Characteristics and resource prospects of tight oil in Ordos Basin, China

Petroleum Research (2016) 1,27-38

Characteristics and resource prospects of tight oil in Ordos Basin, China Hua Yang1,2*, Shixiang Li2,3,4 and Xianyang Liu2,3,4 PetroChina Changqing Oilfield Company, Xi’an 710018, China National Engineering Laboratory for Exploration & Development of Low-permeability Oil & Gas Fields, Xi’an 710018, China 3 College of Energy Resources, Chengdu University of Technology, Chengdu 610059, China 4 Research Institute of Exploration & Development, PetroChina Changqing Oilfield Company, Xi’an 710018, China Received February 8, 2016; Accepted July 1, 2016 1 2

Abstract: Efficient large-scale development of ultra-low-permeability reservoirs (0.3–1 mD) has been achieved in the Changqing Oilfield, Ordos Basin of China. According to unique features of petroleum exploration and development in this basin, tight oil herein refers to petroleum that occurs in oil-bearing shales and interbedded tight sandstone reservoirs adjacent to source rocks with ambient air permeability <0.3 mD. Tight oil in tight sandstone and shale have generally not yet experienced large-scale long-distance migration. In the Yanchang Formation, tight oil has mainly accumulated in the semi-deep to deep lacustrine facies, typically in oil-bearing shales and tight sandstones of the 7th member oil-bearing formation and tight sandstones of the 6th member oil-bearing formation in the center of the basin. Tight oil resource in the Ordos Basin is characterized by wide spatial distribution, excellent source rocks, extremely tight sandstone reservoirs, complex pore throat structures, poor physical properties, high oil saturation, good crude-oil properties, and low reservoir pressure. A fundamental feature of the continuous oil and gas accumulation in tight oil reservoirs is the widespread development of nano-scale pore-throat systems. In the Yanchang Formation, most of connected pore throats in tight sandstone reservoirs have diameters greater than critical pore throat diameter, allowing oil and gas migration in the tight reservoirs. According to contact relationship between tight reservoirs and source rocks, three types of tight oil reservoirs are identified in the Yanchang Formation, i.e., tight massive sandstone reservoir, sand - shale interbed reservoir, and oilbearing shale reservoir. In the Ordos Basin, tight oil is widely distributed in the 6th and 7th members of the Yanchang Formation, with total resources estimated to be 3×109 t. These include >1×109 t of oil resources in shale in the 7th member of the Yanchang Formation and approximately 0.9×109 t and 1.1×109 t of tight sandstone oil resource in the 6th and 7th members of the Yanchang Formation, respectively. These tight oil resources are the realistic resources addition for the oilfield, which can ensure an annual production of 50×106 t of oil and gas equivalent and maintain long-term stable oil production in the Changqing Oilfield, Ordos Basin, China.

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

* Corresponding author. Email: [email protected]

© 2017 Chinese Petroleum Society. Publishing Services by Elsevier B.V. on behalf of KeAi. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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1 Introduction Tight oil is a new hot spot of global exploration and development of unconventional oil and gas resources following shale gas (Liang et al., 2011; Sun et al., 2011; Zou et al., 2011a; Jia et al., 2012a). Tight oil plays a crucial role in the global energy structure, and commercial development of tight oil has been achieved in a number of countries such as the United States, Canada, and Australia (Zou et al., 2011a; Yan et al., Lin et al., 2011). Despite their abundance in China, exploration and development of unconventional oil and gas resources remain in an early stage in this country (Guan et al., 1995; Zou et al., 2010a). Ordos Basin is rich in tight oil resources and has great potential for oil exploration and development. Over the past decade, PetroChina Changqing Oilfield Company has overcome difficulties related to low and ultra-low-permeability and achieved an annual increase of (1–3)×108 t in proved reserves for 10 consecutive years. By the end of 2015, the submitted proved reserves have accumulated to 4.154×10 9 t, including 2.156×109 t in low-permeability reservoirs (<1 mD). Changqing Oilfield Company has successfully achieved efficient large-scale development of tight oil reservoirs with ultra-low-permeability (0.3–1.0 mD) and innovated development technologies, typically in Huaqing Oilfield. In 2015, crude oil production in Changqing oilfield exceeded 25×10 6 t. Presently, field research of oil reservoirs with low permeability of 0.3–1.0 mD is ongoing in Changqing Oilfield. Laboratory researches and field experiments are conducted on shale oil in reservoirs of the 7th member of the Yanchang Formation with permeability <0.1 mD. To date, there has been no consensus on the definition of tight oil. Zou et al consider that tight oil is the petroleum that coexists in source rocks and accumulates in various types of tight reservoirs after short-distance migration, and tight oil reservoirs mainly consist of tight sandstone and tight limestone, with the insitu matrix permeability less than or equal to 0.1 mD (reservoir surface-air permeability <1 mD) (Zou et al., 2012a,b). Jia et al consider that tight oil is the petroleum that occurs in a free or adsorbed state in source rocks or tight reservoir rocks (e.g., sandstone and carbonate rock) interbedded with or adjacent to source rocks, such oil accumulation has not experienced largescale long-distance migration, and the in-situ matrix permeability of the reservoirs is less than or equal to 0.1 mD (reservoir surfaceair permeability <1 mD) (Jia et al., 2012b). As for tight oil represented by Bakken Oilfield in the U.S. (Lin et al., 2011), the insitu matrix permeability of the reservoirs is 0.01–0.1 mD. All the above definitions of tight oil take the surface-air permeability <1 mD as the boundary. According to actual situation of petroleum exploration and development in Ordos Basin, reservoirs with surface-air permeability (herein referred to the permeability in surface atmosphere unless otherwise specified) <1 mD (in-situ matrix permeability <0.1 mD) are referred to as unconventional oil

