Reservoir characteristics and multi-stage hydrocarbon accumulation of the Upper Triassic Yanchang Formation in the southwestern Ordos Basin,NW China

Abstract

The Chang-8 and Chang-6 members of the Upper Triassic Yanchang Formation (lower part) are regarded as the main oil producing members of the Ordos Basin. Recently, new hydrocarbon discoveries have been made in the upper part of the Yanchang Formation (e.g., Chang-3) in the southwestern Ordos Basin, implying that this interval also has a good potential for hydrocarbon exploration. However, studies on the origin of the high-quality reservoir, hydrocarbon migration, and accumulation patterns remain insufficient. In this study, integrated petrological, mineralogical, and fluid inclusion tests are employed to evaluate reservoir characteristics, and reconstruct the history of hydrocarbon migration and accumulation during oil and gas reservoir formation. The results reveal that the Yanchang Formation is characterized by low porosity (8 − 14%), medium permeability (0.5 − 5 mD), and strong heterogeneity; the reservoir properties are controlled by secondary porosity. Two types of dissolution are recognized in the present study. Secondary pore formation in the lower part of the formation is related to organic acid activity, while dissolution in the upper part is mainly influenced by atmospheric fresh water associated with the unconformity surface. The Yanchang Formation underwent hydrocarbon charging in three phases: the early Early Cretaceous, late Early Cretaceous, and middle Late Cretaceous. A model for hydrocarbon migration and accumulation in the Yanchang reservoirs was established based on the basin evolution. We suggest that hydrocarbon accumulation occurred at the early stage, and that hydrocarbons migrated into the upper part of the Yanchang Formation by way of tectonic fractures and overpressure caused by continuous and episodic hydrocarbon expulsion during secondary migration, forming potential oil reservoirs during the later stage.

Keywords

Hydrocarbon accumulation reservoir characteristics inclusions Yanchang Formation Ordos Basin

Introduction

As global exploration of petroliferous basins intensifies, unconventional petroleum resources are receiving greater attention. In particular, tight oil has become the focus of hydrocarbon exploration, and great attention has been given to the formation of these reservoirs (Cao et al., 2017, 2018a, 2018b; Ghanizadeh et al., 2015; Hill et al., 2007; Vinegar et al., 2006; Wang et al., 2017a). However, owing to poor reservoir properties, strong heterogeneities, and complicated distribution of oil and water in these reservoirs, it is difficult to explain their origin and distribution using the theories associated with conventional reservoirs (Fic and Pedersen, 2013; Simpson and Fishman, 2015; Vernik and Landis, 1996; Zou et al., 2013).

The Yanchang Formation of the southwestern Ordos Basin is one of the most important areas for tight oil sandstone exploration (Li et al., 2017; Wang et al., 2017a; Yang and Deng, 2013; Yang et al., 2017a). A series of sedimentological, petrological, and geochemical studies has been conducted on tight sandstones from this interval in recent years (He et al., 2016; Pang et al., 2018; Xie, 2016; Yang et al., 2010; Zeng et al., 2010). Nevertheless, the factors influencing high quality sandstone reservoir formation in this area are still under debate. For instance, some studies conclude that these reservoirs are mainly controlled by secondary porosity formed by dissolution (Dou et al., 2017; Wang et al., 2017a), while others argue that compaction and cementation have had significant effects on reservoir porosity, and that primary pores dominated high-quality reservoir distribution (Liu, 2015; Xu et al., 2017). To date, the hydrocarbon accumulation period and accumulation model of the Yanchang Formation have not been comprehensively investigated. Li (2008) postulated that two large-scale episodes of oil-charging took place in the southwestern Ordos Basin, mainly between the early Early Cretaceous and the mid-Early Cretaceous. On the contrary, Li et al. (2006) argued that oil and gas charging in the Chang-3 reservoir occurred in two stages: the first stage during the Early Cretaceous (122 Ma) and the second during the Late Cretaceous (80 Ma). With respect to the hydrocarbon accumulation model, previous studies have proposed that the Chang-7 member represents the main source rocks and that generated hydrocarbons migrated laterally and vertical to form oil reservoirs in the lower part of Yanchang Formation (Li et al., 2012; Shi et al., 2012; Watson et al., 1987; Xia et al., 2019; Zhang et al., 2019; Zou et al., 2013). Recent discoveries in several exploration wells indicate that the upper Yanchang Formation (Chang-3, 4, and 5) are also potential economic oil reservoirs (Wang et al., 2005; Xiao et al., 2011); however, hydrocarbon accumulation in this model is poorly understood. Therefore, it is necessary to examine how hydrocarbons migrated and accumulated in the upper of Yanchang reservoirs across the southwestern Ordos basin.

