characteristics of tight oil in triassic yanchang formation, ordos basin

RESEARCH PAPER

PETROLEUM EXPLORATION AND DEVELOPMENT Volume 40, Issue 2, April 2013 Online English edition of the Chinese language journal

Cite this article as: PETROL. EXPLOR. DEVELOP., 2013, 40(2): 161–169.

Received date: 22 May 2012; Revised date: 16 Jan. 2013. * Corresponding author. E-mail: [email protected] Foundation item: Supported by the National Science and Technology Major Project "Large Oil and Gas Fields and Coalbed Methane Development" (2011ZX05044; 2011ZX05001-004) Copyright © 2013, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.

Characteristics of tight oil in Triassic Yanchang Formation, Ordos Basin

YAO Jingli1,2,*, DENG Xiuqin1,2, ZHAO Yande1,2, HAN Tianyou1,2, CHU Meijuan1,2, PANG Jinlian1,2 1. Exploration & Development Research Institute of PetroChina Changqing Oilfield Company, Xi'an 710018, China; 2. National Engineering Laboratory for Exploration and Development of Low-Permeability Oil Fields, Xi'an 710018, China

Abstract: By comprehensive study of reservoir and source rock distribution, petrology and geochemistry, the tight oil and its explora-tion potential was analyzed in the Triassic Yanchang Formation, Ordos Basin. The Triassic Yanchang Formation is rich in low permeabil-ity reservoirs. The proved geological reserves of tight oil, with the permeability less than 2×10−3 μm2, is about two billion tons by now. The tight oil mainly occurs in tight sandstone reservoirs of Chang6-Chang8 oil-bearing members which are close to or interbedded with the oil shale layers, without long-distance migration. The large-scale gravity flow sandstone reservoirs of Chang7 and Chang6 oil-bearing members in the center of the lacustrine basin are particularly tight, with the permeability less than 0.3×10−3 μm2 in general. The tight oil in the Yanchang Formation features large scale in sand body complex, tight reservoir, complicated pore throat structure, high content of rigid components, abundant fractures and saturation, good crude property, low fluid pressure and low oil yield. The formation of large-scale superimposed tight oil reservoirs is controlled by the interbeded lithologic combination of extensive source rocks and reser-voirs and the strong hydrocarbon generation and expulsion during geological history. This type of pools is an important potential resource for future oil exploration and development.

Key words: tight oil; Yanchang Formation; Ordos Basin; potential resource

Introduction

In recent years, unconventional oil and gas resources such as tight gas, tight oil and shale oil, have successfully obtained commercial development in the United States, Canada, Aus-tralia and other countries, playing an important role in the global energy structure [1]. Although tight sandstone gas, tight oil, shale oil, and coal bed methane and other unconventional oil and gas resources are abundant in China, their exploration and development is still in the initial stage. The reservoirs in the Triassic Yanchang Formation of the Ordos Basin have a low maturity, strong diagenesis, with fine particles of rock, poor sorting, high cement content, considerable variation in the reservoir space, and great heterogeneity [2]. The reservoirs take on characteristics such as tight reservoirs, being difficult to predict, complex mechanism of accumulation, low produc-tion rate per well [3]. They are typical areas in which low per-meability reservoirs developed in China. In recent years, many new understandings have been obtained through com-prehensive research on favorable sedimentary-diagenetic fa-cies of the clastic rocks, relationship between the reservoir tightening history and hydrocarbon accumulation history, res-

ervoir formation mechanism and the oil accumulation patterns, etc. [4−15] It also effectively guides the exploration and devel-opment of the Mesozoic reservoirs in the Ordos Basin. Con-sequently, large oil fields with reserves of more than a giga-ton-class (109t) have been discovered in Xifeng, Jiyuan, and Hua(chi)-Qing(cheng) [16−19], in which tight oil and gas resources are abundant. Until now, the proved reserves of tight oil with air permeability of less than 2 × 10−3 μm2 are about 2 × 109t. Therefore, there is a great potential for explo-ration and development. The tight oil has now become an important domain for petroleum exploration and development in the Ordos Basin. To strengthen the tight oil study of the Ordos Basin is a great significance for exploration and devel-opment of tight oil resources in China.

