Introduction
Bauxites, sedimentary rocks containing aluminum (Al)-rich hydroxide, contain Al2O3 with mass fraction larger than 40% and mass fraction ratio of Al2O3 to SiO2 larger than and equal to 2 [1-2]. Bauxite is a type of fine-grained sedimentary rock of chemical deposit, and the main minerals include aluminum hydroxides (gibbsite, boehmite, and diaspore), clay minerals, iron minerals, and titanium minerals [2⇓⇓-5]. The parent rock suffered weathering and leaching, leading to a large loss of alkali metal elements and Si, and residual enrichment of Al, resulting in the formation of bauxite. Bauxite is similar to clay rocks, however, the bauxite is harder and denser, without plasticity, and generally exhibits oolitic, pisolitic, and massive textures [4⇓-6]. Based on the parent-rock lithology, bauxite deposits in China can be divided into karstic (sedimentary) and lateritic bauxite [7-8]. The bauxites of the Ordos Basin are the karstic bauxite type, and the formation process includes terrestrial physicochemical weathering, epigenetic weathering and leaching modification, and secondary transport and deposition [1⇓-3]. The bauxite series sequence is controlled by the combination of karst terrains and sedimentary facies, and the sequence changes are complicated [4,6,8 -9].
Based on gas exploration in the Ordos Basin, the bauxite series in the paleo-weathered crust at the bottom of the Carboniferous Benxi Formation has high density and poor physical characteristics, wider distribution, and stronger capping ability. For a long time, it has only been regarded as a regional cap rock and not considered as a reservoir section [10-11]. The section is commonly developed in North China and generally named as G-layer bauxite, usually associated with Shanxi-style iron ores [12-13]. Conventionally, the studies on bauxite series mainly focus on the genesis of bauxite deposits [6,10], with the main focus on exploring metal mines. With the expansion of oil and gas exploration, some exploratory wells showed gas logging anomalies in the section and achieved major breakthroughs in natural gas exploration. In 2020, Well NG-3 in Ninggu area obtained a daily gas production of 13.44×104 m3 in the section [14]; in 2021, the gas testing of Well L47 in Longdong area obtained 67.38×104 m3/d open flow [4-5]; in 2021, the natural gas test of bauxite reservoir in Well LX41 of Linxing block was carried out, with a daily gas production of 5 013 m3, showing a good natural gas exploration and development prospect in the G-layer bauxite [15-16].
A systematic and comprehensive understanding of the reservoir characteristics of bauxite series is an important foundation for sustained breakthroughs in bauxite gas exploration and development. This study focused on the Carboniferous Benxi Formation bauxite series from typical outcrops and two drill cores of Hequ, Fugu, Baode, Xingxian, Linxian, Liulin, Daning-Jixian and Hancheng areas in eastern Ordos Basin. Combining core and well logging data, we conducted hand specimen observation, thin section, whole-rock mineral analysis of X-ray diffraction (XRD), scanning electron microscopy (SEM), low-temperature liquid nitrogen adsorption, carbon dioxide adsorption, and isothermal methane adsorption experiments, to investigate the reservoir characteristics of bauxite series, and to preliminarily clarify the mineral composition, lithology classification, structure, texture and pore characteristics of the bauxite series. We systematically summarized the formation process of bauxite reservoirs in the Ordos Basin and the overall North China Platform, and clarified the characteristics of bauxite gas enrichment, so as to provide technical support for the exploration of natural gas in the bauxite series of North China Craton (NCC).
