Introduction

Extensive studies show that the greater the burial depth, the lower is the porosity, for either clastic or carbonate rocks. Empirical data suggest that the porosity is decreased by 1–3 % per km (Ehrenberg and Nadeau 2005). In theory, reservoirs should not be developed in deeply buried units. However, with the exploration of deeply buried basins, there have been numerous discoveries of oil and gas reservoirs, in deep and ultra-deep carbonate strata, such as the Tahe oilfield in the Tarim Basin, and the Puguang, Yuanba, Longwangmiao and Dengying oil and gas fields in eastern Sichuan. These large oil and gas fields occur at depths of over 5000 m (Ma et al. 2011). These discoveries reveal that deeply buried carbonate rocks, especially dolomite, can be host to valuable reservoir space. Understanding the processes and conditions that lead to the formation and development of oil and gas reservoirs under deep burial conditions is of great theoretical and practical significance for oil and gas exploration in deeply buried zones.

Thus far, only a few of studies have been conducted on deep reservoir formation mechanism in China (Zhu et al. 2008; Pan et al. 2009). Huang et al. (2010b) considered that the reservoir development under deep burial conditions was closely related to the dissolution of carbonate rocks during burial diagenesis. Huang et al. (2010b) suggest, based on geological evidence and thermodynamics, that carbonate dissolution would occur as tectonic uplift and ascending diagenetic fluids via faults, resulting in cooling down of fluid and thereby undersaturation with respect to carbonate. This process has been inferred for the Tarim Basin by various researchers (e.g., Zhu et al. 2008; Jin et al. 2006; Wang et al. 2004; Zhang et al. 2006). Similar processes have been implied in the development of high-quality reservoirs in the middle Devonian Western Canada sedimentary basin (Ihsan and Aasm 2002).

In this paper, core descriptions, thin section petrography and SEM analysis are used to identify petrological and mineralogical features resulting from hydrothermal process of a few wells cored in the Tahe area of the Tarim Basin. Geochemical analyses were carried out on these materials and include carbon and oxygen isotopes, REE, strontium isotopes and fluid inclusion homogenization temperatures, and used to characterize the hydrothermal fluids in the Tarim Basin. Moreover, new hydrothermal dissolution mechanisms are proposed and their impact on hydrothermal dissolution on carbonate reservoirs is discussed.

Geological setting

The Tarim Basin is located south of the South Tianshan Mountains (Fig. 1), northwest China. The study area is the Tahe oilfield, which is located on the northern portion of the Tarim Basin. The Tahe oilfield was the first field in the Tarim Basin, producing oil from marine strata (Kang 2005). The majority of the production is from the Ordovician strata consisting of shallow platform and slope facies carbonate. The Ordovician strata in this area comprised six formations, from oldest to youngest these are the Penglaiba, Yingshan, Yijianfang, Qiaerbake, Lianglitag and Sangtamu formations, from lower to upper. The lithology is described in Table 1.

Fig. 1
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Geology of the Tarim Basin region (modified from PetroChina)

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Table 1 The table of Cambrian–Ordovician strata of the Tahe oilfield
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Structurally, the Tahe oilfield is located in the southwest slope of the Akekule thrust of the Shaya uplift of the Tarim Basin. The Ordovician sequence experienced multi-stage tectonic movement, of which the middle Jialidong (late Ordovician to early Silurian; e.g., Liu et al. 2012) event and Heisey (Devonian to Permian; e.g., Charvet et al. 2007) events are the most important. The uplift during the middle Jialidong changed the sedimentary facies of the Tarim Basin from pure marine carbonates to mixed clastic carbonates or muddy clastic sediments (Table 1). The carbonate reservoirs of the middle Ordovician Yijianfang Formation are predominantly reef (algal) and shoal (ooids), and the upper Ordovician Lianglitag Formation reservoirs are mostly of algal and reef/buildups (Kang 2005).

Exposure and erosion of the carbonates of the Yijiangfang and upper Lianglitag formations correspond to two distinct times in which the sea level decreased during the middle Jialidong movement. The upper Ordovician Qiaerbake and Sangtamu formations were deposited during episodes of global sea level rise. The early Heisey movement caused extensive karst dissolution of the early Paleozoic carbonates, giving rise to the dominant reservoirs in Ordovician carbonates in the north of the Tahe oilfield (Yu 2006).

This study focuses on the Cambrian and Ordovician carbonates (Table 1), in which the Cambrian strata are light gray to reddish-gray massive to medium bedded. The dolomite consists of fine–medium size crystals, algae, stromatolites, and ooids which are all dolomitized, and in places detrital ravel to sand-sized dolomite is found. In places, dolomitized oolites are interbedded with siliceous lenses. The total thickness of Cambrian is 320–400 m. In the Tahe area, only one well, TS1, penetrated the lower Qiulitage.

