4.1 Sandstone composition

The Chang 7 tight sandstone in the study area consists mainly of fine (0.125 mm) to very fine (0.065 mm) grained, moderate to well sorted, and poorly rounded arkoses, lithic arkoses, and feldspathic litharenites, according to the Folk's (1974) sandstone classification scheme (Figure 4).

Figure 4 QFR classification of Chang 7 sandstone (after Folk, 1968). The ternary diagram shows the various types of Chang 7 sandstones with different sediment origins in different parts of the research area. 

Point count data from 1513thin sections show that detrital quartz and feldspar are the most abundant minerals of both Chang7₁ and Chang 7₂ (Table 1).

Table 1 Statistics of detrital components of Chang 7 member sandstones 

Quartz is the most abundant mineral with contents ranges from 25°/0~41°/0 with an average of 36% in Chang7₁, and 25%~40% with an average of 36% in Chang 7₂. Feldspar content of Chang 7₁ is 19%~39%, averaging 24% and 20%~43%, averaging 25% in Chang 7₂. Lithic grains were made up mainly by eruptive rock, quartzite, phyllite, and dolomite, and to a lesser degree from slate, aphanite, schist and metasandstone, and a small amount of limestone, high grain rock. The total content of lithic grains of Chang 7₁ ranges from 10%~23%, averaging 17%, and 12%~25%, averaging 18% of Chang 7₂. Lithic grains content of W, S, and SW districts are higher than that of NE and NW and are characterized by relatively abundant dolomite, phyllite, and quartzite. Mica is familiar throughout the basin, with a content range from 5%~13%, averaging 6% of Chang 7₁ and 3%~6%, averaging 5% of Chang 7₂.

The detrital composition shows little change vertically throughout the Chang 7 unit, but spatial differences are observed. E.g., Chang 7₁ sandstones from NE and NW districts are characterized by a relative higher content of feldspar and mica, and low quartz and lithic grains contrast with that of sandstones from S, W, and SW districts. And sandstones from Chang7₂ show the similar distribution pattern of detrital.

The matrix of the Chang 7 sandstone is mainly fine-grained minerals, mostly clay minerals that fill interstitial spaces amongst the framework grains. Matrix content of Chang 7₁ ranges from 7%~11%, averaging 10%, and 5%~12%, averaging 10% of Chang 7₂. The matrix contents in SW, S and M districts are higher than that in NE, NW and W districts (Table 2).

Table 2 Statistics of matrix and cement composition of Chang 7 member sandstone 

4.2 Diagenetic minerals

The diagenetic minerals in Chang 7 sandstone are mainly included Fe-calcite, ankerite, quartz followed by minor calcite, dolomite, albite, kaolin, and chlorite rims. Total diagenetic minerals content of Chang7₁ range from 4%~10%, averaging 7% and 4%~11%,averaging 7% of Chang7₂. Samples from NE, NW and M districts contain higher cement content than that of sandstones from W, S and SW area.

4.2.1 Quartz cement

Quartz cement, which ranges in average abundance from 0.9% to 1% (average of 1%) in Chang7₁ and mainly 0.8%~1.4% (average of 1.1%) in Chang7₂, occurs as either euhedral quartz crystals that fill intergranular or dissolution pores or as syntaxial overgrowths around quartz grains.

Two generations of quartz cement can be distinguished. First generation quartz cement is covered by chlorite rims and filling primary pores (Figure 5a). Second generation quartz cement grew over the chlorite rims and occurred as small euhedral or pyramidal quartz crystals filling primary and dissolution pores (Figure 5b). Quartz cement iscommon in coarser sandstones with fewer matrix present.

Figure 5 Quartz cement in Chang 7 sandstones. A: early quartz overgrowth been covered by and thus predate chlorite rim, Well Wu 62, 1827.06m. B: authigenic quartz in the intergranular pore, postdate chlorite rims in SEM picture, Well Gao 38, 2125.46m. 

4.2.2 Carbonate cement

The amount of carbonate cement varies from 3% to 6%, averaging 4% in Chang7₁ and Chang7₂, and includes Fe-calcite, ankerite, and minor calcite (~0.2%) and dolomite (generally <0.1%).