28

and gas resources, and those with the permeability of 0.3–1 mD are regarded as ultra-low-permeability reservoir. As of date, efficient large-scale development has been achieved in the abovementioned low-permeability tight oil reservoirs. To focus on our research targets, tight oil in Ordos Basin is defined as the petroleum that occurs in oil-bearing shales and tight sandstone interbed reservoirs adjacent to source rocks and with surface-air permeability <0.3 mD. Such oil resources, including tight sandstone oil and shale oil, have not experienced large-scale long-distance migration. As compared to the explored low- and ultra-low-permeability reservoirs, tight oil reservoirs feature complex accumulation mechanisms, fine pore throats, poor physical properties, and high filling content; as compared to such resources overseas, tight oil in Ordos Basin shows substantial differences in pressure coefficient, oil potential, reservoir brittleness, and natural micro-fracture development. Consequently, well-developed technologies overseas are not fully applicable to the exploration and development of tight oil in Ordos Basin. Based on the latest advances of tight oil exploration in Ordos Basin, this study elaborates major geological characteristics of tight oil and preliminarily assesses and forecasts prospects of tight oil resources in Ordos Basin, in attempt to promote tight oil exploration and related geological research.

2 Characteristics of tight oil in Ordos Basin In Ordos Basin, tight oil of the Yanchang Formation has mainly accumulated in the semi-deep to deep lacustrine facies, typically in oil-bearing shales and tight sandstones of the 7th member in the basin and tight sandstones of the 6th member in the center of the basin. Such tight oil is characterized by wide distribution, superior hydrocarbon source rocks, tight sandstone reservoir, complex pore throat structure, poor physical properties, high oil saturation, good oil quality, and low reservoir pressure coefficient. Typical oil in the shale is identified in the tight reservoirs of the 7th member,where oil-bearing shales are widely distributed with large thickness. Typical tight sandstone oil is identified in sandstone reservoirs of the 7th member and reservoirs of the 6th member in the center of the lacustrine basin. These oil-bearing formations are adjacent to high-quality source rocks and feature excellent source rock conditions, good source - reservoir configuration, and tight reservoirs.

2.1 Wide distribution of oil-bearing shales and high-quality source rocks During deposition of the Late Triassic Yanchang Formation, the Indosinian movement led to the formation of a large-scale inland freshwater lacustrine. During deposition of the 7th member of Yanchang Formation, the basin reached maximum scale and developed major source rocks consisting of black shale, and dark mudstone (Yang and Zhang, 2005; Zhang et al., 20006). In Ordos Basin, oil-bearing shales mainly are developed in the lower part of the 7th member. The oil-bearing shales are NW-

H.Yang et al./Petroleum Research (2016) 1,27-38

SE trending and distributed widely. Their approximate effective distribution area is 100×103 km, approaching western Yan’an - An’sai to the northeast, Majiatan to the northwest, Zhenyuan - Jingchuan to the southwest, and Zhengning - Fuxian to the southeast (Fig. 1). Source rocks are generally 30–60 m thick, and maximum thickness is up to 130 m. Hydrocarbon-generating center lies in the Jiyuan - Huachi - Zhengning area, where distribution area of high-quality source rocks is approximately 50×103 km2. Majiatan

Yanchi Chengchuan

Dingbian

Jingbian

Well Erosion location area

City/ town

Thickness of oil- bear shale / m 0

H210

10

A83

20

Jiyuan

30 40

Wuqi

G295

50

An' sai

Zhidan

Huanxian

W100 L189

Yan' an

Yongning

Huachi Ganquan

Qingcheng

Z86 Z84 Zhenyuan

Fuxian

X41 X21 Qingyan

ZH9 N33

Chongxing Jingchuan Changwu

Huangling Zhengning 0

40 km

Fig. l Distribution of oil-bearing shales in the 7th member of the Upper Triassic Yanchang Formation, Ordos Basin

Black shales of the 7th member of Yanchang Formation are rich in organic matter. Organic geochemical data indicate that the residual organic carbon content of these black shales is generally 6–22% with the maximum level of 30–40% and the average total organic carbon (TOC) content of 13.75% (Yang and Zhang, 2005). Organic maturity (Ro) is 0.85–1.15%, indicating that black shales have entered the peak stage of oil generation (Yang and Zhang, 2005) and mostly had undergone intensive hydrocarbon generation and expulsion (Zhang et al., 2006). Average intensity of hydrocarbon generation is 4.95×106 t/km2, total effective amount of hydrocarbon generation is 247.308×109 t. Average intensity of hydrocarbon expulsion is 2.9×106 t/km2, and total amount of hydrocarbon expulsion is 144.771×109 t. All the data show that the 7th member of Yanchang Formation has good-quality source rocks and is an important resource base for tight oil reservoir.

