There are three types of organic-rich shales in China: the marine shale mainly developed in the Early Paleozoic period, the Carboniferous-Permian marine-nonmarine transitional shale, and the Mesozoic and Cenozoic continental shale^[1,2,3]. Marine-nonmarine transitional shale is a major domain for petroleum exploration and development in China, with wide distribution area and great exploration potential. The shale gas resources in this domain are about 19.8×10¹² m³, accounting for 25% of the shale gas resources in China. As its exploration was started late on the whole, it still has substantial exploration and development potential^[3,4,5]. In recent years, results of drilling and well testing targeting the marine-nonmarine transitional shales revealed high gas potential and bright development prospect. Well Eye 1 drilled in the northwest of the Ordos Basin tested an open flow capacity of 1.95×10⁴ m³/d after fracturing in the Permian Taiyuan Formation shale. In the Yanchuan area in the southeastern Ordos Basin, three horizontal wells tested production from 2.0×10⁴ to 5.3×10⁴ m³/d after fracturing in the shale intervals of the Permian Shanxi Formation. Five vertical wells in Daning-Jixian area tested industrial gas flows in the Shanxi Formation shale, with the maximum open flow capacity of more than 1.0×10⁴ m³/d. In the Qinshui Basin, good shale gas shows were detected in Permian strata of multiple wells, including Well Shixianbei 306, Shouyang Y01 and Wuyuan 01, among which the gas content of Well Shouxian 306 measured from core testing was 0.79-4.03 m³/t. The shale samples from the Permian Dalong Formation-Longtan Formation in Well Xiangye 1 in the Lianyuan basin, Central Hunan, have gas contents between 0.16 and 1.41 m³/t, and the well had a test production of 2300 m³/d after fracturing. Core samples from the Permian Longtan Formation in two wells in the Eastern Sichuan Area, Sichuan Basin, have gas contents from 0.35 to 3.49 m³/t ^[6]. The above drilling results prove that marine-nonmarine transitional shale gas has great exploration and development potential.

The favorable areas of marine-nonmarine transitional shale gas in the Ordos Basin and the Sichuan Basin cover a total area of 13.3×10⁴ km², accounting for 72% in China. The two basins have shale gas geological resources of 13.5×10¹² m³ combined, accounting for 68% of the total in China^{[3,4,5,6,7,8,9]}. The two basins take the majority of marine-nonmarine transitional shale gas resources in China. The exploration results preliminarily show that the east margin of the Ordos Basin is expected to make breakthroughs and achieve large-scale production of shale gas, making shale gas a new strategic alternative resource for natural gas industry in China^{[3, 10-12]}. Although great exploration results were achieved in marine-nonmarine transitional shale gas, geological theory and evaluation system for marine-nonmarine transitional shale gas have not yet been established, high-efficiency drilling and reservoir stimulation need to be tackled urgently, and effective development and production technology need to be explored. The Carboniferous-Permian strata in the eastern margin of Ordos Basin have abundant marine-nonmarine transitional shale sediments. Some researchers evaluated the resource potential of shale gas there, but there is no clear understanding on the distribution of favorable intervals and the prospect of exploration and development, restricting the large scale development. In this study, by examining systematically the geological features of shale gas in the Shanxi Formation in the east margin of the Ordos Basin and comparing with typical marine and marine-nonmarine transitional shale gas plays at home and abroad, geological conditions and exploration and development prospects of the marine-nonmarine transitional shale gas of Shanxi Formation have been made clear. The understandings from this study will provide reference and guide the exploration and development of marine-nonmarine transitional shale gas in other basins.

