Tectonic fractures refer to fractures whose formation and distribution are controlled by local tectonic events or the tectonic stress field. Types, development degrees, and occurrences of tectonic fractures are dependent on the stress field distribution and tectonic position (Hou, 1994; Zeng et al., 2007; Yu et al., 2016). Tectonic fractures are widely developed in field outcrops and mainly include shear and tensile fractures. Some shear fractures present themselves in a conjugate arrangement (typically, X-shaped) in field outcrops (Figures 3A, B), with stable occurrence and long extension. The fracture surface is flat and smooth and is often found with striations. Field observation shows that shear fractures have a length range of 1–5 cm and an aperture range of 0.02–0.1 cm. Tensile fractures are scattered, with limited extensions and zigzag fracture surfaces. Their lengths are mainly 1–2 cm and the aperture is 0.02–0.3 cm (Figure 3C). In cores, some shear fractures only have one part of the expected X-shape (Figure 3E), which may be attributed to the anisotropic compressive strength associated with rock heterogeneity (Zeng et al., 2008). The core observation shows that fillings inside tectonic fractures are mostly quartz and calcite, with sporadic argillaceous fillings.

Diagenetic fractures are near-horizontal fractures generated via geological processes such as pressure solution, compaction, and pressure relief (He et al., 2011; Luo et al., 2017; Zeng et al., 2017). Diagenetic fractures in this study are predominantly bedding seams, with main occurrences at interfaces of bedding. They are parallel to each other and follow the directions of rock beds and micro-bedding. At the outcrop scale, these fractures have long extensions of 5–15 m and yet limited apertures of 0.05–0.1 cm (Figure 3D). At the core scale, they can penetrate the whole core (Figures 3F, G) with apertures of 0.02–0.1 cm. Field outcrops and drilling cores both show that diagenetic fractures are mostly filled with calcite, quartz, argillaceous minerals, and hydrocarbons.

Abnormal overpressure-induced fractures are found in cores, which are mostly drainage fractures formed via hydraulic processes (Liu et al., 2017; Zhang et al., 2017; Zeng et al., 2017). They are mostly observed at the core scale (Figures 3H, I) and are less seen in the field outcrop. They present irregular distribution in the form of vein groups and curved extensions in cores. Their apertures are highly variable—ranging from 0.5 mm to 10 mm, with a maximum of 20 mm. Their lengths are generally several centimeters. Most of them are filled and have low oil content, as seen in cores.

Brittle minerals such as calcite and quartz can normally promote the development of fractures (Zeng and Li, 2010). However, this kind of promotion varies from mineral to mineral. There are three methods of mineral brittleness evaluation (Diao, 2013; Wang et al., 2013; Wan et al., 2016), which are respectively based on i) Young’s modulus and Poisson ratio (elastic parameters), ii) the relative content of brittle and clay minerals, and iii) mineral composition. Particularly, carbonate rocks are found to significantly contribute to the brittleness of reservoirs rich in carbonate minerals (Fridrun et al., 2015).

The studied Lucaogou Formation has a high content of carbonate rocks, with a wide range of 3.6%–87.3% and an average of 45.8%. Therefore, carbonate rocks should be one of the most influential factors of reservoir brittleness. In this study, the brittleness is characterized by the ratio of different minerals:

Quartz brittleness = quartz/(quartz + carbonate minerals + clay minerals).

Carbonate mineral brittleness = carbonate minerals/(quartz + carbonate minerals + clay minerals).

Bulk-rock brittleness = (quartz + carbonate minerals)/(quartz + carbonate minerals + clay minerals).

As shown in Figure 4, the studied reservoir has high brittleness, with bulk-rock brittleness above 0.5. The bulk-rock brittleness presents no notable correlation with quartz brittleness (Figure 4A), while it seems to linearly scale with the carbonate mineral brittleness (Figure 4B). Therefore, carbonate minerals are more important to the bulk-rock brittleness.

As illustrated in Figure 5, feldspar, quartz, pyrite, calcite, and dolomite have absolute controls on the development of tectonic fractures. In contrast, they have no notable control on the development of diagenetic fractures (Figure 6). Specifically, dolomite and calcite are the main controlling factors of tectonic fractures (Figure 5).

In this study, samples are collected from intervals that are far away from faults, belong to the same tectonic belt, and have similar bed thicknesses. The linear densities of tectonic and diagenetic fractures are summarized and plotted for several lithologies, including dolomite, dolomitic sandstone, dolomitic mudstone, limestone, siltstone, fine-grained sandstone, medium-grained sandstone, coarse-grained sandstone, conglomerate, sandy mudstone, muddy sandstone, limy mudstone and mudstone of the upper and lower sweet spots respectively.