In this study, comprehensive tests are employed to reveal the diagenesis and reservoir characteristics of the Yanchang sandstones. These tests include scanning electron microscopy (SEM), physical property tests, and mercury injection. Furthermore, fluid inclusion analysis is used to define the details of hydrocarbon migration and accumulation during the Yanchang period. The results of this study will contribute to the prediction of quality reservoir distribution in the Ordos Basin as well as other basins worldwide with similar sedimentary characteristics.

Geological setting

The Ordos Basin is a multicyclic composite basin with a complex basement, located between the stable zone of eastern China and the western active area (Chen et al., 2018). Structurally, the Ordos Basin is rectangular in shape and is bounded by the Yinshan Mountains to the north, the Qinling Mountains to the south, the Liupan Mountains to the west, and the Lvliang Mountains to the east. The basin spans five provinces in China and covers an area of 250,000 km². Based on the structure and evolutionary history, the Ordos Basin can be divided into six secondary tectonic units, including the Yimeng Uplift, Jinxi Fold Belt, Yishan Slope, Tianhuan Depression, West Margin Thrust Belt, and Weibei Uplift (Figure 1(a); Yang et al., 2005).

The Ordos Basin contains an Archaean and early Proterozoic metamorphic crystalline basement, which is overlain by late Proterozoic, Paleozoic, Mesozoic, and Cenozoic deposits (Chen et al., 2001; Li and Huang, 2013). The entire sedimentary sequence of this basin is more than 6000 m thick, and the geological history of the Ordos Basin can generally be divided into three phases (Figure 1(b)). (1) From the Meso-Neoproterozoic to the Lower Paleozoic, the lithological record in the Ordos Basin is dominated by marine carbonate deposition (Hao et al., 2016; Zhang et al., 1984). (2) During the Caledonian movement, the Ordos Basin was entirely uplifted and experienced a sedimentary hiatus of ∼150 Ma. As a result, there is no record of Silurian, Devonian, or Lower Carboniferous deposits (Hu et al., 2017, 2019; Xie, 2016). (3) Beginning in the early Permian, the Central Asian Orogeny resulted in the conversion from marine to non-marine sedimentation (Liu and Yang, 2000; Ritts et al., 2004). Thus, the deposits of the Ordos Basin are composed mainly of sandstone, mudstone, and thick coal beds deposited during the Permian and Triassic.

Figure 1.

Location, tectonic setting, stratigraphy of the Ordos Basin (modified after Xu et al., 2017; Yuan et al., 2007; Zeng and Li, 2009). C: Chang.

The study area is located in the southwestern Ordos Basin, covering two secondary tectonic units, the Tianhuan Depression and the Yishan Slope (Figure 1(a)). The Upper Triassic Yanchang Formation was deposited in a large inland lacustrine system (Fan et al., 2018; Yang et al., 2017b; Zou et al., 2010), and records a complete sedimentary cycle. The Yanchang Formation can be divided into 10 members based on lithologic characteristics (Yang, 2004). From bottom to top, these members have been named Chang-10 to Chang-1 (Figure 1(c)). The Chang-10, -9, and -8 members are mainly composed of medium- to coarse-grained sandstone interbedded with mudstone, with thick-bedded conglomerate (Wang et al., 2017a). Chang-7 deposits comprise mudstone and oil-shale, and are interbedded with thick-bedded fine-grained sandstone that represent a deep lacustrine sedimentary environment (Xu et al., 2017). The Chang-6, -5, -4, and -3 members are composed of fine to very fine-grained sandstones interbedded with mudstone and coal seams, which represent lacustrine shrinkage (Zhao et al., 2011; Zou et al., 2012). There is almost no record of the Chang-2 and Chang-1 deposits in the study area owing to the telogenetic stage of tectonic uplift (Liu et al., 2013); the Lower Jurassic sediments unconformably overlie the Chang-3 member (Figure 1(c)).