1. Geological setting
The Ordos Basin is located in the western part of the North China Craton (Fig. 1a ). The geological and tectonic evolution mainly experienced the Middle and Late Paleozoic continental rift formation, the Early Paleozoic epicontinental sea basin in the NCC, the Late Paleozoic-Middle Triassic depression in the NCC, Late Triassic-Cretaceous Ordos inland basin development and the Cenozoic peripheral small faulted basin formation period of Ordos Basin [17⇓⇓-20]. At the end of the Ordovician, the Ordos Basin suffered a large-scale regional uplift influenced by the Caledonian orogeny. The Ordovician Majiagou Formation carbonate rocks were widely outcropped on the surface, and underwent strong weathering and karstification for as long as 1.3-1.5 Ga, forming a huge carbonate paleo-weathering crust. The Yinshan Mountains in the northern part of the NCC gradually uplifted and became the main material provenance of Carboniferous bauxite. During the depositional period of the Upper Carboniferous Benxi Formation, the North China Plate descended again to accept deposits, and seawater rapidly intruded from east to west. The range of the central paleo-uplift gradually narrowed, and the Benxi Formation was overlapped from the central part of the basin to the southwest and northeast, forming shallow marine shelf, tidal flat and lagoon sedimentary facies. The NCC Upper Carboniferous bauxite series is in unconformable contact with the underlying Ordovician Majiagou Formation carbonate paleo-weathering crust. The karstification differences of the bottom Majiagou Formation carbonate influenced the stratigraphic relief. Due to the different degrees of weathering and leaching of the bauxite series, there are also differences in lithology combination and thickness (Fig. 1c ).
Fig. 1 Location of the Ordos Basin and the regional stratigraphic column of bauxite series in the Upper Carboniferous Benxi Formation. |
The Upper Palaeozoic in eastern Ordos Basin is the current major area for the development of deep coalbed methane and tight gas, as well as multiple natural gas-bearing formations, including bauxite and marine-continental transitional shale. The eastern Ordos Basin develops four tectonic units, i.e., the Yimeng uplift, the Yishaan slope, the Western Shanxi flexural belt and the Weibei uplift (Fig. 1b ), with strata gradually entering the inner part of the basin from the west to the east, increasing the buried depth and decreasing the dipping angle.
2. Bauxite mineralogic composition and classification in the Benxi Formation
2.1. Mineralogic composition
According to the Analysis method for clay minerals and ordinary non-clay minerals in sedimentary rocks by the X-ray diffraction (SY/T 5163-2018) [21], the 17 bauxite samples from the eastern Ordos Basin were analyzed for whole-rock and clay mineral analyses using the TTRIII multifunctional X-ray diffractometer produced by Rigaku Corporation, Japan. The results show that the minerals are dominated by diaspore, boehmite and clay minerals, with some iron minerals and titanium minerals, and less quartz, feldspar and calcite contents (Table 1 ). The iron-rich layer contains 85.9% hematite and a lower clay mineral content; the aluminum-containing layers contain more clay minerals, together with more aluminum hydroxides (diaspore and boehmite), and lower contents of hematite, goethite and anatase; the bauxite layer is dominated by diaspore and boehmite with over 40% combined content, followed by clay minerals, hematite and anatase. After Majiagou Formation carbonate rocks suffered weathering and leaching, iron ions were filtered out first, and aluminum-containing solutions were transported into semi-enclosed reducing environments (lagoon and tidal flat) to form pyrite, which was oxidized to form hematite, resulting in enriched hematite at the bottom of bauxite series [22].
