Oolitic Sedimentary Characteristics of the Upper Paleozoic Bauxite Series in the Eastern Ordos Basin and Its Significance for Oil and Gas Reservoirs

Abstract

In recent years, great breakthroughs have been made in gas explorations of the Upper Paleozoic bauxite series in the Longdong area of the Ordos Basin, challenging the understanding that bauxite is not an effective reservoir. Moreover, studying the reservoir characteristics of bauxite is crucial for oil and gas exploration. Taking the bauxite series in the Longdong area as an example, this study systematically collects data from previous publications and analyzes the petrology, mineralogy, oolitic micro-morphology, chemical composition, and other sedimentary characteristics of the bauxite series in the study area using field outcrops, core observations, rock slices, cast slices, X-ray diffraction analysis, scanning electron microscopy and energy spectra, and so on. In this study, the oolitic microscopic characteristics of the bauxite reservoir and the significance of oil and gas reservoirs are described. The results show that the main minerals in the bauxite reservoir are boehmite and clay minerals composed of 73.5–96.5% boehmite, with an average of 90.82%. The rocks are mainly bauxitic mudstone and bauxite. A large number of oolites are observable in the bauxite series, and corrosion pores and intercrystalline pores about 8–20 μm in size have generally developed. These pores are important storage spaces in the reservoir. The brittleness index of the bauxite series was found to be as high as 99.3%, which is conducive to subsequent mining and fracturing. The main gas source rocks of oolitic bauxite rock and the Paleozoic gas series are the coal measure source rocks of the Upper Paleozoic. The oolitic bauxite reservoirs in the study area generally have obvious gas content, but the continuity of the planar distribution of the bauxite reservoirs is poor, providing a scientific basis for studying bauxite reservoirs and improving the exploratory effects of bauxite gas reservoirs.

1. Introduction

The Changqing oilfield has obtained a high-yield gas flow with an open flow rate of 673,800 m³/d in the bauxite reservoir of well L47 in the Longdong area, representing the first breakthrough in this area. The bauxite gas reservoir in the Ordos Basin also shows good exploratory potential. [1,2,3,4,5]. This achievement challenges the understanding that bauxite is not an effective reservoir and opens up a new field of study, as it is imperative to analyze the characteristics of reservoirs. In recent years, scholars have paid increasing attention to the analysis of bauxite reservoirs. The mineralogical characteristics and formation mechanisms of bauxite series have been clarified through core tests and analyses, with the composition mainly featuring aluminous and clay minerals of sedimentary origin [2,6]. The reservoir formation process has been analyzed based on the controlling factors of bauxite gas reservoirs and the lithological traps under structural action [7,8,9,10]. The lithology, physical properties, gas-bearing properties, and pore structures of bauxite reservoirs have been analyzed using logging data [5,11,12]. In addition, basic geological problems such as the reservoir characteristics, provenance, and distribution law of bauxite series represent additional research hotspots. Analysis shows that the porosity of bauxite series is 10.6% on average, with permeability of more than 0.3 × 10⁻³ μm² accounting for 36% and good reservoir conditions [2,6,9,10].

At the same time, a large number of oolites have developed in bauxite rocks. According to their particle size, these grains can be divided into bean grains (or giant oolitic grains, generally larger than 2 mm in size) and oolitic grains (generally less than 2 mm in size), all of which are spherical structures. These grains are collectively referred to as oolites in this paper. What is the relationship between oolites and reservoir space (pores)? What is the significance of the geological conditions for oolite formation in oil and gas reservoirs? While these questions are crucial for oil and gas exploration, few scholars have explored this field from a microscopic point of view, especially considering oolites.

In this study, data from previous studies are systematically collected. Taking the bauxite series in the Longdong area as an example, the petrology, mineralogy, oolitic micro-morphology, chemical composition, and other sedimentary characteristics of the bauxite series are analyzed using field outcrops, core observations, rock slices, cast slices, X-ray diffraction analysis (XRD), scanning electron microscope–energy dispersive spectroscopy (SEM-EDS), and so on. The micro-characteristics of the factors controlling the formation of bauxite reservoirs and the significance of oil and gas reservoirs are explored from an oolitic perspective, providing a foundation for bauxite reservoir research and theoretical support for bauxite gas reservoir exploration.

