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Article

Logging Identification and Distribution of Bauxite in the Southwest Ordos Basin

by
Peng Zhang
1,
Xianghui Jing
2,
Renhai Pu
1,*,
Aiguo Wang
1 and
Xueping Huang
3
1
State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi’an 710069, China
2
Research Institute of Exploration and Development, Petro China Changqing Oilfield Company, Xi’an 710018, China
3
The Second Gas Production Plant Changqing Oilfield Company, PetroChina, Yulin 719000, China
*
Author to whom correspondence should be addressed.
Minerals 2023, 13(10), 1253; https://doi.org/10.3390/min13101253
Submission received: 24 July 2023 / Revised: 22 September 2023 / Accepted: 22 September 2023 / Published: 25 September 2023
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Abstract

:
In recent years, with the discovery of oil and gas in the Carboniferous bauxite reservoir in the Ordos basin, the formation and distribution of bauxite and bauxite reservoirs have attracted the attention of oil and gas explorers. Based on the slightly equidistant core testing of minerals and the porosity on the formation and calibration on the logging curves, we established a logging identification method for bauxite in the study area and fitted the formula for calculating the diaspore content and porosity using logging data. By applying this formula and a large amount of logging data, thickness and porosity maps of the Taiyuan formation of bauxite in the southeastern part of the basin were produced. Then, according to the thickness of the earliest deposited Taiyuan formation on the unconformity surface, we analyzed the paleogeomorphology of the bauxite deposition. Finally, a sedimentary facies map of the Taiyuan formation was developed based on the content of sandstone, coal seams, and carbonate rocks contained there. The results show that the diaspore content of bauxite is positively correlated with the natural gamma logging (GR) values. According to the diaspore contents and the GR values, the aluminiferous rocks in the Taiyuan formation can be divided into three categories: ① Bauxite for GR values > 450 API and diaspore content >70%; ② Argillaceous bauxite for GR values = 300–450 API and diaspore content = 25%–70%; and ③ Bauxitic mudstone for GR values = 220–300 API and diaspore content = 0%–25%. Bauxite mainly occurred in the middle and deep lagoon environments in the lower part of the Taiyuan formation. The lagoon is distributed along the paleokarst groove in the NW strike, with a width of approximately 30–40 km and a length of approximately 150–200 km, among which the thickness of pure bauxite is 9 m, argillaceous bauxite 26 m and alumina mudstone 6 m. Bauxite with a high porosity mainly existed in pure bauxite. The lagoon bauxite in the lower part of the Taiyuan formation gradually changes upward into a tidal flat swamp and carbonate platform environment.

1. Introduction

Bauxite is an important metal mineral resource and the formation process and its metallogenic mechanism has been studied extensively [1,2,3,4,5,6,7]. It is generally believed that bauxite can be divided into a karst-type and a laterite-type. Karst-type bauxite exists on the paleokarst surface of carbonate rocks, while laterite-type bauxite exists on aluminosilicate rocks [8,9,10]. Bauxite is widely distributed in the lower part of the Carboniferous Taiyuan formation in northern China, overlying the Ordovician carbonate unconformity [11,12,13,14].
The Ordos basin is in the western part of the North China Block, and the study area is in the southwest part of the basin, also known as the Longdong area (Figure 1). The Carboniferous system has many bauxite rocks, with burial depths of approximately 3500–5000 m, which are generally regarded as the cap layers of the weathering crust gas reservoirs at the top of the lower Paleozoic. Because bauxite tends to be enriched in the grooves of weathering crust, it is also regarded as a marker for the representation of the ancient geomorphology [15,16].
In 2015, during drilling, Well Shaanxi 464 was found to have a low-yielding gas flow of 1849 m3/d in the bauxite reservoir of the Taiyuan formation, and in the same year Well D66-172 was found to have a commercial gas flow of 1.0375 × 104 m3/d in the bauxite of the Taiyuan formation. In 2021, drilling at Well Long47 obtained 67.38 × 104 m3/d of high-yield gas flow in the bauxite of the Taiyuan formation, so it is widely believed that bauxite can be used as an unconventional gas reservoir [17,18].
Bauxite has rarely been studied as a reservoir rock in the past. However, recent discoveries of high yield industrial gas flow rates in bauxitic formations in the southwest of the Ordos basin make understanding the formation and distribution of bauxite and related reservoirs in the overlying strata an important task for natural gas exploration in this area. This is of great significance for oil and gas exploration in the Ordos basin and even in the North China region.