1 Concept of tight oil

Tight oil is a kind of unconventional petroleum resource. At present, there is no uniform definition at home or abroad. In recent years, tight oil has been rapidly developing in the United States, especially the Bakken tight oil in the Williston Basin. It mainly developed in the depression of the Basin,

YAO Jingli et al. / Petroleum Exploration and Development, 2013, 40(2): 161–169

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Table 1 Comparison of characteristics of the tight oil in the Ordos Basin and the Bakken Basin [20−27]

Minerals/% Physical

properties* Tight oil reservoirs

Stratum

Deposi-tional

environ-ment

Depth/m

Shale thick-ness/m

Reservoir rock types

QuartzFeld-spar

ClayPoros-ity/%

Perm./10−3 μm2

Formation pressure factor

Kero-gen type

TOC/ %

Ro/ %

Maximum pyrolysistempera-ture/°C

Source- reservoir

relationship

Bakken Formation

Upper De-vonian—

Lower Car-boniferous

Shallow marine

2 593−3 203

2.4−4.9

Silty dolo-mites, dolo-mitic silt-

stones

47.5 18.9 21.5 6.0 0.04 1.2−1.5 I+II 11.30 0.7−1.0

443−447 interbedded

Yanchang Formation

Triassic Lacus-trine

1 000−2 800

20−60 Fine-grained sandstones, siltstones

41.8 23.5 17.4 9.2 0.43 0.64− 0.87

I+II1 13.75 0.9−1.16

440−455interbedded or immedi-

ately adjacent

Note: The physical property of tight oil reservoirs of the Bakken Formation and the Yanchang Formation are subsurface conditions and surface conditions respectively. All statistics are averages.

with a porosity of generally less than 10% and the in-situ permeability of generally between 0.001 and 0.1 × 10−3

μm2 [20−27] (Table 1). LIN Senhu [28] and JING Dongsheng [29] et al. defined the

tight oil as a continuously distributed oil accumulation with self-source-reservoir in the organic-rich and extremely low permeability dark-colored shales, argillaceous siltstones, or sandstone intervals, in an adsorbed or free state. A large amount of stagnant shale-generated oil and gas occur in the nano-scale pores or micro fractures in the form of free hydro-carbon and absorption hydrocarbon, forming tight oil (gas) available for commercial exploitation, with the exception of parts of the shale-generated petroleum being discharged and migrated into the permeable rocks such as sandstones or car-bonate rocks to form conventional oil and gas reservoirs.

In order to meet the needs of the development of unconven-tional oil and gas resources in China, according to the status quo of China's oil and gas fields, it has been clarified that the tight oil usually refers to the sandstone and carbonate reser-voirs with in-situ matrix permeability of less than 0.2 × 10−3

μm2 or air permeability of less than 2 × 10−3 μm2. Generally the single well has no natural production capacity or the natu-ral productivity is lower than the minimum commercial hy-drocarbon flow, but could obtain commercial oil production [30] through techniques like fracturing, horizontal wells, multi- lateral wells etc. under certain economic conditions.

2 Tight oil distribution

The Mesozoic Yanchang Formation in the Ordos Basin is rich in tight oil resources. Proven geological reserves of about 2 giga tons (109 t) has been submitted from the reservoirs with air permeability of less than 2 × 10−3 μm2, of which the proven geological reserves of tight oil reservoirs with air permeability of less than 1 × 10−3 μm2 outnumber one giga tons. The giga- ton-class tight oil reserves in the Xifeng, Jiyuan, Hua-Qing oilfields with average air-permeability of less than 2 × 10−3

μm2 have been discovered. The Yanchang Formation tight oil is distributed in the "core area" of the play: on plan, in the middle of the lake basin, that is, the range delineated by Huanxian - Wuqi-Zhidan-Zhengning-Ningxian-Qingyang

Fig. 1 The sedimentary facies map of Triassic Chang 6 Forma-tion, Ordos Basin.

