Evolution Mechanism of Differential Diagenesis Combination and Its Effect on the Reservoir Quality in the Tight Sandstone: A Case from the Lower Shihezi Formation in the Hangjinqi Area of Ordos Basin, China

The physical property heterogeneity of tight sandstones was mainly caused by complex alteration of various diagenesis combinations during burial process. However, diagenetic evolution of di ﬀ erent diagenesis combinations which generally result in the strong di ﬀ erence and heterogeneity of physical property and pore structure is rarely well understood. The Middle Permian lower Shihezi Formation is one of the most important tight gas sandstone reservoirs in the Hangjinqi area of Ordos Basin, China. The reservoir heterogeneity of lower Shihezi Formation, which was caused by the di ﬀ erential diagenesis combination, is crucial to e ﬃ cient exploration and development. Evolution mechanism of di ﬀ erential diagenesis combination and its e ﬀ ect on the reservoir quality in the tight lower Shihezi Formation sandstone in the Hangjinqi area of Ordos Basin was investigated by means of thin-section description, cathodoluminescence (CL) imaging, X-ray di ﬀ raction (XRD), scanning electron microscopy (SEM), and homogenization temperature of ﬂ uid inclusions. The lower Shihezi Formation sandstones can be divided into four diagenesis combination types according to the reservoir characteristics and diagenetic relationship. The main diagenetic sequence was mechanical compaction-chlorite rim-early pore- ﬁ lling calcite cementation-dissolution-authigenic kaolinite-quartz cementation-late calcite cementation. Di ﬀ erential diagenesis combination was mainly controlled by the petrological characteristics, microfacies, and fault. Low content of rock fragment and high content of detrital quartz were bene ﬁ cial to the compaction resistance and cementation. The moderate content of pore- ﬁ lling calcite was conducive to pore space protection and feldspar dissolution. The faults control dissolution and di ﬀ erential diagenesis combination by in ﬂ uencing the migration of acid ﬂ uids. Moderate compaction-moderate cementation-moderate dissolution type (BBB type) and weak compaction-moderate cementation-strong dissolution type (CBA type) were in favour of high-quality reservoir development.

The Middle Permian lower Shihezi Formation is one of the most important tight gas sandstone reservoirs in the Hangjinqi area of Ordos Basin, China [24,25]. The tight sandstone of the lower Shihezi Formation has generally undergone complicated diagenetic alterations which have reduced the reservoir quality. Previous studies merely involved deposition, diagenesis, and their impacts on the reservoir quality [26][27][28]. This study is aimed at understanding the evolution mechanism of differential diagenesis combination and its effect on the reservoir quality in the tight sandstone belonging to the Middle Permian lower Shihezi Formation, Hangjinqi area, Ordos Basin, China, in order that the reservoir quality can be forecast ahead of drilling in unexplored regions of the basin.
The Middle-Upper Ordovician to the lower part of Carboniferous are missing owing to widespread uplifting and erosion across the North China block as the result of Caledonian movement in the early Paleozoic [35]. The upper Carboniferous mainly consists of the Benxi Formation. The lower Permian comprises the Taiyuan Formation (P 1 t) and Shanxi Formation (P 1 s). The middle Permian includes the lower Shihezi (P 2 h) and upper Shihezi Formation (P 2 s). The upper Permian comprises Shiqianfeng Formation (P 3 s) [24,25,36] (Figure 2). The lower Shihezi Formation, which mainly consists of tight sandy conglomerates, pebbly coarse-, coarse-, medium-, and fine-grained sandstones, was mainly deposited in fluvial and delta [26,37,38] (Figure 2).
The first member (H1) of lower Shihezi Formation, which was mainly deposited in the braided river, is the main reservoir of study area ( Figure 2) [26,37,38]. The meandering river of the second member (H2) evolved into the delta of the third member (H3) in the Hangjinqi area [26,37,38]. The lower Shihezi Formation has huge potential for producing tight sandstone gas in China [32, 27 28] (Figure 2).

