Genesis of Analcite in Black Shales and Its Indication for Hydrocarbon Enrichment—A Case Study of the Permian Pingdiquan Formation in the Junggar Basin, Xinjiang, China

: This study investigates the genesis of analcite in black shale from continental lakes and its implications for hydrocarbon enrichment, with a case study of the Permian Pingdiquan Formation in the Junggar Basin, Xinjiang, China. As an alkaline mineral, analcite is extensively developed in China’s lacustrine black shale hydrocarbon source rocks and is linked to hydrocarbon distribution. However, the mechanisms of its formation and its impact on hydrocarbon generation and accumulation remain insu ﬃ ciently understood. This paper employs a multi-analytical approach, including petrological observations, geochemical analysis, and X-ray di ﬀ raction, to characterize analcite and its association with hydrocarbon source rocks. The study identi ﬁ es a hydrothermal sedimentary origin for analcite, suggesting that it forms under conditions of alkaline lake water and volcanic activity, which are conducive to organic ma tt er enrichment. The analcite content in the studied samples exhibits a signi ﬁ cant variation, with higher contents associated with hydrocarbon accumulation zones, suggesting its role in hydrocarbon generation and accumulation. This paper reports that anal-cite-bearing rocks display characteristics of high-quality reservoirs, enhancing the permeability and porosity of the rock, which is essential for hydrocarbon storage and migration. In conclusion, this paper underscores the importance of analcite as a key mineral indicator for hydrocarbon potential in black shale formations and provides valuable insights for further geological and hydrocarbon exploration in similar se tt ings.


Introduction
Black shale is a geological formation of profound significance, renowned for its high organic matter content.Through the crucible of geological processes, this organic matter metamorphoses into valuable oil and natural gas, establishing black shale as a pivotal unconventional hydrocarbon resource.Regions such as North America have witnessed the triumphant commercial exploitation of black shale, leading to a marked augmentation of local energy reserves and a surge in energy exportation [1][2][3].
Furthermore, black shale is a repository for critical metal elements.It can be enriched with elements like selenium (Se), tellurium (Te), rhenium (Re), cadmium (Cd), thallium (Tl), and various other dispersed metals, alongside platinum group elements (PGE) and cobalt (Co).These elements hold immense value in contemporary industrial and technological sectors, underscoring the imperative of black shale exploration and development for securing these essential resources [3].
However, the exploration and assessment of hydrocarbons in black shale present considerable challenges due to its fine-grained mineral composition, pronounced rock heterogeneity, low gas-to-oil ratios, and the viscous nature of the oil it contains [4,5].These difficulties are compounded by the scarcity of fundamental geological data on black shale, highlighting the urgent need for innovative theories and technologies.
In China's lacustrine black shale, analcite is a prevalent mineral, exhibiting a synergistic relationship with hydrocarbons [6][7][8][9][10][11].While previous research has predominantly concentrated on the reservoir characteristics of analcite [12][13][14], the connection between analcite and hydrocarbon source rocks has been largely overlooked, and there is a scarcity of research on the analcite's genesis and its role in the enrichment of organic matter.This paper presents a comprehensive case study of the Permian Pingdiquan Formation in the Junggar Basin, a region known for its rich hydrocarbon potential.The abundant analcite present within the geological strata serves as an excellent vector for studying the correlation between the two.This study sheds new light on the dual potential of analcite, suggesting that it is not only a candidate for a superior reservoir rock but also intricately linked to the genesis of high-quality hydrocarbon source rocks.This underscores the critical role of analcite in the enrichment of hydrocarbons.This article attempts to contribute to the knowledge system of unconventional oil and gas resources by conducting a detailed study of analcite to explore the relationship between analcite and the enrichment of oil and gas in black shale.

Geological Background
The study area is situated in the Huoshao Mountain Oilfield in the eastern part of the Junggar Basin, specifically along the northern edge of the Bogda Mountain (Figure 1).During the Late Devonian to Early Carboniferous epochs, the region underwent transformative geological phases, including ocean basin subduction and the intriguing phenomenon of continental-continental weak collision (soft collision) [15].In the subsequent phases, the area was further shaped by post-orogenic extension, which sculpted a multifaceted paleogeographic landscape.This landscape was marked by the persistence of a residual sea basin and the intriguing juxtaposition of marine and continental basins [15].During the period from the Late Carboniferous to the Permian (300-250 Ma), the region experienced extremely intense tectonic activity.The development of intracontinental rifts and faulted basins was widespread, accompanied by extensive volcanic eruptions and profound magmatic activities, including alkaline granitic and mafic magmas (dykes).These dynamics synergistically governed the processes of basin formation, diagenesis, and mineralization in the area [16].Superimposed upon the pre-Carboniferous folded basement, the region boasts a comprehensive sedimentary sequence that evolved from the Carboniferous through to the Quaternary periods.This sequence is neatly stratified, with layers accumulating from the foundational strata to the uppermost deposits.This geological narrative not only provides a snapshot of the region's past but also offers critical insights into its potential for hydrocarbon resources and mineral wealth.
This study delves into the intricacies of the Permian Pingdiquan Formation, which exhibits a spatial variation in its depositional profile within the study area.Characterized by substantial thickness in the northern regions and comparatively thinner strata to the south, the formation spans a thickness gradient from 440 to 1240 m [17].The lithological composition of the Pingdiquan Formation is dominated by fine-grained tuff layers, interspersed with tuffaceous shale, tuffaceous micritic dolomite, and occasional interlayers of micritic dolomite.Through the research conducted by our team, it has been discovered that this assemblage of lithologies is notable for its scarcity of clay minerals, underscoring the formation's distinctiveness as a lacustrine fine-grained sedimentary rock.The rock type is emblematic of a hydrocarbon source rock, intricately shaped by the influence of volcanic activity.This volcanic imprint is not only a testament to the geological forces at play during the formation's genesis but also suggests the potential for rich hydrocarbon endowment within the sedimentary basin.

