As petroleum exploration goes on, the activity of hydrothermal fluid in petroliferous basins has gradually drawn a wide range of attention^{[1,2,3,4,5,6,7,8,9]}. In general, fluid migrating upward from deep inside the Earth with temperature 5 °C above the surrounding strata is regarded as hydrothermal fluid^[10]. Previous studies about identification and geological significance of hydrothermal fluid indicate that areas with hydrothermal fluid activity often have minerals such as fluorite, barite, pyrite, chalcopyrite, penninite, zeolite, anhydrite and dawsonite^{[1, 4]}, as well as abnormal enrichment of F, Ba, S, Fe, Mg, Sr, Ce and Eu^{[5, 8-9]}. Meanwhile, homogenization temperature of primary fluid inclusions developed in hydrothermal fluid associated minerals is often higher than the normal formation temperature^{[3, 7]}. The water-rock interactions between hydrothermal fluids and surrounding rocks can also improve reservoir properties^[5]. Previous studies on hydrothermal fluid activities in Huanghua Depression, Bohai Bay Basin mainly focused on Qikou Sag. The hydrothermal activities in Qikou Sag not only improved reservoir physical properties, but also raised the maturity of source rocks. Hou et al. reported the influence of hydrothermal fluid activities on physical properties of different types of reservoirs in Qikou Sag^[1]. Yang et al. also believed that the activity of hydrothermal fluid has reconstructive effect on carbonate reservoirs of Shahejie Formation in the Qikou Sag^[9]. In contrast, the study on hydrothermal fluid activity in Cangdong Sag is rarely reported.

Cangdong Sag is the second largest petroliferous sag in Huanghua depression, where seven oilfields including Zaoyuan, Wangguantun, and Wumaying etc. have been discovered, with cumulative proved oil reserves of about 3.87×10⁸ t. Among these oilfields, Zaoyuan oilfield located in Kongdian structural belt is the largest one with oil reserves of more than 100 million tons^[11]. In recent years, Wells GD1701H and GD1702H, which were drilled in the second member of Kongdian Formation (Kong 2 Member in short), have made major breakthroughs in shale oil exploration in Cangdong Sag^[12]. Subsequently, Kong 2 Member becomes the new hotspot of exploration and study^[12,13,14]. But different from ordinary lacustrine mudstone, the Kong 2 Member mudstone has rich analcime in laminar, dispersed and pore-filling forms, of 15% on average and up to over 50% in content^[15]. Zeolite is generally formed by alteration of volcanic minerals or volcanic glass^[16]. It is reported that lacustrine argillaceous dolomite of the fourth member of Paleogene Shahejie Formation in the Liaohe Depression contains laminar analcime. The dolostone with analcime has similar rare earth element (REE) distribution pattern with the basalt in the same formation and below, suggesting that the analcime dolostone is the product of joint basalt alteration and hydrothermal fluid activity at the lake bottom^[17]. Analcime is a typical brittle mineral, so enrichment of analcime increases the brittleness of shale, moreover, intergranular pores in analcimes provide effective storage space for shale oil^[14]. In addition, zeolite minerals may act as catalysts in hydrocarbon generation^[18], promoting the thermal evolution of immature-low mature source rocks^[18]. Therefore, study on its origin is of great theoretical and practical significance.

In this study, thin section, electronic probe, laser ablation inductively coupled plasma mass spectrometry, fluid inclusion homogenization temperature and Raman spectroscopy tests were conducted on core samples to identify the hydrothermal fluid activity in Kong 2 Member of Well Z56 of Cangdong Sag, and then the test results were compared with those of Well G108-8 about 20 km southwest of Well Z56 to find out the relationship between magmatic hydrothermal fluid and zeolite minerals.

Cangdong Sag, located in the southwest of Huanghua Depression, Bohai Bay Basin (Fig. 1a), is the regional subsidence center of Cenozoic rifted lacustrine basin developed on the basis of the Mesozoic depression^[19]. In the direction of NE 40°, with an area of about 1760 km² (Fig. 1b), it stretches from the Cangxian uplift in the northwest to Xuhei bulge in the southeast. Well Z56 lies in the Kongdian buried hill structural belt, which is an anticlinal belt controlled by the Cangdong and Xuxi listric faults^[20], and a favorable hydrocarbon accumulation zone in the study area.