As illustrated in Figure 7, the same lithology has the same control on fracture development in the upper and lower sweet spots. Tectonic fractures are more developed in dolomitic rocks (Figure 7A), with linear densities of 2.80 m^-1 in dolomite, 2.46 m^-1 in dolomitic sandstone, and 1.97 m^-1 in dolomitic mudstone for the upper sweet spot; those values for the lower sweet spot are 3.18, 2.25 and 2.29 m^-1, respectively. This is because dolomitic rocks are more brittle—under the same stress conditions, their bearable strains before cracking are smaller and they are more prone to generating tectonic fractures than soft/plastic rocks (Zeng et al., 2008). For both the upper and lower sweet spots, the linear density of tectonic fractures drops in the order of limestone, siltstone, fine-grained sandstone, medium-grained sandstone, and conglomerate, which indicates the control of rock particle sizes on the development of tectonic fractures. With smaller particles and lower pore volumes, rocks are associated with higher rigidity brittleness and are easier to crack under tectonic stress. Muddy rocks tend to absorb more stresses via plastic deformation, and the resultant development of tectonic fractures is relatively low.

The control of lithology on diagenetic fractures is considerably different from that on tectonic fractures. Diagenetic fractures are more developed in argillaceous rocks (i.e., mudstone, dolomitic mudstone, muddy sandstone, and limy mudstone) (Figure 7B), with the maximum linear density in mudstones, reaching 4.81 m^-1. Diagenetic fractures are also well-developed in carbonate rocks (dolomite and muddy dolomite). The plentiful diagenetic fractures in muddy rocks are attributed to the intensive hydrocarbon generation and expulsion of organic matter in such rocks, which can cause dissolution and pressurization.

To conclude, lithology has strong control effects on the development of tectonic and diagenetic fractures. Specifically, tectonic fractures are most developed in dolomite, with an average density of 2.96 m^-1. Compared with siltstone and fine-grained sandstone, argillaceous rocks have lower development of tectonic fractures, as they have larger plastic deformation to absorb more stresses under the same tectonic condition. Meanwhile, medium-grained sandstone, coarse-grained sandstone, and conglomerate have the least developed tectonic fractures, because their larger particles and higher pore volumes lead to lower strengths that, in turn, promote resistance to cracking after elastic deformation. As for diagenetic fractures, they are most developed in mudstone, with an average density of 4.85 m^-1, which is attributed to the organic acid dissolution and pressurization during hydrocarbon generation and expulsion.

Tectonic stress represents a main control on the reservoir fracture development. It is easier to form fractures in more tectonically-active zones. Statistics of field outcrops show that tectonic stress has consistent controls on tectonic and diagenetic fractures. With the same lithology, the tectonic fracture density grows in zones nearer to faults, and the same pattern is also identified for diagenetic fractures (Figure 8), due to the opening of weak bedding planes driven by tectonic stress. It is also noted in field outcrops that the densities of tectonic and diagenetic fractures are higher around larger faults than those near smaller faults. In addition, field outcrops are often associated with conjugate shear fractures, while cores tend to present a single shear fracture. This may be attributed to the underground confining pressure, which in turn reflects the control of tectonic stress on fracture development.

Observation of field outcrops and cores reveals that mudstone and sandstone alternate with each other in forms of interbedding, which leads to the higher development of bedding seams/diagenetic fractures. To eliminate the effects of dissolution, the following analysis involves only sandstone, since dolomite contains higher contents of carbonate minerals. Statistics suggest that tectonic fractures and bedding seams are both affected by bed thickness. Their density climbs up, as the bed becomes thinner. The bed thickness over 3 m is found with the least development of fractures; on the contrary, the highest development of fractures occurs in the case of the bed thickness below 0.5 m (Figure 9).

Under the same stress, organic matter abundance is one of the factors affecting fracture development in mudstones (Liu et al., 2017; Zhang et al., 2017). The control of TOC on fracture development depends on organic matter distribution, organic matter and water consumption by hydrocarbon generation, and hydrocarbon generation-induced pressurization. The existence of organic matter bands becomes more prominent and influential, with higher organic matter abundance, and micro-fractures tend to occur inside and at the edge of such organic matter bands (Zhang et al., 2017).

The Lucaogou Formation in the Jimsar Sag has extensive development of argillaceous source rocks (Kuang et al., 2014), with TOC mostly of 1.29%–13.82% and averaging 5.34%. Fractures are created via the pressurization and dissolution processes during hydrocarbon generation and expulsion.

