Upper Paleozoic Transitional Shale Gas Enrichment Factors: A Case Study of Typical Areas in China

: Based on the shale gas research experience in North America, large-scale geological evaluations have been conducted in China to determine the enrichment characteristics of deep marine shale gas, leading to the discovery of the Fuling, Changning and Weiyuan shale gas fields. However, research on Upper Paleozoic transitional shale gas remains limited, restricting the subsequent exploration and development. Therefore, taking the Lower Permian Shanxi and Pennsylvanian Taiyuan Formations in the northeastern Ordos Basin and the Upper Permian Longtan Formation in southwestern Guizhou as examples, gas logging, gas desorption, thermal simulation, maximum vitrinite reflectance (Rmax), and X-ray diffraction (XRD) were used to study the influence of lithological associations, sedimentary facies, gas generation thresholds, and pore evolution on transitional shale gas, and then Upper Paleozoic transitional shale gas enrichment factors of the northeastern Ordos Basin and southwestern Guizhou were analysed. The results show that carbonaceous mudstone adjacent to coal seams presents a high gas content level, and is primarily developed in swamps in the delta plain environment, and swamps and lagoons in the barrier coastal environment. The gas generation threshold maturity (Rmax) of transitional shale is 1.6% and the corresponding threshold depths of the northeastern Ordos Basin and southwestern Guizhou are estimated to be 2265 m and 1050 m. Transitional shale pore evolution is jointly controlled by hydrocarbon generation, clay minerals transformation, and compaction, and may have the tendency to decrease when Rmax < 1.6% or Rmax > 3.0%, but increase when Rmax ranges between 1.6% and 3.0%, while the main influential factors of pore evolution differ in each period. Continuous distribution of transitional shale gas enrichment areas can be formed along the slope adjacent to coal seams with a moderate maturity range (1.6% – 3.0%) in the northeastern Ordos Basin, and transitional shale gas can be enriched in the areas adjacent to coal seams with a moderate maturity range (1.6% – 3.0%), abundant fractures, and favorable sealing faults in southwestern Guizhou.


Introduction
China has conducted considerable research on marine shale of the Lower Cambrian Qiongzhusi and Wufeng-Longmaxi (Upper Ordovician-Lower Silurian) Formations since 2005, and commercial exploitation has been realized to date [1,2]. The first shale gas exploration and evaluation well (W201) obtained industrial gas from marine shale of the Wufeng-Longmaxi Formations (Upper Ordovician-Lower Silurian) in 2010 [3], and then industrial shale gas production was achieved in Fuling, Changning, Weiyuan, and other regions of the Sichuan Basin [4][5][6][7]. By the end of 2015, accumulative proven geologic reserves, proven recoverable reserves, and production of shale gas in China amounted to 5441.29 × 10 8 m 3 , 1360.33 × 10 8 m 3 , and 60 × 10 8 m 3 , respectively, indicating good prospects for rapid development [3]. Scholars have studied marine shale gas enrichment characteristics in China. Guo et al. [8,9] showed that the deep-water shelf shale gas reservoirs of the Wufeng-Longmaxi Formations in the Sichuan Basin are characterized by high total organic carbon content (TOC) and siliceous content levels due to graptolite development, resulting in a high hydrocarbon generation intensity, and reservoirs are easy to fracture and reconstruct. Moreover, tight top and base layers can effectively prevent hydrocarbon from escaping and serve as significant geological factors for shale gas accumulation. Zou et al. [1] deemed that shale gas enrichment and productivity depend on the sedimentary environment, thermal evolution degree, pore and fracture development, and preservation conditions. Jin et al. [10] found that TOC, pore structure, horizontal fractures, high siliceous content, and high formation pressure control the enrichment and high productivity of shale gas. Zhao et al. [11] observed the geological differences between the Lower Cambrian Qiongzhusi and Wufeng-Longmaxi (Upper Ordovician-Lower Silurian) Formations in South China, and concluded that organic-rich intervals development, moderate thermal evolution, rich organic pores, and good top and base layer preservation conditions are the key parameters of shale gas enrichment.
Compared with marine shale, research on Upper Paleozoic transitional shale gas enrichment characteristics in China remains limited. The organic matter (OM) of transitional shale is mainly derived from terrestrial higher plants, and transitional strata are subject to different hydrodynamic forces. Therefore, mudstone is interbedded with sandstone and coal and lithofacies present significant changes in the vertical and horizontal directions in transitional strata, which has important research value. Taking the Lower Permian Shanxi and Pennsylvanian Taiyuan Formations in the northeastern Ordos Basin, and the Upper Permian Longtan Formation in southwestern Guizhou as examples, this research analysed the influence of lithological associations, sedimentary facies, gas generation thresholds, pore evolution, and structural characteristics on Upper Paleozoic transitional shale gas and then studied the shale gas enrichment factors of the northeastern Ordos Basin and southwestern Guizhou.