Since the Eocene, Kongdian Formation (Ek), Shahejie Formation (Es) and Dongying Formation (Ed) successively deposited in the Palaeogene Cangdong Sag. From bottom to top, Kongdian Formation is divided into Kong 3 (Ek₃), Kong 2 (Ek₂) and Kong 1 (Ek₁) members. Among them, the Kong 2 Member organic-rich shale is the deposit in enclosed inland lake about 400-600 m thick and the most important source rock in the study area^[15]. Ek₂ in Well Z56 at the burial depth of 2274.5-2774.5 m currently consists of mainly dark gray mudstone and brownish gray shale, interbedded with light gray argillaceous siltstone, medium-fine grained sandstone, carbonaceous mudstone and gray-black basalt. According to the drilling/logging data (Fig. 1c), Ek₂ mudstone in Well Z56 contains five sets of basalt interlayers of about 37 m in total.

Huanghua Depression had six stages of intense magmatic activities in Cenozoic, namely the deposition periods of Kongdian Formation, the third member of Shahejie Formation (Es₃), the first member of Shahejie Formation (Es₁), the first member of Dongying Formation (Ed₁), Guantao Formation (Ng) and Minghuazhen Formation (Nm) respectively, and multiple sets of widely distributed igneous rocks were formed correspondingly^[21]. Previous studies showed that all stages of Paleogene basalt in Huanghua Depression were derived from mantle magma without apparent mixture of crust materials during rising and crystallization^[22]. Among them, the igneous rocks in Kongdian Formation belong to alkaline basalt series formed in intraplate environment^[23]. The magma derived from deep mantle migrated upward through deep faults, associated with thermal fluid moved to shallow strata at the same time, affecting the redistribution of energy and material within the basin^[6], and thus resulting in the local maturity anomaly of organic matter in igneous rock areas^[3]. Meanwhile, with the invasion of associated hydrothermal fluids, characteristic minerals such as fluorite, barite, calcite, dolomite, pyrite, anhydrite, dawsonite and zeolite deposited^[1].

In the Kong 2 Member mudstone core from Well Z56, there is a high-angle fracture vein about 2 m long and 1-2 cm wide with relatively straight edges (Fig. 2a). The fracture is filled with black solid bitumen and white minerals including barite, natrolite, analcime and calcite (Fig. 2b-2c). The bitumen belts have clear boundaries from the surrounding rock and minerals (Fig. 2c), and reticular contraction fissures cemented by calcite inside. Two sets of high-angle joints less than 2 mm in aperture longitudinally associate with the fracture (Fig. 2d), in which pyrite and zeolite can be seen in the surface (Fig. 2e). In addition, three mudstone samples from Ek₂ of Well G108-8 were used for comparative study on the origin of zeolite.

Electron probe micro-analyzer (EPMA) was performed on the JEOL JXA-8230 electron probe microanalyzer equipped with four wavelength-dispersive spectrometers (WDS) at the voltage of 15 kV, current of 20 nA, beam diameter of 1 μm or 5 μm, counting time for the peaks of 10 s, and counting time of 5 s on the background locations adjacent to peaks. Thin sections made from the core sample were polished on both sides to measure the homogenization temperatures of fluid inclusions in the fracture vein of Kong 2 Member mudstone in Well Z56 with Axio Scope A1 microscope equipped with MDSG600 heating and freezing stage. Raman spectra of bitumen samples were collected by JY-Horiba LABRAM HR800 Raman system equipped with a dual-frequency Nd-YAG laser (532.06nm) 14 mW in output power at room temperature of 25 °C. The 520.7 cm^-1 band of a polished silicon sample was used to periodically calibrate the Raman peak. Chemical components were analyzed by a Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), with He as carrier gas and NIST610 as standard sample.

Hydrothermal minerals coexist with pyrobitumen in the fracture vein of mudstone in Kong 2 Member of Well Z56 (Fig. 3). The pyrobitumen appears as veins on both walls of the fracture (Fig. 2b), in which contraction fissures are cemented by minerals such as calcite and chlorite (Fig. 3a-3d). The calcite has high crystallinity, with crystals generally more than 50 μm in size. The calcite crystals have features such as polysynthetic twinning, iridescence, rich inclusions and intergranular cementation (Fig. 3a-3b). The chlorite appears as grey-green scaly crystal under plane polarized light (Fig. 3c), and shows "rust color" abnormal interference color under cross polarized light (Fig. 3d). Chlorite rims perpendicular to the bitumen veins can be seen in the transit between the bitumen and minerals (Fig. 3e), the outer edge of the chlorite rim is cemented by barite, when locally enlarged, interactive residual bitumen-chlorite complexes can be seen (Fig. 3f). Electron probe analysis shows that in elemental composition, the chlorite in the fracture vein is similar to the femic chlorite with high silicon, lower aluminum and low calcium contents (Table 1).