2. Regional Geology

The study area is located in the western and southern parts of the North China Craton (NCC) (Figure 1) and developed on the crystalline basement of the Archean–Early Proterozoic boundary [13]. The early Paleozoic was a stable craton basin evolution stage influenced by the Qilian–Qinling Ocean and developed a central paleo-uplift [14,15,16]. The Longdong area was located in the northeastern portion of this large central paleo-uplift during the Early Paleozoic. Due to the influence of Caledonian movement in the Late Ordovician, the study area was uplifted as a whole [17], with sedimentary discontinuities of about 140 Ma. In addition, the sedimentary strata from the Silurian to Early Carboniferous were found to be missing. Since the late Carboniferous, the study area has again experienced overall subsidence and sedimentation due to the southward compression of the Eurasian plate and the closure of the Mongolian Ocean, developing a widely distributed set of bauxite series [18,19,20,21].

3. Sample Collection and Experimental Methods

In this study, we collected core samples of bauxite series from wells L58 and L47-1C in the Longdong area, Ordos Basin (Figure 1), along with 136 sample data on boreholes and profiles from previous studies reporting related results in the Ordos Basin (

Table 1. Data on boreholes and profiles of previous studies on related achievements in the study area.

Number	Borehole/Position	Quantity	Authors	Notes
1	L58	13	Yao et al.	[2]
2	L47	4	Nan et al.	[6]
3	L58	3	Nan et al.	[6]
4	L58	14	Liu et al.	[11]
5	C3	4	Liu et al.	[11]
6	HT2	3	Liu et al.	[11]
7	Eastern Ordos Basin	17	Li et al.	[1]
8	Linxing area	16	Wu et al.	[24]
9	Northeastern Ordos Basin	11	Peng et al.	[25]
10	L58	3	Xu et al.	[26]
11	HT2	7	Xu et al.	[26]
12	HT1	1	Xu et al.	[26]
13	L58	19	Xie et al.	[4]
14	Longdong	21	Liu et al.	[27]
Total		136

). The rock thin section identification method was used to analyze and study the area’s petrological characteristics; this was completed at the Henan Key Laboratory of Biogenic Traces and Sedimentary Minerals, Henan Polytechnic University, Jiaozuo, China. The instrument was a DM4P polarizing microscope manufactured by Leica, Wetzlar, Germany. Microscopic detection of oolitic particles was completed at the National Engineering Laboratory for the Exploration and Development of Low Permeability Oil and Gas Fields. The instruments used for this purpose were a Quanta FEG 450 field emission scanning electron microscope from FEI and a Quantax 200 X-ray energy spectrometer from BRUKER (Billerica, MA, USA). The resolution of the host was set to 1.0 nm@30 kV (SE); the magnification was 200,000–1,000,000; the test voltage was 200 V to 30 kV; and the moving range of the sample table was X = 100 mm, Y = 100 mm, Z = 60 mm, T = −5 to +70°, and R = 360° continuous rotation. The XRD sample test was completed at the Henan Key Laboratory of Biogenic Traces and Sedimentary Minerals, Henan Polytechnic University, Jiaozuo, China. The instruments included a D8 Advance X-diffractometer and CuKα alpha target from Brooke AXS in Germany. The test voltage was 40 kV, the test current was 25 mA, and the scanning width was 3–90°. In addition, the scanning mode was set to continuous scanning, the divergence slit size was 0.6°, the acceptance slit size was 0.1 mm, and the measurement temperature was 25 °C. Before testing, the sample was ground to a 200 mesh size. For specific testing methods and parameters, please refer to the quantitative analysis method for rock minerals using the X-ray energy spectrum (EDS) (SYT 6189-2018) [22] and the analysis method for clay minerals and ordinary non-clay minerals in rocks via X-ray diffraction (SYT 5163-2018) [23].

4. Results and Discussion

4.1. Petrological Characteristics

According to field profile observations, core observations, and thin section identification, the lithology of the bauxite series presents a three-stage structure: bauxitic mudstone is mainly contained in the upper and lower parts, with bauxite series developed in the middle part (Figure 2). At the same time, the area’s lithology also shows cyclicity (

Table 2. XRD analysis results of the samples in well L58.