Geographically, the east margin of the Ordos Basin, spans Shanxi and Shaanxi provinces and is adjacent to Lishi fault to the east and Yellow River and Hancheng-Heyang-Tongchuan area to the west. It is a long and narrow arc-shaped band, about 450 km long from north to south, and 26-100 km wide from east to west, with a total area of more than 4.5×10⁴ km² (Fig. 1a)^{[9, 12]}. It is structurally simple, where the strata incline to the west, and the exposed strata gradually become younger from east to west^[12]. Marine-nonmarine transitional strata, including the Benxi Formation of the Upper Carboniferous, and Taiyuan Formation and Shanxi Formation of the Lower Permian (Fig. 1b) had developed extensively during the Carboniferous-Permian period in the Late Paleozoic, since the erosion surface at the top of the Middle Ordovician strata was formed^[13,14,15]. The stratigraphic characteristics of the study area are closely related to the sedimentary-structural evolution of the Ordos Basin. From the Late Carboniferous to Early Permian, the basin, as a part of the North China Craton, was relatively stable in sedimentation, when a set of marine and marine-nonmarine transitional sedimentary strata deposited, accompanied with coal seams formed by peat accumulation. Influenced by regional tectonic activities, the frequent change of water resulted in multiple stages of sedimentary cycles of littoral neritic facies, delta front facies and inshore shallow lake facies, containing multiple sets of marine-nonmarine transitional organic- rich shale intervals (Fig. 1b), with a cumulative thickness of 43.5-187.3 m and an average thickness of 88.6 m^{[13, 16]}. The shale in the 2nd member of the Shanxi Formation (Shan-2 shale for short) is the most typical, with several thickness centers, including Yulin-Linxian, Shiloubei- Daning-Jixian and Hancheng-Huangling. The shale has a maximum single layer thickness of 50m (Fig. 2) and an average total organic carbon content (TOC) from 1% to 3%, showing huge shale gas potential.

The exploration and development of natural gas in the east margin of the Ordos Basin has a long history. PetroChina, Sinopec, Yanchang Petroleum Group and CUCBM all have carried out exploration in this area, and achieved encouraging results. Since 2008, PetroChina has carried out large amounts of seismic survey and drilling focusing on coal-bed methane and natural gas in coal measures in Hancheng, Linfen, Baode, Hequ, Jungar Prefecture, Sanjiao and other blocks. As a result, natural gas geological reserves of nearly 6000×10⁸ m³ have been proven, an annual gas production of 40×10⁸ m³ has been built and two large development zones of coal bed methane, Hancheng and Baode have been established. With the continuous progress of natural gas exploration and development and the deepening of understandings on the geological laws of natural gas enrichment in the area, the exploration and development of coal-bed methane, tight gas and shale gas have been carried out in multiple formations and fields since 2018, in the hope to realize co-production of multiple types of natural gas and comprehensively efficient resource exploitation. Seven vertical wells in Daning-Jixian area were tested for gas in the shale section of Shanxi Formation after fracturing, of which 5 obtained industrial gas flow, with the maximum test production of over 1×10⁴ m³/d.

The Shanxi Formation in the east margin of the Ordos Basin is a set of shale intervals of continental delta, offshore delta and epicontinental marine-nonmarine transitional facies^[16,17]. During the sedimentary period of the Shanxi Formation, the sea water gradually receded from the east and west of the Ordos Basin, the basin gradually changed from a marine one to a continental one, and the sedimentary environment was relatively stable, which was conducive to the enrichment of shale gas^{[10, 18]}. The Shanxi Formation in the study area is divided into Shan-2 and Shan-1 members. In Jungar Prefecture in the north of the Ordos Basin close to the northern provenance, a set of coarse-grained conglomerate and gravelly sandstone of alluvial fan deposited, coarse sandstone of braided river deposited to the south, and swamps were formed by silting up in inter-fan depressions. Baode-Xingxian area mainly developed braided river channel, natural levee, and flood swamp environments, among which flood swamp was more developed. The fluvial sedimentary system changed to tidal delta sedimentary system to the south, and Sanjiao- Shilou area was a tidal delta plain, where distributary bays were silted into plain swamp. Daning-Jixian area was a transitional zone of the northern and southern provenances, and delta front and littoral, a typical marine-nonmarine transitional sedimentary environment. In Hancheng-Heyang area in the south developed small-scale delta deposits, dominated by delta front.