2.2 Tight sandstone reservoirs and complex pore throat structure In the Ordos Basin, tight sandstone reservoirs primarily consist of continental clastic deposits. The lithology is complex, and dominated by silt-fine sandstones with fine granularity, poor

physical properties, and tight reservoirs. Generally, porosity ranges from 4% to 10% and permeability is less than 0.3 mD, which are typical characteristics of tight sandstones. This kind of reservoir shows large lateral variations in the physical properties and features a number of interbeds vertically. Other characteristics include strong heterogeneity, fine pore throats, and complex pore structure(Wang et al., 2015a). In the 6th member of Yanchang Formation, major types of reservoir rocks are feldspathic sandstone and lithic feldspathic sandstone. Feldspar content is high, averaging 42.2%; quartz content averages 25.2%; metamorphic lithic content averages 4.7%. In the 7th member of Yanchang Formation, major types of reservoir rocks are lithic sandstone and feldspathic lithic sandstone. Average contents of quartz and feldspar are 32.1% and 19.6%, respectively; total lithic content is 27.3%, and dominated by metamorphic lithic debris. Major reservoir fillings are illite, chlorite, and carbonate. The fillings accounted for 15.3% of the 6th member of Yanchang Formation, mainly composed of ferruginous calcite, hydromica, and chlorite; the ferruginous calcite content is relatively high and averages 3.8% (30% of total reservoir fillings). The filling accounted for 17.4% of the 7th member of Yanchang Formation, mainly composed of illite, chlorite, and carbonate (mainly ferruginous calcite and ferruginous dolomite); the illite content is relatively high and averages 7.3% (>45% of total reservoir fillings). Additionally, reservoir fillings include small amounts of kaolinite as well as tuffaceous, siliceous, and feldspathic rocks; illite and ferruginous calcite primarily play a role in destructing reservoirs by reducing their physical properties, whereas widespread development of chlorite clay film in a rim shape effectively improves reservoir properties by exerting a protective effect on the primary intergranular pores. In the tight sandstones of the Yanchang Formation, major pore types are dissolved pore and residual intergranular pore, with tiny pore throats, complex structure(You et al., 2014), poor connectivity and intense diagenetic activity. Overgrowth of quartz and cementation of ferruginous calcite with ferruginous dolomite are observed commonly, and content of clay matrix is relatively high. Throat shapes are mainly laminated-, curved laminated-, and bundled tube-like. Pore throats of tight sandstone reservoirs have good sorting, mainly in the pattern of fine pore - micro throat or medium pore - micro fine throat. Average permeability of tight reservoirs in oil-bearing formations 100 in the center of the lake basin is <0.3 mD, indicating poor physical properties. In the 6th member of Yanchang 95 Formation, porosity of tight reservoirs ranges from 5.6% to15.5% 75(average 10.2%) and permeability varies from 0.05 to 2.22 mD (average 0.22 mD). In the 7th member of Yanchang Formation, porosity of tight reservoirs is from 4.8 to 12.6% (average 7.2%) and permeability ranges from 0.01 25 to 1.35 mD (average 0.18 mD). 5 0

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H.Yang et al./Petroleum Research (2016) 1,27-38

2.3 High oil saturation and good crude oil quality In the Ordos Basin, tight sandstones directly contact with highquality source rocks, thus sufficient oil resources and high charging intensity allow oil and gas generated from source rocks to form reservoirs by short-distance migration. Tight reservoirs of both oil-bearing formations have good oil shows, which generally can be classified as oil-spot to oil-immersion grade by geological logging. Average oil saturation is up to 70%, specifically, average oil saturation is estimated to 72.8% by sealing coring, 69.8% by calculation using the Archie equation, 69.0% by mercury porosimetry, and 70.3% using infiltration method. Surface crude oil from the 7th member of Yanchang Formation has following general properties: density of 0.84 g/cm3, viscosity of 6.12 mPa·s, freezing point of 17. 6°C, and initial boiling point of 76.3°C. Compared with other oilfields, crude oil from the 7th member of Yanchang Formation is of good properties in terms of low density, low viscosity, low sulfur content, and low freezing point.

2.4 Low reservoir pressure coefficients In the Ordos Basin, major Triassic reservoir depth is generally 1200–2300 m with present stratigraphy pressure of 7.0–18.1 MPa. Based on analysis of original stratigraphy pressure and reservoir depth in over 20 developed blocks in the basin, reservoir pressure coefficient is 0.6–0.8, indicating low pressure of the reservoirs(Li et al., 2013). The main reason for formation of low pressure reservoir is that formation temperature experienced a hot-to-cold geothermal conversion during oil and gas accumulation (Xu et al., 2012). In

