Introduction
According to the Specification for Bauxite Mineral Exploration [1], and the classification criteria, genesis and basement rock, the aluminous rock series (ARS) in China can be classified into two types, namely the paleo- weathering crust type (karst) and the laterite type [2⇓-4]. The paleo-weathering crust ARS is dominant and accounts for more than 80% [3-4]. It is caused by ionic crystallization and clastic sedimentation [2,5], and has abundant aluminous minerals (predominantly diaspore). Controlled by karst landform, the paleo-weathering crust ARS presents complicated vertical sequence variation. In North China, the weathering crust ARS is typically concentrated in the Paleozoic, Mesozoic and Neozoic in the North China Craton, especially the Ordovician weathering crust ARS spreads on the largest scale [2,4]. It is reported that extensive Carboniferous-Permian paleo-weathering crust ARS has been found in the Ordos Basin [3].
The exploration area of ARS in the Ordos Basin is 8 000 km2, and the natural gas resources amount to 5 000×108 m3. From the 20th century to the early 21st century, the early practice of drilling exploration in the Ordos Basin demonstrated that the ARS of the Carboniferous Benxi Formation and Permian Taiyuan Formation are impermeable caprocks for the underlying Ordovician weathering crust gas reservoir [6⇓-8]. Nonetheless, with the increasing levels of exploration across the Longdong area in the southwestern basin, 35 wells among the 95 drilled and completed exploration wells presented gas logging anomalies from the ARS of the Taiyuan Formation. Accordingly, some researchers changed their mindset that the ARS can serve as high-quality reservoir rocks for natural gas [9]. Subsequently, formation testing was performed in some exploration wells for the ARS to verify the above idea, which demonstrated low-production gas streams. Based on the test results and well logging evaluation, it is preliminarily confirmed that the thickness and gas logging anomaly of the ARS in the southwestern Taiyuan Formation are both superior to those in the mid-eastern Benxi Formation in the Ordos Basin. After following the exploration strategy of unconventional hydrocarbon resources and carrying out deep research on the ARS gas accumulation model, formation test was performed in 21 wells, of which 14 wells obtained gas production more than 1×104 m3/d, and 4 wells above 50×104 m3/d. A breakthrough has been made in the exploration of the Paleozoic ARS gas reservoirs [10-11].
Currently, regarding the main control factors on gas accumulation of sub-coal (below coal seams) ARS gas reservoirs, few in-depth investigations are reported, and relevant studies are mostly limited to the Longdong area in the southwestern Ordos Basin. Studies targeting the whole Ordos Basin and even the sub-coal ARS gas reservoirs in North China are rarely presented. Based on cores, outcrops, mud logs, well logs, formation test and lab data, this study analyzed the lithological types of the ARS reservoirs from the Carboniferous Benxi Formation to the Permian Taiyuan Formation, described the characteristics and controlling factors of the effective reservoirs, investigated the configuration of coal seams (coaly rock series, CRS) source rocks and ARS reservoirs, and built the gas accumulation model of the Ordos Basin. Finally, the exploration potential of natural gas in the Carboniferous-Permian ARS reservoirs was evaluated, and favorable plays were predicted in North China. The findings are expected to provide technical supports for sub-coal ARS gas exploration in other areas.
1. Geological setting
During the Caledonian Orogeny, the North China Craton was subjected to regional tectonic uplifting. The carbonate rocks in the Ordos Basin were raised above the sea level during the Late Ordovician and experienced physical and chemical weathering-planation for up to 140 Ma, which led to peneplanation and ultra-thick paleo-weathering crust [12⇓⇓-15]. These exposed rocks released various ions due to long-term erosion and decomposition. Alkali metals in the rocks were lost, while aluminum, silicon and iron were retained and separated to form aluminum (Al)-containing iron solutions.
During the Early Hercynian stage, the Ordos Basin in the extensional tectonic setting of the North China Craton experienced regional slow subsidence since the Late Carboniferous. The maximum Late Paleozoic transgression occurred when the Carboniferous Benxi Formation and the Permian Taiyuan Formation were deposited [16-17]. Seawater swarmed from the southeast and took Al-containing solutions to karst lows where Al-containing materials accumulated, which laid a material basis for ARS development. Subsequently, minerals were gradually crystallized and precipitated from the Al-containing solutions. The Carboniferous-Permian ARS reservoirs were formed.