Samples and analytical methods

A total of 320 samples in 31 wells were collected from the Upper Triassic Yanchang Formation in the study area. Among them, 176 casting thin sections were examined under an electron microscope to identify minerals and pore characteristics. Petrographic analysis was performed using a Nikon microscope with a 100-W bulb as the light source, and a Nikon DXM digital photomicrography system captured the images. A total of 20 samples were examined using a field emission SEM equipped with an Oxford Aztec X-Max 150 energy dispersive spectroscopy system to characterize their mineral compositions. The maximum magnification is up to a million and the vacuum degree of the samples was less than 6 × 10⁻⁴–4000 Pa. Mercury injection measurements were carried out on a Pore Sizer 9320 mercury porosimeter; the maximum pressure was 136 MPa and the lower limit of the pore-throat radius was 0.0054 µm. A total of 53 samples were used for conventional porosity and permeability measurements, which were carried out using helium porosimetry and gas permeability. The precision of these measurements were 10⁻⁴ mD and 0.1%, respectively.

A total of 67 samples were obtained from wells Z164, Z337, and Z395, which are located in the west, middle, and east of the study area, respectively (Figure 1(a)). These samples mainly cover Chang-8 to Chang-3, which is considered the deepest general exploration target member in the study area. First, fluid inclusion samples for microthermometric analysis were selected using a microscope. Then, homogenization temperature measurement data were obtained using a THMSG-600 heating-cooling stage designed and manufactured by Linkam (UK). Before the inclusion temperatures were measured, standard samples were used to test the heating and cooling system and to calibrate the Linkam TH-600 program (Wan et al., 2017; Wang et al., 2016, 2017b). Homogeneous temperatures (accuracy: ±0.1°C) were obtained by cyclic testing using a heating rate of 5°C/min with a laboratory temperature of 23.5°C.

All analyses were carried out at the State Key Laboratory of Oil and Gas Reservoir Geology and Exploration (Chengdu University of Technology), Chengdu, China.

Results

Petrological characteristics

Core observation and petrographic analysis (Figure 2) show that the Yanchang sandstones in this area are dominated by feldspathic litharenite (Figure 3(a)), lithic feldspathic sandstone (Figure 3(b)), and litharenite (Figure 3(c)). The sandstones contain a high content of feldspar and rock fragments. The quartz, feldspar, and rock fragment contents vary from 40% to 60%, 20% to 40%, and 10% to 25%, respectively.

Figure 2.

Mineral composition of the Yanchang sandstones in the southwestern Ordos Basin. Q: quartz; F: feldspar; L: lithic fragments.

Figure 3.

Typical thin sections and SEM images showing lithology and pore characteristics of the Yanchang sandstones. (a) Feldspathic litharenite, medium to fine grained, with dissolution fracture. The red arrow indicates fracture. Z277 well, Chang-8. (b) Lithic feldspathic, and feldspar partially dissolved. The red arrow indicates feldspar dissolved pore, Z255 well, Chang-8. (c) Litharenite, feldspar and quartz dissolved (red arrow), Z73 well, Chang-3. (d) Feldspar dissolved intergranular pore (red arrow), Z92 well, Chang-8. (e) Authigenic kaolinite with vermicular or book-page pattern (red arrow), Z236 well, Chang-3. (f) Very fine dolomite fragment between grains (red arrow), Z69 well, Chang-4 + 5. (g) Quartz dissolved intergranular pore (red arrow), Z9 well, Chang-3. (h) Illite with intergranular micropore (red arrow), Z80 well, Chang-4 + 5. (i) Feldspar dissolved (red arrow). X119 well, Chang-6. All photomicrographs were taken under cross-polarized light, except (e), (h), and (i) which was taken under SEM.

The Yanchang Formation sandstones are medium–fine grained, with grain size mainly ranging from 0.1 to 0.25 mm. Most samples have moderate to good sorting and poor psephicity, with angular to subangular shapes (Figure 3(d)). These sandstones primarily exhibit line contact, as well as point contact and concavo-convex contact. Generally, the Yanchang Formation sandstones show moderate to poor textural maturity and low compositional maturity. The cement content ranges from 1% to 30% (mean = 6%) and mainly consists of authigenic clay minerals (Figure 3(e)), calcite, minor silica, dolomite (Figure 3(f)), and siderite.