2. Geologic Background

The Ordos basin belongs to the western part of the North China Block in the Paleozoic, which was transformed into a sedimentary basin and surrounded by mountains in the Meso–Cenozoic. The basin covers an area of 3.7 × 105 km2 and can be divided into six secondary structural units, namely: the Yimeng uplift, western margin thrust belt, Tianhuan depression, Yishan slope, Jinxi flexion fold belt and Weibei uplift. The study area is in the southwestern region of the Ordos basin, most of which is located between the Tianhuan depression and the Yishan slope (Figure 1).
From the early Archean to the late Archean, the prototype of the basement was formed. Into the early Proterozoic, the North China Craton was formed. Then in the middle to late Proterozoic, the cover sequences developed along with the sedimentation of Paleozoic epicontinental seas. Finally, in the Mesozoic to Cenozoic, the basin went through the developmental stage of faulted depression basins and the formation of modern landforms. Throughout this process, the basin experienced multiple periods of tectonic uplift and subsidence as well as migration of the depocenters. The basin is a relatively simple, large, multi-cyclic cratonic basin in its overall tectonic architecture and is presently a west-dipping slope with a dip angle of less than 1 degree. Except for the absence of Silurian–Devonian and Paleogene and Neogene stages, the basin is well developed stratigraphically (Figure 2).
The Ordos basin has experienced marine transgression since the early Cambrian period. In the early Cambrian, shallow marine sediments developed along the southwestern margin surrounding the ancient land of the Ordos. In the early–middle Cambrian, the marine invasion was more significant, covering almost the entire basin area. Marine transgression reached its peak in the middle Cambrian, forming an extensive shallow shelf carbonate deposition. In the late Cambrian, continuous marine regression occurred, with the ancient land area constantly expanding and the structural pattern of uplifts and basins becoming more distinct. In the early Ordovician, marine waters partially re-entered the southeastern margin of the basin, while the main area remained a unified ancient land. In the middle Ordovician, shallow water carbonate facies were widely distributed in the basin, showing a sedimentary pattern of internal subsidence and relative stability along the margins. In the late Ordovician, the depositional range was significantly reduced, with the southwest margin rapidly transitioning to a slope-deep basin environment. In the late Ordovician, marine waters completely withdrew, ending the early Paleozoic marine sedimentary history in this area [19].
In the early Paleozoic, the central Paleo uplift was formed in the southwestern part of the basin due to early regional extension and late compression, and the Longdong area was located partly on the central paleo uplift in the early Paleozoic. During the late Ordovician period, the Ordos basin was entirely uplifted. The middle paleo uplift underwent long-lasting intense erosion for 140 million years, forming an uneven paleo-weathering crust. The Longdong area also experienced long-term weathering and erosion. On the Ordovician weathered surface, bauxite was formed as the ultimate weathering residue derived from the surrounding source uplifts in the basin, which is composed of lower Paleozoic carbonate rocks interbedded with continental clastic rocks. These materials were transported and deposited over short distances into the karst depressions [20,21,22,23].
By the late Carboniferous Benxi period, sedimentation occurred in the Ordos basin again. At this time, there was a differentiation of east–west sedimentation in the Ordos area, forming two sea areas divided by a central ancient uplift. The western sea area was mainly composed of fan delta, lagoon, estuary bay and tidal flat facies. The eastern sea area was characterized by fan delta, delta, lagoon, tidal flat, barrier island and shelf facies. In the Taiyuan period, the central ancient uplift submerged underwater, connecting the east and west sea areas completely. However, the uplift still played a role in separation to some degree. From the early Triassic Shanxi period to the late Triassic Shiqianfeng period, with the closure of the Xingmeng sea channel, the structural pattern changed from east–west differentiation to north–south characteristics, shown as northward tilting southward. At this time, marine waters basically withdrew from the Ordos area, showing a unified continental clastic sedimentation dominated by a fluvial–deltaic–lacustrine depositional system. The differentiated sedimentary environment of the east–west was replaced by a new pattern of north–south differentiation [24].
The Ordos basin, the second largest sedimentary basin in China, is rich in oil and gas resources, and a large amount of oil and gas resources have been discovered and exploited thus far. Its oil and gas are distributed throughout almost the entire basin, mainly in the central part of the basin on the Yishan slope, and its oil and gas resources are mainly found in the Triassic, Jurassic, Carboniferous–Permian and Ordovician formations. The oil-source rocks are mainly the lacustrine shales of the late Triassic Yanchang formation, and the gas-source rocks are mainly coal beds and dark mudstones of the Taiyuan–Shanxi formation [25,26,27,28,29,30,31,32].
Minerals 13 01253 g002
Figure 2. Stratigraphic of the Ordos basin and schematic diagram of the basin’s evolutionary stages (from Zhao, Z. et al., 2009 [33]).
Figure 2. Stratigraphic of the Ordos basin and schematic diagram of the basin’s evolutionary stages (from Zhao, Z. et al., 2009 [33]).
Minerals 13 01253 g002
In addition to the abovementioned traditional oil and gas resources, bauxite oil and gas resources have been discovered in recent years [18,34]. The Longdong area has 50 wells drilled into bauxite, 29 of which have obvious gas content, 10 have been tested and 7 have a gas production greater than 1 × 104 m3/d in the bauxite reservoirs.
The Benxi formation is absent in the study area. The target layer of this paper, the Taiyuan formation, is missing locally in the southwestern region of the study area, and the remaining region is distributed. The lithology of the Taiyuan formation consists mainly of aluminiferous rocks (bauxite, mud bauxite, bauxitic mudstone, mudstone, carbonaceous mudstone, coal, muddy siltstone, siltstone, sandstone, etc.) (Figure 3 and Figure 4). The top of the Taiyuan formation is often bounded by approximately 2–5 m of coal seam with the Shanxi formation, and the bottom boundary is in angular, unconformable contact with the lower Paleozoic carbonate rocks.