the late Early Cretaceous, the Ordos Basin was influenced by the Yanshanian tectonic movement, with frequent volcanic activities in the surrounding areas. During this period, geothermal gradient was up to 3.5–4.5°C/hm (Ren, 1996), major reservoir depth was up to 3000–4000 m, and therefore, formation temperature could reach 100–180°C. High formation temperature leads to intense fluid activity and result in kerogen entering mature stage, thus source rocks began to generate and expulse hydrocarbon. Significant displacement pressure from hydrocarbon generation and expulsion allowed oil and gas accumulation in a large quantity, leading to formation of high-pressure reservoirs. Since the Late Cretaceous, the Ordos Basin was uplifted and denuded as a whole. According to recovery of denuded thickness, the denudation could be >1000 m. Pressure of overlying strata decreased while reservoir porosity rebounded. Meanwhile, magmatic activities in the periphery of the basin were weakened, whereas the geothermal gradient was reduced to 2.8–3.2°C/hm and the formation temperature reduced to 70–100°C (Ren, 1996). The fluid activity was weakened and pore fluid pressure was further reduced, leading to formation of the present low-pressure reservoirs. In the Ordos Basin, tight oil has two key geological characteristics: (1) source reservoir symbiosis with continuously distribution in large areas of banded oil-bearing reservoirs, and no obvious trap or oil-water boundarie, (2) non-buoyant accumulation with continuous charging of oil and gas, no significant influence by hydrodynamic effect, and no uniform oil-water interface and pressure system. Compared with the Bakken Formation in Williston Basin (Table 1) (Schmoker and Hester, 1983; Webster, 1984; Lin et al., 2011), tight oil in the Ordos Basin is characterized by large oil-bearing shale thickness and wide distribution, well development of oil-bearing shale interbedded with clastic rock favour formation of tight oil and

Table 1 Tight oil characteristics between the Upper Triassic Yanchang Formation, Ordos Basin, China, and the Upper Devonialn Mississippian Bakken Formation, Williston Basin, the United States Tight oil in the Bakken Formation, Oil in the oil-bearing shale, Tight oil in the Basin Williston Basin the Ordos Basin Ordos Basin 7th member of Yanchang 7th and 6th members of Strata Upper Devonian - Lower Carboniferous Formation Yanchang Formation Depositional environment Shallow marine facies Lacustrine facies Buried depth/ m 2593-3203 1000-2600 Shale thickness/ m 3-15 20-80 10-80 Quartz/% 36.6 (Oil-bearing shale) 47.5 (Silstone) 26.2 (Oil-bearing shale) 41.8 (Silstone) Mineral Feldspar/% 14.2 (Oil-bearing shale) 18.9 (Silstone) 18.9 (Oil-bearing shale) 23.5 (Silstone) component Clay/% 26.7 (Oil-bearing shale) 21.5 (Silstone) 29.8 (Oil-bearing shale) 17.4 (Silstone) Porosity/% — 6.0 2.08 (Oil-bearing shale) 8.2 (Silstone) Physical properties Permeability/mD — 0.04 0.001 (Oil-bearing shale) 0.02 (Silstone) Oil content of oil-bearing shale/% 10.0 3.5-6.0 Kerogen type I+II I TOC/% 11.30 13.75 Ro/% 0.7-1.0 0.90-1.16 Maximum pyrolysis temperature/oC 443-447 440-455 Relationship between source rock Interbed and symbiosis Interbed and symbiosis and reservoir

30

H.Yang et al./Petroleum Research (2016) 1,27-38

indicate great development potential of tight oil in this basin.

3.2 Accumulation mechanisms

3 Tight oil distribution patterns and accumulation mechanisms

Tight oil and shale oil reservoirs are usually dominated by nanoscale pore-throat systems (Zou et al., 2011b). This is a fundamental pre-requisite for large areas of continuous or quasi-continuous oil and gas accumulation, which determines the continuous or quasi-continuous distribution of oil and gas resources (Zou et al., 2010a; Zou et al., 2012a). Difference between conventional and unconventional large oil and gas fields is size of pore throat. Pore throat diameter of conventional reservoirs range from microns to millimeter, while pore throat diameter of unconventional reservoirs is generally nanometer-scale. High-resolution field emission scanning electron microscopy (FE-SEM) and nano-CT were used to investigate pore throats diameters, nano-scale pore throats (<1000 nm) are widely developed and occupied by petroleum in the unconventional reservoirs in China, indicating nano-scale pore throats play an important role in oil and gas accumulation in unconventional reservoirs (Zou et al., 2012b).

3.1 Distribution patterns In Ordos Basin, tight oil is mainly distributed in the center of the Mesozoic large-scale depression lake basin. Oil-bearing shales of the 7th member of the Yanchang Formation are NW - SE trending and widely distributed, with relatively large thickness (Fig. 1). The hydrocarbon-generating center is located in the Jiyuan - Huachi Zhengning area, covering nearly 50×103 km2 of high-quality source rocks. Tight sandstone oils are primarily distributed in the semideep to deep lake gravity flow sand bodies or delta-front sandstone bodies. Source-reservoir symbiosis is a major factor to control accumulation of tight oil (Yao et al., 2015; Qiu et al., 2016; Zhang et al., 2016). In the 7th member of the Yangchang Formation, tight sandstone oils are well developed in the 1st and 2nd sub-members of the 7th member, and are horizontally distributed in delta-front sand bodies of the Jiyuan area and turbidite sand bodies of the Longdong area (Fig. 2a). In the 6th member of the Yanchang Formation, tight sandstone oils are mainly developed in gravity flow sand bodies of the semi-deep to deep lacustrine and deltafront sand bodies, and are horizontally distributed in the Huachi, Qingcheng, and Heshui areas (Fig. 2b). 0

Dingbian

50 km

Crude oil molecules can migrate through nano-scale pore throats. Fluorescence imagesfrom confocal laser scanning microscopy show that inner microscopic pore structure of tight sandstones in the 7th member of the Yanchang Formation is relatively complex, and pores connectivity is good (You et al,. 2014; Li et al., 2015; Wang et al,. 2015b); most of dissolved pores, residual intergranular pores and micro pores show fluorescence. Molecular diameters 0