A large amount of exploration data confirmed that both the Shan-2 and Shan-1 members in the east margin of the Ordos Basin have shale intervals (Fig. 2). The Shan-1 shale is 9.7-51.5 m thick, 24.6 m on average, and the Shan-2 shale is 21.4 to 92.3 m thick, 41.2 m on average. The Shan-2 shale has a larger thickness, fewer and thinner interlayers, a maximum thickness of single shale layer of 50 m, an area of about 4.5× 10⁴ km², and three major depocenters, Yulin-Linxian, Shiloubei-Daning-Jixian and Hancheng-Huangling. From top to bottom, the Shan-2 member is further subdivided into Shan₂¹, Shan₂² and Shan₂³ three sub-members. In Daning-Jixian area, the Shan₂³ sub-member is composed of typical marine-nonmarine transitional deposits of shallow marine bay-lagoon, delta front, delta plain sedimentary environments (Fig. 3). Three kinds of shale associations have been identified in the Shan-2 member based on observation of cores from Wells Daji 51 and Daji 3-4 etc, namely: (1) shale of shallow marine bay-lagoon facies in the lower sub-member, composed of organic-rich bioclastic shale, charcoal-bearing shale and silty shale deposited in semi-reduction-reduction environment (Fig. 4a, 4b); (2) black shale of tide-dominated lower delta plain facies in the middle sub-member, with flaky charcoal, pyrite/ siderite nodules and bioturbation structures (Fig. 4c, 4d); (3) black shale of upper delta flat facies in the upper sub-member, which is largely swamp and lake deposit between distributary channels, as these depositional environments were open oxidation ones, the shale contains frequent bioturbation signs and carbonaceous shale and coal seams (Fig. 4e, 4f).

TOC value of Shan-2 shales in the study area is relatively high, generally ranging from 1% to 3%, and up to more than 10% (Fig. 5). Analysis results of organic maceral composition, element composition of kerogen and vitrinite reflectance (R_o) of shale core samples show the shale samples have relatively high contents of vitrinites and inertinites, which account for 56%-86% of macerals, mainly types II-III organic matter (Fig. 6), and R_o values from 1.5% to 2.0% (Fig. 7) indicating peak-period of gas generation. The Shan₂³ sub-member is the section in the Shan2 member most abundant in organic matter, with TOC values from logging interpretation ranging between 1.40% and 8.88% (4.91% on average), and measured ones TOC of cores from Well Daji 51 in Daning-Jixian area ranging between 4.53% and 11.68% (7.97% on average).

The above data shows that the Shan-2 member in the study area is a set of typical marine-nonmarine transitional deposits developed in relatively stable regional sedimentary environment with widespread organic-rich shale.

2.2.1. Well-developed micro- and nano-scale pore and fracture systems

Analysis results of FIB-SEM show the Shan-2 shale in the study area has developed micro- and nano-scale pore and fracture systems mainly composed of inorganic mineral pores, organic matter pores and micro fractures (Fig. 8). Among them, inorganic mineral pores and micro fractures are the best developed, while organic matter pores are lower in development degree and heterogeneous to some extent.

Inorganic mineral pores mainly include dissolution pores of mineral, interlayer pores of clay mineral, intercrystal pores of pyrite and marginal pores of mineral (Fig. 8a-8c). The Shan-2 member has a small amount of calcite, dolomite and other carbonate minerals and feldspar and other soluble minerals at the bottom, which would give rise to dissolution pores when dissolved. These dissolution pores are round, oval or irregular in shape, tens to hundreds of nanometers in size, and often distributed between grains (Fig. 8a) or in grains (Fig. 8b). The shale also has a large number of interlayer micro-scale pores in clay minerals, which, occurring in illite of silk-thread or curly flaky form, and book-like or accordion-like kaolinite, are mainly slit-shape, parallel with each other, nano-scale in width and micro-scale in length (Fig. 8c). Pyrite in the shale appears in strawberry-like aggregates or dispersed crystals, and there are irregular intergranular pores between 50 and 200 nm in diameter between the crystals (Fig. 8d).