3. Data and Methods

The study area is in the southwestern region of the Ordos basin, covering an area of approximately 3 × 104 km2. In this study, a total of 96 exploration wells were drilled into the target layer, and logging data were collected, including complete comprehensive logging data and partial well coring data, among which well L58 has complete coring data of the Taiyuan formation. The diaspore content and the porosity of the bauxite were measured using X-ray diffraction and the helium gas method, respectively, at approximately 10 cm, near equally spaced.
The X-ray diffraction (XRD) analysis was conducted using a D8 ADVANCE instrument. The key technical specifications of the instrument include a line focus size of 0.04 × 12 mm and a point focus size of 0.4 × 1.2 mm. The goniometer radius of the instrument is 280 mm, with a 2θ rotation range of −110° to 168°. The instrument provides a minimum readable step of 0.0001° and angle reproducibility of 0.0001°. It also features a Lynxeye_XE-T detector with an active area of 14 mm × 16 mm and an energy resolution of ≤380 eV, enabling continuous automatic scanning for all samples. The testing environment was controlled at a room temperature of 25 °C and a humidity level of 30%. The XRD analysis was carried out according to the X-ray diffraction analysis method SY/T 5163-2018.
Helium porosimetry is a method that utilizes the properties of helium gas to measure the porosity of rock core samples. The measurement principle is that helium gas is introduced into the pore spaces of the core sample. By controlling the pressure of the helium gas, it can be made to fully saturate all of the pores within a sample. The amount of helium gas that occupies the pore volume is then measured. Based on the volume and pressure parameters of the helium gas, the pore volume of the core sample can be calculated. Finally, the porosity can be determined from the pore volume and bulk volume of the sample [35].
In this paper, the relationship between the content of the diaspores and the measured porosity in the bauxite cores was determined and gamma ray, acoustic and neutron logging was constructed. The content and the porosity of all the bauxite in the Taiyuan formation were then each calculated using the logging data of all of the wells. According to the relationship between core, cuttings and logging, the GR-AC, GR-DEN, GR-CNL and GR-RT cross log charts were established to differentiate the types of bauxite, sandstone, limestone, coal seam and carbonaceous mudstone. The thicknesses of the various lithologies and the porosity of bauxite in the Taiyuan formation were quantitatively calculated. Based on this, the thickness and average porosity map of the bauxite rock in the study area was constructed. The Taiyuan formation was divided into upper and lower members based on nearly equal thickness, sedimentary cycles, and lithology. The paleogeomorphological map of the Taiyuan formation was made according to the thickness of the Taiyuan formation that immediately overlayed the unconformity. Based on the analysis of rock and mineral composition, texture, sedimentary structure, and paleontology, the sedimentary facies maps of the lower and upper Taiyuan formations were completed. The contour maps were generated using the interpolation and contouring functions in DF-GVision (version 4.2) software by Beijing GDF Oil & Gas Tech., Inc (Beijing, China). Finally, the relationship between the thickness and the porosity of the bauxite and the sedimentary facies and their paleogeomorphologies were analyzed based on data from the abovementioned maps.