The ARS is widely distributed in the Ordos Basin. It mostly occurs in the Benxi Formation in the mid-eastern region and the Taiyuan Formation in the southwestern region. With the expansion of the transgression during the Late Carboniferous, most of the mid-eastern region was submerged in water and received the Benxi sediments. At the same time, although the scope of the central paleo-uplift in the southwestern region was narrowed, the majority of the structure remained above sea level and was subjected to supergene weathering, and therefore the Benxi sediments rarely went there. Since the Early Permian, the central paleo-uplift was gradually submerged in water and started to receive sediments of the Taiyuan Formation. Hence, the Lower Paleozoic carbonate rocks are overlain by the Carboniferous Benxi Formation in the mid-eastern basin (Fig. 1a ), while in the central-paleo uplift area in the southwestern basin, they are overlain by the Permian Taiyuan Formation, and the Carboniferous Benxi Formation is missing (Fig. 1a ). From bottom to the top, the Paleozoic Erathem consists of the Cambrian Maozhuang, Xvzhuang, Zhangxia and Sanshanzi Formations and Ordovician Majaou Formation of the Lower Paleozoic; the Carboniferous Benxi Formation, and the Permian Tiayuan, Shanxia, Lower Shihezi, Upper Shihezi and Shiqianfeng Formations of the Upper Paleozoic (Fig. 1b ).
Fig. 1. Distribution of the overlying strata of the Lower Paleozoic and the comprehensive stratigraphic column of the Paleozoic in the Ordos Basin. |
2. The ARS reservoirs
2.1. Rock types and characteristics
The XRD (X-ray diffraction) whole-rock analysis shows that there are three predominant mineral components in the ARS in the Ordos Basin, namely aluminum hydroxides, iron (Fe)-containing minerals and clay minerals. Specifically, the aluminum hydroxides mostly include boehmite, diaspore, and gibbsite; the Fe-containing minerals are mainly siderite and pyrite; and the clay minerals are mostly illite, kaolinite and chlorite. Using a triple-end- member classification methodology, the ARS can be sub- divided into seven types [18⇓-20], according to the content of the above three mineral components. They are bauxitic mudstone, muddy bauxite, muddy ferruginous rock, bauxite, ferruginous bauxite, bauxitic ferrolite and ferrolite (Fig. 2 ), and bauxitic mudstone, bauxite and muddy bauxite are dominant. The mineral composition of the bauxitic mudstone is predominated by clay and ferruginous minerals, which jointly account for more than 50%, while aluminum hydroxides (boehmite, diaspore and gibbsite, BDG), occurring as cryptocrystals or cryptocrystal aggregates and distributed in the form of discontinuous laminae, account for less than 50%. The mineral composition of the bauxite is predominated by BDG (over 75%), and clay and Fe-containing minerals jointly account for less than 25%. The bauxite often presents granular textures (detrital, and oolitic), good sorting, positive rhythm and oriented arrangement. The muddy bauxite is also dominated by BDG (50%-75%), and clay and Fe-containing minerals account for 25%-50%. It is characterized by a blocky texture.
2.2. Reservoir space and physical properties
Based on analyses of casting thin sections, SEM (scanning electron microscope) and cores, three major types of reservoir space are identified in the Carboniferous-Permian ARS in the Ordos Basin, namely dissolved pores, inter-crystalline pores and micro fractures. The dissolved pores are mainly residual framework dissolution pores (Fig. 3a , 3b and 3g ) and intra-granular dissolved pores (Fig. 3c , 3d ), and a few matrix dissolution pores (Fig. 3e ). The residual framework dissolution pores are residual inter-granular pores among aggregates with localized dissolution pore growth. Kaolinite and diaspore crystals are often separated out in such pores. The residual framework dissolution pores are mostly flaky and needle-like, with the pore size of 20-150 μm and the plane porosity of 3.0%. The intra-granular dissolved pores often develop in bauxite reservoirs with the pisolitic-oolitic and detrital textures and have the pore size of 100-200 μm and plane porosity of 8.0%. In such pores, needle-like or cylindrical secondary chlorite is often observed to irregularly grow along the normal direction of the pore wall. The matrix dissolution pores are mainly inter-crystalline dissolved pores formed in dissolved loose packs of euhedral micro-crystalline diaspore, precipitated from recrystallization of cryptocrystalline gibbsite [21].