Calcite is stained red in thin section and bright yellow under cathodoluminescence (CL). Two generations of calcite are recognized: calcite I fill large intergranular pores as poikilotopic crystals (>100um) between uncompacted detrital grains. Calcite-II partly or entirely replaces detrital grains and be engulfed by Fe-calcite (Figure 6). Although the content of calcite cement is up to 40%, it is commonly absent and has a mean content of 0.2%.

Figure 6 Calcite-II partly or entirely replaces detrital grains and be engulfed by Fe-calcite under polarizing microscope and CL 

The Fe-calcite is stained purple or blue-violet and dark orange under CL with content ranges from 2%~6%, averaging 2% in Chang 7₁, and from 1%~4%, averaging 2% in Chang 7₂. The Fe-calcite is most abundant in the NE, NW districts ( > 3.5%) and decreases to < 2% in the SW, S and W districts. It usually occurs as micro- to coarse-crystalline (10~100μm) cement that partly or wholy fills isolated intergranular pores, engulfing quartz overgrowth, chlorite rims and calcite (Figure 6, Figure 7a).

Figure 7 Fe-carbonate cement in Chang 7 sandstones under thin section. (a): Fe-calcite grows over quartz overgrowth, Intergranular pores mostly filled by quartz and Fe-calcite, Well Ning 31, 1654.47m. (b): ankerite occurs as overgrowth of dolomite grains and filling the intergranular residue pores after quartz cement and compaction, Well Li 45, 2276.02m. 

Ankerite, stained wathlet blue under the thin section, is micro- to coarse-crystalline (<10~100μm) and generally occurs as overgrowths around dolomite grains, Fe-calcite, and dolomite cement filling intergranular and dissolution pores (Figure 7b). Contrary to the distribution of calcite, ankerite cement mainly occurs in samples from M, S, W, and SW districts with contents ranging from 1%~3% in Chang7₁ and Chang 7₂. And minor ankerite is found in NE and NW districts (0~1%).

4.2.3 Clay cement

Clay cement identified in the studied sandstones include chlorite, kaolin, and illite.

Kaolin is mostly present in the samples from NE and NW districts with contents ranges from 1.9%~3.0% of Chang 7₁ and 1.4%~3.3% of Chang 7₂. Little kaolin is found in samples from M, S and SW areas. It occurs as thin stacks of pseudo-hexagonal plates filling the primary pores and feldspars dissolution pores and partially entrapped by chlorite (Figure 8).

Figure 8 Diagenetickaolin in Chang 7 sandstones. a) kaolinite plates filling the primary pores, Well Hu 211, 2300.4m; b) kaolin with pseudo-hexagonal plates grows over and postdate by chlorite coating, Well Hu 211, 2300.4m. 

Authigenic chlorite occurred in the Chang 7 sandstone as two forms: grain rims and pore-filling. Grain rims chlorite found in about 15% of total samples are thin (3-10μm) and discontinuous with crystals orient perpendicular to the surface of detrital and authigenic quartz crystals (Figure 9). When present, it makes up <0.30% of the whole rock volume. The thickness of chlorite rims on authigenic quartz crystals is much thinner than that on the detrital grains. Chlorite is often observed to wholly or partly completely replace biotite, and in places, its content can be up to ~20% of the whole rock volume.

Figure 9 Chlorite rims on detrital (Ch-1) and diagenetic quartz (Ch-2) surface. The chlorite rims on authigenic quartz crystals are much thinner than that on the detrital grains. a) well Gao18,1917.8m and b) well Huan 54,2661.2m. 

Illite mineral occurs in every sample with three forms under SEM: i) flaky and honeycomb-like crystal with spiny terminations tangentially coating detrital grains (Figure 10a); ii) flaky filaments filling intergranular pores (Figure 10b-e); and iii) fibrous, hair-like illiteoccurring as pore-bridging cement or filling dissolution pores (Figure 10f). And most of the illite occurs as the first two forms. Fibrous illite is rare in Chang 7 sandstones.