Geological Setting
Two typical areas were selected for study: (1) the Lower Permian Shanxi and Pennsylvanian Taiyuan Formations in the northeastern Ordos Basin, and (2) the Upper Permian Longtan Formation in southwestern Guizhou. The Ordos Basin is characterized by a stable tectonic setting which is not heavily folded or deformed, and Paleozoic-Mesozoic strata in the principal part dip to the west at less than 1° [12][13][14]. During the late period of the Pennsylvanian, the sedimentary source area in the northern part of the Ordos Basin was uplifted, and delta plain, delta front and tidal flat-lagoon environments predominated from north to south in the northeastern Ordos Basin. During the early period of the Permian, large-scale regressions occurred and the sedimentary source area in the northern part of the Ordos Basin continued uplifting due to hercynian movement, leading to the acceleration of stratigraphic erosion and southward moving rivers, and fluvial, delta plain and front environments formed mainly from north to south in the northeastern Ordos Basin. Southwestern Guizhou experienced long-term tectonic movements which led to the development of folds, faults, and fractures, forming a complex structural morphology [15][16][17]. Folds are mainly distributed in the northwest direction and anticlines are narrow and steep, while synclines are wide and gentle, forming ejective folds. Faults are mainly distributed in the northeast and northwest directions and show strong heterogeneities. During the late period of the Permian, transgression and regression cycles occurred and the strata was uplifted due to the southward subduction of the Siberia plate. Sedimentary source with a large amount of terrigenous input was deposited in a transitional environment of southwestern Guizhou, forming a delta and a tidal flat-lagoon sedimentary system in the Upper Permian Longtan Formation.

Samples
The locations of wells in the northeastern Ordos Basin and southwestern Guizhou are shown in Figure 1A and Figure 2C, respectively. Cores and gas logging data from wells H1 and H2 of the Shanxi and Taiyuan Formations in the northeastern Ordos Basin, and cores and gas desorption data from well X1 of the Longtan Formation in southwestern Guizhou were used to study the relationship between lithological associations and transitional shale gas content. Then, the influence of sedimentary facies on shale gas enrichment was analysed. Samples M1-1, V1-1 and V1-2 from wells M1 (northeastern Ordos Basin) and V1 (southwestern Guizhou) were used for thermal simulation, Rmax measurement, and XRD to study the hydrocarbon generation and clay minerals transformation characteristics during thermal evolution and then the gas generation threshold maturity and pore evolution tendency were determined. Samples from wells M1, M2, L1, L2 and Y1 (northeastern Ordos Basin), and wells V1 and X1 (southwestern Guizhou) were used to study the relationship between Rmax and depth, and the gas generation threshold depths of the northeastern Ordos Basin and southwestern Guizhou were estimated. Combining the structural characteristics of the northeastern Ordos Basin and southwestern Guizhou, Upper Paleozoic transitional shale gas enrichment factors were analysed. Basic data of the samples are shown in Table 1.