Methodology
The samples mainly come from the Pingdiquan Formation drill cores in the Huoshaoshan area of the Xinjiang Oilfield.Key wells H262, H263, and H2452 exhibit well-developed Pingdiquan Formation strata, which have not been affected by tectonic activity leading to loss of strata.Key wells SD1, SD2, and HQ1 have distinct analcite rock layers, making them special compared to other wells.In the rock and mineral testing section, Sections of the cores from the aforementioned key wells are ground and observed at approximately 0.5 m intervals to clarify the types of rocks in the Pingdiquan Formation.Electron probe sections are prepared for layers with well-developed laminations to observe the characteristics of analcite.The electron probe tests were completed in the electron probe laboratory of the Xi'an Geological Survey Center of the China Geological Survey.The instrument used was a JEOL JXA-8230 from Japan (JEOL, Tokyo, Japan).The experiment voltage was 15 kV, the test current was 1′10-8A, and the beam spot diameter was 1-5 mm.The correction method was ZAF, following the standard GB/T 115074-2008 [19].Randomly selected rock types from the key wells are subjected to X-ray diffraction analysis.They were sent to the Laboratory Testing Center of the Xi'an Geological Survey Center for X-ray diffraction (XRD) analysis.The instrument model used was D/max-2500 (Rigaku, Tokyo, Japan), with parameters set at 40 kV and 200 mA, a scanning step size of 0.020°, a speed of 10°/min, and a range of 5-56°.The DS and SS were set to 1°, and the RS was 0.15 mm.Rare Earth Element Analysis Section: Based on the X-ray diffraction results, rock samples with higher contents of analcite and dolomite are selected for rare earth element analysis.They are conducted at the State Key Laboratory of Continental Dynamics, Northwest University, using a solution method.The instrument model used was an ELAN6100DRC (Perkin Elmer, Waltham, MA, USA) inductively coupled plasma mass spectrometer (ICP-MS).Oil and Gas Evaluation Section: Thin sections from the key wells are observed using a fluorescence polarizing microscope in the Department of Geology, Northwest University.The instrument model used was an Axio Scope.A1 (ZEISS, Oberkochen, Germany).To explore the vertical distribution of analcite and its correlation with oil and gas, field core inspections are combined with well logging data from the Xinjiang Oilfield, Karamay, China.Since well H262 has a well-developed Pingdiquan Formation overall and sufficient oil and gas data, a single-well analysis of mineralogy, phillipsite content, and oil and gas enrichment is conducted for well H262.

Petrological Characteristics
Previous researchers believed that the lithology of the Permian Pingdiquan Formation in the study area mainly consisted of dark mudstone [20], siltstone, and carbonate rocks, with occasional intercalations of tuffaceous layers [20].However, our research team discovered that what was traditionally considered dark mudstone should actually be classified as pyroclastic rock.The crystal fragments in these rocks often have torn, chickenbone-like edges and contain distinctive magmatic minerals.The specific rock types and characteristics are as follows: 4.1.1.Core Descriptions Tuff: It is primarily characterized by horizontal laminations, with the thickness of the laminations ranging between 0.3 and 1 mm.It often appears interbedded with sedimentary tuff and tuffaceous dolomite, and it is commonly associated with lens-shaped structures and other synsedimentary deformation structures (Figure 2a).Sedimentary Tuff: It exhibits a dark gray to black color, which correlates with its high organic matter content.It features numerous synsedimentary deformation structures such as convolute bedding and slump structures, as well as bioturbation structures (Figure 2b).These indicate turbulent water conditions during deposition, characterized by intense sedimentation and significant biological activity.The sedimentary tuff typically displays an alternation of dark layers rich in organic matter and light layers dominated by feldspathic materials.
Tuffaceous Dolomite: It appears in black and dark gray colors, with synsedimentary structures being equally common such as convolute bedding and lens-shaped body structures (Figure 2c).Some tuffaceous dolomites exhibit exploded carbonate particles and oolitic structures (Figure 2d,e).Fish scale fossils are also commonly found (Figure 2f).Pyrite is well-developed and often occurs in the form of lenses (Figure 2g).Dolomite: It is mainly gray-black in color, with a massive structure.Microcrystalline dolomites also frequently appear in thin bedded forms, interbedded with sedimentary tuffs and tuffaceous dolomites, with laminations varying in thickness from 0.3 to 1 mm (Figure 2h).
Analcite Rock (Analcite Content > 25%): Traditionally, analcite-bearing tuff is considered a type of alkaline magmatic rock.However, due to the well-developed analcite in the study area, with the highest content reaching up to 70%, this paper classifies rocks with more than 25% analcite content as a distinct rock type, namely analcite rock.Core observations reveal one type of analcite rock that is overall brown-red, more porous, with a blocky structure (Figure 2i).Another type is gray-white, dense, and hard, often interbedded with microcrystalline dolomite, tuff, etc. (Figure 2j); or locally enriched in the rock, forming gray-white clumps or lens-shaped thin layers (Figure 2k,l).
In addition to the main rock types mentioned above, the Pingdiquan Formation in the study area also includes localized limestones and terrigenous clastic rocks.