Samples and Methods
The well logs and conventional cores were collected from the Exploration and Development Research Institute of North China Branch Company of Sinopec.
Eighty sandstone samples of lower Shihezi Formation were collected from the drill cores of 4 wells (Figure 1(c)), in which 74 thin sections were impregnated with blue epoxy resin and prepared for petrological and diagenetic studies by 300-point count. Porosity of different pore types and volume content of different cements can be calculated under thin section by image quantitative analysis with the Photoshop software [11,12].
Thirteen representative samples, which were coated with gold, were analyzed under a Quanta250 FEG scanning electron microscope (SEM and BSD) equipped with an energydispersive (ED) spectroscope in order to examine authigenic minerals and pore geometry and diagenetic sequence in the sandstones.
X-ray diffraction (XRD) analysis of whole rock was performed on 30 core samples to identify types and contents of major minerals. The relative contents of different clay minerals and I/S mixed-layer ratios of 30 core samples were determined by XRD analysis of quantitative clay minerals. These two experiments were completed by using an Ultima IV X-ray diffractometer under the condition of 25°C temperature and 50% humidity. Cathodoluminescence (CL) analyses were performed on the 4 typical core samples with high content of carbonate cement (>2%) under an Olympus microscope equipped with a CL8200-MKS CL instrument. Homogenization temperature of fluid inclusions within calcite cement of two core samples with carbonate cement (more than 5%) was determined under a petrographic microscope equipped with a Linkam.

Petrology of Sandstones.
The lower Shihezi Formation sandstones in the Dongshen gas field are predominantly litharenite according to Folk's sandstone classification scheme [39], averaged as Q 51.2 F 3.8 R 45 (Figure 3). Detrital quartz (Q), which is the most common detrital composition, varies from 24.7% to 76.6% of the detrital grain volume with an average value of 51.2%. Rock fragments, which consist of volcanic, metamorphic, and minor sedimentary rock fragments (Figures 4(a)-4(f)), vary from 19.5% to 74.1% of the detrital grain volume with an average value of 45% ( Figure 3). Volcanic rock fragments mainly comprise neutral-basic extrusive rock (Figure 4(e)). Feldspars, which range from 1.1% to 8.1% of the detrital grain volume with an average of 3.8% (Figure 3), comprise plagioclase and 2 Lithosphere minor K-feldspar. The detrital grains are poorly to moderately sorted and medium and medium-coarse grained, with some amounts of coarse grained and conglomeratic coarse grained (Figures 4(a)-4(i)).  pores, intergranular dissolved pore, and microfracture approximately occupy 75%, 20%, and 5%, respectively, in the total pore space (Figures 5(a) and 5(b)). The core porosity of lower Shihezi Formation sandstone mainly ranges from 6% to 14% (av. 9.58%), and the core permeability generally ranges from 0.01 to 1 mD (av. 0.63 mD) ( Figure 6). There is only weak correlation relationship between porosity and permeability of lower Shihezi Formation sandstone ( Figure 7).        Table 1). The relative content of smectite in the illite/smectite mixed layer is approximately 20% (Table 2). Authigenic chlorite mainly comprises pore-filling chlorite (Figure 8(e)) and chlorite coating (Figure 8(m)). Transformation processes of clay minerals mainly include the kaolinization of feldspar, illitization of kaolinite, and chlorization of kaolinite (Table 2 and

Dissolution.
Major feldspars and minor volcanic rock fragments were pervasively dissolved in the studied sandstones, which generally produced secondary pores ( Figure 5). The plagioclase and some K-feldspar were dissolved partially or totally ( Figure 5).