Table 1 Lithologic assemblages, whole-rock mineral content and physical properties of Upper Carboniferous bauxite series in eastern Ordos Basin |
| Sample No. | Layer | Lithology | Whole-rock mineral content/% | Clay mineral content/% | Porosity/% | Permeability/ 10−3 μm2 | |||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Quartz | K-feldspar | Plagioclase | Calcite | Diaspore | Boehmite | Hematite | Goethite | Anatase | Clay minerals | Montmorillonite | I/S | Illite | Kaolinite | Chlorite | |||||
| L-1 | Aluminum- containing layer | Kaolinite mudstone | 6.4 | 0.5 | 0.6 | 0 | 7.0 | 0 | 2.7 | 0 | 0 | 82.8 | 0 | 23 | 12 | 59 | 6 | 3.7 | 0.367 |
| L-2 | Aluminum- containing layer | Bauxitic mudstone | 1.4 | 0.5 | 0.8 | 0 | 43.4 | 0 | 8.6 | 0 | 3.7 | 41.6 | 7 | 0 | 43 | 0 | 50 | 7.8 | 0.604 |
| L-3 | Bauxite layer | Clayey bauxite | 2.5 | 0 | 0.9 | 0 | 46.4 | 0 | 20.0 | 0 | 0 | 30.2 | 0 | 0 | 93 | 0 | 7 | 6.7 | 0.528 |
| L-4 | Aluminum- containing layer | Bauxitic chlorite mudstone | 0 | 0 | 0 | 1.6 | 33.6 | 0 | 0 | 0 | 0 | 64.8 | 23 | 0 | 0 | 0 | 77 | 3.8 | 0.458 |
| X-1 | Aluminum- containing layer | Kaolinite mudstone | 0 | 0.6 | 0 | 0 | 10.2 | 0 | 3.0 | 0 | 4.2 | 82.0 | 0 | 4 | 5 | 75 | 16 | 1.8 | 0.161 |
| X-2 | Aluminum- containing layer | Kaolinite mudstone | 0.5 | 1.4 | 0 | 0 | 2.8 | 0 | 0.7 | 0 | 0 | 94.6 | 0 | 4 | 3 | 74 | 19 | 2.1 | 0.192 |
| X-3 | Aluminum- containing layer | Bauxitic kaolinite mudstone | 0.3 | 1.7 | 0 | 0 | 16.7 | 5.1 | 1.0 | 0 | 5.2 | 70.0 | 0 | 4 | 3 | 75 | 18 | 1.6 | 0.173 |
| X-4 | Aluminum- containing layer | Bauxitic kaolinite mudstone | 0.5 | 0 | 0 | 0 | 10.9 | 13.8 | 0.6 | 0 | 4.2 | 70.0 | 0 | 0 | 0 | 89 | 11 | 2.7 | 0.268 |
| X-5 | Aluminum- containing layer | Kaolinite mudstone | 0 | 0 | 0 | 0 | 10.0 | 10.0 | 0 | 0 | 0 | 80.0 | 0 | 0 | 0 | 91 | 9 | 1.5 | 0.153 |
| B-1 | Iron-rich layer | Ferrilite (sedimentary iron ore) | 0 | 0 | 0 | 0 | 0 | 0 | 85.9 | 10.9 | 0 | 3.2 | 0 | 0 | 100 | 0 | 0 | 1.1 | 0.135 |
| B-2 | Aluminum- containing layer | Bauxitic kaolinite mudstone | 0 | 0 | 0 | 0 | 30.0 | 6.0 | 6.0 | 0 | 0 | 58.0 | 0 | 10 | 16 | 67 | 7 | 2.6 | 0.355 |
| B-3 | Bauxite layer | Clayey bauxite | 0.8 | 0 | 0 | 0 | 15.0 | 55.2 | 0 | 0 | 0 | 29.0 | 0 | 0 | 0 | 35 | 65 | 5.5 | 0.539 |
| H-1 | Aluminum- containing layer | Kaolinite mudstone | 2.0 | 0 | 0 | 0 | 2.0 | 0 | 15.3 | 0 | 3.7 | 77.0 | 0 | 0 | 8 | 84 | 8 | 1.8 | 0.182 |
| H-2 | Aluminum- containing layer | Kaolinite mudstone | 0 | 0 | 1.0 | 0 | 0 | 0 | 22.1 | 0 | 0 | 76.9 | 0 | 0 | 8 | 83 | 9 | 1.3 | 0.133 |
| F-1 | Aluminum- containing layer | Kaolinite mudstone | 0 | 0 | 0 | 5.0 | 0 | 0 | 1.0 | 0 | 3.5 | 90.5 | 0 | 0 | 12 | 78 | 10 | 1.9 | 0.264 |
| F-2 | Aluminum- containing layer | Kaolinite mudstone | 0.8 | 0 | 0 | 0 | 0 | 0 | 10.2 | 0 | 3.3 | 85.7 | 0 | 3 | 21 | 70 | 6 | 1.6 | 0.248 |
| F-3 | Aluminum- containing layer | Kaolinite mudstone | 0 | 0 | 0 | 0 | 0 | 0 | 10.9 | 0 | 0 | 89.1 | 0 | 0 | 3 | 90 | 7 | 1.5 | 0.178 |
2.2. Bauxite classification
Dense massive, pisolite-oolite, porous soil (soil or semi-soil) and clastic structures are observed in eastern Ordos Basin bauxite rocks. The dense massive bauxite is generally grey, grey-yellow, dense and hard, with a muddy texture, occasionally with sedimentary bedding, dominated by clay minerals. They were formed by weathering crust products transported to low-energy environments by deposition, in situ residual deposits or rapid slope deposits (Fig. 2a ). The pisolite-oolite bauxite is grey-red with pisolitic and clastic