The Tight Sand Reservoir Characteristics and Gas Source in Coal Measures: A Case Study of Typical Areas in China

Interbedded sandstones, shales, and coal seams were formed in marine-continental transitional environments of coal measures, and the gas source of its tight sand gas has been uncertain, which hinders the progress of natural gas exploration. This article uses the Longtan tight sand gas in west Guizhou as an example to investigate the reservoir features, gas charging history, and main source rock of tight sand gas by examining thin sections, scanning electron microscopy images, X-ray di ﬀ raction results, and the ﬂ uid inclusions with burial and thermal histories and noble gas analysis. The Longtan sandstones in this study are classi ﬁ ed as litharenites which are characterized by low compositional and textural maturities, and high clay contents, and are distinguished petrographically from conventional sandstones by the extensive existence of micropores and microfractures. The results of the homogenization temperatures of the ﬂ uid inclusions show that the Longtan tight sand gas is a single-stage accumulation which occurred during the Late Triassic. Furthermore, through the analysis of the helium and argon isotopes of noble gases, a calculation model was established for determining the shale and coal contributions to the tight sand gas. The research results show that the Longtan tight sand gas mainly belongs to the crust-derived gas, mostly from the Longtan shale-derived contribution and to a lesser extent from the Longtan coal-derived contribution. The measured 40 Ar/ 36 Ar values of the Longtan shales and coals are consistent with the forecasted 40 Ar/ 36 Ar values from the calculation model, demonstrating that the model is viable to calculate the source rock contribution to the tight sand gas. Therefore, the Longtan shales as main source rocks can generate hydrocarbons to charge neighboring tight sandstones to form tight gas accumulation in the Longtan Formation, which provides an opportunity for tight gas development in west Guizhou.


Introduction
Tight sand gas resources in the world are abundant and widely distributed and are an important part of natural gas resources [1,2]. Tight sand gas accumulations have been widely discovered in a number of basins in the United States and China, such as in the Greater Green River and San Juan Basins in the United States and the Ordos, Sichuan, Tarim, and Songliao Basins in China, all of which have been elaborately studied [3][4][5]. These tight gas sandstones were mainly deposited in marine or terrestrial environments [5,6]. However, compared with discoveries in marine and terrestrial tight sandstones, fewer discoveries have been made in marine-continental transitional environments. Marinecontinental transitional formations are widely distributed in China, such as the Upper Permian Longtan Formation in South China and the Lower Permian Taiyuan Formation in the Ordos Basin [7]. These formations consist of tight sandstones, shales, and coals and are considered to be potential targets for unconventional resources [7,8]. However, the gas resources of tight sandstones are poorly understood, which hinders the progress of tight gas development in marine-continental transitional environments.
The resource potential and productivity of tight sandstones can be appraised with a thorough understanding of reservoir characteristics and gas source which contribute to natural gas migration and accumulation [9][10][11]. A variety of techniques (e.g., X-ray diffraction (XRD) analysis, thin sections, and scanning electron microscopy (SEM)) have been used to characterize the reservoir characteristics to provide an early determination for resource potential of tight sandstones [12,13]. To establish the relationships between tight sand gas and source rocks, techniques involving fluid inclusions [14][15][16][17] and noble gas isotopes [18][19][20][21] have been used to evaluate the charging periods and identify the main source rocks of hydrocarbon accumulations. Fluid inclusions have been captured in reservoirs and evolved in a relatively closed system without interference during a certain range of burial, and thus, their composition, temperature, and pressure data can provide important insights into its oil and gas migration and accumulation relative to burial history [17,22,23]. Noble gases (e.g., helium and argon) in reservoirs that originate from different sources (e.g., mantle, source rock, and atmosphere) are isotopically distinct and resolvable, owing to different radioelement concentrations (e.g., uranium) in their sources. Furthermore, noble gases preserve a record of their presence and origin because they do not chemically react with other substances during migration and accumulation history and, therefore, can be used as reliable tracers to correlate source rocks and hydrocarbon accumulations and to resolve different source contributions to noble gases in reservoirs [24][25][26][27].
The assemblages of Longtan sandstones, shales, and coals are widely recognized in west Guizhou, China. Previous research results indicate that tight sand gas [28], shale gas [29,30], and coalbed methane [31] show good potential for unconventional resource development in west Guizhou. A number of wells in west Guizhou have penetrated good gas pays in the Longtan Formation, e.g., the X1 well had gas show values of 20%-30% [8]. However, the ambiguity of tight sand gas potential hinders the development of tight sand gas in west Guizhou. Therefore, the aim of this study is to investigate the reservoir characteristics, charging history, and source rock contribution of tight sand gas in the Longtan Formation in west Guizhou by using various techniques involving thin sections, SEMs, XRDs, fluid inclusions, and noble gas isotopes.