(Figure 1), as a depocenter in the Mid-Late Triassic; vertically, it’s located in the middle of the Yanchang Formation, that is, in the tight sandstone reservoirs interbedded and associated with or immediately adjacent to the oil shale. The oil is with-out large- scale long-distance migration (Figure 2), mainly distributed in Chang 6 – Chang 8 oil reservoir group. The Chang 8 and Chang 6 reservoirs and the Chang 6 oil reservoir in the northern Hua-Qing oilfield are mainly composed of delta front and prodelta sediments, dominated by fine sandstone, and


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Fig. 2 The section of the Triassic Yanchang tight oil, Ordos Basin (Section location shown in Figure 1)

locally developed mid-fine grained sandstones; in the central lake basin, the Chang 6 and Chang 7 oil reservoirs are mainly gravity-flow deposits, dominated by sandstones and silt-stones [31−33]. The reservoirs are particularly tight, with the air-permeability generally less than 0.3 × 10−3 μm2.

3 Tight oil reservoir characteristics

3.1 Sand body distribution characteristics

In the central region of the lake basin, sand body distribu-tion of the Chang 8 oil reservoir group is relatively stable. The lobe- and apron-like sand bodies are distributed in the delta front-end, with a thickness of 10 to 35 m. During the Chang 6

and Chang 7 sedimentary stages, the provenance of source rocks from the northeast, southwest, and western parts con-verged to the depocenter, deepwater area of the lake basin. The sand bodies are compositely contiguous in a large area, distributed in NW-SE orientation, along the line of Huanxian- Heshui roughly parallel to the facies-tract boundaries (the lake basin’s axial) (Figure 1) [31−33]. The sand bodies extend about 150 km and 25 to 80 km wide. The area with a sand-strata thickness ratio of greater than 30% is more than 8 000 km2. In some areas, the ratio between sandstone and stratum thickness is even more than 80%. The sandstone is very thick with a

single sand-layer thickness of 5−40 m, the cumulative thick-ness of 25 to 80 m, and the thickness of continuously distrib-uted sandstone in some areas is up to 100 m.

3.2 The reservoir physical properties

Influenced by deposition and diagenesis, the facies-state change of the sand bodies pinched out or is compounded horizontally; with the argillaceous interlayers and carbonate cementation tight intervals developed vertically, resulting in strongly vertical and horizontal heterogeneity of the reservoirs. The lithology of tight sandstone reservoirs is mainly fine- grained sandstone and siltstone, with line contact and point- line contact between the particles, the pore-section area ratio is generally less than 5.4%. The pore type is generally a dis-solved pore or intergranular pores-dissolved pores, with fine small throats, whose average mid-value radius is only 0.31 μm. The porosity is generally from 6.5% to 14.3% (Figure 3a), and the air permeability is generally 0.01 × 10−3−2.00 × 10−3 μm2 (Figure 3b).

The Chang 6 and Chang 7 gravity-flow deposited sandstone is tighter, with the average porosity generally of about 2%.The main pore types are intragranular dissolved pores of feldspar and debris, dissolved pores of cement, residual intergranular pores and micropores of interstitial matters, with an average


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Fig. 3 The pore throat distribution and physical properties of the Triassic Chang 7 tight oil reservoir in the Ordos Basin

Table 2 Evaluation of the brittleness index of Chang 7 reservoir group of Well Yc2

Depth/m Lithology Brittleness index/% Poisson ratio Young's Modulus/MPa Stress/MPa

1 972−1 993 arenaceous mudstone 35.8 0.26 26 408 18.7 1 993−2 006 sandstone 42.8 0.21 26 620 17.3 2 006−2 013 arenaceous mudstone 36.8 0.25 25 845 18.4 2 013−2 020 mudstone 26.9 0.27 18 732 20.0 2 020−2 042 sandstone 39.5 0.23 26 268 18.0 2 042−2 065 mudstone 28.1 0.28 17 958 20.8 2 065−2 081 mudstone 35.0 0.25 24 859 20.3

aperture of 16.98 μm. With tiny throats, the majority of throat radii of the powder-fine sandstone measured with the conven-tional mercury intrusion porosimetry are less than 0.2 μm (Figure 3c) with the average pore throat ratio of 492. The throat radii of more than 80% of the powder-fine sandstone measured with the conventional mercury intrusion po-rosimetry range from 0.05 to 0.20 μm (Figure 3d) with the maximum connected throat radius of 0.51 μm and the average throat radius of 0.16 μm. Therefore, such reservoirs are dominated by nano-scale throats, mainly small pores and mi-cro throats. The throats are unevenly distributed in the form of a network with poor connectivity, average sorting coefficient of 2.92, mean coefficient of 10.84, and a displacement pres-sure of 2.08 MPa. The complex pore texture makes the perco-lation ability of the reservoir poor, with the air-permeability of generally less than 0.3 × 10−3 μm2.