structures, dominated by diaspore and clay minerals, with fewer internal lamination (Fig. 2b ). The porous soil bauxite is red, with a soil-like structure, loose and porous in surface, suffered intense leaching (Fig. 2c ). The clastic bauxite generally appears grey and light red with clastic structure, and the clastic particles mostly are fine-grained structure with better sorting and roundness, formed by further weathering of protoclastic materials in the residual slope deposits (Fig. 2d ). Moreover, the bottom of bauxite series in the study area has develops ferrilite formed by highly enriched hematite, with the red, grey-red color, massive structure and enrichment of iron minerals.
Fig. 2 Characteristics of typical bauxite series in Upper Carboniferous of eastern Ordos Basin. (a) Baode-Palougou section (B-3), grey-yellow dense massive clayey bauxite; (b) Well LX-98, 2 057.50 m, grey-red pisolitic-oolite bauxite; (c) Linxian-Zhaoxianzhen profile (L-3), grey-red porous clayey bauxite; (d) Well LX-78, 2 344.10 m, grey clastic clayey bauxite. |
Classification of bauxite based on structural-tectonic characteristics cannot adequately represent the lithological and mineralogical characteristics, as well as adapt to the needs of natural gas exploration and development for reservoir lithology classification. The current common rock naming scheme does not explicitly define the rock types of bauxite series, and constraints such as Al-rich minerals and Fe-rich minerals are commonly used, but the word "rich" does not mean the same thing as content. Based on the common naming convention of 10%, 25%, and 50% of a certain mineral content, a triangular diagram was compiled with three end members as aluminum hydroxides and titanium minerals, iron minerals, and clay minerals and other minerals for bauxite series (Fig. 3 ). In which, the aluminum hydroxides are mainly gibbsite, boehmite and diaspore, titanium minerals are mainly anatase, iron minerals are mainly hematite, goethite and limonite, and clay minerals are mainly kaolinite, illite, montmorillonite and chlorite. The other mineral contents in the bauxite series are relatively small, thus the other minerals are combined with the clay minerals into the third end-member. The nomenclature rules are: the mineral with content of 10%-25% is the mineral-bearing type, 25%-50% is the mineral type, and higher than 50% is the main mineral type. Meanwhile, if the single-mineral content of clay minerals in the mudstone end-member is higher than 50%, it is named by one major clay mineral, e.g., kaolinite mudstone; if it consists of two or more clay minerals, use a composite naming, e.g., kaolinite-illite mudstone (Fig. 3a ). In order to further simplify the relevant, bauxitic rock and bauxite, clayey rock and mudstone, etc., are combined to facilitate intuitive understanding of the mineral composition (Fig. 3b ). According to the mineralogy results and triangular diagram, the bauxite series can be divided into nine types, including bauxite, clayey bauxite, ferritic bauxite, mudstone, bauxitic mudstone, ferritic mudstone, ferrilite, clayey ferrilite and bauxitic ferrilite. The bauxite series in the study area are mainly clayey bauxite, clayey-bauxitic rock, bauxitic mudstone, aluminous-bearing mudstone, ferritic-bearing mudstone, mudstone, ferrilite (Fig. 3a ). Further following the simplified classification scheme, the bauxite series in the study area can be classified into four types: mudstone, bauxitic mudstone, clayey bauxite, and ferrilite (Fig. 3b ).
