Study highlights analcite genesis in Liaohe depression
Lei Huang
Tingshan Zhang
Zongyang Dai
Southwest Petroleum University
Chengdu, China
Junfeng Shan
CNPC
Panjin
Huiping Xia
Chuanqing Drilling Engineering Co. Ltd.Chengdu
Analcite minerals have a greater influence on the physical properties of mudstones than carbonate rocks. This article proposes first steps in examining analcite to determine whether they were the product of hydrothermal deposition or transformed through volcanic activity.
Previous studies have shown a direct relationship between the development of analcite and oil and gas enrichment. This study discusses the basic characteristics of analcite-bearing rocks in the fourth section of China's Shahejie formation (Sha 4) in the Paleogene system in the western sag of Liaohe basin.
Research has identified the Shahejie formations as one of the main oil rich reservoirs in northeast China, but few studies have focused on analcite deposits within these formations. Mineral composition, microstructure, and the cause of analcite formation are important to understanding these reservoirs. Researchers used thin section identification, X-ray diffraction, scanned electron microscopy (SEM), and rare earth and trace element analyses to identify analcite minerals' genesis, focused mainly on lamelliferous analcite and its effects on Sha 4.
Analcite genesis
The analysis of analcite-bearing rocks has certain limitations, but this article mainly discusses the genetics of banded analcite rocks. Analcite minerals with higher automorphic degrees could be formed during the diagenesis period. We analyzed samples of analcite-bearing rocks in Sha 4 and applied comprehensive analyses of the major elements, trace elements, rare earth elements (REE), and isotopic elements to determine the relationship between analcite and volcanics and to identify the formation's genesis.
Analcite (NaAl(SiO3)2H2O) has an open frame structure with a density of 2.0-2.2 g/cc and a Mohs scale hardness of 5.5. The mineral belongs to the aluminosilicate and isometric system with the lowest silica-aluminum (Si-Al) ratio among subsilicic zeolites.1 2 Analcite is a fine particle, usually colorless, gray, white, or light red, which makes the mineral hard to recognize in hand specimens.3 Analcite's crystal body is tetragon-trisoctahedron, and it is usually irregular-granular. In the lens, it's transparent with a clear negative low projection and often hexagonal or octagonal. It shows homogeneity, but some larger grains can show very weak birefringence. It registers complete extinction under crossed nicols, and even some abnormal light.1
Analcite's main occurrences are:
• Microlitic lamellar.
• Nodular and flocculent.
• Particulate filling in pores or cracks.
• Stroma or phenocryst in volcanic rocks.
Analcite is distributed globally, both in volcanic clastics and sedimentary rocks. Analcite has been identified in China's Junggar, Qaidam, Nanxiang, Santanghu, and Tarim basins; Niger's Azelik area; the US states of New Jersey and California (west of Marin County); and southwest of Ankara and in Thrace basin in Turkey. Despite analcite's widespread deposition, its genesis remains controversial.
Former studies have cited specific environmental conditions required to form zeolites, such as 30-45° C. temperatures, a hydrological setting with 8-9 pH, a volcanic deposits as material base.4 5 6 Subsequent research has shown, however, that analcite can form independent of volcanic material in a lacustrine deposit, and some scholars believe the formation water and silicate gel in a saline lake are the material bases for formation.4 7-14 Other studies conclude that analcite is formed by direct precipitation or through interaction between intergranular brine and feldspar or clay minerals.15
Accepted thoughts on analcite formation include that it was:
• Altered from volcanic rocks directly derived from the volcanic glass, which is often formed in a saline alkali lake environment.
• Generated by metasomasis nepheline, plagioclase, and montmorillonite.
• Precipitated from formation water in holes and cracks.
• Formed from the hydrothermal sedimentary rocks associated with jet flows in hot springs from the bottom of lakes in continental rift basins.
Some researchers believe that analcite may be converted from other types of zeolite, but there is no evidence to prove this.13
China's Qingxi sag and Santanghu basins have been analyzed for hydrothermal analcite and are rich in silicon dioxide (SiO2), aluminum oxide (Al2O3), and sodium oxide (Na2O), but poor in titanium dioxide (TiO2). This makeup is similar to that of the continental basin's sedimentary exhalative deposits. The concentration of TiO2 and iron (III) oxide (Fe2O3) is lower than that of regional volcanic rocks, while Na2O is slightly higher. Analcite formed by hydrothermal sedimentation has light REE (LREE) enrichment and heavy REE (HREE) loss.
Geologic background
The study area is in the central part of the Liaohe depression's western sag (Fig. 1). The western sag is a typical half-graben depression. Its formation and evolution are controlled by boundary faults. In early Palaeogene, hot mantle from the Liaohe area arched and the crust became thinner under the subduction of the Pacific plate to the Eurasian plate. Axial in the uplift zone cracked and, influenced by north-west tension on the top of the uplift, formed a group of tensional faulting systems with a north-east direction. Magma rose along the tensile surface and then spouted from the earth's crust.