3.3 Rigid components and natural fractures

The content of rigid components of tight sandstone reser-voirs is generally 67% to 81% (with an average content of 41.8% quartz and 23.8% feldspar). The average content of cuttings is 17.4%; analysis of brittle minerals from the well Yc2 shows that the brittleness index of tight sandstones is 39% to 43% (Table 2). The higher brittleness of the reservoirs

is conducive to the reservoir stimulation. During the fractur-ing process, the reservoir rock is prone to produce shear rup-ture that forms a network system of seams or fractures. Natu-ral fractures developed in tight sandstone reservoirs, with about 2.3 natural fractures per 10 m, being mainly structural fractures like high angle fractures and vertical fissures. There are one or more sets of parallel fractures, as well as multiple fracture sets intersecting with each other; both extensional fractures and shear fractures develop. The main direction of these fractures is nearly E-W, accompanied by the N-E and nearly S-N.

3.4 The oil charging degree

In the central lake basin, the Chang 6-Chang 8 oil reservoir group are tight, but the oil charging degree is high. Geological logging indicates that oil is evenly distributed in tight sand-stone reservoirs often at the oil-immersion level. The sand-stone often shows brown gray due to crude oil impregnation (Figure 4a). According to the sealed coring analysis, oil satu-ration in the reservoir generally reaches up to 65% to 85% (Figure 4b). The laser confocal image displays that the pore texture of the tight sandstone is complex, having certain con-nectivity between pores and throats. The pores, throats and fractures are rich in organic matter (Figure 4c).


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Fig. 4 The oil-bearing characteristics of the Triassic Yangchang tight sandstone reservoir in the Ordos Basin

3.5 Formation pressure

In the Late Jurassic - Early Cretaceous, the Yanchang For-mation was at the maximum burial depth. By the end of the Early Cretaceous, influenced by the Yanshan tectonic move-ment, the basin was rapidly uplifted. A series of changes in exposure and erosion of the overlying strata and the tempera-ture drops occurred during the uplifting process which lead to the conversion of the attributes of the reservoir pressure sys-tem. The pressure coefficient of reservoirs in the Ordos Basin is generally low at present mainly ranging from 0.64 to 0.86. Thus the Ordos Basin is a typical low-pressure basin in China. In addition, due to the complicated pore texture of the reser-voirs and poor permeability, the production test shows a gen-erally low-yield or no natural capacity, thus increasing the difficulty of well stimulation and development.

3.6 Physical properties of crude oil

The crude oil property of tight oil is good, having a strong mobility. The density of the crude oil under the surface condi-tions is generally 0.83−0.88 g/cm3, while under the formation conditions, the oil density is about 0.70 to 0.76 g/cm3, with average viscosity of about 1.0 mPa⋅s, the freezing point of 17 to 20 °C, and movable fluid saturation of 47.38% showing good fluidity. Through stimulation of different reservoir types, good results could be obtained, such as the Chang 6 and Chang 7 tight sandstone reservoirs with characteristics of be-ing very thick, having a complex structure and being ex-tremely brittle. During the process of fracturing, technology has been adopted for multistage delivering sands, oriented perforating - multi-fracturing etc. Through making multi-frac-tures and optimizing the fracture length to improve the longi-tudinal conductivity of the reservoirs (which can also increase the degree of natural fracture opening), and increasing the oil drainage volume, the greater thickness of a tight oil reservoir can be effectively utilized. After fracturing, the tight oil pro-duction testing on the vertical well is generally 4−35 t/d. In some relatively high permeability regions, the producing test productivity exceeds 60 t/d. The per-well production of multi-ple horizontal well testing reaches one hundred tons or more.