Geological Setting
Guizhou Province is located in the Upper Yangtze Platform, South China (Figure 1(a)). Tectonically, it can be subdivided into six major structural units, which, from north to south, are the North Yunnan-Guizhou depression, Wuling   [29]) and our study area in the Guizhou Province and (b) well (green-filled circle) locations in the study area.
2 Geofluids depression, Central Guizhou uplift, Southwest Guizhou depression, South Guizhou depression, and Xuefeng uplift (Figure 1(a); [29,32]). Our study area, as shown in Figure 1 (yellow area), mainly includes the Southwest Guizhou depression and part of the Central Guizhou uplift, and it consists of 24 wells (Figure 1(b)). From the Sinian to Permian, a thick marine sedimentary section was deposited in this area, followed by long-term weathering and denudation, which were caused by uplift from the Early to Middle Permian [29,31]. During the Late Permian, a rapid transgression led to the deposition of thick marine-continental sediments in west Guizhou [33]. The Upper Permian Long-tan Formation experienced multiple stages of tectonic movements, such as the Late Yanshanian ca. 140Ma, the Early Himalayan ca. 80 Ma, and the Late Himalayan ca. 3 Ma [34]. The Upper Permian Longtan Formation was widely deposited in west Guizhou and formed under a variety of settings, including an alluvial plain, a delta, and a tidal flat lagoon (Figure 2(a); [35]). The Longtan Formation consists of interbedded tight sandstones, siltstones, shales, and coal seams (Figure 2(b)), which can form multiple sets of source-reservoir-seal assemblages [7,36]. Shales and coal seams in the Longtan Formation have good potential for generating gas because they display high total organic   4 Geofluids carbon contents [30], high thermal maturities, high gas contents, and type III gas-prone organic matter [29,37], which charged neighboring tight sandstones to form tight gas accumulations. Multiple wells (e.g., the X1 well) have encountered tight gas plays in the Longtan Formation [8,28], which have raised expectations for tight sand gas in west Guizhou.