4. Results

4.1. Identification of Bauxite and Its Porosity by Logging

It can be seen from the results of the X-ray diffraction analysis of the fully cored rocks of well Long58 in the Taiyuan formation shows that the mineral composition of the bauxite is mainly diaspore, followed by minerals such as pyrite, anatase, chlorite, kaolinite, illite, montmorillonite, potassium feldspar and quartz (Table 1).
According to the diaspore content, the aluminiferous rocks can be preliminarily divided into three categories: bauxite with a diaspore content >70%; argillaceous bauxite with a diaspore content = 25%–70%; and bauxitic mudstone with a diaspore content = 0%–25%.
In general, the GR values of the sedimentary rocks are between 20–150 API. After the diaspore content of the rocks increases, the GR value increases significantly and can reach more than 800 API (Figure 5). There is an obvious correlation between the GR value of well Long58 and the core bauxite content (Figure 5). Therefore, the total diaspore content of the rocks can be roughly estimated based on the GR value. After core depth correction, a simple linear relationship between the two can be fitted (Figure 6a). It can be inferred that a GR > 450 API indicates bauxite, a GR = 300–450 API indicates argillaceous bauxite, and a GR = 220–300 API indicates bauxitic mudstone.
From Figure 5, it can be seen that the measured core porosity of bauxite is also significantly correlated with the contents of diaspore and CNL, and the porosity of the pure bauxite reaches a value of approximately 28%. A linear relationship can also be roughly fitted between the CNL and the measured core porosity. After defining the bauxite interval, the porosity of the bauxite is found to be POR = 0.2126CNL−7.607 (Figure 6b).

4.2. The Distribution of Bauxite

The diaspore content of the Taiyuan formation in each well was calculated according to the relationship between the GR values and the diaspore content shown in Figure 6a and the well logging curve of 96 wells in the study area within the Taiyuan formation. Then, according to the diaspore content, the types of aluminiferous rocks can be divided, the thickness of each aluminiferous rock can be calculated, and the thickness map of the bauxite, argillaceous bauxite, bauxitic mudstone and aluminiferous rocks in the study area can be drawn (Figure 7).
The three kinds of aluminiferous rocks are beaded in the NW direction. As the diaspore content decreases gradually, the beaded bands become wider and larger. The distribution range of pure bauxite is the narrowest, with the thickest part near wells Long58 and Long51 in the southeastern region of the study area. The central thickness of the lake basin is between 6 and 9 m, and the edge of the lake basin is reduced to 0 m. The bauxite is distributed in five irregular oval circles along the northwest trend, each with a diameter of approximately 35 km (Figure 7a). The thickest part of the argillaceous bauxite is in the southeastern region of the working area, up to 24–28 m, and the oval diameter increases to approximately 50 km (Figure 7b). The thickest part of the bauxitic mudstone is in the southeastern region of the working area, which can be up to 6–7 m. The position and shape of the oval shows little change, and the width of the 0 m isopath reaches 60–70 km (Figure 7c). In general, the thickness distribution of aluminiferous rocks displays a NW extension sag.

5. Discussion

5.1. The Relationship between Bauxite and Ancient Geomorphology

In the study area, the Taiyuan formation is the earliest deposit on the upper unconformity of the lower Paleozoic, which contributes to the filling and leveling of the carbonate karst paleogeomorphology. Therefore, the thickness of the Taiyuan formation indicates the height of the ancient topography. Areas with great thickness represent paleogeomorphologic depressions, while areas with small thickness represent paleogeomorphologic highlands. Based on logging data from the 96 wells, the thinning belt of the Taiyuan formation in the study area presents a slightly left-oblique V-shaped distribution. The Taiyuan formation thickens toward the northwest and northeast from the central part of the study area, and the thickening belt widens northward (Figure 8).
The maximum thickness difference between the karst highland and depression of the V-shape distribution is approximately 40 m. The western highland extends from the center of the southern boundary of the working area northwestward to the center of the western boundary of the working area, with a width of 20–30 km and a length of approximately 150 km. The eastern highland extends and dips northwestward to northward. The central paleo-trough dips NNW and is 30–40 km wide and 150–200 km long. Comparing data shown in Figure 7 and Figure 8, it can be seen that the aluminum-bearing rocks are mainly distributed in the paleogrooves between the V-shaped paleohighlands.