Fig. 3. Reservoir space of ARS in the Ordos Basin. (a) Casting thin section showing the development of residual framework dissolution pores among pisolitic and oolitic grains, 4 119.00 m, Taiyuan Fm., Well L47; (b) Casting thin section showing the development of residual framework dissolution pores among pisolitic and oolitic grains, 4 151.32 m, Taiyuan Fm., Well L47-1C; (c) Casting thin section showing the development of intra-granular dissolved pores of oolitic grains, 4118.00, Taiyuan Fm., Well L47; (d) Casting thin section showing the development of intra-granular dissolved pores of oolitic grains, 4 107.00, Taiyuan Fm., Well L47; (e) Casting thin section showing the development of matrix dissolved pores, 4 107.00, Taiyuan Fm., Well L47; (f) Casting thin section showing the development of inter-crystalline pores, 4 048.50, Taiyuan Fm., Well L58; (g) SEM image showing the development of inter-granular framework dissolution pores, 4 044.99 m, Taiyuan Fm., Well L58; (h) Casting thin section showing micro fractures, 4 611.23 m, Taiyuan Fm., Well HT2; (i) Casting thin section showing micro fractures, 2 134.72 m, Taiyuan Fm., Well M115. |
The inter-crystalline pores typically occur in the inter- crystalline space among boehmite/diaspore, kaolinite, pyrite and anatase, with higher crystallization levels. These pores are generally nano-sized (Fig. 3f ) and smaller than the dissolved pores, and occasionally, dolomite inter-crystalline pores are observed in semi-filled micro fractures.
The micro fractures, including inter-layer micro fractures, shrinkage micro fractures and structural micro fractures (Fig. 3h , 3i ), are mainly distributed in the bauxitic mudstone. The inter-layer fractures attributed to frequent alternating of lithology are in the majority, and the micro fractures are mostly semi-closed under the formation conditions [21].
As observed from the cores, the bauxite is unconsolidated and porous, with a honeycomb texture (Fig. 4 ). It is formed by extensive grain dissolution of pisolitic-oolitic and detrital textures, so porous residual pisolitic-oolitic and detrital textures are often observed. The dissolved pores, large in both quantity and volume and well-connected, jointly constitute the space of the pore-type reservoir system with micro fractures. According to the statistical analysis of 44 samples collected from the ARS reservoir of the Taiyuan Formation in the southwestern Ordos Basin, the porosity is 5.0%-29.7%, with an average of 16.59%, and the samples with porosity above 10% account for 46.16%; the permeability is mostly (0.01- 38.55)×10-3 μm2. The reservoir physical properties are fairly good. The blocky muddy bauxite and the laminated bauxitic mudstone have a low content of BDG and no phanerocrystalline texture; pisolitic-oolitic or detrital textures are seldom seen; dissolved pores are underdeveloped; and existing pores are mostly small inter-crystalline pores among clay minerals, gibbsite, anatase and pyrite. These two types of rock are tight and have inferior physical properties. The porosity and permeability of the laminated bauxitic mudstone are comparable to those of mudstone (Fig. 5 ). In conclusion, the effective ARS reservoir in the Ordos Basin is mainly the honeycomb bauxite, with diaspore over 80% and featured porous residual pisolitic-oolitic and detrital textures, which often occurs in the middle of the ARS [21].
Fig. 4. Core photos of the honeycomb-like bauxite in the Ordos Basin. (a) Bauxite with a porous honeycomb texture, Taiyuan Fm., 4 049.15 m, Well L58; (b) Bauxite with a porous honeycomb texture, Taiyuan Fm., 4 049.50 m, Well L58. |
Fig. 5. Porosity vs. permeability of different types of aluminous rock series. |
3. Controlling factors on effective ARS reservoir development
3.1. Surface planation laying the material basis for the ARS development
The weathering crust ARS in the North China Craton all occur in the Carboniferous or Permian basal strata over the paleo-denudation surface of the Ordovician carbonate rocks. The coming-into-being of the ARS is closely related to the Lower Paleozoic carbonate basement.