Data and Analytical Methods
To assess the reservoir features, charging history, and gas source of the Longtan tight sandstones, 11 representative samples from the Longtan Formation were analyzed: 9 sandstone samples, 1 shale sample, and 1 coal sample (Table 1), with depths ranging from 600 to 1500 m. Thin section, SEM, XRD, and fluid inclusion analyses (Table 1) were conducted at Renqiu Jiechuang Petroleum Technology Co., Ltd., and noble gas (Helium and Argon) analyses were completed at the Laboratory of Gas Geochemistry, Lanzhou Institute of Geology, Chinese Academy of Sciences. Rock property measurements were performed on 1-inch diameter core plugs under the net confining stress of 0.07 Mpa (10 psi) to access the flow capacity (air permeability) and the storage capacity (porosity) of the tight sandstone reservoir.
Thin sections were grinded to a standard of 30-μm thickness to examine the lithology characteristics of the Longtan tight sandstones under the Olympus polarizing optical microscope. All samples were also prepared by vacuum impregnation with blue-dyed resin to facilitate the recognition of porosity. Point-count analysis was performed with respect to detrital framework grains, matrix, cement, interstitial minerals, and porosity, as well as the textural modal grain size and sorting parameters. SEM imaging was performed by using Tescan VEGA-II to observe the characteristics of the pore networks and types of pore-filling minerals of the tight sandstones. Each sample was mounted on an aluminum stub and coated with gold/ palladium (Au/Pd) alloy before imaging. The temperature and humidity of the experiments were set as 24°C and 35%, respectively. X-ray diffraction analysis was carried out using D8-Discover to obtain semiquantitative mineralogical data for whole-rock and clay fraction. Samples were first cleaned to avoid any obvious contaminants and then disaggregated in a mortar. According to the Powder Diffraction File provided by the Joint Committee on Power Diffraction-International Center for Diffraction Data, the relative mineral percentages were estimated quantitatively through Rietveld refinement with customized clay mineral structure models [39] at a temperature of 24°C and humidity of 35%.
The fluid inclusion test was performed via microthermometry in accordance with the test method for fluid inclusions in sedimentary basins [40]. The fluid inclusions were observed by using a Leica microscope (DM 4P); the homogenization temperatures were measured by using a coolingheating stage (LINKAM THMS 600) at a temperature of 20°C and humidity of 30%; and the inclusion compositions were identified by using a microlaser Raman spectrometer (LABH RVIS LabRAM HR 800) at a temperature of 20°C and humidity of 30%.
The noble gas isotope measurements were carried out in accordance with the technique described by Sun [41] for determining noble gas isotopes within natural gas [41]. The samples were first heated to 103°C and then maintained at this temperature for three weeks to remove the adsorbed air inside the samples; then, the samples were placed in an electron bombardment furnace to extract the tight sand gas; the extracted gas samples were heated at 800°C in the sample preparation device by using a titanium sponge to remove hydrocarbon gas, N 2 , O 2 , CO 2 , H 2 S, and H 2 and to retain 100% noble gas; and the components of the noble gas isotopes ( 3 He, 4 He, 36 Ar, 38 Ar, and 40 Ar) were separated     7 Geofluids according to their boiling points by using cryogenic pumps, activated carbon furnaces, and liquid nitrogen. Finally, the contents of the noble gas isotopes were determined by using a mass spectrometer (VG 5400 MS).

Reservoir Characteristics.
The results of thin section analysis show that the Longtan tight sandstones are classified as litharenites according to the classification scheme recommended by Folk [42]. The dominant compositions of the Longtan sandstones are the lithic rock fragments whose contents are greater than 70% (Figure 3). Detailed studies further demonstrate that the lithic rock fragments are primarily made of mud clasts ranging from 48 to 50% and minor volcanic, metamorphic, sedimentary, tuff, and mica rock fragments based on the observations of thin sections. These mud clasts were generally formed in marinecontinental environments [43]. Interstitial material includes matrix with an average content of 7% and cement with an average content of 10% dominated by clay minerals. The grain sizes are mostly very fine to fine sands, exhibiting moderate sorting and low (subangular) to moderate (subrounded) sphericity (Figures 4(a) and 4(b)). Overall, the Longtan sandstones have low compositional and textural maturities.
The term "micropore" in tight sandstones is generally used to describe pore sizes<10 μm and "micropore throats" to describe pore throat sizes<1 μm [44].  Table 2). The clay minerals (e.g., mixed layer illite/smectite) display fibrous, flake-like, and honeycomb-like morphology ( Figure 5(e)) and have a negative effect on reservoir properties of sandstones [45]. Micropores within organic matter have also been observed ( Figure 5(f)). The micropores associated with authigenic clays are treated as dead end pores and have no positive contribution to the fluid flow.