Microfractures often occur inside and at the edge of the organic matter (Figure 10), which may be attributed to the consumption of water and organic matter by hydrocarbon generation or the pressurization also induced by hydrocarbon generation (Zhang et al., 2017). The organic matter adjacent to clay minerals (e.g., illite) is also seen with numerous internal pores and fractures. The reason behind this observation may be that a large volume of fluids is generated during the conversion from smectite to illite to form transitional smectite-illite mixed minerals; such minerals are highly catalytic and promote hydrocarbon generation to create more organic pores and fractures (Zhang et al., 2017).

Here the STD-4 layer of the upper sweet spot and the XTD-1 layer of the lower sweet spot are taken as examples to investigate the correlation between TOC and fracture development. The STD-4 layer is mainly composed of dolomitic mudstone and dolomite, with a total content of calcite and dolomite up to 45.89%, while the XTD-1 layer is predominantly silty-fine sandstone and sandy mudstone, with the total calcite-dolomite content of 32.50%. As revealed in Figure 11, TOC has considerably decisive effects on the development of bedding seams. For both the STD-4 and XTD-1 layers, the bedding seam density grows with the rising TOC. Although the tectonic fracture density also climbs up with the increasing TOC, such a variation trend is weaker than that of bedding seams. To sum up, higher TOC results in the higher development of both tectonic fractures and bedding seams, and yet the control of TOC on bedding seams is stronger than that on tectonic fractures.

The incremental pressure induced by hydrocarbon generation can lead to the failure of rocks and the opening of weak bedding planes to form tectonic fractures and bedding seams respectively (Luo et al., 2017; Fall et al., 2015). In addition, hydrocarbons and acid fluids expelled during hydrocarbon generation may dissolve bedding planes to form bedding seams (Fall et al., 2015). Some fractures have prominent hydrocarbon shows, indicating that the hydrocarbon generation-expulsion is one of the important contributors to fracture formation.

As discussed in Section 4, the development of tectonic fractures varies with layer thicknesses. However, outcrops, cores, and imaging logging all reveal that the Lucaogou Formation reservoir is mostly transitional rocks, with interbedding of centimeter-scale beds of different lithologies. Statistics of bed thicknesses in cores show that beds thinner than 10 cm account for more than 80% of the total (Figure 12). Therefore, it is safe to say that the dependency of tectonic fracture development on bed thickness is rather small for the Lucaogou Formation characterized by the “relatively homogeneous” bed thickness, despite that the bed thickness to some extent restrains the development of tectonic fractures.

Linear positive correlations are found between TOC and tectonic fracture density for both the STD-4 and XTD-1 layers of the upper and lower sweet spots respectively. However, no notable correlation is found in the statistics of the samples of the four layers in the upper sweet spot (Figure 13). It is noted that samples from STD-4 and XTD-1 layers (Figure 11A) have similar brittleness indexes, whereas those from the four layers of the upper sweet spot in Figure 13 have varied brittle mineral contents. Moreover, the overall brittleness index of the upper sweet spot is lower than those of STD-4 and XTD-1 layers. In other words, with varied contents of quartz and carbonate minerals, no notable correlation is found between TOC and tectonic fracture density. This means that the development of tectonic fractures is affected jointly by mineral brittleness and TOC, and the effect of brittle minerals is stronger than that of TOC.

The above analysis indicates that brittle minerals, lithology, and tectonic stress may be the main control factors for fracture development in the Lucaogou Formation tight reservoir among all influential factors. To quantitatively characterize the main control factors for fracture development, the fracture density variation coefficient is introduced in this research (Yang, 2011). V = δ j F a (1.1) δ f = ∑ i = 1 n F i − F a 2 / n (1.2) F a = 1 n ∑ i = 1 n F i (1.3) where V is the fracture density variation coefficient, dimensionless; δ _f is the standard deviation of a variable; f _a is the mean fracture density of all samples; F i is the fracture density of the ith sample; n is the sample quantity.

The coefficient of variation of fracture density can be used to evaluate the impact of a single factor, with a larger value indicating a stronger impact. Tectonic stress and brittle minerals are found to result in the largest coefficient of variation, followed by lithology, TOC, and bed thickness successively (Table 1).

As discussed in Section 4, brittle minerals have no considerable control over the development of bedding seams—bedding seams can be well developed in rocks with high and low calcite-dolomite contents. Similar to the case of tectonic fractures, the “relatively homogenous” bed thickness results in low dependency of the bedding seam development on layer thickness. It is often observed in field outcrops that the bedding seam density grows as it approaches the fault in the case of the same lithology; nevertheless, the bedding seam density of mudstone far away from the fault is higher than that of sandstone nearer to the fault. These demonstrate the higher effects of lithology than tectonic stress on the development of bedding seams.

The above analysis shows lithology and TOC may be the main control factors of the bedding seam development. This is verified by the calculated coefficients of variation of the bedding seam density. In addition, as suggested in Table 2, the effects of tectonic stress and bed thickness on the bedding seam development are rather small.