Fig. 3 Petrological classification scheme for Upper Carboniferous bauxite series in eastern Ordos Basin. |
3. Reservoir pore structure and physical properties
3.1. Pore types
Thin section and scanning electron microscopy show that the pore types of the bauxite series in the study area are mainly intercrystalline pores, dissolution pores, and intergranular pores, with well-developed microcracks (Fig. 4 ). Dissolution pores are mainly distributed in the diaspore crystallite aggregates, with dissolution pores in clay minerals (Fig. 4a ). Intercrystalline pores and dissolution pores are the main storage spaces of the bauxite series, with smaller pore diameters among clay minerals, diaspore, hematite and anatase crystals, generally nanoscale pores (Fig. 4 a-4c). Intergranular pores can be found among larger detrital particles with smaller amount (Fig. 4d ). Microcracks are well developed in the bauxite series and can be found in bauxite and bauxitic mudstone (Fig. 4 e, 4f).
Fig. 4 Reservoir space of Upper Carboniferous bauxite series samples in eastern Ordos Basin. (a) Linxian-Zhaoxianzhen section (L-2), bauxitic mudstone with well- developed dissolution pores; (b) Fugu-Tianqiaoze profile (F-3), kaolinite mudstone with well-developed intercrystalline pores; (c) Linxian-Zhaoxianzhen section (L-3), clayey bauxite, with diaspore and illite, well-developed intercrystalline pores; (d) Xingxian-Guanjiaya section (X-1), kaolinite mudstone, layered structure, with intergranular pores; (e) Baode- Palougou section (B-3), clayey bauxite, with diaspore, pisolitic-oolite particles and microcracks, polarized light; (f) Xingxian-Guanjiaya section (X-4), bauxitic kaolinite mudstone, mud structure, with micro-cracks, polarized light. |
3.2. Pore structure
Low-temperature N2 adsorption/desorption and low-temperature CO2 adsorption experiments were performed on a automatic ASAP 2020 specific surface and pore diameter analyzer. The N2 adsorption/desorption curves of the bauxite series samples present the overall reverse S-shape, similar to the Type IV isothermal adsorption curve defined by IUPAC [23]. When the relative pressure (ratio of adsorption pressure to saturated vapor pressure) is less than 0.45, the adsorption curve rises slowly, slightly convex upward, and basically coincides with the desorption curve. As the pressure rises, the adsorption curve rises gradually when the relative pressure is larger than 0.45 and begins to separate from the desorption curve, forming an obvious hysteresis loop, where nitrogen molecules undergo capillary condensation on pore surfaces, suggesting that mesopores are developed in the sample. When the relative pressure is larger than 0.8, the adsorption curve rises sharply and is nearly upright (Fig. 5 ). According to the classification standard of IUPAC, four types of hysteresis loop shapes can be recognized, corresponding to different pore structures, i.e., open holes at two ends (H1), fine bottleneck-like pores or ink-bottle pores (H2), plate-like pores or wedge-shaped pores (H3) and slit pores (H4) [24-25]. The hysteresis loop shapes of the bauxite series samples in the study area are H3, H4 and mixed type of the two. Among them, the clayey bauxite (L-3) and kaolinite mudstone (X-1) are H3 type, and the kaolinite mudstone (F-3) is H4 type, bauxitic chlorite mudstone (L-4), ferrilite (B-1) and kaolinite mudstone (H-2) are mixed type of H3 and H4, indicating that there are more plate-shaped pores/wedge- shaped pores and slit pores.