Microscopic Observations
The mineral composition in the tuff is predominantly characterized by felsic fragments, with plagioclase, orthoclase, and a small amount of sanidine being the main feldspars.The grain size ranges from approximately 0.01 to 0.1 mm, with some grains exhibiting bay-like and fragmented edges, indicative of a typical tuffaceous texture.Additionally, a significant amount of vitroclastic tuff is observed, where the shards and crystal fragments are irregularly shaped, such as torn and flame-like forms, and in some rocks, they are cemented in a "matrix" manner (Figure 3a,b).
In the tuffaceous dolomite, pyrite is predominantly present in dendritic and globular forms, exhibiting characteristics of biogenic pyritization (Figure 3c,d).Within the micritic dolomite, magmatic hydrothermal calcite veins are commonly developed, and sparry calcite particles from explosive eruptions can be observed [21] (Figure 3c,d).Tuffaceous ooidal dolomite and bioclastic dolomite with special structures often contain analcime.In the ooidal dolomite, analcime appears as cement and as idiomorphic centers within the ooids (Figure 3e,f).The ooids range in size from 3 mm to 1.5 mm, showing a slight positive grading (Figure 2e).
The analcime in thin sections is colorless and transparent, with a distinct negative relief and extinction, and the ideal crystal outline of analcime is hexagonal to octagonal.The types of analcime in the study area are diverse, with significant variations in occurrence and morphology.Therefore, this paper classifies the idiomorphic analcime into three types based on its crystal shape observed under the microscope: euhedral, subhedral, and xenomorphic analcime.On this basis, the euhedral analcime is further divided into granular, patchy, layered, and veined analcime based on its morphology, occurrence, and degree of idiomorphism.pattern, as shown in the backscattered image; (j) H262-105, 1498.3 m, the orthogonal polarized light image of analcite grains in the tuffaceous dolomite, Porphyritic-like, where originally euhedral analcite has been largely dissolved and filled with calcite; (k) SD2-3, 1656.95 m, analcite rock, with clear boundaries of analcite grains and a rough internal texture; (l) is the orthogonal polarized light image of (k); (m) is a local magnified image of (k-l), a backscattered image, showing analcite grains containing quartz and alkali feldspar; (n) H263-42, 1598.8 m, analcite rock, with well-formed analcite grains that have a turbid surface, underlain by calcite; (o) is the orthogonal polarized light image of (n); (p) is the contact part between the analcite rock and the tuffaceous dolomite in (m-o), where the analcite crystals are well-formed and cemented by calcite, surrounded by a mixture of fine-to microcrystalline albite, K-feldspar, and ankerite; (q) HQ1-18, 2221.4 m, analcite grains form semi-euhedral blocky aggregate; (r) is the orthogonal polarized light image of (q); (s) a locally magnified backscattered image of (q-r), containing fine grains of alkali feldspar inside, with analcite cemented by calcite; (t) is an enlarged internal image of the analcite grains in (s); it shows feldspar and quart inside; (u) H262-105, 1498.3 m, analcite fills within the phenocrysts, with the original phenocryst grain morphology still visible; (v) is the orthogonal polarized light image of (u), where analcitedolomite phenocrysts are enveloped by microcrystalline dolomite; (w) H2452-68, 1611.77m, analcite occurs as xenomorphic interstitial material within phenocrysts; (x) is an enlarged backscattered image of the phenocrystic grains in (q), where analcite and dolomite fill the phenocrysts.
Euhedral Analcime: Euhedral analcime refers to analcime grains with well-formed crystals and distinct boundaries, typically exhibiting a dodecahedral shape.In the study area, euhedral analcime can be further categorized based on its occurrence, structure, and relationship with surrounding minerals into dispersed granular, filling granular, and patchy types.
Dispersed granular analcime grains generally exhibit a good degree of euhedralism and are commonly found in tuffs and tuffaceous sediments.These types of analcime grains are often uniformly dispersed in a matrix of tuff or tuffaceous sediment, with grain sizes similar to those of the feldspathic crystals, suggesting that they may have formed during the same depositional or early diagenetic stages as the crystal fragments (Figure 3g,h).
Veined analcime is characterized by the coexistence of analcime grains with calcite, filling veins or pores within the rock, serving a cementing and supporting role.The grain size of this form of analcime typically ranges from 0.05 to 0.2 mm, with clean and smooth surfaces, well-formed crystals, and a susceptibility to dissolution (Figure 3i).
Patchy analcime refers to analcime grains that are significantly larger in diameter than the surrounding grains (usually micritic calcite), displaying a porphyritic texture similar to that of igneous rocks.Patchy analcime is typically found in micritic dolomite, where it is sparsely distributed within the "groundmass" of the micritic dolomite, with clean surfaces and well-formed crystals.Based on its occurrence, this type of analcime may have precipitated concurrently with the micritic dolomite (Figure 3j).
Another type of euhedral analcime grain is often found in rocks where analcime is abundant (mostly analcime-rich rocks), often exhibiting a layered distribution.The grain size of this analcime varies, ranging from 0.1 to 0.4 mm, with good euhedralism and smooth boundaries, although the surface is often rough, and some grains may develop cracks and contain impurities, with the outer rim being relatively clean.The surface of the analcime grains has micropores, and they often contain thin flake-like particles of minerals such as alkali feldspar and quartz.Common base minerals associated with analcime include montmorillonite, feldspar, quartz, and calcite (Figure 3k-p).
Subhedral Analcime: Subhedral analcime grains do not have as distinct boundaries as euhedral analcime but still exhibit the overall morphology of analcime.This type of analcime is also commonly found in a layered distribution, and most of the analcime in grey analcime-rich rocks is predominantly subhedral (Figure 3q-s).The surface of subhedral analcime is generally rough and dirty, and it often contains minerals such as quartz and feldspar (Figure 3t).
Xenomorphic Analcime: Xenomorphic analcime refers to analcime without the characteristic crystal form.Xenomorphic analcime often appears in ooidal dolomite, porphyritic tuffaceous sediments, and tuffaceous dolomite.It typically acts as a matrix component to cement ooidal particles or appears as angular fragments enclosed in the centers of ooids or fills the interstices between larger clasts.This type of analcite lacks an idiomorphic structure (Figure 3e,f,u-x).This may be due to the original minerals undergoing analcite alteration at a later stage.The original minerals are likely to be alkali feldspar, aegirine, and other alkaline minerals.
The analcite is commonly associated with minerals such as quartz, dolomite, calcite, K-feldspar, and alkali feldspar.Analcite grains often contain feldspar and quartz grains (Figure 3m,t), indicating that the formation time of analcite is synchronous with or later than the formation of feldspar, quartz, and other volcaniclastic materials Pyrite is also one of the accompanying minerals, and it often presents special morphologies such as berrylike and needle-like shapes, with obvious biogenic morphologies, growing around the analcite or in a zonal pattern with analcite, iron-manganese calcite, and iron-bearing dolomite (Figure 4a,b).Analcite is also commonly associated with carbonate minerals, and the carbonates often exhibit abnormally high interference colors, typically being magnesium-, iron-, and manganese-rich carbonate minerals, such as magnesite, magnesium-rich calcite, manganese-rich calcite, and ankerite (Figure 4b-d).In addition, intergrowth of analcite with plagioclase can also be observed (Figure 4e).In some samples, analcite is also found to coexist with alkali feldspar, with the alkali feldspar exhibiting a "phenocrystic" appearance on a microcrystalline dolomite base, which may be due to turbidite deposition (Figure 4f).