Zircon age spectrum correlation and whole-rock stable element correlation are two methods frequently used in provenance analysis [22⇓⇓⇓⇓⇓-28]. Whole-rock stable elements, the “DNA” of rock, accurately record the rock source information. Compared with the zircon age spectrum, they are more applicable for the provenance analysis of chemical sedimentary rocks. This is because detrital zircon is an appendage of ARS and is subjected to cycling. Provenance determination based on zircon will not be accurate unless the detrital zircon and the ARS share the same source. The age of detrital zircon only stands for the formation time of the clastics in the ARS, but it is inadequate as evidence for the provenance of the ARS [18⇓-20]. On the contrary, stable elements have a higher stability. They can survive intensive weathering and more importantly, still record some geological characteristics of parent rocks, so they are widely used in studies on the provenance of bauxite [22⇓-24].
With the increasing levels of weathering, stable elements gradually accumulate in weathering materials. Nevertheless, the relative proportions of stable elements are the same in the weathering materials and parent rocks. The zircon/niobium (Zr/Nb) ratio in the study area varies greatly with lithologies from different provenances [27]. The ARS and the underlying Ordovician-Cambrian carbonate rock share a consistent Zr/Nb ratio of 7.52-11.28 (averaging 8.80), which is considerably lower than that of the Permian clastic rock (14.78-17.82, averaging 17.30) and that of the Ordovician tuff (larger than 20).
In addition, aluminum is a major element of the Lower Paleozoic carbonate rock, given the element content. The content of Al2O3 is 0.06%-13.06%, with an average of 2.18%. The Lower Paleozoic carbonate rock has experienced intensive weathering and denudation and released various ions. Aluminum ions managed to remain in solutions and formed Al-containing solutions that were carried to the lower position of the paleo-landform. So there accumulate rich aluminum ions.
The above analysis demonstrates that the original provenance of the ARS in the Ordos Basin is the in-situ Ordovician-Cambrian carbonate rock. Release and enrichment of aluminum ions depend on prolonged weathering of the carbonate rock as a provenance [4], so surface planation is favorable for ARS development. Due to the Caledonian orogeny, the overall Ordos block was uplifted and became onshore. Then, the ultra-thick paleo-weathering crust was formed via planation and peneplanation. The weathering products after erosion and transport are important sources for the Carboniferous-Permian aluminum ions in the Ordos Basin [29].
3.2. Karstic paleo-landform controlling the ARS development
The ARS in the Ordos Basin is paleo-weathering crust deposits, and its development was controlled by paleo- karst landform [20]. The ARS of the Taiyuan Formation in the southwestern Longdong area is superior to that of the Benxi Formation in the mid-eastern basin in terms of thickness and gas logging anomaly. This study takes the Longdong area as a case to demonstrate the control of paleo-karst landform on the ARS development (Fig. 6a ).
Fig. 6. Pre-Carboniferous paleo-landform of the Longdong area in southwestern Ordos Basin and the vertical lithological combinations of aluminous rock series in different paleo-landform units (see |
During the ARS deposition, the paleo-karst landform could be divided into three zones, namely karst platform, karst slope, and basin [30] (Fig. 6a ). According to the variable geomorphic features of the paleo-denudation surface, the karst micro paleo-landform units of the three negative landform zones (i.e., karst platform, karst slope and basin) are underflow crater, terrace and groove, respectively (Fig. 6b ).
In the karst platform, the basement carbonate rock was subjected to a high degree of planation, and weathering resulted in a solid Al-containing material basis. However, in the higher platform zone, Al-containing materials were easily denudated and transported, which is unfavorable for Al preservation. Only the underflow crater at the margin of the karst platform is a lower position in favor of receiving Al gel solutions. The sedimentary environment was mainly closed lagoons, in which Al-containing materials accumulated and were free from loss. Minerals were separated from the Al gel solutions via recrystallization and deposited. The ARS developing in the underflow crater is often funnel-like with a large thickness. The sequences of the vertical lithological combination are pebbly bauxitic mudstone, Fe-containing bauxitic, mudstone, pisolitic-oolitic bauxite, blocky bauxite, muddy bauxite and overlying carbonaceous mudstone from the bottom to the top.