5.2. Relationship between Bauxite and Sedimentary Facies

In the study area, despite the existence of previous studies on the origin of bauxite [36,37,38,39,40,41,42,43], much attention has been given to the sedimentary environment in which bauxite was formed. The lower member of the Taiyuan formation is mainly composed of bauxite and silty mudstone, and carbonaceous mudstone is found locally. The upper member consists of mainly silty mudstone and thin limestone and coal seams. Because coal, limestone and sandstone have important diagnostic facies features, we first quantitatively estimate these lithologies of the Taiyuan formation.
According to the logging responses of different lithologies determined via the core and cuttings data of the Taiyuan formation in the study area, the logging identification threshold values of natural gamma, electric resistivity, sonic time difference and densities of bauxite rock, coal seam, sandstone and limestone can be depicted (Figure 9). The well logging identification thresholds of different lithologies of the Taiyuan formation in the study area are shown in Table 2.
According to Table 2, the thicknesses of the various lithologies of the Taiyuan formation of each well in the study area can be calculated to make thickness maps of the different lithologies. Because some lithologies have important facies diagnostic significance, such as sandstone indicating strong hydrodynamics, micritic bioclastic limestone indicating shallow-water platforms, and coal seams indicating swamp environments, we first made a single lithologic thickness map of the Taiyuan formation in the study area (Figure 10a–c).
Coal, limestone and sandstone are mainly distributed in the upper member, while aluminaceous rocks, mudstone and carbonaceous mudstone are mainly distributed in the lower member. The first lithologic distribution of the bottom Taiyuan formation on the unconformity surface has important characteristics reflecting the earliest depositional environment (Figure 10d).
It can be seen from Figure 10a that limestone is mainly distributed in the northeastern part of the study area, indicating a far distance from the southeastern provenance and the clear water platform. The thickening sandstone belt appears as a low sinuous channel flowing from the southwest to northeast (Figure 10b). The coal is in the southwestern part of the limestone and nearly coeval with the latter (Figure 10c). The first lithology overlying the unconformity surface is only argillaceous bauxite, bauxitic mudstone and mudstone. The center of the paleokarst depression is occupied by argillaceous bauxite and lacks pure bauxite. The sedimentary facies of the early Taiyuan formation should inherit and partly reflect the characteristics of the karst epigenetic period.
Core and thin section data observations show that most of the pure bauxite in the study area has internal clastic textures, and both the debris and the matrix are composed of bauxite, with a variety of grain sizes such as silt, sand and pebbles (Figure 11 and Figure 12). Pyrite nodules are common in bauxite, with sizes ranging from several millimeters to several centimeters and sometimes high concentrations of approximately 5% (Figure 11a), indicating an intensely reducing environment. Siderite is also seen, indicating a moderately reducing environment (Figure 11b and Figure 12d).
Based on the paleogeomorphology and the lithology distribution, it can be inferred that the bauxite and bauxitic mudstone in the lower part of the Taiyuan formation were deposited mainly in the middle–deep lagoon environment of the karst depression, and the surrounding area became shallow mudstone and alumina mudstone in the shallow lagoon. The Taiyuan formation is absent from the paleogeomorphic highland in the southwestern part of the study area, and part of the tidal flat mudstone exists between the pinch-out line and the shallow lagoon (Figure 13a).
The sedimentary facies map of the upper member of the Taiyuan formation can be made according to the thickness ratio contour of the sandstone to the formation, limestone and coal seam distribution. In the middle of the study area, there is a low sinuous channel belt with a width of approximately 20 km, which flows from southwest to northeast, and in which the front transitions to a carbonate platform. The channel bilateral zones are swamp and tidal flat environments, and the maximum coal thickness is approximately 4 m. The coal thickness on the northwestern side of the channel belt is thicker than that on the southeastern side of the channel belt (Figure 13b).
The upper and lower members of the Taiyuan formation reflect the relative secondary rise and fall of sea level. The bauxite is mainly distributed in medium–deep lagoons formed in paleokarst depressions during the early sea level relative rise. At the end of the deposition of the lower member, the sea level fell, so the bauxite deposits were exposed to the surface and leached by fresh water, and then fluvial sandstone and swamp coal and other tidal deposits were gradually deposited. When the upper member of the Taiyuan formation was deposited, relative sea level rise occurred again and flooded the whole study area, forming a clear environment of a distant shallow-water carbonate platform in the northeast, while most other areas of the study area were in lagoon–tidal flat and swamp environments.
In summary, the origin of bauxite can be divided mainly into the following three stages:
Supergene period: continuous karst in the Longdong area, forming multiple karst depressions. In the early depressions, weathered materials from the Majiagou formation were leached for a long time to form bauxite; in the late depression, new weathering materials of lower Palaeozoic rocks were accumulated.
Early sedimentation of the Taiyuan formation: rapid marine invasion covered the entire Longdong area with seawater. A new layer of sedimentary rocks is covered on the karst landform, such as dark mudstone, bauxite mudstone, oolitic chlorite mudstone, siderite bearing mudstone, siderite rock, and colloidal bauxite rock.
Late sedimentation of the Taiyuan formation: as the sea recedes, the surface of karst highlands is exposed and re-karst, and the previously formed bauxite is eroded into debris and transported to nearby lagoons or mountain lakes, forming bauxite in strong hydrodynamic facies zones. With slow regression, depressions generally become swampy, forming carbonaceous mudstone or coal.

