Integrated Diagenesis Study of Tight Gas Sandstone: The Permian Lower Shihezi Formation, Northern Ordos Basin, China

Diagenesis varies greatly from basin to basin and has been considered as the key control of the reservoir quality of tight sandstones. In this study, we analyzed the petrographic characteristics, diagenesis, and pore types and characteristics of the Permian Lower Shihezi Formation in the Ordos Basin. Results show that most of sandstones are litharenites with minor sublitharenites and feldspathic litharenites. The tight sandstones have ultralow permeabilities (averaging 0.67 mD) and porosities (averaging 8.35%), with pore-throat sizes ranging from 0.035 to 13.29 μ m with 94.12% less than 5 μ m. Overall, the tight sandstone reservoirs have undergone complex diagenetic alteration. Compaction and clay mineral cements are the two crucial diagenetic factors that controlled the tightness of the reservoirs. The compaction destroyed most of primary porosity, and the e ﬀ ective pores are mainly dissolved micropores, intragranular dissolved micropores, and micropores in clay minerals and mineral grains. The chlorite coating might have helped preserve a certain amount of porosity, but pore-lining chlorites signi ﬁ cantly obstructed pore throats and reduced permeability. Results of the study provide insights and direct implications for the future success of exploration and production of the tight gas sandstone in the Ordos Basin and other similar tight sandstone reservoirs.


Introduction
Theoretically, tight sandstones refer to those which have been deeply buried and have undergone complicated diagenetic alteration [1][2][3][4][5].In recent decades, tight sandstone gas has become an important unconventional hydrocarbon resource and has contributed to the substantial growth of global hydrocarbon production because of advances in recovery technology [6][7][8][9][10].Diagenesis has been considered as one of key controls of reservoir quality and potential of tight sandstone gas exploration [11][12][13][14][15], but tight sandstones vary greatly in different fields and basins in terms of generating mechanisms and controlling factors.
This integrated diagenesis study of the Shihezi Formation helps document its reservoir characteristics and forming mechanisms.Several upper Paleozoic tight sandstone gas fields have been discovered on the northern slope of the Ordos Basin since the 1980s [11,15,16].Previous studies have focused on the tectonic evolution, regional geology, sedimentology, and the reservoir properties of upper Paleozoic formations [17][18][19].Only limited and local diagenetic studies of those upper Paleozoic tight sandstones have been reported [20][21][22][23].There is a lack of systematic study of diagenesis and its control on the reservoir quality of upper Paleozoic Permian tight sandstones in the Ordos Basin [24,25].
In this study, we chose the Permian Lower Shihezi Formation, in the Hangjinqi area of northern Ordos Basin, to document its diagenetic characteristics through a combination of thin sections, mercury intrusion capillary pressure (MICP), scanning electron microscope (SEM), X-ray diffraction (XRD), and cathode luminescence (CL) testing.The main objectives include the following: (1) analyzing reservoir compositions, diagenetic events and history, and pore systems; (2) revealing favorable reservoir characteristics; and (3) elucidating main controlling factors on reservoir quality of these tight sandstones.Understanding the diagenetic progress and its effects on reservoir quality can provide important references for favorable reservoir prediction in tight sandstones.