Fig. 5 Low-temperature N2 adsorption/desorption curves of samples from Upper Carboniferous bauxite series in eastern Ordos Basin. |
Based on the pore classification scheme proposed by IUPAC, the pore with diameter less than 2 nm is micropore, 2-50 nm is mesopore, and larger than 50 nm is macropore [26]. Low-temperature N2 adsorption/desorption can reveal the pore size distribution from 1.7 nm to 200.0 nm, and the pore size distribution curves are all of single-peak type, dominated by mesopores with the most pores of 2-5 nm, contributing the main pore volume and specific surface area. Low-temperature CO2 adsorption experiments reveal that the pore diameter distribution from 0.4 nm to 1.1 nm, and the pore size distribution curve is multi- peaked, with the most pore diameter of 0.5-0.7 nm. In which, the pore volume and specific surface area of the bauxitic mudstone and mudstone are smaller than those of the clayey bauxite and ferrilite, indicating that the clayey bauxite and ferrilite have the highest micropores content. The mesopores of the bauxite series samples provide the main pore volume and specific surface area, and represent the primary site for gas storage, while the micropores contribute to the specific surface area and also contribute some gas storage space (Fig. 6 ).
Fig. 6 Specific surface area (a) and pore volume proportion (b) of micropores, mesopores and macropores of Upper Carboniferous bauxite series samples in eastern Ordos Basin. |
3.3. Physical properties
Based on GB/T 29172-2012 [27], the porosity and permeability characteristics of 17 samples were tested by porosity and permeability analyzer (VINCI Technology). The results show that the porosity of mudstone is 1.3%-3.7% and permeability is (0.133-0.367)×10−3 μm2; the main porosity of the bauxitic mudstone is 1.6%-3.8%, and the permeability is (0.173-0.458)×10−3 μm2, and the porosity of sample L-2 is 7.8%, and the permeability is 0.604×10−3 μm2, which is influenced by the higher aluminum-bearing mineral content; the porosity of the clayey bauxite is relatively higher in general (5.5%-6.7%), with permeability of (0.528-0.539)×10−3 μm2 (Table 1 ). There is a better positive correlation between porosity and permeability. Clayey bauxite has relatively good physical properties in general, followed by bauxitic mudstone and kaolinite mudstone, and the physical properties of bauxitic mudstone with higher aluminum-bearing minerals are also relatively good. The clastic particles in clayey bauxite are relatively developed, and partial samples have porous structure with microcracks, resulting in relatively good reservoir physical properties. Bauxitic mudstone and kaolinite mudstone have argillaceous/pisolitic-oolite structure, dominated by dissolution pores and intercrystalline pores, and the physical properties can be improved in the reservoir with microcracks.
4. Formation and evolution of bauxite series
The formation of bauxite series is the result of multi factors, mainly influenced by tectonic evolution, sea level change and paleogeomorphology [3,28⇓⇓ -31]. The mineralized materials on the weathered exposure surface migrated downward with the flow of surface water, ground water, etc., forming burrow/funnel filling deposits on karst platforms, lenticular deposits on gentle slopes, and layered and massive deposits in lagoon and swamp environments on low-lying areas (Fig. 7a ). Bauxite series at the platform were mainly distributed in the higher terrain area, with strong vertical leaching effect. During the process of ground water seepage, cyclic changes in the mineral layer occurred within the formation, and the bauxite series was dominated by the primary pisolitic-oolite and porous structures. Aluminum hydroxides minerals on gentle slopes are transported over a short distance, and the pisolitic-oolite was broken during the transportation process, associated with the gravel-grade argillaceous debris. Low-lying depositional position is not above the water table, dominated by clastic and dense structures (Fig. 7a ). Paleogeomorphological units such as platforms and gentle slopes with relatively high terrain have strong dissolution transformation, well-developed pores, and large thickness of bauxite reservoirs with better physical properties.