5.3. Distribution of the Bauxite Reservoir in the Taiyuan Formation

The bauxite pores and vugs may be formed by dissolution. Many dissolved pores can be seen on both the core and casting thin sections (Figure 14), which may be contributed to by freshwater dissolution during sea level drop in the lower member of the Taiyuan formation and organic acid dissolution during deep buried phases when hydrocarbons migrated in the early Cretaceous. According to the relationship between the measured core porosity and CNL, the calculated average porosity of the Taiyuan formation bauxite ranges from 2% to 12%, showing a NW—SE extension (Figure 15). The high value (>10%) area is in the southeast of middle–deep lagoon deposition, and the median zone (8%–10%) and low value zone (<8%) are distributed in a circular pattern around the high porosity zone. The high porosity bauxite is mainly distributed in the pure bauxite of the ancient groove landform.

6. Conclusions

According to the relationship between the diaspore content and well logging, the aluminiferous rocks in the southwestern Ordos basin can be divided into three types: bauxite, with GR values > 450 API and diaspore content >70%; argillaceous bauxite, with GR values = 300–450 API and diaspore content = 25%–70%; and bauxitic mudstone, with GR values = 220%–300 API and diaspore content = 0%–25%.
The bauxite distribution is controlled by the unconformity karst paleogeomorphology and sedimentary environment. Pure bauxite is mainly distributed in the middle–deep lagoon environment of the paleokarst groove.
Bauxite reservoirs with pores and vugs are mainly distributed in pure bauxite, and the porosity is positively correlated with the diaspore content. The average porosity value of the bauxite reservoirs ranges from 2% to 12%, and is mainly formed by various dissolution processes.

