractures/pores, due to the source-reservoir pressure difference generated along with the pressure induced by hydrocarbon generation of the source rock. No obvious trap was found. Shale oil is continuously distributed in a large area on the plane, with sweet spots locally [25-26].
The below-source tight oil in the whole petroleum system in the shallow-medium strata in northern Songliao Basin is mainly distributed in Fuyu and Yangdachengzi reservoirs, which form an assemblage with the overlying Qingshankou Formation source rocks. The petroleum is accumulated mainly in gentle nosing structure zones, gentle slope zones, and paleo-uplifts, and locally in uplifts between depressions. Long-term inherited ancient slope zones and ancient nosing structures are the main destinations for lateral petroleum migration and accumulation (Fig. 13 ).
Fig. 13. Petroleum distribution and reserves superimposition in Fuyu and Yangdachengzi reservoirs in shallow-medium strata in northern Songliao Basin. |
The pressure difference between source and reservoir is the main driving force for tight oil accumulation below the source. The three-dimensional transport system composed of faults and sand bodies is the critical support for petroleum migration. The mature source rocks of Qingshankou Formation served as the main source of petroleum. Four elements, i.e. oil source, structure, fault and sand body, were coupled to control the accumulation of tight oil in the Fuyu and Yangdachengzi reservoirs. The simulation of pressure history shows that the source rocks of Qingshankou Formation experienced three hydrocarbon expulsion peaks at the end of Nenjiang Formation sedimentation, the end of Mingshui Formation sedimentation, and the end of Paleogene. Fluid inclusion analysis in previous studies indicates that the end of Nenjiang Formation sedimentation and the end of Mingshui Formation sedimentation are two important periods for oil charging in the Fuyu oil reservoir [27]. In fact, the Fuyu oil reservoir tended to be tight during the deposition of Nenjiang Formation, and became completely tight in the early deposition stage of Mingshui Formation. The source rock had a maximum overpressure of 20 MPa or higher when it was matured at the end of the Mingshui Formation sedimentation, while the average displacement pressure of the Fuyu reservoir was generally less than 5 MPa [17]. Driven by such source- reservoir pressure difference, the petroleum entered the underlying Fuyu and Yangdachengzi reservoirs from the source rocks along the source-connecting faults, and finally accumulated at favorable positions (Fig. 14 ). Three models of below-source tight oil enrichment were established based on the elements such as driving force, transport system and hydrocarbon supply mode. The first model is the vertical or lateral expulsion of petroleum, with source and reservoir connected, that is, the source and reservoir are adjacent, and driven by overpressure, petroleum generated from the source rocks is transported downward vertically or laterally to the reservoir rocks. The second model is fault transport, with source and reservoir separated, that is, the source and reservoir are separated, and driven by overpressure, petroleum generated from the source rocks is migrated downward via faults to the sand bodies and accumulated there. The third model is fault-sand body configuration, with source and reservoir separated, that is, the source and reservoir are separated, and driven by overpressure, petroleum generated from the source rocks is transported laterally via faults to the sand bodies and accumulated there.
Fig. 14. Mechanism of below-source tight oil and gas charging in shallow-medium strata in northern Songliao Basin (see the section position in |
5. Conclusions
The shallow-medium strata in the northern part of the Songliao Basin have good conditions for forming the whole petroleum system. A set of high-quality source rocks formed during the deposition of Qingshankou Formation in semi-deep lake and deep lake provided sufficient petroleum source. Multiple types of reservoirs provided favorable storage space for petroleum accumulation. Faults and sand bodies together formed a petroleum transport system, providing pathways for petroleum migration to reservoirs. The exploration practices show that multiple sets of oil-bearing layers are developed in the shallow-medium strata in northern Songliao Basin, forming a whole petroleum system centered on the source rocks of the Qingshankou Formation.
Different types of reservoirs are significantly different and also correlative in sedimentary systems, physical properties, lithologies and other aspects. The whole petroleum system in the shallow-medium strata in northern Songliao Basin is characterized by orderly coexistence of conventional oil, tight oil and shale oil. Vertically, conventional oil reservoir, tight oil reservoir, shale oil reservoir, and tight oil reservoir are arranged in an orderly pattern, from top to bottom. On plane, conventional oil reservoir, tight oil reservoir, interlayer shale oil reservoir, and shale type shale oil reservoir are orderly developed from the edge of the basin to the center of the depression.
Based on the types of petroleum, source-reservoir relationship, and petroleum accumulation dynamics, three models were established for the whole petroleum system in the shallow-medium strata in northern Songliao Basin. The models include: conventional oil accumulation by buoyancy-driven charging above the source, shale oil accumulation by retention inside the source, and tight oil accumulation by source-reservoir pressure difference- driven charging below the source. Conventional oil above the source was driven by buoyancy and induced by structures to transport through faults vertically and sand bodies laterally to multi-layer reservoir rocks and accumulate there. Shale oil inside the source includes shale type shale oil, which is generated and stored in situ both in the shales, and interlayer shale oil, which is driven by overpressure to transport by a short distance to and accumulate in siltstone, dolomite and other interlayers. Tight oil below the source follows three enrichment models. The first model is the vertical or lateral expulsion of petroleum, with source and reservoir connected, that is, the source and reservoir are adjacent, and driven by overpressure, petroleum generated from the source rocks is transported downward vertically or laterally to the reservoir rocks. The second model is fault transport, with source and reservoir separated, that is, the source and reservoir are separated, and driven by overpressure, petroleum generated from the source rocks is migrated downward via faults to the sand bodies and accumulated there. The third model is fault-sand body configuration, with source and reservoir separated, that is, the source and reservoir are separated, and driven by overpressure, petroleum generated from the source rocks is transported laterally via faults to the sand bodies and accumulated there.