Figure 10 The illite minerals with different crystal shapes of Chang 7 sandstones. (a) flaky illites covering the detrital grains, well Geng 292, 2481.01m; (b) flaky illites disorderly filling the intergranular pores, well Geng 292,2481.01m; (c) flaky to hair-like illite partly filling the intergranular pores, well Huan 54, 2650m; (d) Honeycomb illite covering the detrital grains, well An72, 2237.43m; (e) illite with no significant crystalline morphology, well Zhuang40, 1824.16m; (f) pore-bridging silk-thread illite, well Gao 38, 2125.46m. 

4.4. Porosity and permeability

Average porosity of thin sections from different districts ranges from 1.7%~4.7% and averages 1.9% in Chang 7₁ and from 0.7%~4.9%, averages 2.2%in Chang72 (Table 3). Highest thin section porosities are found in samples from W, NE districts with porosity up to 4.9%.

Thin section and SEM analyses show that there are three types of pores: intergranular pores, which make up a substantial amount of the total porosity (0.4%~3.5%, av.0.8% in Chang 7₁ and 0.3%~3.3%, av.0.9% in Chang 7₂); secondary pores (0.7%~1.3%, av.1.0% in Chang 7₁ and 0.8%~1.4%, av.1.2% in Chang 7₂) and micropores. Secondary pores are mostly formed by the dissolution of feldspar and a small number of rock fragments and carbonate cement. The micropores with a pore radius of 0.5 μm or less (^{Pittman, 1971}; Jefrin et al., 2018) can be of primary or secondary origin (^{Loucks & Dutton, 2007}), and are associated with the clay minerals and weathered or altered rock fragments.

Although microporosity cannot be quantified in thin section, it is an important pore type of Chang 7 sandstones. The scatter plot of thin section porosity, core plug porosity versus depths shows that the thin section porosity variation trends with depth are overall in agreement with that of core plug porosity (Figure 11). Whereas the point count thin section porosity (mainly macropores) is significantly less than core plug porosity (0.9%~20.4%, av.9.3% in Chang 7₁ and 01.4%~19.5%, av.9.6% in Chang 7₂) indicating that abundant micropores in the Chang 7 sandstones are not counted in thin section.

Figure 11 Plots of thin section porosity, core porosity versus depth 

5.1. Paragenetic sequence

The main diagenetic processes affecting Chang 7 sandstones include mechanical compaction, cementation of quartz, Fe-calcite and ankerite and authigenesis of chlorite, kaolin, and illite.

Grain-coating clay formed on detrital grain surfaces in Chang 7 sandstones occur as flaky texture, tangentially arranged around, and are typically originated by postdepositional infiltration (^{Matlack et al., 1989}).

Dominantly long and concavo-convex contacts combine with flexible grain deformation in Chang 7 sandstones, suggests a moderate to intense mechanical compaction. In sandstones containing significant content of ductile components, mechanical compaction is characterized by grain rearrangement and plastic grain deformation at relatively shallow depths (^{Pittman and Larese, 1991}).

Quartz cement (>1%) are standard in Chang 7 sandstones and have several sources (McBride, 1989; Worden and ^{Morad, 2000}; Hussin et al., 2017). The presence of syntaxial quartz overgrowth near the sites of intergranular dissolution and on tightly packed detrital quartz grains indicates a mesogenetic origin (McBride, 1989; ^{Sibley & Blatt, 1976}; Worden & Morad, 2000; Sunny et al., 2018). In Chang 7 sandstones, concavo-convex contacts between detrital quartz are widely observed. It seems that in situ pressure solution provide materials for the bulk of the cement. No apparent correlations are found between average contents of detrital contents with quartz cement indicating that: 1) the silica sources, including the situ pressure solution of quartz, feldspar dissolution, and smectite illitization, exceed the demand for quartz precipitation in Chang 7 sandstone; and 2) the available pore space for silica precipitate and temperature as well as pH condition are more important factors for quartz cement.