Jingbian

30 km Zhidan

Huanxian

Wuqi

Huachi Zhidan

Huanxian Huachi

Qingcheng Qingcheng Heshui Heshui Zhenyuan

Qingyang

Qingyang

Ningxian

Zhenning

(a)Tight oil distribution of the 7th member of the Yanchang Formation

Delta plain facies

Delta front facies

Deep lacustrine facies

Major sand body

(b)Tight oil distribution of the 6th member of the Yanchang Formation

Secondary sand Boundary between Boundary between body delta plain and delta front and delta front deep lacustrine

Proved reserve area

Favorable target area

City or town

Fig. 2 Tight oil distribution of the 7th and 6th members of the Yanchang Formation, Ordos Basin, China

31

H.Yang et al./Petroleum Research (2016) 1,27-38

Bou

Where d is the thickness of bound water film, mm; φ is the porosity of core, %; Sw is irreducible water saturation, %; A is specific surface area, m2/g; ρr is density of core, 103kg/m3.

ater

nd w ater film

According to bound water saturation of different samples, relevant thickness of bound water film are calculated, ranging from 34-53 nm (average 43 nm) (Xiang et al., 1999). Therefore, critical pore throat radius that allow asphaltene of maximum molecular diameter of 4 nm to pass through is 45 nm, that is, the critical pore throat diameter is 90 nm (Fig. 3). Statistics of nano-scale pore throat diameter distribution of tight oil reservoirs in the Ordos Basin shows that median pore throat diameters of tight oil reservoirs in the Yanchang Formation are 20–300 nm, mainly in the range of 50–200 nm; maximum pore throat diameters are 300–2000 nm, mainly in the range of 500– 1000 nm(Fig. 4). Pore throat diameter of tight oil reservoirs varies in different areas. In Jiyuan area, median pore throat diameters of tight oil reservoirs are 40–300 nm, mainly in the range of 80–200 nm; maximum pore throat diameters are 300–1300 nm, mainly in the range of 550–1200 nm. In Heshui area, median pore throat diameters of tight oil reservoirs are 13–300 nm, mainly in the range of 40–100 nm; maximum pore throat diameters are 300–2000 nm, mainly in the range of 300–600 nm. In Shangliyuan area, median pore throat diameters of tight oil reservoirs are 20–300 nm, mainly in the range of 50–200 nm; maximum pore throat diameters are 300–900 nm, mainly in the range of 500–800 nm.

film

B

nd w

(1)

d = 7142φ S w /(A⋅ ρ r )

In the tight oil reservoirs of the Yanchang Formation, the median pore throat diameters mainly range from 50 to 200 nm, most of the connected pore throats diameters are greater than the critical pore throat diameter, which are controlled by hydrocarbon expulsion pressure. These conditions meet requirements of oil and gas migration in tight reservoirs.

Bou

of major hydrocarbons from crude oil are 0.38–4 nm. Molecular diameters of CH4, C2H6, C3H8, iC4H10, and nC5H12 are 0.38, 0.44, 0.51, 0.53, and 0.58 nm, respectively. Maximum molecular diameter of asphaltene is 4 nm. Based on numerical simulation, thickness of bound water film in the cores can be calculated by equation 1 (Xiang et al., 1999):

Framework grains

Framework grains

C

A

30µm A

Molecular diameter / nm

B

CH 4

0.38

C2H6

0.44 Adamantane

H2O

0.7~2

C3H8

0.51

n- Alkane

0.4~2

iC 4 H 1 0

0.53

0.8~3

nC 5 H 1 2

0.58

Arene Maximum asphaltene

4 Major reservoirs types and exploration progress of tight oil There are abundant tight oil resources in the Ordos Basin (Zou et al., 2010b; Jia et al., 2012). Through researches on vertical distribution of oil-bearing shales and tight sandstones in the 7th Sandstone

Tight sandstone

Sample number=53 Samples from Jiyuan

Samples from Heshui

0.1

1

Median throat diameter

Maximun asphaltene

Maximum pore throat diameter

Samples from Shangliyuan

10 100 Pore throat diameter / nm

1000

10000

Fig. 4 Distribution of pore throat diameter of tight oil reservoirs in the Yanchang Formation, Ordos Basin, China

32

4

Fig. 3 Critical pore throat diameters of tight sandstones in the Yanchang Formation, Ordos Basin, China, A represents the critical pore throat diameter of 90 nm; pore throat diameter at B is approximately 200 nm; C represents average thickness of bound water film of 43 nm.

Shale

Arene n-Alkanes Adamantane nC 5 H 12 iC 4 H 10 C3H8 C2H6 CH 4 H2O

0.3

H.Yang et al./Petroleum Research (2016) 1,27-38

member of the Yanchang Formation, and different assemblage relations of sand - shale interbeds, adaptive technology program can be explored and tight oil resources can be used effectively.