4 Controlling factors of tight oil

4.1 Source rocks

The deposition period of the Yanchang Formation Chang 7

oil reservoir group is the biggest fan-lake period, with a strong depression of the lake basin. Distribution of the lake water is wide (more than 10 × 104 km2), with a set of organic-rich oil shales and dark-colored mudstones deposited, thickness of 20 to 60 m and the regional area of 5 × 104 km2. Source rocks have a good type of organic matter are dominated by low aquatic-organisms, and rich in ferrum, sulfur, phosphorus and other life elements. The average TOC value is 13.75%, and the main kerogen types are I and II1. The proportion of kero-gen in the rock is high, approximately 15% to 35%, with the condition of forming the kerogen network [12−13]. The Early Cretaceous was at the maximum burial depth stage (about 3000 m), with a moderate maturity, Ro value generally ranging from 0.9% to 1.1%, entering the peak stage of hydrocarbon generation and expulsion. The Chang 7 oil shale has charac-teristics of high hydrocarbon-generating rate (oil production rate of 400 kg/t), high HC-expulsion efficiency (generally 55% to 90%) [12−13], and widespread hydrocarbon-generation, being a set of high-quality source rocks. In addition, the or-ganic carbon contents of the dark-colored mudstone of Chang 4+5, Chang 6 and Chang 9 oil reservoir groups are 1.67%, 2.18% and 5.3% respectively. The main kerogen type is II1 (part of kerogen in the oil shales of Chang 9 oil reservoir group is Type I), having a certain hydrocarbon-generation ability.

4.2 Reservoir body

Under the control of the five major sedimentary sources, deltas were developed in the Triassic Yanchang Formation, Ordos Basin. The Northeast Delta is the largest, with the most extensive coverage, followed by the Southwest Delta and Northwest Delta. The depositional periods of the Chang 9, Chang 8, Chang 6 and Chang 3 were important delta con-struction periods. The sand bodies extend stably, with a large thickness. Ephemeral lacustrine transgression occurred in the Chang 4+5 sedimentary stage with a limited scale. Overall there was proportionally enough space for the deposits, with locally developed delta sand bodies in certain size. In the dif-ferent depositional periods, the delta scale presented charac-teristics of rising here and subsiding there. With the advance and retreat of the shoreline, delta sand bodies of different types such as distributary channels, underwater distributary channels and mouth bars, superimposed vertically and com-


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positely distributed horizontally, forming a large-scale delta group stretching around the depocenter of the lake basin. In the lake basin’s central region, large-scale gravity flow com-posite sedimentary sand bodies (including sandy debris flow, slump and turbidite deposition type) developed in the upper part of Chang 6, Chang 7 pay sets. The sand body has large longitudinal superposition thickness, parallel to the fa-cies-tract boundaries in the plane or distributed stably around the delta front-end. These sets of different genesis in different directions spreading sand bodies constitute an intertwined huge reservoir body in space and form a “full basin of sand” reservoir distribution pattern, providing a favorable reservoir space for oil accumulation.

4.3 The causes of the reservoir layer densification

The lithology of tight sandstone reservoirs mainly consist of arkose, lithic arkose and feldspathic lithic sandstone. Fine sandstone and siltstone are dominant. The average content of metamorphic debris, mica and other plastic components is 11.7%, resulting in poor anti-compaction capability of reser-voirs which are vulnerable to deflection, deformation, and occurrence of pseudo-matrix under external forces. The rigid particles have close interaction being arranged in certain di-rections, mainly connected by lines and point-line between the particles (Figure 5a). The loss of primary intergranular pores is as high as 36% to 41%. The interstitial matter content in tight sandstone reservoirs is high, with an average of 13.6% to 17.4%, mainly composed of water mica, iron-bearing carbon-ate and other later stage cement (Figures 5b, c and d). Sand-stone reservoirs in the gravity flow sedimentation of Chang 6,

Chang 7 oil reservoir groups are tighter, with interstitial mate-rial contents of 15.3% and 17.4%, respectively. Illite contents of Chang 6, Chang 7 were 7.0% and 10.4%, respectively, in a hair-like and fibrous shape, turning the effective pores into invalid micro-pores and greatly reducing the percolation ca-pacity of the reservoirs. The iron-bearing carbonate fills inter-granular pores and feldspar dissolution pores and the cemen-tation are tight [8, 15], with the average content of 5.2% and 4.1%, respectively [8]. Strong compaction, cementation and clay mineral transformation, and other diagenesis lead to tight sandstone reservoirs with small pore throats, complex textures and poor connectivity.