The shale has a lower development degree of organic matter pore. The organic matter pores include a small number of primary textural pores, hydrocarbon generating pores, and marginal pores of organic matter (Fig. 8e, 8f). Some organic matter contains a large number of nano-scale hydrocarbon generating pores, which are circular, oval or irregular in shape, tens to hundreds of nanometers in diameter, in good connection and heterogeneous (Fig. 8e). Marginal pores and fractures are seen at the connecting area between organic matter and mineral grains of the shale samples. Occurring in slit, band and irregular shapes, and different sizes, these pores and fractures may be resulted from the hardness difference between organic matter and mineral particles or the pyrolytic contraction of organic matter (Fig. 8g). The interlayer fractures in clay minerals and fractures between mineral grains are well developed, which are tens of nanometers to hundreds of nanometers wide and several microns long (Fig. 8h, 8i). In the Shan-2 shale, illite and kaolinite take the majority of clay minerals; in the process of diagenesis, montmorillonite was transformed into illite through mixed-layer illite/smectite formed after dehydration, generating micro-scale pores and fractures with the decrease of volume. In addition, the acidic diagenetic conditions in the process of hydrocarbon-generation evolution made feldspar minerals alter into kaolinite which has a large number of micro-scale pores between crystals (Fig. 8h). The organic macerals of the Shan-2 shale samples are mainly vitrinite, indicating strong gas generating capacity. The abnormal pressure created during the thermal evolution made organic matter break, generating micro-fractures (Fig. 8g).

2.2.2. Good reservoir physical properties

The logging-interpreted porosity of the Shan-2 shale in 17 wells in the east margin of the Ordos Basin is 4%-6%, which shows a positive correlation with permeability. The high- quality shale section of Shan₂³ sub-member has a logging-interpreted porosity of 5.1%-5.8%, 5.3% on average, core-measured porosity of 1.25%-4.85%, 3.80% on average, and core- measured permeability of 0.01-0.10×10^-3 μm², 0.04×10^-3 μm² on average (Fig. 3), indicating the shale layers in the study area have relatively high storage capacity.

Both marine and nonmarine shales are complex in rock assemblage and mineral composition^{[10, 19-20]}, and the marine-nonmarine transitional shale is no exception. Shale intervals in the second member of Shanxi Formation in the eastern margin of the Ordos Basin are complex in lithology, and composed of carbonaceous shale, calcareous shale, silty shale, argillaceous siltstone, with fine sandstone, siltstone, coal streak and seam interbeds (Figs. 1 and 3). The analysis results of X-Ray Diffraction show mineral components of the Shan-2 shale samples mainly include quartz (24%-54%, 38.6% on average), clay (22% to 72%, 55.3% on average), and a small amount of calcite, dolomite, feldspar and pyrite (Fig. 9). In the clay minerals, the kaolinite, chlorite, illite and illite account for 61.50%, 20.58%, 15.75% and 2.17% respectively. Except a few samples, most samples have lower contents of carbonate minerals of 3.3% on average. The shale samples contain pyrite, siderite and other authigenic minerals, with a content of less than 5% generally. The brittle minerals in the Shan-2 shale samples are quartz and carbonate minerals, with an estimated content of 40%- 65%. The Shan ₂³ high quality shale has an average quartz content of 55%, and contents of clay minerals of 20.0%- 41.0% (28.9% on average), and contents of brittle minerals of 59%-80% (71% on average).

The brittleness index of mineral is used to characterize the fracability of shale reservoir, which is generally calculated by the equation: brittleness index = (quartz + carbonate)/(quartz + carbonate + clay). Calculated by the equation, the brittleness indexes of the Shan-2 shales in the east margin of the Ordos Basin are up to 85%, and 51.2% on average; and those of high- quality shales in the Shan₂³ sub-member are 72% on average. After analyzing well logging data of three shale gas wells, Well Ji 2-4, Ji 41 and Ji 36, in Daning-Jixian area, the Shan-2 shales show favorable rock mechanic properties, with young's modulus of 20-44 Gpa and Poisson's ratio of 0.2-0.27, indicating good reservoir properties conducive to fracturing.