In the karst slope zone that is slightly low, weathering was less intensive on the basement carbonate rock, so the material basis for the ARS development is inferior to that of the karst platform. With the deeper waters, the sedimentary environment became semi-closed lagoons and tidal flats. Shallow and fluctuating waters facilitated accumulation of Al-containing materials, but some have been lost. The ARS occurring in the terrace is lenticular, but its thickness is smaller than in the underflow crater. The sequences of the vertical lithological combination are pebbly bauxitic mudstone, Fe-containing bauxitic mudstone, pisolitic-oolitic bauxite, blocky bauxite, muddy bauxite and overlying coal seam or low-quality coal seam from the bottom to the top.
In the karst platform margin and slope that are steep, the original ARS sediments attributed to ion crystallization were transported a short distance to the slope base after intensive mechanical fragmentation. There they accumulated into pebbly bauxitic mudstone. This may explain why pebbly bauxitic mudstone typically occurs at the base of the ARS in underflow craters or terraces.
In the basin zone, the basement carbonate rock was less affected by surface planation, and weak weathering could not provide a good material basis for ARS development. The sedimentary environment was neritic (open water) shelf zones where only a small quantity of Al-containing materials could accumulate in the lower grooves closer to the paleo-land, but most of them were lost. High-quality bauxite with high content of BGD cannot be formed. The ARS developing in grooves presents thin-layer distribution. From the bottom to the top, the sequences of the vertical lithological combination are Fe-containing bauxitic mudstone, bauxitic mudstone and overlying coal seams and limestone, with no development of bauxite.
The above analysis demonstrates the control of paleo-karst landform on the ARS development, resulting in three types of vertical lithological combinations. The bauxite featuring pisolitic-oolitic and detrital textures mainly occurs in the relatively closed underflow crater or terrace at the karst platform margin and slope zones (Fig. 6 ). Moreover, in such bauxite, the ARS reservoir with bead-string distribution along the circumference of the paleo-uplift was formed, due to the dissolution in the karstic drainage channels.
3.3. Land surface leaching improving the storage capability
Early-deposited bauxite with pisolitic and oolitic granular textures has experienced three important diagenetic stages, namely supergene, burial and deuterogene stages. It was reconstructed by surface leaching, recrystallization and organic acid dissolution, resulting in a honeycomb reservoir texture that greatly improved the reservoir storage capability.
Supergene stage: The pisolitic and oolitic grains in bauxite are diagenetic products formed on the grain surface via repeated and alternating flocculation of gels such as aluminum hydroxides and silicic acid. Therefore, there are abundant soluble siliceous components with unstable physicochemical properties inside the BDG frameworks. After pisolitic and oolitic bauxitic materials accumulate directly above the carbonate weathering crust and are semi-consolidated, they go through supergene weathering and leaching by the surface water and meteoric water with a high content of carbon oxide. The massive soluble siliceous components inside the BDG frameworks tend to be leached and carried out through the drainage channels such as dolines and fractures at the karst sag floor, which results in a large quantity of residual framework dissolution pores and intra-granular dissolved pores.
Burial stage: The crypto-crystalline gibbsite in the bauxite matrix was first dehydrated to produce boehmite, which then became micro-crystalline diaspore via textural adjustment. Finally, the post-recrystallization diaspore was developed into loose packs and dissolved into numerous matrix dissolved pores.
Postdiagenetic stage: The overlying carbonaceous mudstone and coal seam generated and expelled a large volume of organic acid after maturation. Coal-based gas production simulation suggested that the water with dissolved coal had an organic acid concentration of 1 402.38-16 061.22 mg/L [31]. With burial, the organic acid generated by the CRS persistently seeped downward into the ARS, which strengthened desilication and dissolution, and consequently expanded the volume of dissolved pores.
Land surface leaching is the key to the formation of the pore-type reservoir. The pores attributed to meteoric water leaching during supergene exposure and drainage channel dissolution account for nearly 77.8% of the reservoir space [31]. In the underflow crater of the karst platform or terrace of the karst slope above the water table, bauxite with pisolitic-oolitic and detrital granular textures was found, and moreover, numerous fluid flow channels such as caverns and fractures at varied scales are present in the underlying carbonate rock. Unstable siliceous components inside the BDG frameworks tend to be dissolved and carried away, leaving plentiful pores that provide a favorable condition for the development of effective ARS reservoirs. However, such bauxite was not developed in the karst groove in the basin zone below the water table, and the underlying formation was tight without considerable seepage channels, so it is unfavorable for the development of effective ARS reservoirs.