4.1 Reservoir types Evolution of basin controls assemblages of depositional facies and development characteristics of sand bodies. Delta-front and gravity flow sand bodies are mainly developed in the 7th member in the basin and the 6th member in the center of the basin (Zou et al., 2009; Li et al., 2010; Li et al., 2011), and thick laminated sand bodies are formed in the center of the lake basin (Fu et al., 2010). The area of lake basin reached the largest during deposition of the 3th sub-member of the 7th member (Yang et al., 2010), then its gradually shrank thereafter. During deposition of 7th to 6th member of the Yanchang Formation, the lake basin was dominated by lake regressive - sand trangressive deposits, and experienced multiple small-scale oscillations, resulting in fluctuations of the lake level. With oscillations of the lake basin, the lake underwent periodic trangressive and regressive processes, leading to deposition of multiple sets of favorable reservoir-cap rock assemblages of sand - shale interbeds. Vertical distribution patterns of oil-bearing shales and tight sandstones in the Ordos Basin shows that tight oil reservoirs can be divided into 3 major types, including tight massive sandstone reservoir, sand - shale interbed reservoir, and oil-bearing shale reservoir (Fig. 5). Tight massive sandstone reservoirs are mainly distributed in the middle - upper 7th member and the bottom 6th member of Yanchang Formation, characterized by multi-stage superposition of gravity flow slump sandstones with less clay content. The sand bodies are integral with large thickness, contiguous complex planes and large scale. Reservoir spaces are mainly dissolved pores and micro pores (Fig. 6a), with average porosity of 9.5%, permeability of 0.21 mD and good pore throats connectivity (Fig. 6b). The sand - shale interbed reservoirs contain a number of interlayers, which are characterized by multiple sets of superimposed development of sandstone, silty mudstone, muddy siltstone and dark mudstone. The reservoirs heterogeneity is strong, with high clay content, fine grain size, developed micro pores, poor physical properties (average porosity of 8.1%; average permeability of 0.1 mD) and poor connectivity (Fig. 6c and 6d). In oil-bearing shale reservoirs, major reservoir spaces are micro pores and micro fractures in oil-bearing shales. Oil-bearing shales of the 7th member of the Yanchang Formation are mainly composed of quartz, feldspar, carbonate, clay and pyrite, with high content of rigid materials and high quality and abundance of organic matter. The oil-bearing shales are interbedded with tuff (Fig. 6e and 6f) and dissolved pores are developed in the tuff (Fig. 6f). Horizontal micro fractures can be observed in the oil-bearing shales (Fig. 6e and 6f). The oil-bearing shales

are of certain reserve ability because of micro fractures and its residual liquid hydrocarbon.

4.2 Exploration progress Due to poor physical propertie, tight oil reservoirs should be modified by effective technological measures to achieve industrial production. In the Changqing Oilfield, process tests that focus on 3 types of tight oil reservoirs have been carried out, and tight oil reservoirs of massive sandstones, sand - shale interbeds, and oilbearing shales are modified by fracturing, leading to production of industrial oil flow and new advances in tight oil exploration. Thick massive tight oil reservoirs of the 7th member of the Yanchang Formation are fractured by pumping slippery fluid and surfactant gel and slick mixture fluid in large fluid volume, large drainage volume with small sand ratio. This process forms reticular fractures and dramatically expands drainage volume, thereby improving the single-well production of tight oil. This technology has been successfully applied in 18 wells in the Ordos Basin, high-yield oil flow over 20 t are obtained from 5 wells, especially, in the Well H210 (Fig. 5a), high yield of test oil flow reach to 21.91 t/d after injection of 60.0 m3 of ceramsites with a sand ratio of 18.5% and a drainage volume of 6.0 m3/min. For sand - shale interbed and oil-bearing shale reservoirs, Well L189 and Well G295 are optimised from areas with large thickness of oil-bearing shales to test. Water injection through oil tube and casing pipe can increase capacity of fracturing fluid, and then further expand drainage volume and improve sing-well production. In the sand - shale interbed reservoirs, ,oil-bearing sandstone formation in the Well L189 is 4.3 m thick and yield of test oil flow is up to of 13.18 t/d after injection of 80 m3 sand with sand ratio of 12.4% and drainage volume of 12.0 m3/min (Fig. 5b). In the oilbearing shale reservoirs, oil-bearing shale formation in the Well G295 is 28.0 thick and yield of test oil flow reaches 20.40 t/d after injection of 22 m3 ceramsites with sand ratio of 6.3% and drainage volume of 5.0 m3/min (Fig. 5c). With technological applications in tight sandstone reservoirs, a number of large-scale oil-rich zones have been discovered in the Ordos Basin. Presently, field-testing has been accomplished in approximately 600 wells for tight sandstone reservoirs of the 7th mumber of the Yanchang Formation. Of these, nearly 350 wells have yielded industrial oil flow and test oil production of approximately 150 wells are >10 t/d. These wells are mainly distributed in the gravity flow and delta-front sand bodies in adjacent hydrocarbon-generating sags, and are mainly spread in the Longdong and Jiyuan areas; presently, more than 10 largescale oil-rich zones are identified (Fig. 2a), covering total area of approximately 1400 km2 with implemented reserves >400×106 t initially. Large-scale reserves of tight oil have been found in the sandstone reservoirs of the 6th of the Yanchang Formation. The Huaqing Oilfield is rapidly and efficiently identified based on