Fluid Inclusions Characteristics.
The Longtan tight sandstones are characterized by low porosities and permeabilities with the existence of micropores and microfractures. The results of fluid inclusion analysis show that the fluid inclusions are mostly located in the microfractures of quartz minerals and hardly observed in cements ( Figure 6). Overall, the fluid inclusions have low abundance and are mainly divided into three types: primary gas-liquid inclusions, secondary brine inclusions, and secondary gas-liquid inclusions ( Figure 6). The original gas-liquid inclusions of the YV-1-1 and YV-1-3 samples are scattered and disordered (Figures 6(a) and 6(b)) and are mainly located in quartz minerals with a wide range of sizes (5-20 μm). The original gas-liquid inclusion shapes are oval, and their liquid ratios are approximately 10% (Table 3). The secondary inclusions   Table 3). The gas-liquid ratio of the secondary gas-liquid inclusions of the HV-3-1 sample is 25% (Table 3), and they are characterized by observing a brown fluorescence. Laser Raman spectra of the secondary gasliquid inclusions have shown that the fluid compositions of some inclusions are rich in saturated hydrocarbons (Figure 7(a)); some inclusions mainly contain methane with low contents of carbon dioxide (Figure 7(b)); and other inclusions are rich in methane (Figure 7(c)). As a result, according to the distributions, colors, and compositions of the inclusions, the secondary gas-liquid inclusions within the Longtan sandstones are mainly hydrocarbon fluids captured during the sandstone diagenesis stage.
The homogenization temperatures of the hydrocarbonrich gas-liquid inclusions can reflect the paleotemperatures of when hydrocarbons are captured [22]. The homogenization temperature results of the gas-liquid inclusions within the Longtan sandstones ( Figure 8) show that the homogeni-zation temperatures of the inclusions range from 105°C to 140°C and present a unimodal distribution with a single peak at 120°C-130°C, indicating that the charging processes of oil and gas within the Longtan sandstones were continuous.