Author Contributions

Conceptualization, P.Z. and R.P.; methodology, P.Z.; R.P. and A.W.; validation, X.J. and R.P.; formal analysis, P.Z. and A.W.; investigation, P.Z. and A.W.; resources, X.J., R.P. and X.H.; data curation, P.Z.; writing—original draft preparation, P.Z.; writing—review and editing, R.P., A.W.; visualization, P.Z. and A.W.; supervision, X.J. and X.H.; project administration, X.J. and X.H.; funding acquisition, X.J.; R.P. and X.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by National Natural Science Foundation of China: No. 41390151. and The APC was funded by Pu, R.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) Geomorphic map of the Ordos basin; (b) Ordos basin structural map and study area location map.
Figure 1. (a) Geomorphic map of the Ordos basin; (b) Ordos basin structural map and study area location map.
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Figure 3. Section diagram of the connecting wells of the Taiyuan formation aluminiferous rock series in Longdong area. (a) A-a section diagram; (b) B-b section diagram.
Figure 3. Section diagram of the connecting wells of the Taiyuan formation aluminiferous rock series in Longdong area. (a) A-a section diagram; (b) B-b section diagram.
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Figure 4. Comprehensive columnar profile of Taiyuan formation depositional facies in well Long83.
Figure 4. Comprehensive columnar profile of Taiyuan formation depositional facies in well Long83.
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Figure 5. Comprehensive log column profile of Taiyuan formation in well Long58 and X-ray diffraction changes in the content and porosity of diaspore.
Figure 5. Comprehensive log column profile of Taiyuan formation in well Long58 and X-ray diffraction changes in the content and porosity of diaspore.
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Figure 6. (a) The relationship between the content of bauxite and GR; (b) the relationship between the porosity of bauxite and CNL.
Figure 6. (a) The relationship between the content of bauxite and GR; (b) the relationship between the porosity of bauxite and CNL.
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Figure 7. (a) Thickness map of pure bauxite; (b) thickness map of argillaceous bauxite; (c) thickness map of bauxitic mudstone; (d) cumulative thickness of aluminiferous rock series.
Figure 7. (a) Thickness map of pure bauxite; (b) thickness map of argillaceous bauxite; (c) thickness map of bauxitic mudstone; (d) cumulative thickness of aluminiferous rock series.
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Figure 8. Thickness map of the Taiyuan formation in the Longdong area (paleogeomorphological map by impression method).
Figure 8. Thickness map of the Taiyuan formation in the Longdong area (paleogeomorphological map by impression method).
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Figure 9. Cross plot of log curves of different lithologies of the Taiyuan formation in the Longdong area; (a) GR-AC cross plot; (b) GR-RT cross plot; (c) GR-DEN cross plot; (d) GR-CNL cross plot.
Figure 9. Cross plot of log curves of different lithologies of the Taiyuan formation in the Longdong area; (a) GR-AC cross plot; (b) GR-RT cross plot; (c) GR-DEN cross plot; (d) GR-CNL cross plot.
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Figure 10. Single lithologic distribution of the Taiyuan formation in the study area; (a) thickness map of limestone in the upper member of the Taiyuan formation; (b) sand–land ratio map of the upper member of the Taiyuan formation; (c) thickness map of the coal seam in the upper part of the Taiyuan formation; (d) the first lithologic distribution map at the bottom of the lower member of the Taiyuan formation.
Figure 10. Single lithologic distribution of the Taiyuan formation in the study area; (a) thickness map of limestone in the upper member of the Taiyuan formation; (b) sand–land ratio map of the upper member of the Taiyuan formation; (c) thickness map of the coal seam in the upper part of the Taiyuan formation; (d) the first lithologic distribution map at the bottom of the lower member of the Taiyuan formation.
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Figure 11. Core photos of bauxite; (a) bauxite containing pyrite (Py) nodules; (b) bauxitic mudstone containing speckled siderite (Sd); (c) internal clastic bauxite of sand grade, sub-round, poor sorting; (d) internal clastic bauxite of fine gravel grade, sub-round, middle sorting.
Figure 11. Core photos of bauxite; (a) bauxite containing pyrite (Py) nodules; (b) bauxitic mudstone containing speckled siderite (Sd); (c) internal clastic bauxite of sand grade, sub-round, poor sorting; (d) internal clastic bauxite of fine gravel grade, sub-round, middle sorting.
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Figure 12. Thin section photo of bauxite; (a) the internal clastic bauxite in the middle sand grade, sub-round, poor sorting, and both the particle and the matrix are bauxite; (b) Bauxite, oolitic structure; (c) Unequally sized detrital internalclastic argillaceous bauxite, sub-angular to sub-round shape, poor sorting; (d) bauxitic mudstone containing siderite (Sd) nodules.