33

The thrid sub-member

The 7th member of Yanchang Formation The second sub-member The first sub-member

34 Member Sub-member

75

SP/mV

15

150

GR/API

50

2250

2240

2230

2220

2210

2200

2190

2180

2170

2160

2150

2140

Depht/m

Fine sandstone

2.2

Siltstone

100

Delta

Sub-facies

Delta front

Member Sub-member

26

500

SP/mV

16

20

GR/API

Oil-bearing shale

The first sub-member

The second sub-member

2250

2240

2230

2220

2210

2200

2190

2180

2170

2160

2150

Dark mudstone

Mud content/% AC/( μ s·m -1 ) DEN/ 100 0 203 363 -3 (g·cm ) Sand Lithology AT/( Ω ·m) content/% 2.7 2 760 16 0 100

(b)

Mud - sand content

Lacustrine

Sub-facies

Deep lacustrine

Oil layer

The first sub-member

The second sub-member

500

85

50

60

SP/mV

GR/API

2700

2690

2680

2670

2660

2650

2640

2630

2620

2610

Poor oil layer

(c)

Mud content/% 100 0 Sand content/% 0 100

Dry layer

AC/( μ s·m -1 ) DEN/ 385 (g·cm -3 ) AT/( Ω ·m) 2 5 2000 2.7

Lithology 185

Fig. 5 Major types and characteristic of tight oil reserviors in the Yanchang Formation, Ordos Basin, China

0

content/%

Mud content/%

DEN/ 100 0 (g·cm -3 ) Sand

AT/( Ω ·m) 170 2.8 5

AC/( μ s·m -1 ) 350

Lithology 100

(a)

Interpretation

Facies Perforation section Test oil production

Oil: 20.91t / d, water: 0 m 3 The 7th member of Yanchang Formation The thrid sub-member

Depht/m

Interpretation

Facies Perforation section Test oil production

Oil : 13 . 18 t / d, water: 0 m 3

Member Sub-member

The 7th member of Yanchang Formation The thrid sub-member

Depht/m

Interpretation

Sub-facies

Semi-lacustrine

Deep lacustrine Lacustrine

Facies Perforation section Test oil production

Oil : 20 . 49 t / d，water : 0 m 3

H.Yang et al./Petroleum Research (2016) 1,27-38

H.Yang et al./Petroleum Research (2016) 1,27-38

(a)

(b)

(c)

(d)

(e)

(f)

Fig. 6 Microscopic pore structure characteristics of different reservoirs in the Yanchang Formation, Ordos Basin, China, (a) Dissolved pores and intergranular pores in the tight massive sandstone reservoir; surface porosity 3.8%, porosity 8.1%, and permeability 0.28 mD; 2nd sub-member of the 7th member of the Yanchang Formation, 2180.11 m of Well A83. (b) Laser confocal photomicrograph of the tight massive sandstone reservoir mainly consisting of dissolved pores and micro pores; good pores connectivity; 2nd sub-member of the 7th member of the Yanchang Formation, 2180.11 m of Well A83. (c)Micro pores in the sand - shale interbed reservoir; porosity 10.0%, permeability 0.065 mD; 1st sub-member of the 7th member of the Yanchang Formation, 1658.96 m of Well N33. (d) Laser confocal three-dimensional image of the sand - shale interbed reservoir, bright yellow indicates good development and connectivity of pore throats, followed by green; blue areas indicate poor development and connectivity of pore throats; 3rh sub-member of the 7th member of the Yanchang Formation, 1658.96 m of Well N33. (e) Tuff - oil-bearing shale interbed, with dissolved pores in the tuff and horizontal micro fractures in the oil-bearing shale (dark-brown banded zones in horizontal direction); 3rh sub-member of the 7th member of the Yanchang Formation, 2015.06 m of Well N33. (f) Tuff interbedded with oil-bearing shale, with dissolved pores in the tuff; 3rh sub-member of the 7th member of the Yanchang Formation, 2015.06 m of Well 100.

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H.Yang et al./Petroleum Research (2016) 1,27-38

integrated exploration and assessment, the proved reserves are 522×106 t. Additionally, 4 large-scale oil-rich zones have been discovered in the periphery of the Huaqing Oilfield as well as the Heshui and Ta’erwan areas (Fig. 2b), with controlling area of 1100 km2 and initial implemented reserves of nearly 600×106, and the center of the lake basin becomes a favorable target for tight oil exploration and evaluation.

5 Resource potential Based on assessment of hydrocarbon source rock and analysis of tight oil accumulation conditions and mechanisms, some key indices, such as trap area, reservoir thickness, oil saturation are used to preliminarily estimate tight sandstone and shale oil resources with a variety of methods (Guo et al., 2011).

5.1 Tight sandstone oil resource potential In this study, tight oil resources of Ordos Basin are calculated using a volume method. This method should use reservoir area and effective reservoir thickness. Some other parameters for calculation are derived from blocks of submitted proved reserves. The equation of the volume method is expressed as:

N  100 Ah So  oTr / Boi

(2)

where N is amount of resources, 104 t; A is the trap area, km2;h is the oil layer thickness, m; φ is the average effective porosity,%; S o is the average initial oil saturation,%; ρo is the average surface crude oil density, t/m3; Tr the success rate of exploratory wells,%; and Boi is the average initial crude oil volume factor. Reservoir area (trap area) boundary is generally determined by both sides of the main sand body, sand body boundaries of 4- or 10-m-thick oil layers are used as the oil-bearing boundary, when the oil-bearing boundary is not identified in the extensional direction of the sand body, the trap area boundary can be determined by extending the oil flow well with test oil production >2.0 t or stable oil production >0.65 t by 1–1.5-time development well spacing. With this method, trap areas of the 7th and 6th members of Yanchang oil-bearing formations are estimated to be 8461.20 km2 and 4872.54 km2, respectively. Low limit of effective reservoir thickness is mainly determined by investigating lithology, oil saturation and physical properties of the effective reservoir. Low limits of lithology and oil saturation are identified from core observations, analysis of physical properties and oil testing results. Empirical statistics combined with single-layer oil testing data are used to calculate low limit of physical properties of the effective reservoir. The average tight oil thickness of 7th and 6th members of Yanchang oil-bearing formations are 6.0 and 9.3 m, respectively. The average effective porosity can be obtained by a arithmetic mean method. The porosity of 7th member of Yanchang oil-

36

bearing formation is generally 5–10% (average 8.1%), the porosity of 6th member of Yanchang oil-bearing formation is generally 9–15% (average 11.3%). The average oil saturation of reservoirs can be obtained by examining relationship between tight reservoir porosity and oil saturation from sealing core drilling. The average oil saturations of 7th and 6th members of Yanchang oil-bearing formations are 73% and 71%, respectively. The crude oil density is used 0.85 t/m3. The formation crude oil volume factor is 1.341 which is from the 3rd sub-member of 6th member of Yanchang Formation in Huaqing area. Risk factor represent degree of implementation of traps, due to restriction of geological understanding and exploration degree, exploration of tight oil resources in the Ordos Basin is still in an early exploration stage, and current understanding of tight oil accumulation mechanism needs to be improved. Therefore, recent success rate of exploratory wells is 47.3%. Results of preliminary assessment show that tight sandstone resource in Ordos Basin are approximately 2×109 t, including 0.9×109 t of the 7th member of Yanchang oil-bearing formation and 1.1 ×109 t of the 6th member of Yanchang oil-bearing formation.

5.2 Tight oil resource potential in oil-bearing shale The buried depth of oil-bearing shale in the 7th member of Yanchang Formation is mostly >1000 m. Tight oil are mainly developed in 3rd and 2nd sub-members of the 7th member of Yangchang Formation. Variations of the formation thickness are very rapid laterally, and single-layer thickness of oil-bearing shale mainly ranges from 3 to 15 m. The accumulative thickness of oil-bearing shale in the best developed area is up to 40 m and ranges from 10 m to 40 m in most areas. The tight oil resources in oil-bearing shale can be calculated using the following equation:

(3) N  GOc Bo 4 where N is the amount of resources, 10 t; G is the weight of oil-bearing shale, 10 4 t; O c is the content of retained liquid hydrocarbons, mg(hydrocarbon)/g (oil-bearing shale); and Bc is the effective oil-bearing shale ratio.

In the 7th member of Yanchang Formation, oil-bearing shale which accumulative thickness is more than 20 m are distributed in the NW SE direction along the Jiyuan - Huachi - Zhengning area (Fig. 1), with distribution area of 25×103 km2, average thickness of 11.2 m, density of 2.21 g/cm3, and weight of 994×109 t. Retained liquid hydrocarbon content of the oil-bearing shale is generally 1–3 mg/g and averages 2.64 mg/g. Testing results show that oil content >3.5% meets standard of industrial grade oil. In the 7th member of Yanchang Formation, ratio of effective oil-bearing shale that meet standard of industrial grade oil is approximately 40%. Tight oil resource in shale of the 7th member of Yanchang Formation is estimated to be >1×109 t.

H.Yang et al./Petroleum Research (2016) 1,27-38

6 Conclusions (1) In the Yanchang Formation of the Ordos Basin, tight oil has mainly developed in the semi-deep to deep lacustrine facies, and has typically accumulated in oil-bearing shales and tight sandstones of the 7th member of Yanchang oil-bearing formation as well as tight sandstones of the 6th member of Yanchang oil-bearing formation in the center of the basin. (2) Tight oil are characterized by wide spatial distribution, excellent hydrocarbon source rock conditions, tight sandstone reservoirs, complex pore throat structures, poor physical properties, high oil saturation, excellent crude oil quality, and low reservoir pressure. thus providing favourable geological conditions for tight oil accumulation and having good producing potential. (3) Tight oil reservoirs have median pore throat diameters of 20–300 nm, mainly in the range of 50–200 nm. The maximum pore throat diameter is 300–2000 nm, mainly in the range of 500–1000 nm. The critical pore throat diameter that meets requirements of hydrocarbon migration is 90 nm. Diameters of most connected pore throats of the Yanchang Formation are greater than this critical value, thereby satisfying requirements of oil and gas migration in tight reservoirs. (4) Major types of tight oil reservoirs are tight massive sandstone reservoir, sand - shale interbed reservoir, and oil-bearing shale reservoir. (5) Total tight oil resources in the Ordos Basin are approximately 3.0×109 t, includiing >1 × 109 t of tight oil in shale reservoirs and approximately 0.9 × 109 t of tight sandstone oil in the 7th member of Yanchang Formation, and nearly 1.1×109 t of tight sandstone oil in the 6th member fo Yanchang Formation.

Acknowledgements T h i s wo r k wa s s u p p o r t e d by Na t i o n a l S c i e n c e a n d Technology Major Project of China (Grant No. 2011ZX05044, 2011ZX05001), and National Key Basic Research Program (973 Program) of China (2014CB239003).

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