Main minerals in the center of fracture are natrolite and analcime (Fig. 3g-3i), with rare residual barite (Fig. 3g). The columnar natrolite is yellow-white interference color in first-order under cross polarized light (Fig. 3g), and high in euhedral crystal degree, indicating it was formed by early hydrothermal fluid crystallization. Later, it was dissolved and cemented by calcite (Fig. 3h-3i). The calcite crystals in the middle of the fracture are clean and transparent, and different from those in contraction fractures, contain no cloudy inclusions (Fig. 3a).

The joints and laminae of mudstone in Kong 2 Member all have analcime (Fig. 3j-3l). Analcime is very similar to natrolite in elemental composition (Table 1), but the former is optically isotropic mineral with complete extinction under cross polarized light (Fig. 3j-3k), which is obviously different from natrolite with yellow-white interference color of first-order (Fig. 3g). The analcime joint veins are cut and broken by slip deformation parallel to the direction of the mudstone laminae, indicating that when the veins were formed, the mudstone of Kong 2 Member was shallow in burial depth and in the stage of early compaction diagenesis. Analcime microcrystals also occur in the region with fibrous chlorite and pyrite complexes (Fig. 4). The chlorite in mudstone shows pleochroism under plane polarized light from yellow-light to brownish yellow (Fig. 3j), and grey-white interference color of first-order under cross polarized light (Fig. 3k). Druses of the chlorite are in lenticular shape on the whole, and the chlorite crystals grow perpendicular to mudstone laminae, even extend into and interact with the pyrite aggregates (Figs. 3j, 3k and 4). In addition, the chlorite druses contain a small amount of spotted dark minerals (Fig. 3j), which is identified as chalcopyrite by electron probe (Fig. 4). Clearly, analcime, chlorite, pyrite and chalcopyrite are symbiotic, and are speculated to be products of magmatic hydrothermal fluid and mudstone interaction.

Previous studies show that Kong 2 Member is mainly composed of organic-rich lacustrine shale depositing in semi-deep to deep water environment with low energy^[13]. As basalt is formed by cooling of high-temperature molten magma erupting to the surface, it is inferred that when the magma in Kong 2 Member erupted, the Kong 2 Member in the study area was only about over ten meters deep, and the argillaceous sediments haven’t been compacted and were easy to generate synsedimentary deformations such as fractures, joints and bedding slip (Figs. 2-4). The later magmatic hydrothermal fluid probably moved along the early fractures and slip surfaces and had reaction with Kong 2 Member mudstone, generating hydrothermal mineral assemblages such as barite, chlorite, natrolite, analcime, pyrite and chalcopyrite.

In recent years, the role of ferric hydrothermal fluid in magma eruption has drawn the attention of researchers^[24,25,26]. Pyrite can be formed in low oxygen reduction hydrothermal channel with a temperature higher than 250 °C^[27,28], chalcopyrite is also a kind of major product of magmatic hydrothermal fluid mineralization^[29], and chlorite is resulted from intermediate-low temperature hydrothermal alteration^[30]. Based on the intimate connection between the formation temperature, ion substitution and site occupancy, chlorite can be regarded as geological thermometer^[31]. The formula proposed by Rausell-Colom et al.^[33] and modified by Nieto^[32] is as follows:

The chemical structural formula of chlorite was assumed to have 14 oxygen atoms. Then formation temperature of the chlorite was calculated by the formula between basal spacing and formation temperature^[34]:

It can be seen from Table 2, the chlorite in Kong 2 Member mudstone of Well Z56 was formed at temperatures between 124 °C and 166 °C, much higher than the temperatures of the formation at the maximum burial depth now (about 88 °C at 2404 m, Fig. 5). From the analysis above, it is speculated that the formation of chlorite is closely related to magmatic hydrothermal fluid activity.