Distribution of Analcite
According to the X-ray diffraction results (Table 1), the content of analcite in different rock types is as follows: Tuff: The distribution of analcite in tuff is notably uneven, with an average content of only about 3.9%.Out of 21 tuff samples, analcite was detected in only 8, and its content varied significantly among these samples, ranging from a high of 23.1% to a low of 1.3%.
Sedimentary Tuff: Analcite is relatively well-developed, with an average content of about 7%.Among 22 sedimentary tuff samples, only two samples were devoid of analcite, with its content varying from 1.4% to 24.3%.
Tuffaceous Dolomite: Analcite is less developed, with an average content of about 1.4%, and out of four samples, two did not detect Analcite.However, tuffaceous oolitic dolomite with special structures and bioclastic dolomite often contain analcite, with average analcite contents of 10.2% and 5.9%, respectively.Dolomite: Analcite content is minimal, at only 0.7%.Analcite Rock: The average analcite content in four analcite rock samples is 42.2%.In the brownish-red analcite rock, the analcite content reaches as high as 70.7% (Table 2).

Electron Probe Results for Analcite
Probe analysis of analcite samples from the study area (Table 3) reveals that the main composition elements of analcite include: SiO2 content ranges from 56.37% to 62.04%, with an average of 59.63%, higher than the standard value of 54.58%.Al2O3 content ranges from 18.72% to 21.88%, with an average of 20.08%, slightly lower than the standard value of 23.16%.Na2O content ranges from 5.45% to 12.8%, with an average of 8.89%, lower than the standard value of 13.05%.Both Na and Si, as well as Al elements, exhibit significant variations in content, and the Si/Al ratio shows a range from 2.22 in low-silica analcite to 2.83 in high-silica analcite.

Rare Earth Element Results for Shale Containing Analcite
The range of total rare earth element (REE) content in analcite-bearing rocks is quite broad, ranging from 96.79 to 435.20 ppm(Table 4).Except for the sample SD2-4, which has a unique content, the majority of the samples have a total REE content slightly lower than the PAAS standard values.The LaN/YbN and CeN/YbN ratios are in the ranges of 0.41 to 0.8 and 0.45 to 0.85 ppm, respectively.The LREE/HREE ratio is between 5.51 and 7.72, indicating a relative enrichment of light rare earth elements (LREE) compared to heavy rare earth elements (HREE).The δEuN values range from 0.31 to 1.06, and the REE distribution patterns show a clear bimodal trend through Eu anomalies (Figure 6).Analcite rocks and tuffs containing analcite exhibit a significant negative Eu anomaly, whereas the rocks mixed with lake sediments such as analcite-bearing sedimentary tuffs and analcite-bearing dolomites display a slight positive Eu anomaly.The δCeN values are between 3.29 and 3.84, demonstrating a pronounced positive anomaly.