Analytical Methods
During the gas logging process, drilling fluid was degassed and dried as it circulated to the ground, and hydrocarbon content was measured with a hydrogen flame ionization detector (version, manufacture, city, country) [19][20][21].
Shale gas desorption was carried out to determine desorbed gas, lost gas, and residual gas contents. A core was placed in closed desorption equipment at the reservoir temperature, and desorbed gas content was measured with a gas metering device, while lost gas content was calculated using the U.S. Bureau of Mines (USBM) method [22]. Then, residual gas content was measured with ground grains of shale sample after desorption. Shale gas content is the sum of desorbed gas, lost gas, and residual gas contents under standard temperature and pressure (STP) conditions (0 °C, 101.325 KPa).
A thermal simulation was conducted to simulate shale maturation through geological time [23][24][25], and hydrocarbon generation and clay minerals transformation characteristics during thermal evolution were determined. Dried samples were divided into 11 parts, and TOC, kerogen type, and Rmax were measured for 1 part, while thermal simulation was performed on the other 10 parts using a hydrous closed system. Samples for thermal simulation were heated from room temperature to 200-650 °C at intervals of 50 °C with a heating rate of 20 °C/h, respectively [26]. Then, hydrocarbon production (liquid and gaseous hydrocarbons), Rmax, and XRD were obtained with the samples after thermal simulation.
Rmax was used to characterize shale maturity. Kerogens were separated from the shale sample and made into polished sections for measurement using a Zeiss microscope and a TIDAS microspectrophotometer. Fluorescence Alteration of Multiple Macerals (FAMM) technology was used for the correction of reflectance [27]. The polished section on the stage was rotated for 360° slowly and then Rmax was determined based on the principle of photoelectric effect.
XRD was used to calculate the relative contents of minerals. The samples were ground into powders of 325-400 mesh sizes and performed with an X-ray diffractometer. Then, the mineral percentages were estimated semi-quantitatively by the area under the curve for the major peak with correction for Lorentz Polarization.

Lithological Associations and Sedimentary Facies
Transitional strata natural gas mainly includes shale gas, coal bed methane (CBM), and sandstone gas. Shale gas occurs mainly as adsorbed and free gas [28][29][30], and CBM occurs mainly in the adsorption state [31,32], while sandstone gas occurs mainly in the free state and reservoir space and cap rock control gas accumulation [33]. As three types of gas reservoirs are superposed onto each other in the vertical and horizontal directions within transitional strata, lithological associations have an important influence on shale gas enrichment characteristics. According to the cores and gas logging data from wells H1 and H2 of the Shanxi and Taiyuan Formations in the northeastern Ordos Basin (Figures 3 and 4), and the cores and gas desorption data from well X1 of the Longtan Formation in southwestern Guizhou ( Figure 5), carbonaceous mudstone adjacent to coal seams presents a high gas content level. Carbonaceous mudstone is rich in OM and has a strong gaseous hydrocarbon generation capacity. In addition, the migration of CBM to adjacent shale can increase the shale gas content levels. Together, these characteristics result in shale gas enrichment. Carbonaceous mudstone adjacent to coal seams is primarily developed in swamps in the delta plain environment, and lagoons and swamps in the barrier coastal environment ( Figures  3-5).   A delta forms at the interface of marine and continental environments and is related to river channel and overbank settings and occurs alongside wave and tidal action of the shallow marine realm. It consists of the delta plain, delta front, and prodelta [34]. The swamp is mainly distributed in the delta plain, which is a low-lying area that is periodically submerged in water and in a reducing environment. The lithologies mainly include carbonaceous mudstone and coal, and massive and horizontal beddings are developed. The swamp is characterized by warm and humid climatic environments and abundant plants [35], leading to the enrichment of OM and mudstone interbedded with coal ( Figure 6). The barrier coast is developed in a transitional environment within which barrier island-tidal flat-lagoon-swamp systems can be formed [34]. The lagoon is a coastal body with very limited connections to the open ocean. The sediments are mainly fine-grained, and horizontal bedding is developed. The lagoon is in a quiet and low-energy environment in which large quantities of sediments are carried by rivers under rainy weather conditions. Anaerobic bacteria reproduce in the lower part of lagoon to form a reducing environment [36], which is favorable to the preservation of OM, forming an organic-rich shale and shale gas enrichment area (Figure 7). The swamp is widely distributed in the supralittoral zone, which presents a relatively weak hydrodynamic environment and fine-grained sediments. Abundant plants in the swamp lead to OM enrichment, and frequent alternations of high and low water levels result in mudstone interbedded with coal ( Figure 7).