Diagenesis Combination.
The compaction can be divided into weak (point contact), moderate (spot-line act), and strong (line contact) types according to grain contact relationship. Cementation can be divided into weak, moderate, and strong types according to content of calcite cement. Dissolution can be divided into weak, moderate, and strong types according to thin-section porosity of secondary pore. The lower Shihezi Formation sandstones can be divided into four diagenesis combination types according to the reservoir characteristics and diagenetic relationship.
(1) Strong compaction-weak cementation-weak dissolution type (ACC type) Strong compaction was indicated by the dominance of long and some concave-convex grain contacts and deformation of mica and plastic rock fragments. Weak cementation was manifested by the low content of pore-filling calcite cement (less than 3 vol%) ( Table 3). The thin-section porosity of secondary pore is generally less than 2%, which suggested that weak dissolution ( Table 3). The relative quartz contents of ACC sandstones vary from 24.7% to 67.4% with an average of 46.4% (Table 4 and Figure 12(a)). Feldspar contents range from 1.1% to 5.6% with an average of 3.3% (Table 4 and Figure 12(b)). Rock fragment contents vary from 30.4% to 74.1% with an average of 50.3% (Table 4 and Figure 12(c)). The tight ACC sandstones comprise poorly to moderately sorted detrital grains. The matrix contents are generally more than 5% (Table 3). The ACC type mainly occurs in the distributary channel and minor braided channel microfacies ( Table 4). The ACC types were mainly observed in the well J77, well J104, and well J51 ( Figure 13).
(2) Weak compaction-strong cementation-weak dissolution type (CAC type) Weak compaction was indicated by the dominance of point and point-long grain contacts, grain-support, and  partial matrix-support. Strong cementation was manifested by the high content of pore-filling calcite cement (more than 10 vol%) ( Table 3). The thin-section porosity of secondary pore is generally less than 2%, which suggested weak dissolution ( Table 3). The relative quartz contents of CAC sandstones vary from 72% to 76.6% with an average of 74.3% (Table 4 and Figure 12(a)). Feldspar contents range from 3.7% to 3.9% with an average of 3.8% (Table 4 and Figure 12(b)). Rock fragment contents vary from 19.5% to 24.4% with an average of 21.9% (Table 4 and Figure 12(c)). The tight CAC sandstones comprise moderately to wellsorted detrital grains moderately and minor poorly sorted detrital grains. The CAC type mainly occurs in the distributary channel microfacies (Table 4).
(3) Moderate compaction-moderate cementationmoderate dissolution type (BBB type) Moderate compaction was proved by the major pointlong and some long grain contacts and grain-support. Moderate cementation was manifested by the moderate content of pore-filling calcite cement (3-10 vol%) ( Table 3). The thinsection porosity of secondary pores generally varies from 2% to 4%, which suggested moderate dissolution ( Table 3). The relative quartz contents of BBB sandstones vary from 40.4% to 61.4% with an average of 52.5% (Table 4 and Figure 12(a)). Feldspar contents range from 2.2% to 6.9% with an average of 4.2% (Table 4 and Figure 12(b)). Rock fragment contents vary from 34.5% to 56.2% with an average  (Table 4 and Figure 12(c)). The BBB sandstones comprise moderately and minor poorly sorted detrital grains. The BBB type mainly occurs in the distributary channel microfacies ( Table 4). The BBB type mainly occurs within the first member (H1) of lower Shihezi Formation of the well J24 and well J104 (Figure 13).  Weak compaction was indicated by the dominance of point and point-long grain contacts, grain-support, and partial matrix-support. Moderate cementation was manifested by the moderate content of pore-filling calcite cement (3-10 vol%) ( Table 2). The thin-section porosity of secondary pores is generally more than 4%, which suggested strong dissolution ( Table 2). The relative quartz contents of CBA sandstones vary from 48.8% to 75.5% with an average of 61.6% (Table 4 and Figure 12(a)). Feldspar contents range from 1.1% to 7.5% with an average of 4.7% (Table 4 and Figure 12(b)). Rock fragment contents vary from 21.3% to 47.2% with an average of 33.8% (Table 4 and Figure 12(c)). The CBA sandstones comprise moderately to well-sorted detrital grains. The matrix contents are generally less than 2% ( Table 3). The CBA type mainly occurs in the distributary channel and minor braided channel microfacies ( Table 4). The CBA type mainly occurs within the first member (H1) of the well J51, well J24, well J94, and well J104 (Figure 13). In general, the BBB type and CBA type were mainly distributed in the wells near the fault (Figures 1 and 13).