The burial depth of the bottom of the Shan-2 shale in the east margin of the Ordos Basin ranges between 800m and 2600 m, that is 1200-2600 m in Daning-Jixian block, and 1600-2600 m in Shiluoxi area. The shale intervals have pressure coefficients from 0.95 to 1.05, indicating largely normal pressure^[21]. Gas content is the most important parameter to evaluate the development potential of shale gas reservoir^[22,23]. The gas contents of the Shan-2 shales in 17 wells of Daning-Jixian block from logging interpretation are 1.38-5.66 m³/t, and 2.63 m³/t on average, those of the high-quality Shan ₂³ shale section at 2295-2298 m in Well Daji 51 are 1.55-3.72 m³/t, 2.15 m³/t on average, and the measured gas contents of core samples are 0.75-3.71 m³/t, 2.15 m³/t on average (Fig. 3). It is worth noting that the coal interlayers in the Shan-2 shales in the study area have gas contents of 10.98-16.98 m³/t. The shale gas samples from the study area have CH4 contents of 95.15%-99.03%, 96.60% on average, tiny amount of heavy hydrocarbon gas and non-hydrocarbon gas, indicating the gas is typical dry gas.

Compared with marine shale, marine-nonmarine transitional shale has the following geological characteristics: (1) Marine-nonmarine transitional shale intervals often interbed with coal seams (streaks) and dense siltstone (sandstone) and have rapid lateral change; (2) they largely have types II-III organic matters, high abundance and maturity of organic matter, and strong gas generation capacity; (3) they have complex mineral composition and higher content of clay minerals^{[3, 5, 8]}. As a set of typical marine continental transitional shale intervals, the Shan-2 member in the east margin of the Ordos Basin has largely types II-III organic matters, and TOC of 1%-3%, in which the Shan₂³ sub-member has an average TOC of up to 7.97%. This shale member has high maturity (with R_o values ranging from 1.5% to 2.0%), and relatively high gas content of over 2.0 m³/t on average^{[1, 24]}, slightly lower than typical marine shale (Fig. 10a). This shale member is characterized typically by well-developed inorganic pores and microfractures, and relatively undeveloped organic pores. It has a porosity range of 4%-6% from logging interpretation, and an average measured core porosity of 3.8%, which is lower than the typical marine shale^{[1, 25-26]}. For example, major marine gas-bearing shale formations in the United States have a porosity range from 2% to 14%, mainly from 4% to 7%, and porosity range of 4%-12%, 5.2% on average from logging interpretation^[27,28]; and marine shale intervals in the Lower Paleozoic in South China have porosities of 3.0%-9.1%, 6.95% on average. Development of pores in shale reservoir is influenced by multiple factors. Organic carbon content, kerogen type, thermal evolution degree, type and content of clay minerals all affect the development of nano-scale pores to various degrees. Many studies have confirmed that the type and content of organic matter in shale are important factors affecting the development of organic pore^{[18, 29-30]}. The Shan-2 shale shows a weak negative correlation between the total organic carbon content and the total pore volume calculated by the BJH (Barrett-Joyner-Halenda) model, which is obviously different from the positive correlation of marine shale (Fig. 10b), and is consistent with the observed non-development of organic pores by SEM (Fig. 8). The Shan-2 shale has a higher content of clay minerals, which shows a poor positive correlation with the total pore volume calculated by the BJH model (Fig. 10c). Compared with the typical marine shales in South China, the Shan-2 shale has a significantly smaller pore volume (Fig. 11), but still has good storage capacity in general.