Pore and fracture features

Observation of thin sections and scanning electron micrographs show that residual primary intergranular pores and micro-pores, dissolved intergranular pores, and dissolved intragranular pores are the major pore types in the Yanchang sandstones. Residual primary intergranular pores refer to the pores that remain after compaction and partial filling during deposition. The pore sizes vary, being generally larger than 10 µm, and often triangular or irregular polygonal (Figure 3(g)). This type of pore is the most common in the study area, accounting for more than 35% of all pores. Residual primary intergranular micro-pores are developed among fine cement minerals. Such pores are formed after mechanical compaction and filling by various cements, and are less than 10 µm in size (Figure 3(h)). The content of such pores is small and their contribution to the reservoir is relatively limited.

Intergranular dissolution pores are enlarged pores formed from primary intergranular pores that were modified by dissolution. They are characterized by irregular pores, serrated or embayed edges, coarse pore throat structures, and good connectivity (Figure 3(d)). The pore diameter is generally 10 − 600 µm, accounting for 35% of the total volume of pores in the Yanchang sandstones. Intragranular dissolution pores, accounting for ∼10% of all pores, mainly exist in rock fragments and feldspar, and are formed by partial dissolution of these particles (Figure 3(b) to (i)). They are generally irregular in shape (e.g., embayed edges) and of different sizes (10 − 100 µm), with low planar porosity.

There are three types of fractures developed in the study area, representing just 10% to 15% of the total volume of pores; however, they are significant for the development of the reservoir. The first type forms when mineral particles crack or along cleavage cracks formed by mechanical compaction (Figure 3(a)). The second type is tectonic fractures that form when rocks are compressed or stretched. The third type is bedding fractures that formed during sediment deposition. In this study, the number of cracks and their plane porosity are generally small. The unfilled cracks contribute greatly to the permeability of rocks. In particular, tensile fractures can effectively improve the reservoir space and are effective seepage channels for fluids.

Pore-throat structure

The pore-throat structure refers to the geometric shape, size, distribution and connectivity of pores and throats in rocks. Generally, mercury injection tests and capillary pressure curves are used to analyze pore structure characteristics (Schlueter et al., 1997). The capillary pressure curves from two typical Yanchang sandstones (Figure 4) show that the entry pressure is relatively low, ranging from 0.18 to 0.44 MPa (average 0.29 MPa). The median pressure varies from 1.86 to 2.65 MPa. The pore throat sorting is not very good, with the sorting and variation coefficients in the range of 2.13 − 2.45 and 0.21 − 0.26, respectively. The maximum mercury saturation is 85.67 − 92.23%. Mercury removal efficiency is moderately low, ranging from 18.16% to 33.31%. On the whole, the pore-throat condition of sandstones in Yanchang Formation is medium, belonging to the fine pore throat−fine pore micro throat type. Besides, some samples develop small pores and medium throats, with poor–medium pore-throat connectivity.

Figure 4.

Capillary pressure curve of the Yanchang sandstones from Well Z202 (2143.20 m) and Z29 (2013.60 m) in the southwestern Ordos Basin.

Reservoir porosity and permeability

The porosity (φ) and permeability (K) results of the sandstones in the Yanchang Formation are presented in Figure 5; the porosity is mainly concentrated around 8 − 14%, which is consistent with results from the Jiyuan, Huanjiang, and Longdong areas of the southwestern Ordos Basin (Dou et al., 2018; Xi et al., 2019; Xu et al., 2017; Zhou et al., 2016). The permeability ranges from 0.5 to 5 mD, which is obviously higher than that found in other recent studies of the Yanchang Formation (Dou et al., 2018; Xi et al., 2019; Xu et al., 2017). According to the evaluation criteria of oil and gas reservoirs by National Energy Administration of China (SY/T 6285–2011; 10 < φ < 15, 1 < K < 50 mD), the Yanchang reservoirs are classified as ultra-low porosity and permeability (Zeng and Li, 2009). However, recent research suggests that the permeability of the Yanchang sandstones (>0.1 mD) should not be classified as low permeability (Jia et al., 2012). It is generally believed that the Yanchang Formation sandstones of the Ordos Basin represent a tight oil reservoir, with some “sweet spot” high permeability reservoirs distributed throughout owing to diagenetic heterogeneity (e.g., Dou et al., 2018). Thus, the reservoir in the present study is generally characterized by low porosity and medium permeability, with local low permeability reservoirs. Furthermore, the porosity and permeability cross-plot (Figure 6) show that there is a relationship between porosity and permeability, with a correlation coefficient of 0.65. This shows that the reservoir has strong heterogeneity and that secondary pores contribute greatly to the reservoir properties.