Besides chlorite, homogenization temperature of primary fluid inclusions in hydrothermal minerals can also reflect the fluid temperature at the time of crystal precipitation. Primary fluid inclusions are geological fluids encapsulated in lattice defects during the process of crystallization with phase boundary from host mineral till now^[35]. Secondary fluid inclusions are geological fluids trapped during recrystallization, secondary enlargement and fracture healing of host minerals. It is generally considered the geological fluid trapped in mineral is homogeneous, so the homogenization temperature of inclusions can reflect the active period and temperature of the geological fluid. In the Kong 2 Member of Well Z56, the homogenization temperature test results of inclusions in minerals indicate that (Fig. 5) isolated primary fluid inclusions developing commonly in natrolite have homogenization temperatures from 80 °C to 196 °C, banded secondary inclusions commonly seen in calcite have homogenization temperatures between 60 °C to 189 °C, distinctly elongated secondary inclusions in barite have homogenization temperatures of 84-109 °C. The inclusions in different minerals differ significantly in homogenization temperature, indicating the mudstone of Kong 2 Member in Well Z56 may have experienced multiple stages of hydrothermal fluid activities, which is consistent with multiple sets of basalt interlayers developing in the strata more than ten meters above the sampling location (Fig. 1b).

In addition, laser Raman spectroscopy is an effective means to study organic matter. Bayssac et al.^[36] believe that Raman spectroscopy could be regarded as geological thermometer of carbonaceous substances including kerogen, pyrobitumen and graphite, and put forward a calculation formula:

where R₂=D₁/(G+D₁+D₂).

The formation temperatures of bitumen in Well Z56 calculated by overlapping peak resolving of Raman spectroscopy are 324-354 °C (Table 3). In contrast, the present temperature of Kong 2 Member in Well Z56 (at the maximum burial depth) is about 88 °C (Fig. 5), indicating the pyrobitumen formed under abnormally high temperatures of 324-354 °C is obviously related to magmatic hydrothermal fluid activity.

Comprehensive analysis of chlorite geological thermometer, homogenization temperatures of fluid inclusions in barite, calcite and natrolite, and the formation temperature of bitumen calculated by overlapping peak resolving of Raman spectroscopy implies that the mudstone of Kong 2 Member in Well Z56 experienced multiple stages of hydrothermal fluid activities, and the hydrothermal fluids of these stages were 124-166 °C, 84-109 °C, 68-189 °C, 89-196 °C and 324-354 °C respectively. However, most fluid inclusions in barite and calcite have homogenization temperatures lower than the present formation temperature (88 °C), indicating barite and calcite are diagenetic product of normal formation fluid or crystallization products of one magmatic hydrothermal activity from high to low temperatures, that is pyrobitumen → natrolite → chlorite → barite → calcite, in which pyrobitumen, natrolite and chlorite are products of magmatic hydrothermal fluid and mudstone of Kong 2 Member interaction.

It is well-known that the separation of hydrothermal fluid from magma chamber containing volatile matter and water as magma cools is usually associated with precipitation of barite, zeolite, chlorite, pyrite, chalcopyrite, fluorite, anhydrite and dawsonite^{[1, 4, 37]}. Especially for barite, the main elements Ba and S usually come from magmatic hydrothermal fluid. In the early stage of magma-fluid conversion, high oxygen fugacity environment makes S exist in the form of SO₄^2-[38], and SO₄^2- is easy to combine with Ba to form barite. The coarse-grained barite vein filling in the fracture (Fig. 3e, 3f) is also usually considered as a sign of hydrothermal fluid origin^{[1, 39]}. The drilling data of Well Z56 shows there are multiple sets of basalt interlayers more than ten meters above the sampling location in Kong 2 Member (Fig. 1b), confirming geological background and material basis of magmatic hydrothermal fluid activities. Test results of laser ablation inductively coupled plasma mass spectrometry show the barite has rich light rare earth elements (REE) and obvious positive anomaly of Eu (Fig. 7).

As REE enter the mineral lattice through isomorphism, ion radius is one of the main factors limiting their entry into minerals. With the increase of atomic number, the ion radii of REE gradually decrease (0.086 1-0.103 2 nm)^[40]. The ion radius (0.135 nm) of Ba²⁺ of barite differs widely from the ion radius of REE, making it difficult for REE to enter the lattice of barite, so the barite has low REE abundance^[40]. Eu is regarded as a sensitive indicator for redox conditions, as Eu²⁺ ions are likely to be oxidized into insoluble Eu³⁺ ions, leading to enrichment of Eu³⁺ ions under high temperatures, the positive anomaly of Eu can be taken as a symbol of hydrothermal fluid activity^[41]. The coarse-grained vein barite in the fracture of the Kong 2 Member mudstone of Well Z56 may be directly precipitated from the magmatic hydrothermal fluid (Fig. 7).