Genesis and Sequence of Diagenesis
(1) Genesis of Diagenesis. The pore-filling calcite ranges from 2% to 18 vol% with an average of 6.55 vol%, which suggests that the pore-filling calcite was mainly precipitated from the pore fluids. The homogenization temperature of fluid inclusions within calcite cement also indicates that the pore-filling calcite was mainly precipitated at the eodiagenetic and early mesodiagenetic stages. Calcite replacing detrital grains generally occur with dissolution (Figures 5(f) and 5(h)), which indicates that replacement of calcite was related with dissolution.
The dissolution was mainly determined by feldspars, minor volcanic rock fragments, and acid fluids ( Figure 5). Besides, the BBB type and CBA type were mainly distributed in the wells near the fault (Figures 1 and 13), because the fault is the main migration path for acid fluids [3,5,11,12]. Besides, the dissolution can provide diagenetic environment and material basis for the calcite replacing detrital grains.
(2) Diagenesis Sequence. The clay mineral characteristics (20% smectite in the illite/smectite mixed layer) and burial depth (2000 m-3000 m) suggested that the lower Shihezi Formation sandstones were mainly at mesodiagenetic stage.
Relative diagenetic sequence can be determined according to petrographic evidences from thin section, cathodoluminescence (CL), and scanning electron microscope (SEM) analysis. Besides, the formation period of calcite cement can be calculated by homogenization temperature of fluid inclusions within calcite cement. The main diagenetic sequence was mechanical compaction-chlorite rim-early pore-filling calcite cementation-dissolution-authigenic kaolinite-quartz cementation-late calcite cementation (Figure 14). On the basis of previous burial and thermal history studies, the diagenetic sequence of the lower Shihezi Formation sandstones can be reconstructed and illustrated in Figure 13 [27,28].

Controlling Factors of Differential Diagenesis Combination
(1) Petrological Characteristics of Sandstones. The average quartz content of ACC type is obviously less than the BBB, CAC, and CBA types, whereas the rock fragment of ACC type is more than other three types ( Figure 12). This suggests that the mechanical compaction was mainly influenced by the contents of detrital quartz and rock fragment (Figure 15). CAC and CBA have less rock fragment and more detrital quartz than the ACC and BBB types, which indicate that low content of rock fragment and high content of detrital quartz were beneficial to the compaction resistance and cementation ( Figure 15). The CBA and BBB have more feldspar than the ACC and CAC, which suggests that feldspar was the important dissolution object (Figure 12(b)). The ACC sandstone has poorly sorted grains than the other three types, which indicates that mechanical compaction was also controlled by the textural characteristics (Table 4).
(2) Microfacies. The CAC, BBB, and CBA types mainly occur in the distributary channel microfacies of delta plain. Besides,  12 Lithosphere ACC type mainly occurs in the distributary channel and minor braided channel microfacies. These suggest that distributary channel was in favour of preservation of pore and cementation, and distributary channel and braided channel microfacies may enhance the mechanical compaction (Table 4).
(3) Fault and Fracture and Source Rock. The shorter distance between the fault and well, the CBA and BBB, were more developed (Figures 1 and 13). Besides, the CBA type and BBB type mainly occur within the first member (H1) sandstone, which is closed to the underlying source rock of Shanxi Formation (Figure 2). This indicates that the dissolution and differential diagenesis combination were influenced by the fault and source rock, because the fault is the key migration path for acid fluids released by source rock [3,5,11,12].

Genetic Mechanism of Differential Diagenesis Combination
(1) Strong Compaction-Weak Cementation-Weak Dissolution Type (ACC Type). The strong compaction of ACC type sandstone almost destroyed all primary pores, so that there were not enough pores for the cementation. The tight ACC type sandstone, which was mainly caused by the strong compaction and weak cementation, was not beneficial for the organic acid flow and dissolution ( Figure 14).
(2) Weak Compaction-Strong Cementation-Weak Dissolution Type (CAC Type). Many primary pores can be preserved by chlorite coating, the weak compaction as result of high content of detrital quartz and low content of rock fragment of distributary channel. Many residual primary pores provided enough space for strong calcite cementation, so that almost all primary pores were occupied by the calcite cement. The strong calcite cementation generally restrained organic acid flow and dissolution. Therefore, the densification of CAC type sandstone was mainly influenced by the strong cementation and weak dissolution ( Figure 14).
(3) Moderate Compaction-Moderate Cementation-Moderate Dissolution Type (BBB Type). Some primary pores can be preserved by the chlorite coating, moderate compaction as result of middle content of detrital quartz, middle content of rock fragment, and moderately sorted detrital grains of distributary channel. These residual primary pores can provide some space for the moderate calcite cementation. Besides, moderate content of calcite cements can protect primary pores from destruction of mechanical compaction, which can also provide channel for organic acid flow and moderate dissolution ( Figure 14).
(4) Weak Compaction-Moderate Cementation-Strong Dissolution Type (CBA Type). Many primary pores can be preserved by the weak compaction as result of high content of detrital quartz and low content of rock fragment of distributary channel. Many residual primary pores provided enough space for calcite cementation, so that many primary pores were occupied by the calcite cement. However, moderate Long and some concave-convex grain contacts and deformation of mica and plastic rock fragments, matrix contents are more than 5% Pore-filling calcite cement content is less than 3%.
Thin-section dissolved porosity is less than 2%.
CAC type Point and point-long grain contacts Pore-filling calcite cement content is more than 10%.
Thin-section dissolved porosity is less than 2%.
BBB types Point-long and minor long grain contacts, matrix contents vary from 2% to 5% Calcite cement, which mainly comprises pore-filling calcite and minor replacement, varies from 3% to 10%.