Marine-nonmarine transitional shale gas reservoirs are different from marine ones to some extent in distribution, mineral composition, organic matter type, burial depth and pressure system. The Shan-2 shale in the east margin of the Ordos Basin is thick, with a maximum single layer thickness of 50 m, and has three depocenters, Yulin-Linxian, Shiloubei-Daning- Jixian, and Hancheng-Huangling (Fig. 11a). This shale member is at moderate burial depth of mainly from 800 to 2600 m (Fig. 11b), and gradually increases in maturity from north to south, with R_o values between 1.5% and 2.0% (Fig. 11c), indicating it has reached the peak of gas generation. In summary, compared with different types of shale gas reservoirs, the Shan-2 shale reservoir in the east margin of the Ordos Basin has moderate basic geological conditions, including burial depth, thickness, organic content, thermal evolution degree, gas content, and pressure coefficient etc. The Shan-2 shale is similar with the transitional Lewis shale in the San Juan Basin, Untied States in TOC value, thermal evolution degree, gas content, and porosity etc. key parameters, but is higher in clay mineral content^[31,32,33]. Lewis shale once was one of the five traditional shale gas-producing plays in the United States, so the Shan-2 member in the east margin of the Ordos Basin is believed to have favorable geological conditions and exploration potential in terms of depth, thickness and gas content.

Based on the basic geological characteristics of the Shan-2 shale in the east margin of the Ordos Basin, the evaluation indexes of gas potential and fracability of the favorable interval of marine-nonmarine transitional shale gas are preliminarily proposed, by referring to the evaluation indexes of typical marine shale gas reservoir^{[1, 3, 18]}. The evaluation indexes of gas potential include TOC value of greater than 2%, porosity of more than 2%, and gas content of higher than 2 m³/t. The evaluation indexes of fracability include content of brittle minerals of higher than 50% and development of micro-fractures. On this basis, shale thickness of more than 25 m and buried depth greater than 1500 m are added into the evaluation of favorable shale gas zone (Fig. 11b). According to the "Regulation of shale gas resources/reserves estimation" issued by the former Ministry of Land and Resources in 2014^[34], the volume method was used to estimate the shale gas resources in the Shan-2 member of the Shanxi Formation. The following parameters were used in the calculation, favorable shale zone of 22800 km², reservoir thickness of 37-59 m, shale density of 2.6-2.7 g/cm³ and gas content of 2.15-2.63 m³/t. The geological resources were estimated at (3.3-5.2)×10¹² m³ from the burial depth of 1000 m. There are three favorable blocks (Fig. 11d), Yulin-Linxian, Shiloubei-Daning-Jixian and Hancheng-Huangling, in which Yulin-Linxian block has an area of about 4600 km² and geological resources of (6670-10580)× 10⁸ m³; Shiloubei-Daning-Jixian block has an area of about 6000 km² and geological resources of (8700-13800)×10⁸ m³; Hancheng-Huangling block has an area of about 2200 km² and geological resources of (3200-5060)×10⁸m³.

Shale gas development effect depends on gas-bearing properties of shale reservoir, physical properties, mechanical properties, pressure and fracability of the reservoir^{[25, 35-37]}. In the 2nd member in the east margin of the Ordos Basin, the Shan₂³ shale has the largest thickest between 20 m and 40 m in general, stable distribution and few interlayers (Figs. 1 and 2). With high gas content and high content of brittle minerals, this section is the most favorable sweet spot interval of shale gas, and also the major fracturing test interval at present.

From 2018 to 2019, the sweet spot section at 2295-2298 m in Well Daji 51, a vertical well in Daning-Jixian area was fractured, production tested for 1612 h, and then shut in for pressure buildup test for 1073 h. The section had an initial formation pressure of 17.86 MPa, and produced 33.9×10⁴ m³ at average daily gas production of 0.6×10⁴ m³ during the production test period. During the production test, the average rate of pressure drop was 0.11 MPa/d, and the bottom gas production per unit pressure drop was 4.7×10⁴ m³/MPa. The rate of pressure recovery was 0.59 MPa/d during the shut-in of 7 d. According to the well testing interpretation, the effective formation permeability was 0.06×10^-3 μm², the open flow (AOF) was 2.3×10⁴ m³/d, and the extrapolated formation pressure was 17.25 MPa, which is 0.6 MPa lower than the initial formation pressure, showing high pressure-maintaining level. This indicates that the formation energy was high and the pressure recovery was rapid. The well can effectively produce at 0.6×10⁴ m³/d when 1/5 to 1/3 of AOF is used for prorating production, while it can produce at 0.5×10⁴ m³/d when the average production during the production test period is used. Compared with the average production of 0.8×10⁴ m³/d of shale gas of fractured vertical wells in the United States, this well shows a good development prospect. It is estimated based on the results of well test that the controlled dynamic reserves of this well are as high as 882×10⁴ m³. A fracture with a half-length of about 66 m was generated near the well, resulting in a conductivity of about 75×10^-3 μm²•m and a well control area of 418 m×700 m.