Genesis of Analcite
Luhr and Kyser have categorized analcite into a quintet of types: magmatic analcite (P-type or I-type), metamorphic analcite (X-type or L-type), hydrothermal analcite (Htype), metamorphic analcite (M-type), and sedimentary analcite (S-type) [23].Moreover, a distinctive variant known as hydrothermal sedimentary analcite has been identified, which is intricately linked to the ejection of hot springs in the depths of lacustrine rift basins [6,[24][25][26][27].Given the distinctive tectonic backdrop and sedimentary milieu of the study area, the genesis of analcite here is likely influenced by a confluence of factors:

Magmatic and Hydrothermal Activities
During the Permian period in the study area, there was an intracontinental rift evolution phase in the late Paleozoic era.The Pingdiquan Formation encapsulates a lacustrine sedimentary sequence that was intermittently laid down amidst two significant volcanic episodes: the Karagang and Tiaohu periods of the early and mid-Permian, respectively [28,29].The extensive development of tuffaceous sediments indicates a tranquil lacustrine depositional setting on the surface, paradoxically juxtaposed with an underlying high geothermal activity.
In our study, we encountered a multitude of minerals that serve as indicators of intense hydrothermal activity.These include carbonate hydrothermal veins, exploded calcite particles, and pyrite.Moreover, the proliferation of syn-sedimentary deformational structures within the rock formations vividly attests to the frequent magmatic-hydrothermal activities in the research area.This geological narrative is further enriched by the presence of these minerals and structures, painting a dynamic picture of the geological past shaped by the interplay of molten rock and hot fluids.
Additionally, in China, a unique type of analcite-bearing phonolite has been discovered only in the Dangxiong area of Tibet [30].The hand specimens exhibit a flesh-red color with feldspar phenocrysts embedded within.Similarly, in our study area, analcite-bearing rocks have been identified, featuring sanidine crystals or intergrown with analcite, indicating that during the evolution of alkaline magma, sanidine may have crystallized before analcite or they may have crystallized concurrently.
Furthermore, the electron probe results of the xenomorphic analcite with replacement residual structures observed in the study area reveal a lower Si/Al ratio (around 2.2) compared to other types of analcite, which is more akin to the magmatic origin.This suggests that the analcite may have formed through late-stage hydrothermal alteration.
Zhang et al. [21], through their research, concluded that the tuff in the study area likely originates from alkaline intermediate-acidic magma.The elemental composition of the analcite, particularly its high silicon content, aligns with the intermediate-acidic tuff present in the study area.The substantial variation in the content of Na, Si, Al elements, and the Si/Al ratio in the analcite suggests significant differences in the material sources and the temperature-pressure conditions during the formation of the analcite.Comparative studies have revealed that xenomorphic zeolite with a filling pattern often exhibits a lower Si/Al ratio and a lower SiO2 content, whereas idiomorphic to subhedral analcite tends to have a higher Si/Al ratio and a higher SiO2 content.This indicates that the filling pattern analcite may have formed at higher temperatures, potentially directly from magmatic-hydrothermal fluids, while the analcite within analcite rocks may have formed at lower temperatures, possibly as a result of the mixing of acidic hydrothermal fluids with lake water.
The relationship between analcite and associated minerals indicates a close relationship between analcite and biological and hydrothermal activities, and it also shows a certain sedimentary sequence, that is, iron-bearing dolomite/magnesite → albite (alkali feldspar) → quartz → analcite → organic matter → iron-manganese-bearing calcite.