According to the chlorite classification standard proposed by Melka^[42], the chlorite in the mudstone and fracture vein of Kong 2 Member in Well Z56 belongs to delessite-pennine-high siliceous pennine series (Fig. 8). The radial chlorite in the vein has silica content much higher than that of fibrous chlorite in the mudstone, and the increase of silica content of chlorite from surrounding rock toward vein inside is one of the characteristics of magmatic hydrothermal fluid activity. Meanwhile, the special crystal shape of radial chlorite with high silica content is also a sign of hydrothermal fluid origin^[43]. It can be concluded that the complex of chlorite, pyrite and chalcopyrite in the mudstone and fracture vein of Kong 2 Member of Well Z56 is closely related to magmatic hydrothermal fluid. Furthermore, chlorite exists stably at pH values of 8-10, and zeolite stably exists at pH values of 7-10^[44], coexistence of chlorite and zeolite, and the absence of quartz in Kong 2 Member of Well Z56 indicate that the hydrothermal fluid was alkaline fluid rich in metallic elements such as Fe and Mg, which is consistent with alkaline basalt occurring in the Kongdian Formation of the study area^[23].

Test results of laser ablation inductively coupled plasma mass spectrometry show that the radial chlorite is characterized by weak enrichment of light rare earth elements and obvious Eu positive anomaly (Fig. 7), suggesting the chlorite is related to hydrothermal fluid. But the chlorite is different from natrolite and basalt in Kongdian Formation in REE distribution, indicating that the metallogenic source material of chlorite is not from magmatic hydrothermal fluid directly. According to the analysis of elements and crystallization characteristics of the chlorite mentioned above (Figs. 3. and 4), it is concluded that the chlorite in the mudstone and fracture vein of the Kong 2 Member of Well Z56 is formed by metasomatism between hydrothermal fluid and mudstone matrix.

Besides barite and chlorite filling the fracture and replacing mudstone, the interaction between magmatic hydrothermal fluid and mudstone of Kong 2 Member also produced natrolite and analcime (Figs. 3 and 4). Zeolite is a kind of framework silicate mineral, which is generally the alteration product of volcanic glassy feldspar, feldspathoid or hydrothermal product filling basalt pores, cracks and geodes. It is proved by experiment that natrolite and analcime can crystalize and precipitate from alkaline hydrothermal fluid at temperatures of 200-300 °C^[45,46]. Based on electron probe microanalysis, analcime in mudstone of Kong 2 Member of Well Z56 has a Si/Al radio of 1.97-2.18, very similar to zeolite precipitated directly from alkaline hydrothermal fluid^[47,48].

Test results of laser ablation inductively coupled plasma mass spectrometry show the natrolite is characterized by weak enrichment of light rare earth and weak Eu positive anomaly (Fig. 7). The natrolite is similar to basalt in Kongdian Formation but different from Mesozoic basalt in REE distribution pattern, implying that the material source of natrolite comes from magmatic hydrothermal fluid of basalt in Kongdian Formation. According to the above analysis of crystallization characteristics (Figs. 3 and 4), it is concluded that the automorphic natrolite and vein analcime in fractures and joints of mudstone of Kong 2 Member in Well Z56 precipitate directly from the magmatic hydrothermal fluid after volcanic activity.

It is worth noting that the shale oil reservoir of Kong 2 Member in Cangdong Sag is also rich in analcime^[14]. Laminar analcime similar to that in Well Z56 is more common in shale of the Kong 2 Member of Well G108-8, and reaches the content of over 50% in local parts^[15]. Test results of laser ablation inductively coupled plasma mass spectrometry show the laminar analcime in Kong 2 Member of Well G108-8 is characterized by enrichment of light rare earth elements, but only one sample has slight positive Eu anomaly (Fig. 9).