Figure 12. Thin section photo of bauxite; (a) the internal clastic bauxite in the middle sand grade, sub-round, poor sorting, and both the particle and the matrix are bauxite; (b) Bauxite, oolitic structure; (c) Unequally sized detrital internalclastic argillaceous bauxite, sub-angular to sub-round shape, poor sorting; (d) bauxitic mudstone containing siderite (Sd) nodules.
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Figure 13. Sedimentary facies plan of the Taiyuan formation in the study area; (a) sedimentary facies plan of the lower member of the Taiyuan formation; (b) sedimentary facies plan of the upper member of the Taiyuan formation.
Figure 13. Sedimentary facies plan of the Taiyuan formation in the study area; (a) sedimentary facies plan of the lower member of the Taiyuan formation; (b) sedimentary facies plan of the upper member of the Taiyuan formation.
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Figure 14. Photos of dissolution pores in the core and cast thin sections of bauxite; (a) bauxite core dissolution pore; (b) the dissolution hole of the cast thin slice of Figure 14a, where pink is the dissolution hole, with the same below; (c) the bauxite particles and the matrix are dissolved, where blue represents the pore, with the same below; (d) kaolinite (Kln) is dissolved; (e) siderite (Sd) is dissolved; (f) diaspore (Dsp) is dissolved.
Figure 14. Photos of dissolution pores in the core and cast thin sections of bauxite; (a) bauxite core dissolution pore; (b) the dissolution hole of the cast thin slice of Figure 14a, where pink is the dissolution hole, with the same below; (c) the bauxite particles and the matrix are dissolved, where blue represents the pore, with the same below; (d) kaolinite (Kln) is dissolved; (e) siderite (Sd) is dissolved; (f) diaspore (Dsp) is dissolved.
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Figure 15. Pore contour plan of the Taiyuan formation bauxite in the study area.
Figure 15. Pore contour plan of the Taiyuan formation bauxite in the study area.
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Table 1. X-ray diffraction statistics of the bauxite member of the Taiyuan formation in the Long58 well.
Table 1. X-ray diffraction statistics of the bauxite member of the Taiyuan formation in the Long58 well.
NumberDepth
(m)		Content (%)
QuartzPotash FeldsparChloriteIlliteClayPyriteAnataseHeavy MineralDiasporeTotal
KaoliniteSmectite Mixed Layes
14040.000.600.4059.3035.0094.700.001.601.603.10100.00
24040.620.400.7947.2027.6075.597.302.309.6014.3099.89
34041.740.800.5411.005.7017.242.201.904.1078.00100.14
44042.540.200.540.902.103.540.002.202.2094.0099.94
54042.970.200.3819.5029.8049.680.003.003.0047.1099.98
64044.990.000.530.703.404.630.001.801.8093.60100.03
74044.99 0.10 0.37 0.20 0.80 1.37 0.00 1.60 1.60 96.90 99.97
84045.24 0.10 0.60 0.10 1.10 1.80 0.00 2.10 2.10 96.10 100.10
94045.90 0.10 0.51 0.10 1.00 1.61 0.00 1.90 1.90 96.40 100.01
104047.40 0.10 0.69 0.10 0.80 1.59 0.00 1.80 1.80 96.50 99.99
114048.25 0.10 0.96 0.10 1.30 2.36 0.00 2.20 2.20 95.40 100.06
124049.58 0.10 0.94 0.10 0.70 1.74 0.00 2.50 2.50 95.60 99.94
134050.76 0.60 1.39 4.30 14.60 20.29 0.00 5.70 5.70 73.50 100.09
144051.10 0.30 0.67 1.90 8.90 11.47 0.00 2.70 2.70 85.70 100.17
154051.71 1.30 1.19 8.30 31.90 41.39 0.00 6.30 6.30 51.00 99.99
164052.39 7.90 1.10 6.00 48.80 55.90 0.00 5.50 5.50 30.80 100.10
174053.29 11.60 1.85 2.20 62.50 66.55 0.00 5.20 5.20 16.60 99.95
184053.99 11.60 1.80 2.20 62.60 66.60 0.00 5.20 5.20 16.50 99.90
194055.26 5.50 1.10 1.70 39.00 41.80 0.00 4.50 4.50 48.00 99.80
Table 2. Well logging identification thresholds of different lithologies in the Taiyuan formation in the study area.
Table 2. Well logging identification thresholds of different lithologies in the Taiyuan formation in the study area.
Logging CurveLithology
BauxiteArgillaceous
Bauxite		Bauxitic
Mudstone		CoalSandstoneLimestoneMudstone
GR
(API)		GR > 450300 < GR < 450220 < GR < 300GR < 110GR < 120GR < 100120 < GR < 220
RT
(Ω·m)		1 < RT < 1021 < RT < 10310 < RT < 10310 < RT < 10410 < RT < 103103 < RT < 10520 < RT < 120
AC
(μs/m)		100 < AC < 300100 < AC < 300100 < AC < 300300 < AC < 500150 < AC < 250150 < AC < 200150 < AC < 280
DEN
(g/cm3)		2.3 < DEN < 32.3 < DEN < 32.3 < DEN < 31 < DEN < 1.752.3 < DEN < 1.752.4 < DEN < 2.82.0 < DEN < 3.0
CNL
(pu)		40 < CNL < 10020 < CNL < 8020 < CNL < 6040 < CNL < 800 < CNL < 20 pu0 < CNL < 5 pu5 < CNL < 40
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Zhang, P.; Jing, X.; Pu, R.; Wang, A.; Huang, X. Logging Identification and Distribution of Bauxite in the Southwest Ordos Basin. Minerals 2023, 13, 1253. https://doi.org/10.3390/min13101253

AMA Style

Zhang P, Jing X, Pu R, Wang A, Huang X. Logging Identification and Distribution of Bauxite in the Southwest Ordos Basin. Minerals. 2023; 13(10):1253. https://doi.org/10.3390/min13101253

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Zhang, Peng, Xianghui Jing, Renhai Pu, Aiguo Wang, and Xueping Huang. 2023. "Logging Identification and Distribution of Bauxite in the Southwest Ordos Basin" Minerals 13, no. 10: 1253. https://doi.org/10.3390/min13101253

APA Style

Zhang, P., Jing, X., Pu, R., Wang, A., & Huang, X. (2023). Logging Identification and Distribution of Bauxite in the Southwest Ordos Basin. Minerals, 13(10), 1253. https://doi.org/10.3390/min13101253

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