Depth (m)	Lithology	Boehmite	Clay	Other	Notes
4040.00	bauxitic mudstone	3.1	94.3	2.6
4040.12	bauxitic mudstone	0.9	94	5.1
4040.62	bauxitic mudstone	14.3	74.8	10.9	Repeat
4041.00	bauxitic mudstone	18.8	69.6	11.6
4041.74	bauxite	78	16.7	5.3
4042.54	bauxite	94	3	3
4042.82	bauxitic mudstone	62.9	28.2	8.9	Test
4042.82	bauxitic mudstone	62.9	12.8	24.3	Data
4042.97	bauxitic mudstone	47.1	49.3	3.6
4044.63	bauxitic mudstone	29	19.8	51.2
4044.99	bauxite	93.6	4.1	2.3	Repeat
4045.24	bauxite	96.1	1.2	2.7	Repeat
4045.55	bauxite	89.7	2.3	8
4045.90	bauxite	96.4	1.1	2.5	Repeat
4047.10	bauxite	93.1	0.7	6.2
4047.10	bauxite	93.1	0.7	6.2
4047.40	bauxite	96.5	0.9	2.6
4048.25	bauxite	95.4	1.4	3.2	Repeat
4049.58	bauxite	95.6	0.8	3.6	Repeat
4050.76	bauxite	73.5	18.9	7.6
4051.10	bauxite	85.7	10.8	3.5
4051.71	bauxitic mudstone	51	38.6	10.4	Repeat
4052.20	bauxitic mudstone	36.4	50.6	13
4052.20	bauxitic mudstone	36.4	40.2	23.4
4052.39	bauxitic mudstone	30.8	54.8	14.4
4053.29	bauxitic mudstone	16.6	64.7	18.7	Repeat
4053.99	bauxitic mudstone	16.5	64.8	18.7
4054.92	bauxitic mudstone	20.9	57	22.1
4055.26	bauxitic mudstone	48	40.7	11.3

).

Oolite-rich bauxite is generally located in the middle of the bauxite series and is light grey in color. Compared with the lower gray-black bauxitic mudstone, the compactness of this layer is poor, with pores visible inside the oolitic and clastic structures (Figure 3a,b). These pores serve as the main reservoir spaces for bauxite reservoirs.

4.2. Mineralogical Characteristics

Based on the XRD analysis combined with thin section identification and the in situ analysis of scanning electron microscope (SEM) images, the mineral composition of the bauxite series can be divided into four categories: aluminum minerals (Figure 4a), clay minerals (Figure 4b,c), titanium minerals, and other minerals (Table 2).

Figure 5 shows that there is little difference in the composition of similar minerals on the same horizon. However, there are obvious differences in mineral composition on different horizons. These differences indicate that illite and kaolinite are the main minerals in the lower bauxite mudstone, diaspore is the main mineral in the middle bean oolitic bauxite, and kaolinite is the main mineral in the upper bauxitic mudstone.

4.3. Microscopic Characteristics

4.3.1. Oolitic Characteristics

Oolites are mostly round or elliptical with different particle sizes and generally less than 3 cm in diameter. According to their density and degree of mineral crystallization, oolites can be divided into two categories. In the first type, there are many dense layers with more than ten layers composed of dark- and light-colored minerals. These minerals are dark in color, poor in mineral crystallization, and large in particle size; they also present significant shear plastic deformation (Figure 6a). The other type includes a few sparse layers (generally only three) composed of dark- and light-colored minerals, with different colors, good mineral crystallization, and weak shear plastic deformation (Figure 6b).

The ring structures of most oolites are obvious (Figure 7a). Through local amplification and scanning analysis of the energy spectrum distribution, we found that the chemical composition mainly features O, Al, and Si (Figure 7c–h). Moreover, the ring structure is caused by differences in composition, which can also be verified by the results of the energy spectrum analysis.

4.3.2. Oolitic Pores

Bauxite pores in the outcrop profile and core are generally present in the study area. Observing the cast slice using a scanning electron microscope and other techniques indicated a clear dissolution phenomenon. Generally, oolitic pores in the area are about 8–200 μm in size and irregularly shaped (Figure 8a,b), showing obvious leaching characteristics. After the oolites are dissolved, the subsequent solution migrates to the dissolution space to form clay or aluminum minerals in the reducing environment (Figure 8a). The oolites are semi-filled or filled. When the oolites are not filled, dissolution pores develop in the oolites or debris (Figure 8c). Due to the poor crystallization and small crystals in the clay minerals formed during later periods, intergranular pores developed in the filled oolites or debris. The intergranular dissolved pores are small, about 3 μm, and irregularly shaped (Figure 8b,c). The dissolved pores of the granules are generally distributed in the particles, with uneven sizes ranging from 6 to 95 μm (Figure 8c). The intergranular dissolved pores are distributed between the oolites, with sizes exceeding 1 mm. Fractures occasionally occur, which play a certain role in connecting pores but are not the main reservoir space. Instead, dissolved pores provide the main space for bauxite reservoirs.