To sum up, effective ARS reservoirs are the result of planation controlling the provenance, paleo-karst landform controlling the diagenesis, and land surface leaching controlling the reservoir space.
4. CRS-ARS spatial configuration and natural gas accumulation model
4.1. Spatial configuration and controlling factors
Field outcrops and available exploration data show that the vertical lithological combination sequences of the ARS and its overlying strata are composed of the lower Fe-containing bauxitic mudstone, middle bauxitic mudstone and bauxite or muddy bauxite, and upper carbonaceous mudstone or coal seams, which represent a typical three-segmented “coal-Al-Fe” stratigraphic structure [3-4]. This is related to the Carboniferous-Permian hot humid climate in the North China Craton and the global eustasy during the glacial-interglacial period, as the Gondwanaland drifted to the South Pole.
4.1.1. Hot and humid climate
ARS and CRS develop in the similar climate. During the Carboniferous-Permian, the whole North China Craton was between 30°S and 30°N; the annual average temperature was 23-26°C; the climate was warm and humid (tropical and subtropical); the sedimentary environment was paralic. Those paleo-geographic and paleo-climate environments were highly favorable for the extensive development of ARS and coal seams [2⇓-4,32]. The Carboniferous-Permian is a vital Al- and coal-forming stage in the Ordos Basin, and even North China, so CRS and ARS spatially accompany each other [4].
Since the late Benxi Period, the Ordos Basin gradually shifted from slow subsidence to regionally structural stability [12]. The paralic sedimentary environment along the paleo-land margin was associated with peneplain terrain, warm and humid climate, and extensive coverage of flourishing plants. The ever-flourishing plants provided abundant organic matter and also suppressed the denudation of basement rock, which reduced the supply of detrital provenance, prevented the loss of meteoric water and formed a coal-accumulating peat bog environment. Inorganic deposition transitioned toward organic deposition, and plant remains replaced terrestrial debris. Accordingly, CRS was deposited above the ARS.
4.1.2. Eustasy
The relative content and relevant parameters of tricyclic terpane (TT) compounds C19-23 can be used to clarify the sedimentary environment and relative changes in the sea level [5⇓-7,33 -34]. A shallow-water sedimentary environment is favorable for the generation and distribution of TT compounds with low carbon numbers, and correspondingly the relative content ranks as C19TT > C20TT > C21TT > C22TT > C23 TT. The case of a deepwater environment is the opposite. Given the above, the higher ratios of C23TT/C21TT and C21TT/C20TT represent deeper water and higher salinity.
Here, a case study of Well L18 at the karst slope zone of the central paleo-uplift in the southwestern Ordos Basin is presented to illustrate how the sea level change controls the formation of the coal-Al-Fe three-segmented stratigraphic structure. The initial deposition of the ARS took place when transgression was going on. Both the ratios of C21TT/C20TT and C23TT/C21TT were large, the water was deep and the salinity was large. The sedimentary environment was reductive, and pH higher than 8. SiO2 dissolution was suppressed, and Fe and Al were barely soluble. Fe- and Al-containing silicate minerals were first precipitated, so Fe- and Al-containing bauxitic mudstone, hematite, and pyrite often occur in the basal ARS. As regression started, the ratios of C21TT/C20TT and C23TT/C21TT started to decline, and so did the salinity and depth of the water. During the initial regression, the pH of the sedimentary environment was 5-8. In that environment, the solubility of SiO2 was 10-20 times that of Fe, and the dissolved SiO2 peaked. Muddy elements were lost rapidly, and the dissolution rate of Fe also climbed up. However, Al remained stable and accumulated in solutions, which gradually led to bauxite and muddy bauxite in the mid-upper part of the ARS [4]. During the late regression, the sedimentary environment became relatively acidic (pH less than 5), due to the decay of organic matter. Dissolved Fe and Al considerably exceeded dissolved SiO2, and bauxite was converted toward silicate rock. Consequently, carbonaceous mudstone and coals occurred at the top of the ARS (Figs. 7 and 8 ).