Gas Generation Thresholds
Hydrocarbon generation contents (liquid and gaseous hydrocarbons) during thermal evolution were determined by thermal simulation, hydrocarbon production and Rmax measurements using samples M1-1, V1-1, and V1-2 (Table 1). Hydrocarbon generation contents increase with increasing Rmax values during thermal evolution, while liquid hydrocarbon contents first increase when Rmax < 1.6% and then decrease and gaseous hydrocarbon contents begin to increase rapidly when Rmax > 1.6% due to kerogen pyrolysis and liquid hydrocarbon cracking. Therefore, the gas generation threshold maturity (Rmax) of transitional shale is 1.6%. Previous studies indicated that relations between vitrinite reflectance and depth show upper and lower segments, and the reflectance in the lower segment increases more rapidly than in the upper segment [37,38]. Therefore, the variation tendency of Rmax with depth was established with the Rmax values and depths of the Shanxi and Taiyuan Formations in the northeastern Ordos Basin and Longtan Formation in southwestern Guizhou, and then the corresponding threshold depths of the northeastern Ordos Basin (about 2265 m) and southwestern Guizhou (about 1050 m) were estimated. Relationship between Rmax and depth is shown in Figure 8, while hydrocarbon generation characteristic and gas generation thresholds of transitional shale are shown in Figure 9.
When Rmax < 0.6%, transitional shale is in the submature stage, while hydrocarbon generation and clay minerals transformation are not obvious. Compaction is the main factor decreasing pore space [44,46], and macropores are more easy to compact than micropores and mesopores [45].
When 0.6% < Rmax < 1.6%, transitional shale enters the "oil window" [49,50] and kerogen pyrolysis generates a large number of liquid hydrocarbons, but few gaseous hydrocarbons are generated. Moreover, clay minerals transformation is not obvious, indicating that the transformation occurs mainly in the middle diagenesis B and late diagenesis stages. Micropores and mesopores are infilled by liquid hydrocarbon and bitumen due to large surface areas [51,52] and macropores are compacted during the period, leading to a decrease in the pore space. Figure 9. Hydrocarbon generation, clay minerals transformation, and pore evolution characteristics based on thermal simulation. When 1.6% < Rmax < 2.5%, transitional shale enters gas generation threshold. Gaseous hydrocarbon content begins to increase rapidly due to kerogen pyrolysis and cracking of liquid hydrocarbon and bitumen, while liquid hydrocarbon content reaches its maximum value and then decreases. Kaolinite begins to convert to illite rapidly and generate illite/smectite (I/S) as an intermediate product [53], leading to the decrease in kaolinite and increase in illite and I/S, while smectite in I/S converts to illite during the period. Kerogen pyrolysis, liquid hydrocarbon and bitumen cracking, and volatile matter release generate nearly circular or bubble-like pores in OM [54,55] and gaseous hydrocarbon generation forms high pore pressure resisting further compaction [56,57], while OM shrinkage forms fissures [58]. Organic acids originating from kerogen decarboxylation, hydrocarbon cracking, and minerals oxidation dissolve feldspar and clay mineral to form secondary pores [59,60]. Illitization of kaolinite dissolves feldspar [61,62], while the Fe 3+ from the conversion of smectite to illite in I/S promotes the release of the peripheral dicarboxylic acid group from kerogen and improves the formation of carboxylic acids and phenols [63], which contribute to the development of secondary dissolved pores. Moreover, the smectite dehydration enhances the formation of fissures [64,65]. Together, these processes increase pore space.
When 2.5% < Rmax < 3.0%, transitional shale hydrocarbon generation tends to the end, while clay minerals transformation is still proceeding rapidly. During this period, the increase in pore space is jointly controlled by a graphitic-like structure with abundant nanopores which is generated in solid OM [54], illitization of kaolinite which dissolves feldspar [61,62], and smectite dehydration which forms shrinkage cracks [64,65].
When Rmax > 3.0%, transitional shale hydrocarbon generation and clay minerals transformation come to the end. Increasing external pressure leads to pore collapse [44,57] and decreases pore space.
Therefore, transitional shale pore space may have the tendency to decrease when Rmax < 1.6% or Rmax > 3.0% but increase when Rmax ranges between 1.6% and 3.0%. The main influential factors of pore evolution differ in each period.
According to the transitional shale gas generation threshold maturity (Rmax = 1.6%) and pore evolution tendency (increase when Rmax ranges between 1.6% and 3.0%), shale gas exploration should focus on the areas with a maturity range of 1.6%-3.0%.