Fig. 7 Formation and evolution model of Upper Carboniferous bauxite series in NCC (a) and stratigraphic columnar section in eastern Ordos Basin (b) (see |
Systematic comparison of field outcrops and core data in the NCC suggests that the section structure and lithologic types of bauxite series are similar. It is divided into five sections from bottom to top (Figs. 1c and 7b): (1) Section A, ferrilite layer, Shanxi-style iron ore, mineral composition dominated by hematite, goethite, etc., honeycomb structure, fine crystallization; (2) Section B, aluminum-containing layer, bauxite mudstone, well-developed lamination, mineral composition dominated by kaolinite, illite, chlorite and diaspore, etc.; (3) Section C, bauxite layer, bauxite/clayey bauxite, dominated by porous, pisolitic-oolite and massive structures, with mineral composition dominated by diaspore and boehmite; (4) Section D, aluminum-containing layer, bauxitic mudstone, debris-containing, mud structure, mineral composition dominated by clay minerals such as kaolinite, illite, chlorite, and sometimes quartz mineral grains and other clastic debris are seen; (5) Section E, dark mudstone section, with dark mudstone and coal bed deposits, reflecting peat swamp environment in marine transgression environment during the late depositional period. Section A is developed in some areas, generally referred to as the Shanxi-style iron ore. Affected by water level and depth fluctuations under the background of marine transgression and regression, one or more cycles can be developed in sections B, C and D.
From the perspective of mineralization mechanism, the mineralization process of bauxite series can be summarized into three stages. The first stage is karstification and material input. The parent rock is decomposed and destroyed under weathering, and the alkaline earth elements are migrated and leached, providing material basis for the mineral formation and elemental enrichment. Iron, aluminum and silicon ions have not leached, and the large aluminum-containing mineral enrichment provides material basis for the thicker bauxite layers under epigenetic leaching later. The second stage is migration and in-situ of mineral-forming materials. Alkaline earth elements (mainly K+) are leached out after further stronger weathering, and the downward migration of iron and aluminum ions with surface water and ground water leads to higher bottom sediment activity, favoring the formation of iron and aluminum oxides. K-feldspar and other primary minerals in the parent rock are transformed into illite during weathering, and then into kaolinite with alkaline earth elements precipitated, representing the migration and accumulation process of Al-rich primary sediments. The third stage is epigenetic leaching and late modified diagenesis, including epigenetic weathering and leaching, late diagenesis as well as possible metamorphism and resilicification [22]. The leaching process results in higher-purity bauxites with a greater relative proportion of iron minerals (hematite, goethite, etc.). During the weathering and leaching process, the iron-bearing minerals are gradually oxidized as the submerged level gradually decreases.
The three evolutionary stages of bauxite series formation can be briefly summarized as: (1) The parent rock material is transported to form weathered residual deposits or colloids rich in Al, Si and Fe; (2) During the syngenetic period of marine transgression and regression, the sediments are transported in short distance and accumulated in semi-consolidated state, forming sediments with pisolitic-oolite and clastic structures; (3) During epigenetic period exposed to atmospheric freshwater leaching, Si is washed away to form porous structure, and pore fluid precipitation during diagenesis is favorable to authigenic mineral cementation.
The water level of the lagoon decreased after the marine regression during the depositional period of the Benxi Formation. The aluminous sediments formed in the reduction phreatic semi-enclosed area were leached by surface water and were rich in Fe and Al. Large amounts of soluble components were lost to form dissolution pores, forming an effective bauxite reservoirs [3,32]. On the one hand, karst platforms and gentle slopes are conducive to deposition of thick bauxite sediments. On the other hand, the higher terrain is conducive to surface water leaching, karst modification is stronger, and leaching is easy to occur in the aluminum mineral lattice. The soluble components are partly lost, and the dissolution pores provide more storage space. Thus, the bauxite reservoir at the karst platform is thicker, with well-developed dissolution pores and good physical properties. The thickness of bauxite reservoir at gentle slope is controlled by karst depression with thinning thickness and better physical properties. The bauxite reservoir in the low-lying area is thicker and has relative continuity and stability, but dominated by mudstone or bauxitic mudstone (debris and dense massive structures). During later stage, the thickness is reduced by compaction, and the physical properties become worse.