Sedimentary Processes
Depositional analcite, as an authigenic mineral, can emerge in a variety of geological settings [31,32]: (1) saline-alkali lakes; (2) saline-alkali soils; (3) marine sediments; (4) percolating water deposits under open hydrologic systems; (5) burial diagenesis; and (6) hydrothermal alteration.The study area, characterized as a relatively enclosed brackish to saline, semi-deep to deep freshwater lake basin [33,34], is accompanied by intense magmatic-hydrothermal activities.The genesis of analcite in this region can be categorized based on the stages of sedimentation and diagenesis into the following types:
Li et al. [27] discovered multiple layers of white banded analcite rocks within the thick dark mudstones of the Permian Lucaogou Formation (P2l) in the Santanghu Basin, Xinjiang, which are similar to the bedded analcite rocks discussed in this paper.She posits that these represent a hydrothermal sedimentary rock assemblage associated with the activity of hot springs on the floor of a continental rift basin lake.
By comparing and analyzing the compositional content of analcite in the study area with that in hydrothermal sedimentary rocks from the Lucaogou Formation of the Santanghu Basin in Xinjiang, the Xiagou Formation of the Qingxi Depression in the Jiuxi Basin in Gansu, and the analcite phonolite from the Dangxiong area of Tibet [27,30,44,46] (Figure 7), it can be observed that the analcite in the study area generally shares a consistent correlation with the analcite in the other two hydrothermal sedimentary rocks.The content of alkali metal ions falls between that of the analcite in the Lucaogou Formation of the Santanghu Basin and the Xiagou Formation of the Qingxi Depression, and is closer to that of the analcite in the Dangxiong area.A notable point is that the analcite in the study area has a higher FeO content compared to the analcite from the other two regions, suggesting a greater influence from magmatic-hydrothermal activities.Furthermore, through plotting, we have found that the Si/Al ratio in the study area exhibits a strong negative correlation with the content of alkali metal ions Na, K, and Ca.A lower silicon content indicates a higher content of alkali metal ions, which also implies that the analcite may originate from an environment richer in alkali metal ions.
By comparing the trace element results of analcite-bearing rock samples from the study area with those from the analcite rocks of the Lucaogou Formation in the Permian System of the Santanghu Basin in Xinjiang [27], and the micritic sodium-rich ferro-dolomite of the Lower Gully Formation in the Cretaceous of the Qingxi Depression in the Jiuquan Basin on the Qinghai-Tibet Plateau [44,46] (all samples normalized to PAAS), the following observations can be made: The total rare earth element content in analcite-bearing rocks in the study area falls between those of analcite-bearing rocks in the Permian Lucaogou Formation of Santanghu Basin, Xinjiang, and the dolomite of the Xiagou Formation in Qingxi Depression, Qinghai-Tibet Plateau.The REEs in the study area generally show a slight left-leaning trend, whereas the REEs in analcite-bearing rocks from the Lucaogou Formation of Santanghu Basin and the dolomite from the Xiagou Formation in the Qinghai-Tibet Plateau show a more gentle left-leaning and right-leaning trend.It indicates that the tendency of REE distribution does not reflect the genetic characteristics of the analcite rocks well.The variation range of LaN/YbN and CeN/YbN values in the samples from the study area is smaller than that in the Santanghu Basin and Jiuquan Basin, but the degree of fractionation between light and heavy REEs is significantly less pronounced than in the other two regions.Figure 6 illustrates a comparison of rare earth elements among the three locations.The δEuN of the samples from the study area exhibits a clear bimodality, a feature not observed in the other two regions.The dolomites from all three areas show a slight positive Eu anomaly, which is considered a typical characteristic of hydrothermal sedimentary rocks [39].The negative Eu anomaly in the tuff and analcite rocks from the study area reflects a magmatic stage that has undergone plagioclase crystallization separation, possibly related to acidic magma or silica-rich post-magmatic fluids.The δCeN of the samples from the study area shows a significant positive anomaly, while the other two regions show no significant depletion or enrichment, which may reflect differences in sedimentary and diagenetic environments.During the sedimentation and diagenesis processes in the study area, the water body was relatively rich in Mn, reducing Ce 4+ into the water body, resulting in a positive anomaly.The presence of a large amount of high-manganese calcite and dolomite in the study area confirms this sedimentary environment, which also serves as evidence that the lake bottom was a hot brine environment with strong reducing and alkaline conditions [47,48]. Figure 8 shows that the ∑REE is well correlated with Ce/δCeN and that the Ce/δCeN is well correlated with Eu/δEuN.This indicates that the samples arecontrolled by diagenetic processes which may also have affected the Ce anomaly and the overall enrichment of REEs in the sample SD2-4.
In summary, the findings of this study indicate that the rocks containing analcite in the study area share similar characteristics with hydrothermal sedimentary rocks, which are mainly reflected in the following aspects: (1) A substantial amount of volcanic ash material is observable in the study area, along with well-preserved crystal and glass shards, as well as an alkali feldspar and dolomite micrite base combination that resembles the "porphyritic" texture of magmatic rocks, indicating the material source and thermal conditions for hydrothermal sedimentation [49]; (2) The banded dolomite and tuffaceous rock types are rich in albite and dolomite, with iron-enriched dolomite, coexisting with quartz, analcite, and pyrite, which constitutes a typical hydrothermal sedimentary mineral assemblage [50].The clear zonal arrangement of the mineral assemblage demonstrates a trend of changing sedimentary minerals with variations in environmental temperature and physicochemical conditions; (3) Various types of syn-sedimentary deformational structures are common, suggesting that the sedimentation phase was influenced by external forces [51,52], likely due to the action of hydrothermal fluid venting; (4) The Si/Al ratio of analcite varies significantly and is consistent with the Si/Al ratio and alkali metal ion trends observed in analcite from two other reported hydrothermal sedimentary rocks; (5) The REEs pattern of rocks containing analcite is similar to that of hydrothermal sedimentary rocks in two other locations, and exhibits a distinct positive anomaly of δCe, proving that the hydrothermal sedimentation of rocks containing analcite in the Pingdiquan Formation occurred in an environment characterized by deep water, high reducibility, and alkalinity [26].Through the aforementioned research, it is further inferred that the analcite commonly found in the rock types where tuffaceous materials are mixed with lake sediments in the study area is very likely to have been formed from the deposition of tuffaceous materials, such as volcanic glass, under hydrothermal conditions after coming into contact with alkaline lake water.The several special analcite rocks observed in the study area are chemically uniform and exhibit a distinct negative Eu anomaly, which may suggest that relatively large-scale post-magmatic silicic hydrothermal fluids, after being ejected into the lake water, combined with the abundant Na + in the lake water to form analcite that then precipitated.Subsequently, these were rapidly covered and compacted by new tuffaceous materials, with parts in greater contact with lake water being cemented by calcite later on.In the later stages, the rocks were influenced by diagenetic processes, leading to the overall migration and enrichment of rare earth elements.

Diagenetic Stage
The formation of analcite is often associated with the burial stage, primarily formed in sedimentary rocks through diagenesis and chemical alterations.In the Permian of the northwestern margin of the Junggar Basin, there is a vertical zoning of zeolite minerals [53,54].With increasing depth of burial, there is an evolutionary process from volcanic glass to clinoptilolite, then to mordenite, analcite, and finally to chabazite.The study area also exhibits analcite that formed after the depositional period, where volcanic glass or other tuff materials undergo chemical alteration and subsequently crystallize and precipitate through the pore waters of sandstones.However, the study area only has analcite among the zeolite family minerals, indicating that during the diagenesis stage, the environment was relatively stable, and the presence of alkaline lake water allowed analcite to be preserved.The abundant Na + from the lake water and tuff materials prevented the transformation of analcite into other zeolite minerals.Analcite formed during the diagenesis stage often fills intergranular spaces with idiomorphic granular crystals, clean surfaces, and good crystal forms.