Taian-Dawa fault is a product of this period, and several secondary sags formed under the strong function of Taian-Dawa and its derived faults. Controlled by Taian fracture, west Taian fracture, and the Gaosheng uplift, the northeastern portion of Liaohe's western sag is narrow while the southwestern portion broadens. The nose-shaped formation is higher in the north. The brown-gray, brown-yellow carbonates are predominantly argillaceous dolomite and argillaceous limestone, interlayed with gray and dark gray mudstone, calcerous shale, dolomitic mudstone, and limy mudstone in Sha 4. Rocks in Sha 4 consist of analcite minerals, as well as some secondary minerals such as hematite, pyrite, and siderite.
Analcites transformed through volcanic rocks are easier to distinguish than those in depositional environments. Their formation is related to the dissolution of volcanic material by smaller hypidiotopic spherical particles. Development of zeolites, such as clinoptilolite and erionite, is the most notable feature. Sherd, crystal, and volcanic quartz are also common in volcanic-formed analcite.
Analcite formed through other means displays a general lack of glass or crystal chips, volcanic quartz, feldspar, or other types of early formed zeolite minerals. The lack of exposure markers, such as secondary leaching or hematitization, indicates the formation remained buried. Analcite is unique among the non-volcanic zeolites. Al2O3 and Na2O are slightly positively correlated, while SiO2 is positively correlated with Al2O3.13
Sha 4 analcite characteristics
X-ray diffraction and microscopy studies display a variety of minerals in analcite-bearing rocks in the Sha 4 section of the western sag. Dominant minerals include dolomite, quartz, K-feldspar, plagioclase, calcite, and analcite. Secondary minerals are hematite, siderite, pyrite, and anhydrite. Sha 4 lacks other types of zeolite minerals, such as euzeolite, clinoptilolite, and erionite (Table 1). Among these, analcite minerals are interlayed with rich carbonate or mudstone formations or as filling materials in pores or fractures (Fig. 2).
The interlays have a lamellar structure and consist of analcite, microcrystalline dolomite, and small amounts of microcrystalline feldspar, mudstone, dolomite, and organic matter. The layer is 0.2-1 mm thick with local deformation. The filling examples feature subhedral-automorphic analcite in full or half-filled microfractures with calcite which could have formed during the diagenetic stage.
Analcite in fractures and some section layers shows characteristics of multiple phases. Some early-stage analcite was completely or partially dissolved from formation water or organic acid, becoming late analcite and growing along the edge in the appropriate geological conditions. X-ray diffraction analysis of the entire rock and microscopy observation show no residual volcanic material or structure.
Comparing composition of analcite-bearing rocks in the western sag region with those of the Permian strata in Santanghu basin, Xiagou formation in Jiuxi basin's Qingxi sag, and southwest of Ankara, Turkey, reveals similar mineral composition lacking other types of zeolite minerals.
Element composition
Table 2 shows oxide content analysis for 11 samples. Among them, Z2-1 and Z2-2 are mud-bearing micritic dolomite and carbonate shale. Z3-1-Z3-3 each contain analcite (17.4-54.9 wt %) (Table 2). V1-1, V2-1, and V3-1 are tephrite, trachybasalt, and basalt. By comparison, the amount of SiO2, Al2O3, and manganese oxide (MnO) in analcite-bearing rocks does not obviously differ from regional volcanic rocks. SiO2, Al2O3, and Na2O in micritic dolomite quantities, meanwhile, were lower than found in other rocks. TiO2 and total iron (TFe2O3) contents were lower in micritic dolomite and analcite-bearing rocks than in volcanic rocks.
The analcite-bearing samples in Table 2 contain Na2O (4.34%), Al2O3 (13.555%), and SiO2 (42.825%), the percentage representing mean values. Being rich in SiO2, Na2O, Al2O3, and poor in TiO2 are similarities the analcite-bearing samples share with terrestrial hydrothermal sediments. The samples' Al2O3 content positively correlates with SiO2, while the correlation with Al2O3 and Na2O is poor and the overall correlation less positive (Fig. 3). These results are similar to the correlation of oxide-informed studies from non-volcanic materials near Ankara.
Isotope characteristics
Carbon and oxygen isotope analysis on 17 samples with different lithology from Sha 4 determined via the distribution of δ13C and δ18O that these rocks were products of a closed environment. The δ13C in dolomite is more positive and the equilibrium temperature of analcite-bearing rocks differs from dolomite's (Fig. 4). The analcite-bearing rocks' higher equilibrium temperature suggests production by hydrothermal deposition, while only parts of the dolomites were affected by hydrothermal activities.