In the Shanxi Formation, multiple shale layers are superposed vertically, high-quality gas reservoirs should be stimulated in priority in the process of shale gas development. Considering the mineral and pore compositions, additional amount of pad-acid can be used to dissolve the filling minerals and carbonate rocks to increase the stimulation volume and get rid of potential problem of drainage. The fracturing curves of Wells Daji 27, Daji 36, Daji 41 and Daji 2-4 earlier show that the closure stress of the formation is 43-53 MPa, and the calculated effective closure stress acting on proppant is more than 25 MPa. Accordingly, the proppant of quartz sand of 70 140 mesh (0.106-0.212 mm) and proppant combination of quartz sand of 40/70 mesh (0.212-0.425 mm) and medium-strength ceramsite are recommended. The higher the net injection pressure is, the more complex the fracture network will be created and the higher the single well production will be. Increasing injection rate is the most important means to increase net pressure. Through analysis of the early fracturing treatments, it is found that treating pressure is stable when injection rate is more than 12 m³/min. The shale intervals in the study area have small pore size and high static starting pressure, so intermittent production would result in large energy loss, and the speed of flowback during fracturing should be kept low by controlling the nozzle throughout the whole process to reduce the risk of sand-out and ensure continuous flowback. In general, the Shan-2 member in the study area has abundant shale gas resources, favorable recoverability and fracability, and bright development prospect.

In addition, two major coal seams in the Shanxi and Taiyuan Formations occur in the whole east margin of the Ordos Basin. The one in the Shanxi Formation is 1-15 m, generally more than 2.5 m thick, and the other one in the Taiyuan Formation is 2-20 m, generally more than 3.5 m thick^[21]. The coal seams have R_o values from 0.59% to 2.35%, and increase gradually in thermal evolution degree from north to south. The coal seams have cumulative quantity of generated gas calculating from the long-flame coal of more than 50 m³/t^{[21, 38]}. Therefore, the study area has multiple types of natural gas in the shale and adjacent intervals. With multiples shale intervals superposed vertically, co-production of multiple types of natural gases can be a good idea in the future development.

The Carboniferous-Permian systems in the east margin of the Ordos Basin are composed of marine-nonmarine transitional deposits, with multiple thick layers of shale. The shale intervals have favorable TOC, thermal evolution degree, porosity, permeability, gas content, structural preservation conditions and burial depth, so this area has favorable geological conditions for generation, storage and sealing of shale gas. The 2nd member of the Shanxi Formation in the study area has four major geological features: (1) relatively stable sedimentary environment and widely distributed organic rich shale; (2) well developed micro- and nano-scale pore and fracture systems with good storage capacity; (3) abundant brittle minerals such as quartz, favorable for reservoir fracturing; and (4) moderate reservoir pressure and relatively high gas content.

Based on the basic geological characteristics of the Shan-2 shale in the east margin of the Ordos Basin, and the evaluation indexes for favorable interval of transitional shale gas, geological resources of the Shan-2 shale in the study area were estimated at (3.3-5.2)×10¹² m³. Three favorable blocks, Yulin- Linxian, Shiloubei-Daning-Jixian and Hancheng-Huangling, were sorted out, with a total area of about 1.28×10⁴ km² and resources of (1.8-2.9)×10¹² m³, indicating great exploration potential.

In the Shan-2 shale in the east margin of the Ordos Basin, wells have stable gas production and high controlled geologic reserves, indicating favorable recoverability and fracability of the formation. Systematic well testing shows the formation has sufficient energy and stable production capacity, implying bright development prospects. The Shanxi and Taiyuan Formations in the study area have several shale intervals interbedded with several coal seams vertically, thus, co-production of multiple types of natural gases from multiple intervals can be good choice.