Compositions of the Noble Gas Isotopes.
The results of the noble gas isotopic analysis within the Longtan tight sand gas samples show that the contents of helium (He) are 2:91 × 10 −6 mL/g and 2:45 × 10 −6 mL/g, respectively, which are lower than that in the atmosphere at 5:24 × 10 −6 mL/g; the contents of argon (Ar) are 9:72 × 10 −6 mL/g and 9:95 × 10 −6 mL/g, respectively, which are 3 orders of magnitude lower than that in the atmosphere at 9340 × 10 −6 mL/g; the 3 He/ 4 He values are 3:08 × 10 −7 and 2:30 × 10 −7 , respectively, which are lower than that in the atmosphere of 1:4 × 10 −6 ; the 4 He/ 20 Ne values are 29 and 25, respectively, which are approximately 2 orders of magnitude greater than that in the atmosphere at 0.288; the 40 Table 4). The drastic differences in the isotopic ratios (e.g., 4 He/ 20 Ne) among the samples and the air indicates that the compositions of tight gas samples are hardly influenced by those isotopic ratios in the air. Stable isotopes of He include 3 He and 4 He, where 3 He is a primordial nuclide, and 4 He is a radioactive nuclide [46]. The 3 He/ 4 He values in air, mantle-derived gas, and crustderived gas are approximately 1:4 × 10 −6 , 1:0 × 10 −5 , and 1:0 × 10 −8 , respectively [47,48]. He isotopes within tight sand gas originate from air, mantle, and crust. Their relative contents determine the 3 He/ 4 He values within the Longtan tight sand gas. The 3 He/ 4 He values (Table 4) within the Longtan tight sand gas samples are much smaller than those in the air and mantle-derived gas, indicating that the He isotopes within the samples are mainly crust-derived. In addition, researchers have demonstrated that the 3 He/ 4 He ratio of the sample (R) to the atmospheric 3 He/ 4 He ratio (Ra) can effectively represent the gas origin in the sample (Figure 9). The R/Ra values of crust-derived gas range from 0.01 to 0.1 and those of mantle-derived gas are greater than 8 [47,49,50]. The R/Ra values of the Longtan samples are slightly greater than 0.1 (Table 4), which indicates that the He isotopes are primarily crust-derived with a small contribution from mantle-derived helium (Figure 9). According to Equation (1) 36 Ar, 38 Ar, and 40 Ar, where 36 Ar and 38 Ar are primordial nuclides and 40 Ar is a radioactive nuclide that is produced by the decay of potassium (K) [52]. The 40 Ar contents are highly dependent on the K content and decay time [21]. There is a wide range of crustderived and mantle-derived 40 Ar/ 36 Ar values, and they cannot be used to differentiate the gas origin of Ar. However, a couple studies show that the crust-derived 40 Ar/ 36 Ar values decrease as the 3 He/ 4 He values increase (e.g., the Kela2 gas field), and the mantle-derived 40 Ar/ 36 Ar values show the opposite trend (e.g., the Xushen and Changshen gas fields). The 40 Ar/ 36 Ar values of the Longtan tight sand gas samples behave the crustal-derived feature ( Figure 10). This indicates that Longtan Ar has a crustal origin, which is consistent with the results from the 3 He/ 4 He analysis.  [14,15,17,22]. Pet-roMod was used to generate the burial and thermal histories of the YV-1 and HV-3 wells (Figure 11), which are based on thermal maturity (Ro) data and the pioneering studies of Dou et al. [31] and Wang et al. [53]. The results of the burial and thermal histories (Figure 11) of the study area show that the paleowater depth of the LV-1 and HV-3 well areas in west Guizhou gradually increased from the Late Permian 11 Geofluids to the Middle-Late Triassic, and the sedimentary environments transferred from marine-continental transitional settings to marine carbonate depositions [53]. The burial depth of the Longtan Formation gradually increased, the thermal maturity of organic matter increased, and Ro reached approximately 1.3% in the Late Triassic. After that, multiple stages of tectonic movements (e.g., the Indosinian, Yanshanian, and Himalayan) resulted in uplift with intense   faulting, fracturing, and folding, and the thermal evolution of organic matter in the Longtan Formation ceased ( Figure 11). Until the Quaternary, thin layer residues and slope deposits were formed, and the burial depth and thermal maturity of the Longtan Formation slightly increased ( Figure 11). The homogenization temperatures outlined above for the gas-liquid inclusions within the Longtan tight sandstones range from 105°C to 140°C and show that the Longtan tight sand gas is a single-stage accumulation. Based on the thermal histories of the study area, the main charging time of the tight sand gas in the Longtan Formation occurred in the Late Triassic (215-205 Ma).