Diagenetic chlorite can have multi origins but typically forms at relativelyhigh temperatures (^{Spoetlet al., 1994}). Chlorite may form by (i) the alteration of eogenetic, grain-coating berthierine or odinite (^{Hillier & Velde 1992}; ^{Humphreys et al., 1989}; ^{Kugler & McHugh, 1990}), kaolinite and smectite precursor in deltaic deposits (^{Ehrenberg, 1993}; ^{Ryan, 1997}; ^{Stokkendal et al., 2009}); (ii) by the dissolution of Fe- and Mgrich detrital grains (^{Anjos et al., 2000}; ^{Blackbourn & Thomson, 2000}; ^{Ros et al., 1994}; ^{Remy, 1994}; ^{Valloni et al., 1991}). Diagenetic chlorites in Chang 7 sandstones were most probably formed by (i) massive biotite alteration, (ii) the transformation of precursor clay, and (iii) direct precipitation from pore fluids. Petrographic examination showed that authigenic chlorite, especially pore filling chlorite, is often related to degradingbiotite (Figure 12) and, accordingly, high authigenic chlorite contents are associated with biotite-rich sandstones. The chlorite-coats grow both on detrital and diagenetic quartz surface with different thickness (Figure 9), and when present on detrital grains, the chlorite coating is thicker than that of the diagenetic quartz, indicating that chlorite coating on diagenetic quartz is direct precipitates from pore water, while chlorite on detrital grain probably is firstly transformed from precursor clay coating and then continue to grow as thicker rims.

Figure 12 Biotite alteration and associated products. Biotite altered to chlorite and dolomite by releasing magnesium, well Zhuang191, 2498.6m. 

The positive correlation of high-grade metamorphic rock fragments (HMRFs) versus chlorite contents reflects that the HMRFs could be the additional source of ions for the chlorite rims (Figure 13).

Diagenetic kaolin usually occurs in mesogenetic stage and mostly occurred in NE and NW districts where sandstones contain abundant feldspar. SEM observations showed that kaolinite filling the dissolution and intergranular pores and, thus, likely formed by the alteration of detrital feldspars. The formation of kaolinite would require acidic pH condition and sufficient Al and Si. The acidic fluids may have been derived from adjacent shales during the maturation of organic matter (^{Surdamet al., 1984}). Negative correlations between matrix content, mainly illite, versus kaolin and other cement contents suggesting that precipitation pores, which are closely related with matrix content of sandstone, are essential for diagenetic kaolin (Figure 13a).

Figure 13 Content scatterplots of (a) matrix with kaolin, (b) mica with Fe-calcite, (c) feldspar with Fe-calcite and (d) dolomite rock fragments with ankerite. 

The flaky and honeycomb-like illite is wide spreading in the Chang 7 sandstones, indicating that illite contains smectite mixed-layer. The presence of grain coating, flaky and honeycomb-like illite crystals with sharp edges suggest a diagenetic origin (Lemon & Cubitt, 2003; ^{Moradet al., 2000}). The extensive illitizaiton of smectite depend on high temperatures (90-130°C) and sufficient K supply (^{Bj0rlykke, 1998}; ^{Ehrenberg, 1993}; Moradet al., 2000; Morad & De ^{Ros, 1994}). The K and Al ions needed for illite precipitation may have been derived from the feldspar dissolution.

Authigenic carbonates in Chang7 sandstones occur as micro to coarse crystalline poikilotopic, pore-filling blocky and isolated pore-filling, indicating precipitation in different stages.

The early calcite cement occupies an intergranular grain volume (IGV) of ~40% and blocks large intergranular pores between loosely packed detrital grains indicating it occurred before intense mechanical compaction.

The Fe-carbonate cement typically occurs as isolated pore filling, engulfing and thus postdating quartz overgrowth, chlorite rims, and calcite cement and were mainly formed at moderate to a deeper depth and have a mesogenetic origin. Homogenization temperatures of fluid inclusions reveal that the precipitation of Fe-carbonate cements at temperatures of 70°C~150°C and mainly focused on 100°C~110°C(Zhuet al., 2015).

Positive correlations between feldspar and mica versus Fe-calcite (Figure 13b-c) suggest that feldspar acid dissolution and mica alteration provided the bulk of Fe and Ca ions for late-stage Fe-calcite precipitation. Wang reported that light δ¹³C PDB values (3.23960- ~ 8.02°/00־) in Fe-calcite (Wang et al., 2007), indicating carbon of Fe-calcite was most likely originated from the maturation of organic matter during burial process (^{Morad, 1998}).