Geological Setting
The Ordos Basin is a typical cratonic depression located in North-Central China.Internally, it can be subdivided into six tectonic units: the Yimeng Uplisft, the Western Thrust Belt, the Tianhuan Depression, the Yishan Slope, the Jinxi Flexture Belt, and the Weibei Uplift (Figure 1).The basin has Archean-Proterozoic metamorphic basement and is filled with Paleozoic to Cenozoic aged sedimentary units, with a total thickness of over 4000 meters to the southwest (Figure 2).It experienced six tectonic stages, including (1) Archean to Paleoproterozoic basement formation stage, (2) the Meso-to Neoproterozoic formation of an intracratonic rifting trough (or depression trough), (3) formation of a Sinian to early Paleozoic epicontinental basin on the North China block, (4) Late Paleozoic to early Mesozoic depression of the North China Block, (5) formation of the Middle to late Mesozoic great Ordos inland basin, and (6) formation of a peripheral fault basin.
The study area is in the Hangjinqi region, the border between the Yimen Uplift and the Yishan Slope, in the northern Ordos Basin (Figure 1).In the Hangjinqi area, the basement is directly overlain by the Upper Carboniferous, coal-bearing Taiyuan Formation [26] (Figure 2).Permian to Triassic strata, including the Shanxi, the Lower Shihezi, the Upper Shihezi, the Shiqianfeng, the Liujiagou, the Heshanggou, the Ermaying, and the Yanchang formations (in ascending order), are composed of alluvial fan, deltaic, and lacustrine deposits [27][28][29].In the Hangjinqi area, the Late Triassic to Early Jurassic strata are either eroded or missing due to uplift related to Early Jurassic subduction along the eastern margin of Asia [26].The Middle Jurassic strata, including the Yan'an, the Zhiluo, and the An'ding formations, are mainly fluviolacustrine clastic deposits [30].The Early Cretaceous strata (the Zhidan Formation) are characterized by fluvial and eolian red beds overlain by Quaternary loess deposits [31,32].This region lacks upper Cretaceous-Neogene strata [26,33].
Two hydrocarbon systems, a Jurassic-Triassic petroleum system and a Paleozoic gas system, exist in this basin [34].The Paleozoic gas system is mainly found in northern Ordos Basin.Within the Paleozoic gas system, coals from the Taiyuan and Shanxi formations serve as source rocks.Lacustrine mudstones of the Shiqianfeng Formation and Upper Shihezi Formation are the main cap rocks.Deltaic sandstones of the Shanxi and the Lower Shihezi formations are the main reservoir rocks (Figure 2).This study is mainly focused on sandstones of the Lower Shihezi Formation.This area experienced multiple burial events and two main uplifts after the deposition of the Lower Shihezi Formation (Figure 3).The first uplift after depression developed during the Jurassic with an average burial depth of about 2500 m, with the formation temperature reaching 90 °C to 100 °C [35].The second uplift developed in the Late Cretaceous with an average burial depth of about 3000 m with the formation temperature reaching 115 °C to 130 °C [36].The mod-ern burial depth is about 2000 m to 3000 m with formation temperatures from 85 °C to 95 °C.

Methods and Data
We observed and described cores from 22 wells and collected 110 tight gas sandstone plugs (2.5    Figure 3: Burial history and paleogeothermal field evolution of study area (changed from [37]).were utilized to study mineral composition, diagenesis, and pore characteristics.Scanning electron microscopy (SEM) of 40 samples and mercury injection capillary pressure (MICP) analyses from 86 core plug samples were applied to study pore throat structure, observe the mineralogy, and document diagenetic events.Cathodoluminescence (CL) and X-ray diffraction (XRD) analyses of 30 samples were also used to identify and evaluate the mineral content.Additionally, results of porosity and permeability analyses of 1188 samples from cores of 11 wells and 86 samples of MICP were conducted by the Research Institute of China Sinopec North China Company (RICSNCC) and were incorporated into this study to further evaluate the storage and potential of reservoirs.The test types and numbers are listed in Table 1 as follows and well locations in Figure 1(b).

Results
4.1.Composition.The results of 115 thin sections show that the Lower Shihezi sandstones are mainly litharenite with minor sublitharenite and feldspathic litharenite according to the sandstone classification of Folk et al. [37] (Figure 4).Framework grains include 51-81% quartz (average~67%), 16-48% rock fragments (average~30%), and less than 10% feldspar (average~3%) (Figure 5).Core observation and grain size analyses show that Lower Shihezi tight sandstones range from pebble coarse sandstone, coarse sandstone, to fine sandstone (Figure 5) and are poorly to well-sorted with subrounded to rounded grain shape.There are also small amounts of conglomerate and siltstone.