Tests show abundant stromium (Sr) ratios of 87Sr and 86Sr in Sha 4's analcite-bearing rocks. The ratios range from 0.707501 to 0.711304, with a mean value of 0.7101319. This factor is slightly higher than that of seawater from the same period (0.70778) and mantle-derived mafic rocks (0.7035), but lower than that of crust-originated sial rocks (0.72 ± 0.005). Considering that εSr is higher, with a mean value of 79.7510, the lithogeneous components must be from the crust and mantle.16
Trace elements
This analysis compared Clarke values of crustal elements to show the lack of deep-source, mantle-derived trace elements represented by chromium (Cr), cobalt (Co), and nickel (Ni) in Sha 4's analcite-bearing rocks. Ni distribution varies widely but its content is low as a whole (Table 3). Strontium (Sr) and barium (Ba) were more abundant, which is related to the adsorption capacity of the rock and sedimentary environment. This observation can also be produced in an abnormal sedimentary environment.17
REE analysis data show the content of rare earth in analcite-bearing rocks from Sha 4 as 127.09-204.08 μg/g, slightly higher than that of underlying basalt and similar to the tephrite in the same layer. But all samples share similar REE distribution patterns (Fig. 5), suggesting a consistent source and formation mechanism. From the REE distribution pattern it follows that analcite-bearing rocks have richer ΣLREE than ΣHREE, with an obvious right leaning.
Sha 4 is similar to other continental hydrothermal sedimentary rocks including both the Xiagou formation in the Qingxi sag and Permian thermal liquid sedimentary rocks in Santanghu basin. Analcite-bearing rocks in Sha 4 have a weak europium (Eu) abnormity and their supply-material components are similar to those of basalt and tephrite. These characteristics may represent the typical REE composition of hydrothermal fluid in a lake.
Reservoir effect
Reservoirs in western China feature analcite development. Junggar, Qaidam and Tarim basins contain clastic rocks with analcite-bearing cement formed in the early diagenetic stage.1 18 The reservoir's close proximity to source rocks caused acidic fluid to enter the clastic rocks and form analcite with corrosive properties, improving reservoir storage performance.
Analcite minerals in Sha 4 have only a small constructive role on the carbonate material. Analcite has a direct influence on the calcareous mudstone but can become rock if the concentration is too high, worsening the reservoir's physical properties (Fig. 6).
Dissolution, brittleness
Aluminosilicate, especially the alkaline series, is most vulnerable to organic acid. Its analcite lattice cavity is large and the mineral can absorb and filter different sizes of molecules from other substances.
Compared with other common authigenic minerals, analcite easily dissolves. The dissolution of organic acid is unique to analcite forming in the diagenetic stage, but has almost no effect on layered colloid analcite (Fig. 7).
Uniaxial mechanics experiments show only a small difference in brittleness between carbonate-bearing analcite rocks and analcite-bearing carbonate rocks (Fig. 8). There are no obvious boundaries between their compressive strength, confirming that analcite minerals have no significant effect on brittleness of carbonate rocks. Analcite and carbonate minerals greatly improve brittleness in mudstone formations, making it easier for the mudstone to produce cracks and improving the performance of mudstone reservoirs.
Formation analysis
Thin-section observations show analcite as the only type of zeolite mineral without any hyaline, indicating that it formed in Sha 4's lacustrine without transformation from previous zeolite minerals or volcanics in the western sag of Liaohe depression.
Analcite can be formed directly by chemical precipitation or interaction between montmorillonite and oligoclase. But the analcites in Sha 4 were formed by hydrothermal deposition.
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The authors
Lei Huang ([email protected]) is a PhD candidate in geological resources and geological engineering at the Southwest Petroleum University, Chengdu, China. She holds a BS in resource reconnaissance engineering and an MS in mineral prospecting and exploration from Southwest Petroleum University.
Tingshan Zhang ([email protected]) is a professor at the School of Geoscience and Technology, and State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University. He holds a PhD from Nanjing University, Nanjing, China.
Zongyang Dai ([email protected]) is an associate professor in the School of Geoscience and Technology, Southwest Petroleum University. He holds a BS and an MS in geological resources and geological engineering from Southwest Petroleum University.
Junfeng Shan ([email protected]) is a senior engineer in Liaohe Oilfield at CNPC. He holds a PhD in mineral prospecting and exploration from China University of Geosciences, Beijing.
Huiping Xia ([email protected]) is an engineer at the Geological Exploration and Development Research Institute of Chuanqing Drilling Engineering Co. Ltd, Chengdu, China. She holds a BS in resource exploration engineering and an MS in geology engineering from Southwest Petroleum University.