The REE distribution pattern of analcime from Well G108-8 is consistent with that of natrolite from Well Z56, and the former fluctuates up and down with the latter as the baseline, reflecting that the analcime from Well G108-8 and natrolite from Well Z56 have close affinity on one hand, but on the other hand the analcime from Well G108-8 is subjected to diagenetic transformation (such as recrystallization, metasomatism, etc.) or has impurity minerals mixed in. In comparison, the relatively stable REE distribution of natrolite from Well Z56 is consistent with the previous inference that natrolite is derived from the precipitation of hydrothermal fluid. In general, laminar analcime from Well G108-8 and automorphic natrolite from Well Z56 have close affinity with Kongdian Formation basalt. Since laminar structure is commonly regarded as typical sign of sedimentary rock, it is speculated that the laminar analcime in Kong 2 Member has two kinds of origins, one is sedimentation of weathered volcanic material from Kongdian Formation or underlying strata, the other is that the magma with origin same to the basalt in Kongdian Formation provided Na⁺, Al³⁺, Si²⁺ ions by eruptions at the bottom of the lake or hydrothermal exhalation, and then mixed with the normal lake sediments to form hydrothermal-peperite. But there are two doubtful points in the above speculation: (1) Although Kongdian Formation has widespread volcanic eruptive rocks, as the strata subsided continuously during the deposition of Kongdian Formation, there was no geological conditions for large-scale weathering of volcanic rocks erupting in the early stage (such as basalt in the Kong 3 Member). (2) Although the igneous rocks in Kong 2 Member are large in scale, most of the igneous rocks are hypabyssal intrusive diabase, indicating there wasn’t geological basis for continuous magma eruption at the bottom of the lake. Finally, the eruption of magmatic hydrothermal fluid associated with basalt of the Kongdian Formation might provide some materials for zeolite formation, but whether it plays the major role in the formation of the widespread analcime in the mudstone of Kong 2 Member needs further investigation.

The peperite rich in analcime of Kong 2 Member is hybrid sedimentary rock of magmatic hydrothermal minerals formed by magmatic eruption or hydrothermal exhalation at bottom of the lake and normal lake sediments, and is the major source rock of Cangdong Sag. The nutrients carried by the magmatic hydrothermal fluid certainly had effects on the initial biological productivity and the chemical properties of the lake water, thus affecting the mineral composition and quality of the source rock of the Kong 2 Member. Different from ordinary lacustrine mudstone, the mudstone samples from the Kong 2 Member in Cangdong Sag have an analcime content of about 15% on average and 50% at maximum^[15]. Meanwhile, source rock of the Kong 2 Member is rich in organic matter, with TOC from 1.2% to 8.4% and up to 12.9%^[13]. The characteristic of rich analcime and organic matter in mudstone of Kong 2 Member not only increases the brittleness but also the oil and gas content of the mudstone, which is conducive to the exploration and development of shale oil.

The mudstone of the Kong 2 Member in Well Z56 of Cangdong Sag contains multiple sets of basalt interlayers and apparent signs of magmatic hydrothermal fluid activity. In mineral assemblage, the mudstone contains characteristic minerals of hydrothermal fluid, such as barite, calcite, chlorite, pyrite, chalcopyrite, natrolite and analcime. In terms of fluid temperature, chlorite geological thermometer, homogenization temperatures of primary fluid inclusions in natrolite, and calculated results of Raman spectroscopy of bitumen all show that the mudstone of Kong 2 Member experienced hydrothermal fluid activities with wide variations in temperature, but all higher than the normal ground temperature. In terms of REE, the positive Eu anomaly of barite and chlorite and the similar REE distribution of natrolite and basalt of Kongdian Formation all indicate the existence of hydrothermal fluid activities associated with basalt of the Kongdian Formation. Together with mineral crystallization characteristics, it is concluded that the natrolite in the mudstone fracture of Well Z56 directly comes from the crystallization and precipitation of the magmatic hydrothermal fluid of the basalt of Kongdian Formation. The consistency of REE distribution of laminar analcime from Well G108-8 and natrolite from Well Z56 indicates the analcime and natrolite have the same material source—basalt magma of Kongdian Formation. It is preliminary speculated that sublacustrine magma eruption or hydrothermal exhalation is one of the reasons for rich zeolite in the Kong 2 Member.

d—basal spacing of chlorite, nm;

D₁, D₂, D₃, G—peak fitting area, dimensionless;

GR—natural gamma-ray, API;

R₂—ratio of peak area, dimensionless;

R_LLD—deep lateral resistivity, Ω•m;

T—temperature, °C;

n_Al—aluminum atoms number calculated by electron probe microanalysis, in which the structural formulas of chlorite is based on 14 oxygen atoms, dimensionless;

n_Fe—iron atoms number calculated by electron probe microanalysis, in which the structural formulas of chlorite is based on 14 oxygen atoms, dimensionless.