Figure 5.

Distribution of porosity and permeability of the Yanchang sandstones in the southwestern Ordos Basin.

Figure 6.

Plot of porosity vs. permeability of the Yanchang sandstones in the southwestern Ordos Basin.

Fluid inclusions

Based on the occurrence of diagenetic minerals and fluid inclusions, three types of petroleum inclusions were identified in this study. All fluid inclusions in study area are predominantly two-phase saltwater (Figure 7). Type 1 fluid inclusions are distributed in early cracks of quartz particles (Figure 7(a) and (b)). They are usually light in color and elliptical in shape, with a size of 2 × 2 to 5 × 5 µm. This type is mainly found in the Chang-7 and Chang-8 members, with a few such fractures are found in the Chang-4, 5, and 6 members. Type 2 fluid inclusions are distributed along the secondary overgrowth edges of quartz (Figure 7(c) and (d)) and carbonate cements. Their color is light gray to light yellow under fluorescence. They are generally smaller than Type 1 inclusions. These represent the most abundant inclusions and they are distributed in each member of the Yanchang Formation. Type 3 fluid inclusions are distributed in late cracks of quartz particles (cutting through the earlier cracks; Figure 7(e)), and liquid oil can be observed (Figure 7(f)). Their shape is usually elliptical with a maximum size of 8 × 8 µm. These represent the least abundant inclusion type. They are concentrated mainly in the Chang-3 member, with small numbers found in the Chang-4, 5, and 6 members.

Figure 7.

Photomicrographs of representative hydrocarbon-bearing fluid inclusions from Yanchang sandstones.

Distribution characteristics of homogenization temperature

A total of 219 test points of fluid inclusion thermometry were obtained from 67 samples. The Yanchang Formation sandstone samples had mean homogenization temperatures ranging predominantly from 80°C to 140°C (Figure 8). The continuous distribution of the homogeneous temperature values indicates that the study area has the characteristics of continuous charging. At certain points, the temperature was found to be higher than 140°C, which might indicate that those inclusions are primary. Therefore, such values are not used in the following analysis and discussion. According to the distribution of the homogenization temperature values, the inclusions can be divided into three assemblages.

Figure 8.

Histograms showing homogenization temperatures measured from different types of fluid inclusions.

As a whole, Type 1 fluid inclusions have homogenization temperatures ranging from 80°C to 100°C with a peak at 83.6°C (Figure 8(a)), Type 2 inclusions have homogenization temperatures ranging from 110°C to 130°C with a peak at 121.6°C (Figure 8(b)), and Type 3 fluid inclusions have homogenization temperatures ranging from 90°C to 105°C with a peak at 100.1°C (Figure 8(c)). These results suggest that the inclusions represent fluids belonging to three stages. In addition, the distribution of the homogenization temperatures is different in each member. In the Chang-7 and Chang-8 members, fluid inclusions mainly consist of Type 1 and Type 2 inclusions, with a few Type 3 inclusions (Figure 9(a)). In the Chang-4, 5, and 6 members, most inclusions belong to Type 2; Type 3 inclusions are slightly less abundant than Type 1 inclusions (Figure 9(b)). However, in the Chang-3 member, most inclusions are Type 3, with just a few inclusions belonging to the other two types (Figure 9(c)).

Figure 9.

The proportion of three types of fluid inclusion. (a) Chang-7 and Chang-8 members; (b) Chang-6 and Chang-4 + 5 members; (c) Chang-3 member.

Discussion

Origin of secondary pores

Detailed petrological observations and analyses indicate that diagenesis was the key factor in the improvement of the quality of the Yanchang Formation sandstone reservoirs; however, some diagenesis may have had opposite effects (e.g., compaction and cementation). The thin sections and scanning electron micrographs demonstrate that there are various diagenetic processes occurring in the Yanchang sandstones, including compaction, cementation, dissolution, and metasomatism (Figure 10). The distribution of effective reservoir spaces appears to have a close relationship with four main factors. (1) Mechanical compaction: strong compaction with minor pressure dissolution will result in the drastic loss of primary pores, but also generates a small number of secondary pores. (2) Cementation: early calcite cement, siliceous cement, and late ferrodolomite cement significantly reduced the secondary pores. (3) Dissolution: the dissolution of feldspar and lithic fragments in the Yanchang sandstones, which produced a large number of secondary dissolution pores, improved the quality of the reservoir (Al-Areeq et al., 2016). (4) Clay minerals: kaolinite, illite, and chlorite are commonly present in the study area and affect the development of secondary pores. For instance, chlorite may have protected primary porosity by preventing the growth of quartz cement (Bloch et al., 2002).