Clay Mineral Cementation.
Four main clay minerals, kaolinite, illite, chlorite, and mixed-layer illite/smectite, were identified based on the SEM morphological characteristics and XRD mineralogical analysis.The content of total clay minerals ranges from 9.9% to 57% with an average of 21.35% in all the tight sandstone samples (Figure 6).In all tested tight sandstone samples, chlorite is the most abundance compared to the other clay minerals clay with averages of 8.62%.And the kaolinite has an average content of 5.49%, the illite-smectite complex has an average content of 4.63%, and illite has an average content of 2.60%.
Most of the authigenic kaolinite occur as vermiculite booklets stacked as pseudohexagonal crystals (Figure 7(a)).The illite mainly occurs as flaky (Figure 7(b)) and fibrous types (Figures 7(d) and 7(f)).There are two types of authigenic chlorite: the pore filling type and the coating type (Figure 7(e)).There is no authigenic quartz where the chlorite coating developed.The flaky illite mainly occurs as porefilling habits, and fibrous illite mainly surrounds detrital  refer to reservoirs with a porosity of ≤10%, and in-situ formation permeability of ≤0.1 mD or air permeability of ≤1 mD [16,38].In this study, we collected porosity and permeability from a total of 1188 core samples.Porosity ranges from 0.69% to 21.45% with an average of 8.35% (Figure 10(a)), and the horizontal air permeability ranges from 0.006 mD to 8.42 mD with an average of 0.67 mD (Figure 10(b)).Approximately 79.21% of the 1188 core sandstone samples of the Lower Shihezi Formation are tight sandstones with permeabilities of less than 1 mD, with only eight core samples having permeabilities higher than 8 mD, which is likely related to microfractures and strong dissolution.Overall, porosity and permeability show a positive correlation (Figure 11).12(d) and 13(d)).These tight gas sandstones also feature a wide range of pore sizes (Figures 13 and 14) ranging from the maximum pore size of 0.54 mm to the minimum of 10 μm.The micro-and nanopores, which are developed in intergrain minerals as pore filling matrix due to diagenesis or dissolution, are the key storage spaces for the tight gas sandstones.Based on the MICP curves of the 136 core samples, the maximum pore throat size ranges from 0.035 to 13.29 μm, 94.12% are less than 5 μm (Figure 14(a)), and 97.79% of the pore throats are less than 1 μm (Figure 13(b)).
The tight sandstone reservoirs were divided into three types based on capillary pressure curves and pore throat radius distributions (Figure 15).Type I usually has low displacement pressure (< 0.1 MPa), low median pressure (1-3 MPa), and large pore throat (0.16-2.5 μm) size.And the sandstones have good reservoir quality and are the best reservoirs.Type II usually has low displacement pressure (< 1 MPa), high median pressure (20-50 MPa), and small pore throat (< 1 μm) size.This type sandstones are mainly 8 Lithosphere fine sandstones.Type III has the highest displacement pressure (> 10 MPa), and the maximum mercury saturation is less than 50% (Figure 15).That means this type sandstones cannot be reservoirs.In general, the capillary pressure curves of type I generally show a platform with left concave shape (Figure 15), but the capillary pressure curves of the type II and III are steep or have right convex shapes (Figure 15).The course-grained sandstones and some medium-grained sandstones usually have the type I pore type.The sandy conglomerates and some mediun-grained sandstones mainly have the type II pore type.And the fine-grained sandstone has type III pore type (Figure 15).