Whereas evident correlation of diagenetic ankerite with dolomite rock fragments proposes dolomite grains is likely to be the primary source for ankerite cement (Figure 13d). This is consistent with the heavier 513C PDB values (-1.92/ ~ -0.84/) in ankerite, which indicating dolomite rock fragments may serve as the chief carbons source for later ankerite precipitation.

The relative timing of diagenetic minerals of sandstones of the Chang 7 member in the study area was inferred from the textural relationship as observed from thin section, SEM and a paragenetic sequence is present in Figure 14.

Figure 14 Diagenetic sequences of sandstones of Chang 7 members, Ordos basin 

Sandstone reservoir quality is a function both of the depositional conditions and diagenetic processes (^{Ajdukiewicz et al., 2010}).

The effect of mechanical compaction on the Chang 7 sandstones is evidenced by concavo-convex contacts and the deformation of micas and plastic grains. A plot of IGV versus cement volume indicates that compaction contributed to the major primary porosity loss of Chang 7 sandstones (Figure 15). Fine grain size, abundant mica, plastic grain as well as clay minerals, and rapid burial after deposition all contributed to compaction of the sandstone.

Figure 15 The plot of intergranular volume (%) versus cement (%) for the Chang 7 member sandstones, Ordos Basin (after Houseknecht, 1987) 

Authigenic minerals, especially illite, quartz and carbonate minerals occupy an average of ~6% rock volumes and also played an essential role in further reducing porosity.

Carbonate cement, mainly Fe-calcite and ankerite, are the significant cement during the late stage, and these fill the residual intergranular and dissolution pores. The contribution of illite to the reduction in reservoir quality, by filling pore space and blocking pore throats (Worden & ^{Morad, 2009}), is more significant for fine sandstones than coarse sandstones (^{Armitage et al., 2010}). Quartz overgrowth is avital reservoir quality reducing factor for many deep burial reservoirs (Worden & Morad, 2000). However, the influence of silica and albite cement on reservoir quality is limited due to their scarcity in Chang7 sandstone.

Though the very low reservoir porosity, thin section porosity of most Chang7 sandstones is still largely secondary (>50%) and thus is mainly controlled by the accessibility of acid fluid. Acidic fluids generated from the maturation of organic matter within the underlie Chang7₃ shales can directly migrate into the sandstones of Chang 7₂, then Chang7₁ generate generally upward decreasing of dissolution pores (Table 3).

Table 3 Statistics of thin section porosity of Chang 7 member sandstones (%) 

No apparent correlation is found between feldspar content and dissolution porosity, but high content of plastic component, especially mica and clay minerals facilitate the reduction of primary porosity and permeability during compaction and make acid fluids accessing to sandstone difficult. Sandstones from M districts, where thickest source rock developed, contain lower dissolution porosity than that of other areas due to the high matrix and mica contents.

The best reservoir sandstones with relatively high core testing porosities (10%~20%) mainly distribute in W district and at depth ranges of2300m-2800 m (Figure 16). These sandstones porosities are primary and show relatively coarse-grained, lower matrix and well preserved primary pores. And thus reservoir quality is mainly dependent on the textural maturity of the sandstones, controlled principally by depositional conditions (^{Tucker, 2003}).

Figure 16 Distribution of thin section porosity, primary porosity and dissolution porosity with the depth of Chang 7 member sandstones, Ordos basin. 

These sandstones with well-preserved primary pore, from W, NE and M districts, are also characterized by chlorite coating and restively low quartz and carbonate cement (Figure 17). It is considered that chlorite coating improves reservoir quality in most cases, especially in deeply buried sandstones (^{Dowey, 2012}; ^{Ehrenberg, 1993}; ^{Tayloret al., 2010}; Worden & ^{Morad, 2009}). The presence of chlorite coating inhibits quartz overgrowth, as well as carbonates cement by changing the grain surfaces to be oil wetting and preventing the access of water to detrital surfaces, thus stopping cementations (^{Al-Ramadan et al., 2004}).

Figure 17 Scatter plots of chlorite contents (including pore filling chlorites and chlorite rims) versus primary porosity of Chang 7 member sandstone, Ordos basin