4.4 The petroleum play

The water body of the Ordos Basin in the Yanchang Forma-tion depositional period has a complicated evolution, which was specifically manifested in two aspects: one was the mul-tiple lake transgression and lake regression, while the other one was the migration of the depocenter. These two aspects controlled the sedimentary assemblage relationship between source rocks and reservoir sand bodies, pinching out horizon-tally or varying laterally, and superimposed vertically. The most significant lake transgression occurred at the end of the sedimentary stage of the Chang 8 oil reservoir group, and widely distributed high quality source rocks of the Chang 7 oil reservoir group were formed at that time. Delta and gravity flow sedimentary sand bodies of the Chang 6 oil reservoir group were developed and overlay the high-quality source rocks of the Chang 7 in the central region of the lake basin.

Fig. 5 The diagenesis characteristics of the Triassic Yangchang tight sandstone reservoir in the Ordos Basin


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The delta sand-bodies were developed in the underlying strata Chang 8 oil reservoir group. These two sets of sandstones, which are the closest to the Chang7 high quality source rocks, have a preferential location to trap the oil and gas and act as the major Mesozoic reservoir of the basin. In other words, the Chang 7 high quality source rocks, when matured, could pro-vide hydrocarbon downward and upward. Under stable tec-tonic settings, it is conducive to the formation of large-scale lithologic Chang 4+5, Chang 6, and Chang 8 oil reservoir groups. In addition, large-scale gravity flow reservoir sand bodies were developed in the Chang 7 oil reservoir group of the depocenter, being an important and typical suite of oil-bearing strata of tight oil (Figure 2).

4.5 The main oil accumulation period and oil charging

The central part of the Yanchang Formation is dominated by tight sandstone reservoirs. Only buoyancy cannot meet the demand of driving forces for large-scale oil migration and accu-mulation. Hydrocarbon-generating pressurization is the main driving force of strong hydrocarbon expulsion and charging.

Burial history studies show the following: in the Jurassic - Early Cretaceous, the Yanchang Formation was quickly buried, with the maximum burial depth in the Early Cretaceous. The burial depth of the Chang 6-Chang 8 oil reservoir group is generally 2500 to 3200 m. The tectonic thermal events that occurred in the late Mesozoic led to the high geothermal field, with a geothermal gradient of 3.5 to 4.0 °C/100 m [34−35]. The increase of the ground temperature promotes the maturity of

high quality source rocks and the formation of the tight oil in the Ordos Basin. Inclusions of Chang 6-Chang 8 reservoirs recorded the thermal event and its influence on the oil reser-voir formation. The uniform temperature of the salt-water inclusions in the same period with the hydrocarbon inclusions is generally from 83 to 126 °C, with the peak temperature from 100 to 118 °C. A large number of research projects in recent years show that the abundance of organic matter in the Chang 7 high quality source rocks is high, with a large amount of hydrocarbon generation. It’s easy to increase the volume of fluid in transformation of organic matter to petro-leum hydrocarbons. The accumulative volume of the crude oil is 8.0 % to 18.7 % of the rock volume, or even higher [13], with a strong hydrocarbon-generating pressurization. The Ordos Basin today is a low pressure basin, but in the geological his-tory period it was an overpressure basin. At the maximum burial depth stage (the Early Cretaceous), an abnormally high pressure prevailed in the Chang 7 oil reservoir group in the central region of the lake basin. The overpressure was gener-ally from 8 to 20 MPa. The coefficient of formation pressure was generally from1.5 to 1.8, with the overpressure decreas-ing upward and downward. The residual fluid pressure of the Chang 6 and Chang 8 oil reservoir groups was generally 4−8 MPa [36−37]. Large pressure differences existed between the source rocks and the upper and lower adjacent reservoirs (Figure 6). The average efficiency of two-way strong hydro-carbon expulsion reached as high as 72% [13]. The extent of oil charging in the tight sandstone reservoirs is high under the

Fig. 6 The section of the reservoir fluid power systems in the southwest of the Ordos Basin (Section location shown in Figure 1).