5. Natural gas enrichment
Coal and mudstone developed above the bauxite reservoir in Carboniferous Benxi Formation can provide a gas source for the bauxite reservoir, and serve as a regional cap layer to effectively seal the gas from escaping upward. Based on the well logging curve characteristics of bauxite in the Benxi Formation of the study area, the reservoir resistivity is lower overall, and the bauxite series is characterized by three-high and two-low well logging responses, i.e., ultra-high GR, high U content, high Th content, low AC value, and low K content. Due to the different degrees of epigenetic drenching, there are differences in the type and permeability of bauxite in regional distribution. The study area as a whole is a monoclinic structure that is higher in east and lower in west, and bauxite reservoir has good sealing conditions, forming a continuous type of natural gas reservoir [15]. Bauxite has experienced strong weathering and leaching, with good storage space and pore types dominated by dissolution pores. Coal-measure source rocks are in direct or indirect contact with bauxite layers, and faults and fractures further improve the transmission capacity of the reservoir, forming bauxite gas reservoir of an upper-generation and lower-storage type jointly controlled by source rock, reservoir rock and fractures [12], which has certain exploration and development potentials (Fig. 8 ).
Fig. 8 Natural gas reservoir section of Upper Carboniferous bauxite series in eastern Ordos Basin (see |
6. Conclusions
According to the common and typical mineral assemblages of bauxite series, a petrological nomenclature and classification scheme for bauxite series based on three end members, i.e., iron minerals, aluminum hydroxides and titanium minerals, clay minerals and other minerals, is proposed. There are nine rock types including bauxite, clayey bauxite, ferritic bauxite, mudstone, bauxitic mudstone, ferritic mudstone, ferrilite, clayey ferrilite and bauxitic ferrilite. The bauxite series in eastern Ordos Basin mainly include mudstone, bauxitic mudstone, clayey bauxite and ferrilite.
The iron-rich layer of the bauxite series in the study area has a high hematite content of 85.9% and a lower content of clay minerals. Aluminum-containing layer has a high content of clay minerals and more aluminum hydroxides. Bauxite layer has the highest content of aluminum hydroxides (larger than 40% overall), followed by clay minerals, hematite and anatase. Bauxitic mudstone is mainly dense massive and clastic, and clayey bauxite is mostly dense massive, pisolitic-oolite, porous and clastic textures. The porosity of the bauxite reservoir is generally less than 10%, and the physical properties of the clayey bauxite are better, dominated by dissolution pore and intercrystalline pore, with microcracks, and the mesopores are the main gas storage space.
The section structure and rock types of the bauxite series in the study area are divided into five layers from bottom to top, i.e., iron-rich layer, aluminum-containing layer, bauxite layer, aluminum-containing layer and dark mudstone layer. Individual layers may be missing due to tectonic evolution, sea level change and paleogeomorphology.
Bauxite series formed burrow/funnel filling deposits on karst platforms, lenticular deposits on gentle slopes, and layered and massive deposits in lagoon and swamp environments on low-lying areas. The formation and mineral evolution process of the bauxite series involve the input of material sources, the migration and in-situ stages of mineral-forming materials, and the epigenetic leaching and late modified diagenesis. The paleogeomorphology controls the bauxite types: relatively thick clayey bauxite is developed on karst platforms and slopes, and mudstone and bauxite mudstone are mainly developed on gentle slopes. Gas enrichment in bauxites is jointly controlled by source rock, reservoir rock and fractures.