Fig. 7. Vertical distribution of aluminum oxide, silicon dioxide and tricyclic terpane in the Taiyuan Formation in Well L58. |
Fig. 8. Schematic diagram of formation mechanisms of the representative three-segmented ARS structure (refer to |
The coal-Al-Fe three-segmented stratigraphic structure was developed in a transgression-regression cycle, in which bauxite was deposited when regression took place. As the first regression semi-cycle ended, the sea level dropped to the lowest and then started to rise, and the second transgression semi-cycle started. The ARS occurring in the second cycle developed in the same way as in the first semi-cycle. Due to the Carboniferous-Permian multi-stage eustasy, the coal-Al-Fe three-segmented stratigraphic structures developing in different stages stack over each other vertically [35].
Based on the above analysis, it is believed that the sedimentary environment that facilitates intensive desilication and de-iron acid-alkaline transition and presence of abundant organic matters should be a paralic- shore-shallow sea facies. It is the frequent changes in the sea level that cause the complex and variable physicochemical environment in the study area, and the more. frequent and stronger such changes are, the more developed the bauxite is [32]. The above demonstrates that although the bauxite and its underlying Fe- and Al-containing silicate minerals and overlying CRS are spatially associated with each other, their development scales and degrees are negatively correlated with those of each other [2].
4.2. Under-source natural gas accumulation model of the ARS
In the Ordos Basin, the natural gases in the Carboniferous-Permian ARS, Ordovician weathering crust and Upper Paleozoic tight sandstone share a consistent source-mainly from the Carboniferous-Permian CRS source rock [36⇓⇓⇓-40]. The Upper Paleozoic CRS source rock has a large distribution area, abundant organic matter, and high thermal maturity. It has a high hydrocarbon generation capability and can provide sufficient gas for the ARS gas reservoir [41⇓-43]. The ARS reservoir directly contacts the Upper Paleozoic CRS source rock, forming a good source-reservoir assemblage characterized by the upper source rock and lower reservoir rock and facilitating near-source gas accumulation.
The sealing conditions of the trap decide that the ARS gas reservoir is a lithological gas reservoir. On the one hand, the ARS reservoir is confined by surrounding mudstone which is tight and serves as a lateral barrier. On the other hand, the overlying CRS functions as source rocks to supply gas and also as caprocks to prevent gas escape (Fig. 9 ).
Fig. 9. Natural gas accumulation model of the ARS in the Longdong area of the southwestern Ordos Basin (see |
5. Exploration suggestions for natural gas in the Carboniferous-Permian weathering crust ARS in North China
Due to the special Late Paleozoic paleo-geographical setting of the North China Craton combined with the overall effects of the tectonic movements such as Caledonian and Hercynian orogeny and paleo-climate, the ARS in the different areas have highly similar diagenetic and reservoir-forming characteristics and natural gas accumulation patterns [4,44].
Exploration breakthrough in the ARS gas reservoir is expected to provide a reference for natural gas exploration of the Carboniferous-Permian weathering crust ARS across North China. The above analysis of the factors controlling the ARS effective reservoir in the Ordos Basin shows that the geology for the ARS reservoir development in North China has the following similarities:
(1) Before the deposition of the ARS, the North China Craton was slowly uplifted and became a land during the Middle Ordovician. It presented positive and negative structures due to nonuniform uplifting, and the degrees of denudation varied in different positions [26]. The paleo-land, or paleo-island and its periphery were denudation zones. The Lower Paleozoic carbonate rock experienced long-term and large-area intensive weathering denudation, and elemental separation and concentration occurred after alteration and decomposition, which provided a solid material basis for the ARS development in North China.
(2) In the sedimentary slope break belt between denudation and low-lying zones, the paleo-karst landform units were platform margin-slope zones, inside which the sedimentary environment in the underflow craters or terraces at relatively lower positions covered semi-closed and closed lagoons and tidal flats. They were close to the denudation provenance and had sufficient Al supply, which contributed to the distribution of high-quality bauxite (Fig. 10 ).
Fig. 10. Carboniferous-Permian coal accumulation centers and predicted favorable ARS gas reservoir zones in the North China Craton. |
(3) The platform margin-slope zones in the paleo-land were generally above the water table. The bauxite developed in the underflow craters or terraces and featured by pisolitic and oolitic granular textures was prone to supergene meteoric water leaching and drainage channel dissolution to create plentiful pores and form honeycomb ARS reservoirs. Hence, the ARS reservoirs in North China are mostly developed in shallow-water area at the edge of ancient land or ancient island in the form of discrete clusters, rather than the deepwater zones far away from the paleo-land [30].