4.3.3. Porosity and Permeability Characteristics

We obtained 181 porosity readings from reservoir samples in the study area (based only on the number of samples specified in the analysis data) [2,6,7,8,12,24,25]. Porosity ranged from 0.2% to 29.7% and was generally greater than 10%. We also obtained 173 permeability samples, with the permeability ranging from 0.004 × 10⁻³ μm² to 38.55 × 10⁻³ μm², whose average permeability was generally greater than 4 × 10⁻³ μm². Additionally, the reservoir conditions were found to be good, with conditions in oolitic bauxite reservoirs generally better than those of other reservoirs.

The logging and core physical property testing data indicate that the porosity and permeability curves are consistent and have similar variation patterns across the overall profile. There is also a clear positive correlation between porosity and permeability, especially in the oolitic bauxite reservoirs. The porosity and permeability of the bauxitic mudstone in the upper and lower parts are very low, while the porosity and permeability of the oolitic bauxite reservoirs are very high. The physical property tests achieved values of 27.6% and 20.18 × 10⁻³ μm² for porosity and permeability, respectively, far exceeding the corresponding values of the Upper Paleozoic sandstone and Lower Paleozoic carbonate rock.

4.4. Brittleness Index

The fracturing ability of the reservoir, i.e., the brittleness index of the reservoir, is one of the most important parameters to be evaluated in later reconstructions of the reservoir. The higher the brittleness index, the more favorable it is for subsequent fracturing of the reservoir [6,26,28]. The reservoir brittleness index is generally calculated using the mineralogical method. The main minerals in the bauxite series are diaspore and clay minerals, so the brittleness index of the bauxite series can be calculated according to Formula (1), that is, the diaspore can be calculated based on brittle minerals [6]. According to the calculation, the brittleness index of the study area can reach 99.3%. The brittleness index of well HT2 is 56.5% to 93.7%, while that of well L58 is 16.4% to 99.3% (Figure 9) [2,4,6,26].

$$\mathrm{B}\mathrm{r}\mathrm{i}\mathrm{t}\mathrm{t}\mathrm{l}\mathrm{e}\mathrm{n}\mathrm{e}\mathrm{s}\mathrm{s} \mathrm{I}\mathrm{n}\mathrm{d}\mathrm{e}\mathrm{x}={\displaystyle \frac{{\mathcal{w}}_{\mathrm{d}\mathrm{i}\mathrm{a}\mathrm{s}\mathrm{p}\mathrm{o}\mathrm{r}\mathrm{e}}+{\mathcal{w}}_{quartz}}{{\mathcal{w}}_{\mathrm{d}\mathrm{i}\mathrm{a}\mathrm{s}\mathrm{p}\mathrm{o}\mathrm{r}\mathrm{e}}+{\mathcal{w}}_{quartz}+{\mathcal{w}}_{Carbonate minerals}+{\mathcal{w}}_{clay minerals}}}\ast 100\%$$

(1)

Importantly, the brittleness index reflects the fracturing quality of the reservoir, which affects the difficulty of fracturing, among other factors. The higher the brittleness index, the more complex the fracture network that will be formed. The brittleness index of the bauxite series in the study area is high overall. Most notably, the average content of oolitic bauxite (a brittle mineral) in well L58 is 90.82% (Table 2, Figure 4 and Figure 5). About 50% of the samples in these oolitic series have a brittleness index exceeding 95%. Mechanical rock experiments also show that the bauxite series have the characteristics of Young’s modulus (36.4 GPa) and Poisson’s ratio (0.35) [6], which meet the standards of reservoirs rich in brittle minerals and are suitable for the subsequent fracturing and exploitation of natural gas reservoirs. These factors are conducive to improving the initial single-well productivity.

4.5. Influence of Geological Processes on Reservoir Space

The bauxite rock series in the Longdong area is a product of strong chemical weathering and laterization of the original rocks [2,4]. The mineral composition and structures of the original rocks have been thoroughly transformed. The formation of bauxite involves a process of rich iron and aluminum mineralization, as well as the formation of oolites [18,20]. Studies have confirmed that bauxite series are formed under warm/high temperatures in humid/rainy climates [18,19,21,29,30,31,32,33,34,35,36,37,38]. During the Middle Carboniferous, the NCC (where the study area is located) became separated from the Gondwana continent and drifted to the area near the equator, yielding a high temperature and rainy climate [29,39,40,41]. Under these climatic conditions, the chemical weathering degree of parent rocks is strong, which provides a sufficient supply of aluminum-bearing materials and parent materials for oolitic formation in the study area [42].