Tight Sand Gas
Source. The 40 Ar contents within the gas samples are determined by the amount of Potassium (K) contained in the gas source rocks of the same period. The K contents in shales and coals show a large difference (Table 5), which provides a basis for using 40 Ar/ 36 Ar to calculate the source rock (e.g., shales and coals) contribution of tight sand gas [21,54]. Potassium sources of 40 Ar within the tight sand gas include detrital minerals, mantle, air, and source rocks (shales and coals). Potassium in detrital minerals is mainly present in potassium feldspars (orthoclases). According to X-ray diffraction analysis, the mineral compositions of the Longtan tight sandstone are mainly clay min-erals and quartzes and do not consist of potassium feldspars (Table 2). Therefore, 40 Ar contents within the Longtan tight sand gas includes no contribution from the detrital minerals. According to the results discussed above, 40 Ar within the Longtan tight gas is mainly crust-derived; thus, the mantle contribution is negligible. Therefore, the calculation model of the shale and coal contribution for the Longtan tight sand gas was established based on the pioneering studies of Kaneoka and Takaoka [18], Ballentine et al. [24], Zhang et al. [21], and Liu et al. [9]. First, the 40 Ar/ 36 Ar values and K contents of source rocks at the same age show a positive correlation, and the 40 Ar/ 36 Ar values of the Longtan coals can be calculated under the circumstance of determining the 40 Ar/ 36 Ar values of the Longtan shales and the K contents of the Longtan coals and shales via Equation (2) [21]. The potassium contents of the Longtan shales are set as the average K content (2.67%) of shales in China (Table 5); the contents of potassium in the coal seam are based on the average content (0.214%) of coals in China (Table 5)  13 Geofluids significantly from the measured 40 Ar/ 36 Ar value of the Longtan coal sample (Figure 12), which indicates that the Longtan tight sand gas mainly originates from the Longtan shales. This understanding, in addition to the results from the calculation model, signifies that the Longtan shales are the main source rocks that can generate gas to charge the tight sandstones to form tight gas accumulations in the Longtan Formation. As a result, the measured 40 where R coal is the coal-derived contribution for tight sand gas and R Shale is the shale-derived contribution for the tight sand gas. The gas expulsion capacity from an organic-rich shale or coal is controlled by both gas generation and adsorption capacity [57][58][59]. The gas generation capacity is determined by the thickness, type, richness, and maturity of organic matter [34,60]. The YV-1 and HV-3 wells featured thick organic-rich shales that were 49.8 and 54.3 m, greater than those of the coals, but the maturity and richness are almost the same, and therefore, the gas generation capacity of shales at the YV-1 and HV-3 wells is greater than that of coals in west Guizhou. The adsorption capacity is determined by kerogen type, richness, maturity, mineral composition, pore structure, temperature, and pressure [59,61]. Xia et al. [62] showed that the adsorption capacity of coals is much greater than that of shales ( Figure 13), mainly owing to the differences in chemical structures, maceral composition, and pore structure [62,63]. During burial history, maturities of organic matter increase with depth, and the generation of gas from source rock begins at levels of thermal maturity greater than 0.6% R o [64]. The generation of gas was first stored in source rocks and started to migrate into reservoirs after full saturation within the source rocks [57]. Therefore, the gas expulsion capacity of shales the YV-1 and HV-3 wells is speculated to be greater than that of coals, and the shale contribution to the tight sand gas is prevailing, which is consistent with the result from the calculation model. Uncertainty exists in the calculation model due to the limitation of the number of tight gas samples in west Guizhou. The calculation model from He and Ar isotopes should be optimized with the acquisition of more gas samples, and integration of source rock thickness, richness, maturity, and burial history, etc. to understand hydrocarbon generation and expulsion characteristics are import for assessing the result of the calculation model.

Conclusions
Sandstones, shales, and coal seams are interbedded in the Longtan Formation in marine-continental transitional environments of coal measures, and the potential of the tight sand gas are unclear. This paper analyzes the reservoir characteristics, charging history, and source rock contribution for the Longtan tight sand gas in west Guizhou, and the following conclusions can be drawn.  [62]). The results show that the adsorption capacity of shales is greater than that of coals at a given maturity and pressure.
14 Geofluids (1) The Longtan sandstones in this study are classified as litharenites which are characterized by low compositional and textural maturities, and high clay contents. The clay minerals are dominated by illite/ smectite (I/S) mixed layers. High clay contents are one of the key factors leading to the poor reservoir quality of the Longtan sandstones (2) Constrained from homogenization temperatures of the gas-liquid inclusions and burial and thermal histories, the main charging time of the tight sand gas in the Longtan Formation occurred in the Late Triassic (215-205 Ma). The helium and argon isotope analysis of the Longtan tight sand gas shows that the tight sand gas is primarily crust-derived with a very low content of mantle-derived gas. The occurrences of gas charge into the Longtan sandstones indicate the potential for tight gas development in West Guizhou (3) The calculation model of the shale and coal contribution for the Longtan tight sand gas was established. The results of calculation model and expulsion capacity show that the main source rocks of the Longtan tight sand gas are the Longtan shales; thus, the coal-derived contribution for tight sand gas in the Longtan Formation is minor, signifying that the Longtan shales are the key contributors for tight gas in the Longtan Formation

Data Availability
The data used to support the findings of this study are included within the article.

Conflicts of Interest
The authors declare that they have no conflicts of interest.