The Indicative Significance of Analcite in Black Shale for Oil and Gas Enrichment
Through the analysis of the characteristics, distribution, and genesis of analcite presented in this paper, it is evident that the analcite in the study area is characterized by high content, extensive distribution, and a close relationship with magmatic-hydrothermal activities.As a common mineral in these black shales, it holds significant indicative meaning for the enrichment of organic matter in black shales.

Source Rock Quality
The hydrocarbon source rocks in the Huoshaoshan Anticline (within the Permian Pingdiquan Formation) have a thickness ranging from 50 to 230 m [55,56].Research on the hydrocarbon source rocks of the Pingdiquan Formation in the study area by Li [17] indicates that the black shales contain a high abundance of organic matter, predominantly of the humic-sapropelic type (Type I1), with a sapropelic component reaching up to 90%.The total organic carbon (TOC) content is relatively high, ranging from 0.17% to 20.19%, and the maturity of the organic matter is in the low to mature stage.The hydrocarbon generation potential (S1 + S2) varies from 0.01 to 131.65 mg/g, leading to an overall assessment that the Pingdiquan Formation's source rocks possess considerable hydrocarbon generation potential.
According to the study by Zhou et al. [57] on the source rocks of the Fengcheng Formation in the Junggar Basin, it is believed that tuffaceous rocks themselves are a type of oil-prone rock, with an organic carbon content of 1% to 3%, and the kerogen type is either Type I or Type II, demonstrating a high capacity for hydrocarbon generation.Research by Li and Meng [58,59] on the Lucaogou Formation in the Junggar Basin also shows that tuffaceous shales, banded dolomites, and carbonate tuff ("porphyritic" carbonate rocks) are all high-quality hydrocarbon source rocks with considerable potential for hydrocarbon generation.Although the organic matter content in the tuffaceous limestone is low, it exhibits good reservoir properties.
The results from the observation of fluorescent thin sections indicate that under the excitation of a blue light source, the laminated hydrocarbon source rocks of the Pinglequan Formation exhibit strong fluorescence(Figure 9).Notably, the laminations with higher analcime content display a more intense yellow fluorescence, which suggests a robust oil-generating potential and a relatively low degree of organic matter maturation.In conjunction with the organic geochemical studies conducted by other researchers on the hydrocarbon source rocks, it is evident that rocks containing analcime possess a strong capacity for hydrocarbon generation.Moreover, the organic matter is predominantly derived from aquatic algae, with a portion also attributable to bacteria and terrestrial plants, albeit in smaller proportions for the latter [50].
Based on the genetic study of analcime in the research area, we believe that analcime formed in a saline lake basin with intense volcanic hydrothermal activity.Alkaline lakes are among the most productive ecosystems on earth, and the alteration of volcanic glass can promote the evolution of the lake basin toward high alkalinity [60].Previous research has found that there are many types of algae that can thrive and reproduce in saline lakes, such as coccolithophores, dinoflagellates, chlorophytes, and euglenoids, as well as a large number of halophilic bacteria, such as Halobacterium and Halococcus [61].The appropriate salinity promotes the reproduction of algae and bacteria.
Volcanic activity brings a large amount of material from deep sources, such as various nutrients like nitrates, phosphates, trace metal elements like Fe, Cu, Mn, Zn, U, and magmatic gases such as H2S, SO2, CO2, NH3, etc., laying a material foundation for the enrichment of organic matter.Magmatic hydrothermal activity also plays a positive role in the enrichment of organic matter [62], and modern submarine volcanic hydrothermal activities are often accompanied by a large number of bacterial microorganisms [63,64].The large number of berry-shaped pyrite and the radiolarian siliceous rocks found in the research area may serve as evidence of the massive proliferation of organisms [65].The hydrothermal environment also provides favorable conditions for the growth and prosperity of organic matter.Halbach et al. [66] found that the closer to the hydrothermal activity area, the number and activity intensity of organisms in the water body will increase, which is one to three times of magnitude higher than the ordinary marine surface.In addition, we believe that the formation of analcime in the research area is related to acidic hydrothermal fluids, and acidic hydrothermal fluids can make the sedimentary environment anoxic, which is conducive to the preservation of organic matter [67].These factors have promoted the formation of high-quality hydrocarbon source rocks in the research area.

Reservoir Quality
In the study area, analcite has played a significant role in the formation of high-quality reservoirs.Through thin-section observations and analysis of the physical properties of the oilfield, a certain correlation and complementarity between analcite and oil and gas storage and migration have been observed.Figure 10 illustrates that, longitudinally, the sections where analcime is relatively enriched correspond to better permeability and richer oil and gas responses.Concurrently, it can be seen from the figure that dolomites (tuffaceous dolomites) and sandstones are the better reservoir rock types in the study area, with numerous analcime dissolution pores observed under the microscope (Figure 11).Therefore, analcime has played a positive role in oil and gas reservoirs for the following reasons: (a) The solubility of analcime actively improves the physical properties of the reservoir.The dissolution of analcime can form secondary pores, which helps to increase the porosity and permeability of the reservoir, thus improving its storage capacity and permeability.As Jia et al. mentioned [11], analcime has a cage-like structure in three-dimensional space, and the internal channels of the structure are conducive to the occurrence of dissolution, which may have a positive effect on the formation of secondary pores in the reservoir.(b) During the early diagenesis process, the cementation effect of analcime has an anti-compaction effect, allowing the pores to be preserved, providing a basis for subsequent dissolution and improvement.The hardness of analcime is 5.5; thus, it has a greater resistance to compaction than carbonate and sulfate minerals, and under the same conditions, the permeability provided by analcime cementation is greater than that of carbonate and sulfate cementation [68].Therefore, the strong cementation effect in the early burial alkaline environment played a key role in the anti-compaction of the clastic framework during the later stages of diagenesis, laying the foundation for the subsequent dissolution and transformation dominated by analcime cementation [69,70].