CBA type
Point and point-long grain contacts, grainsupport, and partial matrix-support, matrix contents are less than 2%-5% Calcite cement, which mainly comprises pore-filling calcite and minor replacement, varies from 3% to 10%.
Thin-section dissolved porosity is more than 4%.  13 Lithosphere content of calcite cements can protect primary pores from strong destruction of mechanical compaction and calcite cementation. Therefore, the residual primary pores were beneficial for the organic acid flow and strong dissolution ( Figure 14).

Effect of Differential Diagenesis on the Reservoir Quality.
The higher content of detrital quartz, the higher calcite cement (Figure 14(a)) or higher thin-section porosity (Figure 15(b)). Besides, the moderate content of porefilling calcite (3-10%) can protect the primary pore from  14 Lithosphere destruction of mechanical compaction (Figures 14 and  16(a)). However, calcite replacing detrital grains have positive relation with thin-section porosity (Figure 16(b)), which indicates that replacement of calcite was related with dissolution ( Figure 14). Besides, the calcite cement generally appears with secondary pores and feldspar dissolution ( Figures 5(b)-5(k)). This indicates moderate content of pore-filling calcite was beneficial for pore space protection and feldspar dissolution ( Figure 14). Therefore, CAC type was characterized by the high content of detrital quartz and calcite, which resulted in weak dissolution.
The BBB type has experienced moderate compaction due to middle content of detrital quartz and calcite cement, which resulted in the moderate dissolution. The CBA type has more dissolved pores due to the moderate calcite cement and strong dissolution. In a word, the BBB and CBA type sandstones were in favour of reservoir development ( Figure 14).

Conclusions
This study of the lower Shihezi Formation sandstones in the Hangjinqi area of Ordos Basin, China, yields important clues about differential diagenetic combination and its effect on the reservoir quality in the tight fluvial sandstone, including the following: (1) The lower Shihezi Formation sandstones mainly comprise four diagenesis combination types: strong compaction-weak cementation-weak dissolution type (ACC type), weak compaction-strong cementationweak dissolution type (CAC type), moderate compaction-moderate cementation-moderate dissolution type (BBB type), and weak compaction-moderate cementation-strong dissolution type (CBA type) (2) The main diagenetic sequence was mechanical compaction-chlorite rim-early pore-filling calcite cementation-dissolution-authigenic kaolinite-quartz cementation-late calcite cementation (3) Differential diagenesis combination was mainly controlled by the petrological characteristics, microfacies, and fault. Low content of rock fragment and high content of detrital quartz were beneficial to the compaction resistance and cementation. The moderate content of pore-filling calcite was conducive to pore space protection and feldspar dissolution. The faults control dissolution and differential diagenesis combination by influencing the migration of acid fluids (4) CAC type was characterized by the high content of detrital quartz and calcite, which resulted in weak dissolution. The BBB type has experienced moderate compaction due to middle content of detrital quartz and calcite cement, which resulted in the moderate dissolution. The CBA type has more dissolved pores due to the moderate calcite cement and strong dissolution. The BBB and CBA type sandstones were in favour of reservoir development

Data Availability
The data that support the conclusions of this study are available from text and the corresponding author upon reasonable request.  16 Lithosphere