Discussion
5.1.Diagenetic Sequence.According to the national standards of the Oil and Gas Industry in China (SY/T5477-2003), the diagenetic stage was divided to eodiagenesis (A and B subage stage), mesodiagenesis (A and B subage stage), and telodiagenesis according to vitrinite reflectance (Ro), homogenization temperature of the aqueous inclusions, petrographic evidence including evolution degree of clay minerals (mixed-layer illite/smectite), crosscutting relationships of cements, and dissolution-filling relationships of diagenetic minerals and pore types.9 Lithosphere Chlorite coatings are commonly developed around quartz grains where there usually have remaining pores and no quartz cements, which indicates that chlorite formed before quartz cement and strong compaction (Figures 8(d) and 10(a)).The type I quartz cementation of quartz overgrowth (stage-I quartz cement, Figure 8(a)) is earlier than the type II of authigenic quartz (stage-II quartz cement, Figure 8(d)).
During the eodiagenesis A stage, the sediment underwent a rapid settling process after deposition (Figure 3).In this process, the sandstone is strongly compacted, and the reservoir primary pore space is rapidly reduced.And the coating chlorites and stage-I quartz cements were formed at the stage.During the eodiagenesis B, the maximum burial depth is around 2000 m.The ground temperature reaches 65 °C to   10 Lithosphere 85 °C, and organic matter begins to produce organic acids, causing dissolution of feldspars and rock chips in the reservoir.And the resulting dissolution products, such as quartz, kaolinite, and calcite, are further cemented to fill the reservoir pores.During the mesodiagenesis A stage, the maximum burial depth is around 2000 m, and the ground temperature reaches 130 °C; organic matter produces large amounts of organic acids to dissolve feldspars and rock chips, forming dissolution pores and at the same time, the dissolution products become colloidal to form colloidal materials such as quartz and iron calcite (Figures 3 and 16).Therefore, the main diagenetic progresses that tight sandstones experienced include mechanical compaction, chlorite coating, stage-I quartz cement, chemical compaction, dissolution, stage-II quartz cement, and calcium pore-filling cementation.The burial depth of the tight sandstones of the Lower Shihezi Formation is generally 2600-3300 m.The vitrinite reflectance value of the Lower Shihezi Formation ranges from 1.01% to 1.27%.The homogenization temperature of the aqueous inclusions ranges from 90 °C to 130 °C [32].The percentage of S in I/S is 15-26% indicating that the illite/montmorillonite interlayer minerals are highly ordered illite among the clay 11 Lithosphere minerals of sandstone.In summary, the Lower Shihezi Formation is in mesodiagenesis A substage to mesodiagenesis B substage (Figure 15).Firstly, the recovery of the original porosity is mainly based on the relationship between sandstone sorting coefficient and original porosity proposed by Beard and Weyl [39].

Diagenetic Controls on
where Φ0 is the original porosity, and So is the sandstone sorting coefficient.The current pore volume is formed by the further reduction of the remaining intergranular pore volume and the increase of some small pore volume after cementation.Therefore, the residual porosity after compaction can be calculated as following: where Φ1 is the residual porosity after compaction, C is the content of cementation, P1 is the measured intergranular porosity, P2 is the measured microporous surface porosity based the thin section, P3 is the air measurement porosity, and P4 is the total pore surface porosity.
where Φ2 is the residual porosity after cementation.
where Φ3 is the porosity increased by dissolution, and P5 is the dissolution surface porosity based on the thin section.
where Φ4 is the calulated current porosity.
Based on the above equations, the porosity evolution of 30 wells in the study area was quantified, and the results were calculated as follows (Table 2).IGV values range from 8.4% to 24.3% (Figure 17).The primary porosity of the Lower Shihezi sandstone reservoirs was mainly destroyed by mechanical compaction, and the cementation destroyed primary porosity less than 33% vol % (Figure 17).The mechanical compaction accounted for 44.2%-79.81%(with an average of 61.32%) of the total intergranular pore volume loss (PVL) during burial (an initial porosity of 34% calculated based on the Beard and Weyl, 1973).