Upper Paleozoic Transitional Shale Gas Enrichment Factors
Taking the Lower Permian Shanxi and Pennsylvanian Taiyuan Formations in the northeastern Ordos Basin, and the Upper Permian Longtan Formation in southwestern Guizhou as examples, Upper Paleozoic transitional shale gas enrichment factors were analysed. Lithological associations, sedimentary facies, gas generation threshold, pore space, and structural characteristics are the main controlling factors of Upper Paleozoic transitional shale gas enrichment. As discussed in Section 4.1-4.3, carbonaceous mudstone adjacent to coal seams, which is primarily developed in swamps in the delta plain environment, and lagoons and swamps in the barrier coastal environment, presents a high gas content level. The gas generation threshold maturity (Rmax) is 1.6%, and pore space has the tendency to increase when Rmax ranges between 1.6% and 3.0%, so shale gas is mainly distributed in the areas with a maturity range of 1.6%-3.0%. Upper Paleozoic strata of the northeastern Ordos Basin and southwestern Guizhou present different structural characteristics. The Ordos Basin is a stable multicycle cratonic basin which is not heavily folded or deformed, and Paleozoic-Mesozoic strata in the main section dip to the west at less than 1° [12,13], which leads to the continuous distribution of shale gas enrichment areas along the slope (Figure 10). Long-term tectonic movements lead to the development of folds, faults, and fractures, forming a complex structural morphology [15][16][17]. Fractures enrich the reservoir space, favoring shale gas storage and reservoir reconstruction, and sealing faults can prevent shale gas migration to adjacent formation. Therefore, shale gas can be enriched in the areas with abundant fractures and favorable sealing faults in southwestern Guizhou ( Figure 11).

Conclusions
1. Gas logging and desorption revealed carbonaceous mudstone adjacent to coal seams presents a high gas content level due to abundant OM and gas migration from coal seams, and is primarily developed in swamps in the delta plain environment, and swamps and lagoons in the barrier coastal environment. 2. Gas generation threshold maturity (Rmax) of transitional shale is 1.6%, and corresponding threshold depths of the northeastern Ordos Basin and southwestern Guizhou are about 2265 m and 1050 m. 3. Transitional shale pore evolution is jointly controlled by hydrocarbon generation, clay minerals transformation, and compaction. When Rmax < 1.6%, pore space may decrease due to compaction and filling of liquid hydrocarbon and bitumen. When Rmax ranges between 1.6% and 3.0%, pore space may increase due to kerogen pyrolysis, liquid hydrocarbon and bitumen cracking, volatile matter release, organic acids dissolution, OM shrinkage, high pore pressure, graphitic-like structure formation in solid OM, illitization, and smectite dehydration. When Rmax > 3.0%, transitional shale pore space may decrease due to increasing external pressure. 4. The continuous distribution of transitional shale gas enrichment areas can be formed along the slope adjacent to coal seams with a moderate maturity range (1.6%-3.0%) in the northeastern Ordos Basin, while transitional shale gas can be enriched in the areas adjacent to coal seams with a moderate maturity range (1.6%-3.0%), abundant fractures, and favorable sealing faults in southwestern Guizhou.
Author Contributions: F.W. conceived the research plan, analysed the research data, designed the figures, and wrote the original manuscript. S.G. designed the experiment and was responsible for the project administration, supervision and validation. All authors participated in writing and revising the manuscript. All authors have read and agreed to the published version of the manuscript.

Funding:
The research was financially supported by the National Major Science and Technology Project: Study on the Upper Paleozoic transitional shale gas occurrence and enrichment mechanisms (2016ZX05034001-004).