According to the zircon geochronology, regional structure, and stratigraphy, transgression occurred during the formation of the bauxite series [19,20,21,29,30,31,32,33,34,35,36,37,38]. The Ordos Basin was affected by the central paleo-uplift [14,15,16], and its eastern part was a limited epicontinental sea [7], which belonged to a typical subtidal low-energy sedimentary environment. Due to the hydrodynamic conditions, aluminum-bearing minerals here mostly feature clastic and oolitic structures, with oolites widely distributed.

Due to the effect of the central paleo-uplift and the existence of the ancient Qingyang land, the bauxite series were transported to the lower part of the paleogeomorphology, thereby enriching the material basis of oolitic ore-forming materials due to the low and relatively closed terrain [7]. At the same time, the relatively low paleogeomorphology facilitated surface water leaching, with high karstification intensity [33,41]. Consequently, the soluble components in the area were more easily lost, and leaching led to the formation of minerals such as boehmite (Table 2, Figure 4 and Figure 5) and the development of a large number of oolites.

The pores in the bauxite series were generated during the intense chemical weathering processes of the original rock, which were closely related to physical and chemical factors. Pores formed through physical processes are often modified by later chemical reactions. As mentioned previously, a large number of oolitic pores developed relatively deep into the paleogeomorphology, providing more space for the reservoir. Indeed, the pores in the bauxite series are closely related to the formation process of the bauxite series, whose pore formation and storage were primarily informed by related chemical processes. The types of pores formed were primarily dissolution pores and intergranular pores. Therefore, the sedimentary paleogeomorphology of the Ordos Basin controls the quality of the Paleozoic bauxite reservoirs in the Longdong area.

In the Early Paleozoic, the whole study area existed in a stable craton evolution stage. By the late Cambrian, the Qingyang area was uplifted to form a paleocontinent, which established the regional tectonic background of the Longdong area located in a large central paleo uplift [14,15]. Since the Middle Ordovician, the study area has experienced an overall uplift through Caledonian tectonic movement, resulting in sedimentary discontinuity and stratigraphic loss. In the late Paleozoic, the craton depression structure settled steadily, and frequent transgression occurred, leading to thicker coal seams and carbonaceous mudstone deposits in the upper part of the bauxite series. The humic acid formed in the diagenetic process further reformed the bauxite reservoir and promoted the development of numerous dissolution pores in the bauxite reservoir [43].

Comparing and analyzing the carbon isotope data of natural gas in various gas strata of the Paleozoic indicates that the bauxite, Shihezi Formation tight sandstone, and pre-Carboniferous weathered crust gas reservoirs are homologous. These reservoirs were primary formed from Upper Paleozoic coal measure hydrocarbon source rocks [7,44,45]. Upper Paleozoic coal measure source rocks are widely distributed and have a high thermal evolution degree. These rocks also offer strong hydrocarbon generation capabilities, which can produce sufficient gas resources for the Paleozoic bauxite gas reservoir [7].

The coal seam and carbonaceous mudstone in the upper part of the bauxite series are not only superior source rocks but also good sealing layers, forming a source reservoir configuration with “upper generation and lower storage”. The bauxite series in the study area features a gas reservoir combining source storage and sealing [2,7]. The development of faults and fractures during the Indosinian period effectively connected the Shanxi Formation source rocks with bauxite reservoirs, diverted oil and gas, and facilitated oil and gas accumulation. Therefore, the bauxite series in the study area generally contain significant gas content. However, the formation of the aforementioned oolitic mineral series (or the intensity of aluminum enrichment or chemical weathering) is closely related to the drainage conditions of karst processes. The bauxite series appears most developed at the karst funnel and is positively correlated with the depth of the funnel. Paleokarstification of the ancient rock also controls the planar distribution of the bauxite series.

5. Conclusions

• The lithology of the bauxite series was mainly bauxite and bauxitic mudstone, and the mineral composition was mainly aluminum minerals, clay minerals, titanium minerals, and other minerals. The content of boehmite in oolites ranged from 73.5% to 96.5%, with an average of 90.82%. The ring structures of most oolites were obvious and caused by differences in composition.
• Oolite-rich bauxite with corrosion pores and intercrystalline pores about 8–20 μm in size was most common. The dissolved pores provided the main reservoir space for the bauxite reservoirs. The brittleness index was high, making the area suitable for subsequent fracturing of natural gas reservoirs.
• The main gas source rocks for the oolitic bauxite rock and Paleozoic gas series were coal measure source rocks of the Upper Paleozoic. The oolitic bauxite reservoirs generally contained obvious gas content, but the continuity of the planar distribution was poor.