Clay Mineral Cement Control on Reservoir Quality.
The clay mineral cements have significant effect on Lower Shihezi sandstone pore and throat size distributions (Figures 5 and 11).The pore throat systems primarily control the porosity and permeability of the tight sandstone resulting in positive correlation between porosity and permeability [42,43].The correlation coefficient between porosity and permeability in the Lower Shihezi tight sandstone reservoirs is about 0.52 (Figures 10 and 11) as the result of the clay cementation, but the different kinds of clay mineral cements have different effects on tight sandstone porosities.The relationship between I/S, kaolinite, illite content, and porosity and permeability show that the porosity and permeability decrease with the increase of these mineral cements (Figures 18(a)-18(f)).The inverse correlation suggests that these three types of clay mineral cements occlude both pores and pore throats, but the relationship between  [40]; Ehrenberg [41]).14 Lithosphere chlorite content and porosity shows that porosity decreases with the content of chlorite less 10%, and then when the chlorite content is greater than 10%, the porosity becomes larger as the chlorite content increases (Figure 18(g)).This implies that chlorite coatings have preserved the porosity from quartz overgrowth as show in Figure 7(e), that reducing of pore space due to cementation.In contrast, permeabil-ity decreases as the chlorite content increases (Figure 18(h)).This indicates that, despite the chlorite coating effect of preserving porosity, the pore-lining chlorites may significantly obstruct pore throats.15 Lithosphere warm and humid climate by braided river systems [17,18], and all Lower Shihejie tight sandstones share similar depositional environments.There is no obvious relationship between the content of the quartz and rock fragments and reservoir quality (Figure 19).With an increase of quartz content or rock fragments, porosity and permeability show no increase or decrease.However, data from this study suggests that tight sandstone granularity shows some effect.In the study area, the coarser sandstones (Cog, Gcs, CS) have better reservoir quality than the medium and fine sandstones [44] (Figures 10 and 14).We interpret that this is caused by higher discharge during the deposition of the coarser sandstones, which can flush the clay and siltstone away resulting in less cementation.

Conclusions
(1) The sandstones of Lower Shihezi are mostly litharenite and minor sublitharenite and feldspathic litharenite, which are characterized by abundant quartz (2) The Lower Shihezi sandstones have ultralow porosity and permeability, small pore throats, and high displacement pressures.The pore types include intergranular dissolved micropores, intragranular dissolved micropores, micropores in clay minerals and mineral grains, and microfractures.The tight sandstones were divided into three types based on capillary pressure curves and pore throat radius distributions, which the type I sandstones are the best reservoirs (3) The Lower Shihezi tight sandstone reservoirs have undergone complex diagenetic alteration.Compaction and clay mineral cements are the two crucial diagenetic factors that controlled the tightness of the reservoirs.The compaction has destroyed most of primary porosity, and the effective pores are mainly dissolved micropores, intragranular dissolved micropores, and micropores in clay minerals and mineral grains.The chlorite coating might help preserve certain amount of porosity, but pore-lining chlorites significantly obstruct pore throats and reduce permeability

Figure 2 :
Figure 2: Simplified regional stratigraphic succession and tectonic history of the Hangjinqi area, Ordos Basin.

Figure 10 :
Figure 10: Porosity and permeability distribution of the tight sandstone.

Figure 11 :
Figure 11: Scatter diagram of the relationship between porosity and permeability of sandstone with different grain size.Key: Cog: conglomerate; Gcs: gravelly coarse sandstone; CS: coarse sandstone; MS: medium sandstone; FS: fine sandstone (FS).
Reservoir Quality 5.2.1.Compactional Controls on Reservoir Quality.Compaction occurred widely in the Lower Shihezi sandstones (Figure 5).A quantitative evaluation of intergranular volume (IGV) was used to clarify the compaction and cementation impacts on Lower Shihezi tight sandstone reservoir quality.
cm in diameter and 5 cm in length).110 standard thin sections were used to analyze grain size, and a total of 110 polished thin sections

Table 1 :
The wells, test types, and numbers.

Table 2 :
Relationship between reservoir diagenesis and pore evolution.