Conclusions
In the eastern part of the Junggar Basin in China, specifically in the Huoshaoshan area of the Permian Pingdiquan Formation, we have discovered a significant amount of analcite in the black shale, which exhibits a coupling relationship with the distribution of oil and gas.We endeavor to investigate its formation mechanism and how it influences the generation and accumulation of oil and gas.Our research suggests: (a) The rock series of this formation is a lacustrine fine-grained sedimentary rock system rich in analcite and volcanic detritus, primarily characterized by interbedded tuffaceous shale and microcrystalline dolomite, with local intercalations of tuff, and an overall scarcity of clay minerals.(b) Analcite is enriched within this formation, appearing as stratiform or lenticular interlayers.The single crystals of analcite predominantly exhibit euhedral, subhedral, and anhedral forms.Analcite commonly coexists with minerals such as quartz, dolomite, calcite, alkali feldspar, and pyrite, and displays a crystallization sequence of ankerite/magnesite → albite (alkali feldspar) → quartz → analcite → organic matter → iron-manganese calcite.(c) In the study area, the content of SiO2 in analcite ranges from 56.37% to 62.04%, Al2O3 from 18.72% to 21.88%, and Na2O from 5.45% to 12.8%, with Si/Al ratios between 2.22 and 2.83, indicating a distinct provenance of the analcite-bearing materials.After normalization against Post-Archean Australian Shale (PAAS), the rare earth elements (REEs) show a relative enrichment of light rare earth elements (LREEs) compared to heavy rare earth elements (HREEs) and a significant positive δCe anomaly.By comparing the analcite-bearing rocks in the study area with those of hydrothermal sedimentary origin, we conclude that the analcite in the study area is predominantly of hydrothermal sedimentary and burial sedimentary types.(d) In vertical profiles, layers enriched in analcite often correspond to hydrocarbon accumulation zones; microscopic fluorescence characteristics also indicate the coexistence of analcite and organic matter.This may be related to the abundant organic matter brought by hydrothermal sedimentation and the beneficial effects of analcite on reservoir quality.
The research delves into the genesis of analcite in black shale formations, which is crucial for understanding hydrocarbon enrichment in these geological settings.It provides insights into the relationship between analcite and the quality of source rocks, which is vital for hydrocarbon exploration and development.The study contributes to the existing body of knowledge by offering a detailed analysis of analcite in the context of unconventional hydrocarbon resources.Future researchers engaged in basin oil and gas studies, upon encountering a significant presence of analcite, should consider it as an indicator of favorable conditions for oil generation and storage.

Citation:
Bai, Y.; Jiao, X.; Liu, Y.; Li, X.; Zhang, X.; Li, Z. Genesis of Analcite in Black Shales and Its Indication for Hydrocarbon Enrichment-A Case Study of the Permian Pingdiquan Formation in the Junggar Basin, Xinjiang, China.

Figure 1 .
Figure 1.Location map of the study area (adapted from Wang et al. [18]).The red box indicates a detailed map of the study area located in the Junggar Basin.

Figure 3 .
Figure 3. Microscopic characteristics of the minerals.(a) H2452, 1602.68 m, sedimentary tuff, containing a large amount of glass shards, acicular in shape, distributed in a stratified and lens-shaped pattern; (b) orthogonal polarized light image of (a), volcanic glass is completely extinct; (c) H2-35, 1751.37 m, exploded calcite particles, intergrown in a crystalline pattern, with internal development of acicular pyrite; (d) the orthogonal polarized light photo of (c) shows that calcite exhibits abnormal interference colors; (e) H262-62, 1557.4 m, oolitic dolomite, both the matrix and the ooids are cemented by analcite; (f) is the orthogonal polarized light image of (e), analcite fills the interior of ooids; (g) H262-54, 1567.1 m, tuff rock, dominated by feldspathic volcanic detritus particles, contains organic matter, with dispersed granular analcite; (h) orthogonal polarized light image of (g), analcite is completely extinct; (i) H262-123, 1458.62 m, euhedral analcite and calcite occur in a vein-like

Figure 5 .
Figure 5. Average content of analcite in different rock types.

Figure 6 .
Figure 6.Rare earth element distribution pattern maps of analcite-bearing rocks in the study area and comparative regions.

Figure 7 .
Figure 7. Plot of Alkali Metal Ions and Si/Al Ratio of Analcite.

Figure 8 .
Figure 8. Correlation plots of Ce/δCeN with ∑REE and Ce/δCeN with Eu/δEuN for analcite-bearing rock samples in the study area.

Table 2 .
Whole-rock X-ray diffraction results for analcite rock in the study area.

Table 3 .
Electron microprobe analytical data of analcite of research area and other areas (wt%).

Table 4 .
Analytical data of REE compositions (ppm) of rocks in research area and other compatible areas(a).