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strong hydrocarbon-generating pressurization. Both the oil- bearing level and oil saturation were high. Therefore in the peak period of hydrocarbon expulsion, overpressure has played an important role in oil migration and accumulation process.

The comparative analysis of the Yanchang Formation tight oil in the Ordos Basin and the Bakken Formation tight oil in the Williston Basin of North America shows (Table 1) that both of them have similar superimposed relationship of source and reservoir. That is, they are similar in parameters or in-dexes such as types, maturity, abundance of the source rocks, and brittleness degree and physical properties of reservoirs. The thickness of source rocks and reservoirs of the Yanchang Formation in the Ordos Basin is obviously greater than that of the Bakken Formation tight oil, but the heterogeneity of the nonmarine reservoirs and source rocks in the Ordos Basin is relatively strong. The Ordos Basin today as a low pressure basin is significantly different with the overpressure of the tight oil of the Bakken Formation in the Williston Basin.

5 The resource potential of tight oil

Tight oil resources of the Yanchang Formation in Ordos Basin are abundant. With air permeability of generally 0.3− 1.0 × 10−3 μm2, the reservoirs of recently-found big oilfields such as Jiyuan, Hua-Qing etc. with hundred-million-ton re-serves had been commercially developed. In recent years, they are important source to enhance petroleum production rate of the Changqing Oilfield.

The air permeability of the Chang 6 gravity flow deposition reservoirs and Chang 7 delta, gravity flow clastic reservoir layers in the central part of the lake basin is generally less than 0.3 × 10−3 μm2. They are still at the development trial stage without submission of proven reserves. There are 1 400 km2 tight oil enrichment area determined in the Chang 7 oil reservoir group. The Chang 6 tight oil enrichment regions have been found in the peripheral area of Hua-Qing Oilfield, Heshui and Tarwan areas, covering 1200 km2, well spacing of 3 to 7 km. Commercial oil flows, even high-yield of commer-cial oil flows, have been obtained from hundreds of wells after fracture stimulation. The reserves scale of such reser-voirs is estimated to be 0.8 to 1.0 giga-tons (109 t). Recently, significant results have been obtained through hydraulic frac-turing, volume fracturing, and horizontal-well development on the tight reservoirs with air permeability of about 0.2 × 10−3

μm2. It demonstrates good prospects of exploration and de-velopment of this kind of oil reservoirs.

6 Conclusions

The tight oil reservoir usually refers to sandstone and car-bonate reservoirs with in-situ matrix permeability of less than 0.2 × 10−3 μm2 or air permeability of less than 2 × 10−3 μm2. The single well has generally no natural production capacity or the natural productivity is lower than the minimum of commercial oil and gas flow, but could obtain a commercial oil production under certain economic conditions and fractur-

ing, horizontal wells, multi-lateral wells and other techniques. The Mesozoic Yanchang Formation in the Ordos Basin is rich in tight oil resources. On the plane, the Yanchang Formation tight oil reservoirs located at the middle of the lake basin, that is, the range delineated by Huanxian - Wuqi - Zhidan - Zhengning - Ningxian - Qingyang, as a depocenter in the mid-late Triassic; vertically, it’s located in the central horizons of the Yanchang Formation, that is, in the tight sandstone res-ervoirs interbedded and associated with or immediately adja-cent to the oil shales. Without large-scale long-distance mi-gration, the tight oil is mainly distributed in the Chang 6 – Chang 8 oil reservoir groups. Tight oil reservoir sand bodies are stably distributed at large scales. Sandstone reservoirs are tight, with complex pore texture. The content of rigid compo-nents is high, with developed natural fractures. The degree of oil charging is high. The formation fluid pressure is generally low, with low natural production capacity or no natural pro-ductivity. Physical properties of crude oil are fine, which is conducive to significant results of well stimulation. High quality source rocks were interbedded with widespread thick reservoir bodies. In the geologically historic period, strong hydrocarbon-generating pressurization and strong hydrocar-bon-expulsion control the formation of the widespread su-perimposed tight oil reservoirs in the Yanchang Formation. The tight oil reservoirs of the Ordos Basin have a huge resource potential, and have become targets for recent production capac-ity construction and an important domain for future oil explora-tion and development.

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characteristics of tight oil in triassic yanchang formation, ordos basin

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