The Carboniferous-Permian CRS source rocks in the North China Craton are characterized by large-area distribution and wide hydrocarbon generation [45]. Besides the Ordos Basin, the weathering crust ARS in the other areas in Northern China also possesses gas accumulation potential. (1) The climate and paleo-geographic setting required by the formation of the ARS were just those required by the formation of coal seams. The sea level in the North China Craton facilitated multiple acid-alkaline transitions and abundant organic matters in the paleo-land or at the margin of the paleo-island [35], and the stratigraphic structure of the “upper coal, middle Al, and lower Fe” was formed during the desilication and de-iron process. The CRS and ARS are spatially associated with each other and constitute the favorable source-reservoir assemblage with the featured upper source and lower reservoir. (2) The coal accumulation center of the Carboniferous-Permian CRS source rocks in the North China Craton lies on the eastern side of the Qinling paleo-land and southern side of the Inner Mongolia paleo-land (Fig. 10 ). It is several tens to hundreds of meters thick, and the hydrocarbon generation conditions are similar to those in the Ordos Basin, representing great hydrocarbon generation potential [43].
The slope break belts between denudation and low-lying zones in the east of the Qinling paleo-land, south of the Inner Mongolia paleo-land, and west of the Lüliang and Wutai paleo-islands have paleo-karst landform units represented by platform margin-slope zones, inside which high-quality ARS reservoirs were developed in the underflow craters or terraces. Moreover, near the major Carboniferous-Permian coal accumulation centers, these ARS reservoirs [44⇓⇓-47] were finally developed into excellent ARS lithological gas reservoirs. The clustered ARS gas reservoirs are favorable targets for the exploration of sub-coal ARS gas reservoirs in Northern China. ARS gas reservoir, as a novel type of unconventional gas reservoirs, breaks through the boundary between conventional hydrocarbon accumulation and metal accumulation and will be a new field of future natural gas exploration.
6. Conclusions
As per the content of aluminum (Al) hydroxides, iron (Fe)-containing minerals and clay minerals, the aluminous rock series (ARS) can be divided into seven types via a triple-end member classification approach, namely bauxitic mudstone, muddy bauxite, muddy ferruginous rock, bauxite, ferruginous bauxite, bauxitic ferrolite and ferrolite. Bauxitic mudstone, bauxite and muddy bauxite are dominant rock types of the ARS in the Ordos Basin. The effective ARS reservoir in the Ordos Basin is mainly honeycomb-like bauxite with porous residual pisolitic-oolitic and detrital textures. It usually appears in the middle of the ARS, with reservoir space featured by dissolved pores, and high porosity and permeability.
The formation model of the ARS effective reservoir is summarized below: Planation provides a material basis for the ARS development, paleo-karst landform controls the ARS development, and land surface leaching improves the storage capability. The bauxite with pisolitic-oolitic and detrital textures typically occurs in the relatively closed underflow crater or terrace at the karst platform margin or in the karst slope zone. The dissolution of karst drainage channels facilitates the bead-string distribution of the ARS reservoir along the circumference of paleo-uplifts.
The coal-Al-Fe three-segmented stratigraphic structure is under the joint control of warm and humid climate and sea level change. The CRS source rocks and the ARS are spatially associated with each other and form a favorable source-reservoir assemblage featuring an upper source and lower reservoir. The CRS source rocks have large-area distribution, high thermal maturity and strong hydrocarbon generation capability. Natural gas accumulates in the ARS reservoirs below coal seams, which represents an under-source hydrocarbon accumulation.
Exploration breakthrough in the ARS gas reservoir in the Ordos Basin can effectively guide natural gas exploration of the Carboniferous-Permian weathering crust ARS in Northern China. The sedimentary slope break belts in the east of the Qinling paleo-land, south of the Inner Mongolia paleo-land, and west of the Lüliang and Wutai paleo-lands in the North China Craton have both high-quality ARS reservoirs with bead-string distribution and major Carboniferous-Permian coal accumulation centers. With good gas accumulation conditions, the clustered ARS gas reservoirs are expected to be favorable plays for sub-coal (below coal seams) ARS natural gas reservoirs in Northern China.