Figure 10.

Diagenetic processes of the Yanchang sandstones in the southwestern Ordos Basin.

In addition, recent studies have emphasized that reservoir quality is significantly affected or even controlled by the content of diagenetic minerals (Pallatt et al., 1984; Peltonen et al., 2009). Figure 11 depicts the relationships between authigenic mineral content and porosity. There are no obvious correlations among dolomite cements, siliceous cements, and porosity (Figure 11(a) and (b)), indicating that the impact on porosity improvement is unclear in this study. Chlorite and kaolinite contents are positively correlated with porosity (Figure 11(c) and (d)), because kaolinite is produced when feldspar dissolves, which is accompanied by the development of dissolution pores. Early authigenic chlorite coatings prevent the growth of quartz cement, which is conducive to the preservation of primary pores.

Figure 11.

Plots of porosity and (a) dolomite cements; (b) siliceous cements; (c) chlorite cements; and (d) kaolinite cements.

As discussed, dissolution is the primary factor controlling the distribution of high-quality reservoirs. However, the main reason for large-scale dissolution remains a subject of debate. Some scholars suggest that the organic acids generated during oil and gas migration are the main reason for the occurrence of dissolution pores throughout the Yanchang Formation (Yuan et al., 2013; Zhou et al., 2016). However, the evidence obtained in this study does not fully support this model. Plots of the vertical distribution characteristics of feldspars, kaolinites, porosity, and dissolution pores are presented in Figure 12. The feldspar content increases with distance from the unconformity in the upper part of the Yanchang Formation; whereas the kaolinite content, dissolved pores, and porosity have the opposite trend, indicating that most dissolved secondary pores were strongly associated with the Indosinian unconformity (Dou et al., 2017; Yang et al., 2013). In low temperature and open systems, kaolinite is the major diagenesis mineral rather than illite as the dissolution product of feldspar (Ding et al., 2014). This tectonic uplift event forms numerous kaolinites by the dissolution of feldspars near the unconformity, as described in equation (1)

\begin{array}{l} {2KAl}_{2} {Si}_{3} O_{8} + {16H}_{2} O = {Al}_{2} {Si}_{2} O_{5} {(OH)}_{4} + {2K}^{+} + {4SiO}_{2} + 2 {(OH)}^{-} + {13H}_{2} O \end{array}

(1)

Figure 12.

Plot of the vertical distribution characteristics of feldspars, kaolinites mass fraction, porosity, and dissolution pores.

Thus, atmospheric freshwater associated with the unconformity will lead to a positive correlation between depth and feldspar content. In addition, dissolved pores and porosity decrease with distance from the unconformity, indicating that this unconformity and the subsequence effects (i.e. atmospheric freshwater) play an important role in dissolution. Similar relationships between mineral contents and depth have been observed elsewhere (Ding et al., 2014), which also confirms the existence of atmospheric freshwater in the upper part of the Yanchang Formation. In summary, large-scale dissolution controlled by atmospheric freshwater should have been synchronized with the Indosinian tectonic movement.

It is noteworthy that the porosity is relatively stable below the Chang-4 and 5 members, and shows no correlation between porosity and distance from the unconformity (Figure 12) in the lower Yanchang Formation. This indicates that the formation of secondary pores caused by dissolution is mainly related to organic acids in the middle and lower part of the Yanchang Formation, as described by equation (2)

{2KAl}_{2} {Si}_{3} O_{8} + {2H}^{+} + H_{2} O = {Al}_{2} {Si}_{2} O_{5} {(OH)}_{4} + {4SiO}_{2} + {2K}^{+}

(2)

In fact, the Chang-8 and Chang-6 sandstone reservoirs are adjacent to the Chang-7 source rocks (Shi et al., 2012; Yang et al., 2010), which could have released sufficient organic acids to dissolve minerals during hydrocarbon generation and migration.

Episodes of hydrocarbon migration

Since the discovery and development of the Yanchang Formation reservoirs in the Ordos Basin began in the 1980s, many scholars have investigated the hydrocarbon accumulation periods in this formation. However, the timing of accumulation remains a topic of debate (e.g., Li et al., 2006, 2008). In summary, most previous studies only involved a single member and, to our knowledge, there has been no analysis on the accumulation period of the whole Yanchang Formation. This may also be the reason why the problem of oil and gas charging has not been resolved to date.

In this work, based on results of the homogenization temperature tests above the Chang-8 member from Well Z337 and previous research on the reconstruction of both the burial and thermal histories (Ren et al., 1994; Zhao et al., 1996), we established an oil and gas charging history diagram of the Yanchang Formation in the study area. As shown in Figure 13(c), the first episode of oil and gas charging of the Yanchang Formation occurred from the Late Jurassic to the early Early Cretaceous (∼135–125 Ma). The second episode of hydrocarbon charging occurred at the end of the Early Cretaceous (∼125–100 Ma). During this period, the basin continued to subside and the Yanchang Formation was at its maximum burial depth (Liu and Yang, 2000; Yang et al., 2005). The third episode of oil and gas charging occurred during the middle Late Cretaceous (∼88–65 Ma). At that time, Late Yanshanian tectonism caused renewed uplift of the basin (Li et al., 2012).

Figure 13.

Diagram of the burial–thermal evolution and the oil and gas infill history of the Yanchang Formation in the southwestern Ordos Basin. C: Chang; Fm: Formation.

Hydrocarbon accumulation model

To establish a hydrocarbon accumulation model in the Yanchang stage, we first addressed the oil source problem. A relatively consistent understanding of the hydrocarbon source rocks of Mesozoic reservoirs in the Ordos basin has been obtained. It is generally believed that the oil of the Yanchang Formation mainly comes from the high-quality source rocks of the Chang-7 reservoir member (e.g., Watson et al., 1987; Zou et al., 2013). However, the hydrocarbon accumulation model of the Yanchang Formation remains unclear owing to a lack of research. At present, existing research based on a study related to a single member among the Yanchang sandstones suggests that there are three modes of reservoir formation. (1) The Chang-8 member reservoir is in direct contact with the source rock for hydrocarbon accumulation (Li, 2008). (2) Chang-6 member hydrocarbon accumulation occurred in a meandering river delta under lacustrine regression. (3) Chang-8 member hydrocarbon accumulation occurred in a braided river delta under lacustrine transgression (Yang, 2004).

In this study, based on the inclusion temperature data and regional tectonic evolution, we suggest that the accumulation process of the Yanchang Formation in the southwestern Ordos basin can be divided into three stages (Figure 14). Firstly, hydrocarbons accumulated in a nearby reservoir (Figure 14(a)) owing to the low maturity of the early source rocks (Ro ≈ 0.7%; Guo, 2017). The vertical fractures that run through the entire Yanchang Formation had not yet formed during this period. The main source rocks of the lower Chang-7 member cannot communicate with non-neighboring strata. Therefore, the generated hydrocarbons from the Chang-7 oil shale could migrate upward or downward into the neighboring reservoirs as a result of buoyancy and overpressure (Xu et al., 2017). However, the obliquity of the Yanchang Formation across the southwestern Ordos Basin is gentle. Thus, the buoyancy energy is weak in study area (Guo, 2017; Zhao et al., 2003). Furthermore, the permeability of the Yanchang sandstones is generally low, except for local medium permeability “sweet spot” areas. Therefore, there is little possibility of long-distance oil migration. According to the results of the inclusion analysis, we believe that the target of the oil-charging in this time was the Chang-7 self-generating reservoir and the Chang-8 reservoir, which had sufficient contact with the Chang-7 source rocks. As a result, the amount of oil and gas generated during this period was limited because of low maturity source rocks, and in turn, only a small portion of the oil and gas charging occurred.

Figure 14.

Hydrocarbon migration and accumulation model of the Yanchang reservoir, southwestern Ordos Basin. (a) Early stages of the Early Cretaceous: the maturity of the source rocks was low, hydrocarbon primarily from neighboring migration and accumulation. (b) Late stages of the Early Cretaceous: mass generation and expulsion of hydrocarbons from the source rock, large-scale oil and gas migration and accumulation. (c) Middle of the Late Cretaceous: tectonic uplift occurred, and the oil and gas charging process stopped temporarily, hydrocarbons migrated into the Chang-3 reservoirs.

As the maturity of the source rocks increased (Ro ≈ 1.0%) under a deep burial depth, the generated hydrocarbons from the Chang-7 source rocks began to accumulate in large-scale reservoirs during the late Early Cretaceous (Figure 14(b); Guo, 2017). During the burial process, rock densification caused by diagenesis (e.g., compaction) in the Yanchang sandstones formed a typical sandstone reservoir of low porosity and low–medium permeability (Dou et al., 2018; Zeng and Li, 2009). The extremely high pressure produced by expansion linked with hydrocarbon generation in the Chang-7 member becomes the driving force for hydrocarbon migration (Yang et al., 2007; Xu et al., 2017). Although the basin was tectonically stable during this period, the buoyancy and overpressure exerted an important effect on large-scale hydrocarbon migration at this time. In particular, abnormally high pressure caused by large-scale episodic expulsion (Xu et al., 2017) is observed in the southwestern Ordos Basin (Guo et al., 2015). Generated hydrocarbons not only charged the neighboring Chang-8 to Chang-7 reservoirs, but also migrated along the vertical fractures to the upper Chang-4, 5, and 6 reservoirs.

Finally, the accumulated oil and gas in the reservoirs was readjusted because of tectonic uplift, and hydrocarbon generation ceased in the middle Late Cretaceous (Figure 14(b)). Tectonic uplift of the entire strata resulted in a series of NW–SE and NE–SW cracks in the Ordos Basin (Watson et al., 1987; Xie, 2016). These fractures served as migration pathways and facilitated vertical communication of hydrocarbon in different members of the sandstone reservoirs. Wang and Cao (2019) proposed that the Chang-2 to Chang-5 members are structural-lithologic reservoirs, while Chang-6 and the lower members are lithologic reservoirs. Consequently, oil and gas that accumulated primarily in the reservoirs from Chang-8 to Chang−4+5 could secondarily migrate into the upper part of the Yanchang Formation (e.g., the Chang-3 member) due to tectonism and overpressure.

Conclusions

In this study, comprehensive test methods including SEM, mercury injection, and fluid inclusion tests were employed to demonstrate the diagenesis and reservoir characteristics of the Yanchang sandstones in the southwestern Ordos Basin. Three main conclusions can be drawn from the results of this study.

The Yanchang sandstones in the study area are mainly composed of fine-grained feldspar lithic sandstone, lithic feldspar sandstone, and lithic sandstone. These sandstones generally have a residual primary intergranular and intergranular dissolution pore and medium pore structure with low porosity and low–medium permeability.

The Yanchang sandstones have experienced strong diagenesis, and dissolution is the most important factor in high-quality reservoir distribution. The dissolution of feldspar and rock lithic fragments is caused by meteoric water associated with the unconformity surface in the upper Yanchang Formation, which mainly formed owing to organic acid in the lower part.

The distributions, homogenization temperatures, and physical characteristics of hydrocarbon inclusions reveal that the Yanchang Formation experienced three episodes of hydrocarbon charging. The first stage occurred during the early Early Cretaceous (135–125 Ma), mainly forming the initial Chang-7 and Chang-8 reservoirs, with small-scale hydrocarbon generation and accumulation. The second stage occurred in the late Early Cretaceous (125–100 Ma) when hydrocarbon charging covered the Chang-8 to Chang-4 reservoirs. Finally, the accumulated oil and gas in the reservoirs was readjusted owing to tectonic uplift, with hydrocarbon generation ceasing in the middle Late Cretaceous (88–65 Ma). Secondary hydrocarbon was charged and accumulated in the Chang-3 reservoir by tectonism and overpressure caused by large-scale continuous and episodic expulsion.

Footnotes

Acknowledgements

Careful reviews and constructive suggestions of the manuscript by associate editor Prof Qin Shenjun and anonymous reviewers are greatly appreciated.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This paper was jointly sponsored by the National Natural Science Foundation of China (Nos. 41872109, 41602107, and 41702129), Open found of State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (Chengdu University of Technology; No. PLC20180307), Open Research Fund Program of Hunan Provincial Key Laboratory of Shale Gas Resource Utilization, Hunan University of Science and Technology (No. E21820), Chongqing Research Program of Basic Research and Frontier Technology (No. cstc2017jcyjAX0448), and the Science and Technology Research Program of Chongqing Municipal Education Commission (No. KJQN201800115).

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