Materials and methods

Samples were collected from cores of the S85, S88, S15, S65, and TS1 wells. Thin section samples were used for petrographic examination, doubly polished slabs were used of fluid inclusions temperature measurements, and powder samples were extracted for elemental REEs, carbon–oxygen, and strontium isotope analyses. The calcite in pores and cavities were sampled by micro drill to separate it from surrounding rock. Then the samples were crushed into small pieces of several millimeters, and further ground into powder less than 200 meshes in agate mortar for the purpose of REE and isotope analyses.

The element composition is obtained through EPMA-1600 at the Southwest Mineral Resources Supervision and Inspection Center of Ministry of Land and Resources. When analyzing, the voltage was 15kv and current 20 nA. Fluid inclusion temperature measurements were carried out on a Linkam Coo1ing System THMS600 in the State Key Laboratory of Chengdu University of Technology. The precision of temperature measurement is ±1 °C. The carbon and oxygen isotope analysis was carried out on MAT252 Gas Isotope Ratio Mass Spectrometer in the Geological Laboratory of PetroChina Southwest Oil and Gasfield Company. Strontium isotope analysis was conducted in the Institute of Geology and Geophysics of Chinese Academy of Sciences.

Results

Petrographic evidence

Hydrothermal fluid migration along faults and fractures resulted in the precipitation of a variety of minerals, such as fluorite, dickite, barite and dolomite. The precipitation of these minerals resulted from changes in temperature and pressure, as fluids ascended and chemical changes took place as they interacted with the surrounding rocks.

Presence of fluorite

There is a lot of controversy about the genesis of fluorite ore in this region. Most scholars (Zhu et al. 2008; Wang et al. 2004) considered that fluorite ore was derived from magmatism and its formation was related to the Hercynian–Late Permian magmatic thermal events that occurred in the Tarim Basin. However, other scholars argued that the ore-forming hydrothermal fluids were derived from heated meteoric waters (Zhang et al. 2006; Pan et al. 2012). We have observed fluorite along fractures in the Yingshan Formation in the S65 well (Fig. 3a). Fluorite in the Yijianfang Formation in the S85, S76 and S86 wells has also been reported by others (e.g., Li et al. 2008). Analysis of the fluid inclusions homogenization temperatures showed that the temperature of fluorite precipitation in the Yijianfang Formation in the S85 well was high, with a narrow range from 208.5 to 218.5 °C, and an average of 213.6 °C (Table 2). This range is higher than the expected temperature of 201 °C if we assume a geothermal gradient of 3 °C/100 m and surface temperature of 25 °C. Fluorite is thus considered to be of hydrothermal origin.

Table 2 Homogenization temperature of the primary brine inclusions in fluorite in the S85 well (5872.24 m)
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Presence of dickite

Dickite is widely distributed in various hydrothermal sedimentary rocks, but it is rarely present in carbonate formations. Huang et al. (1995) reported that dickite was widespread on the top of Ordovician paleo-weathering crusts in the Ordos Basin. Zhao (1997) reported dickite presence in the Lower Ordovician weathering crust in the southwestern Tarim Basin. The provenance of dickite in the above occurrences was related to weathering and most likely it was converted from kaolinite (Huang et al. 1995).

Dickite was found at 6408.79 m in the S88 well in the Tabei area, identified by its hexagonal-platelet morphology with assemblages in booklet or pillared layered morphology as seen in SEM (Fig. 3c, d) and X-ray diffraction pattterns (Fig. 2). The dickite discovered in the S88 well was far away from the Ordovicians exposure unconformity and 925 m below the closest overlying erosional unconformity (i.e., the surface between the carboniferous Bachu Formation/Ordovician Yingshan Formation). Cleary, the dickite origin in the S88 well was unrelated to low-temperature weathering or conversion from kaolinite during the burial diagenetic process. Furthermore, the dickite discovered in the S88 well occurred in brecciated limestone (Fig. 3c, d), suggesting that its origin was associated with the hydrothermal activity producing the breccias. It is likely that the dickite might be associated with the deep hydrothermal fluids that went through strata rich in aluminum silicates.

Fig. 2
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X-ray diffraction graph of the dickite sample. Di is the X-ray diffraction of dickite and Do is that of dolomite, most of which is not marked. 6408.79 m, S88 well

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Presence of barite

Pore-filling barite is found at the same depth as fluorite in the Yingshan Formation in the S65 well (Fig. 3b). Barite has been reported in other localities of the Tarim Basin, such as in the Tazhong 4 well at a depth of 3609.96 m (Jin et al. 2006) and in the Tazhong 12 well (Cai et al. 2009; Li et al. 2008). The barite involved in this study showed fasciculate shape, with a set of cleavage extensively distributed in the long axis direction. It is Sr rich, with BaO and SO3 content of 63.47 and 35.76 %, respectively, and SrO content as high as 0.62 %. The association of barite and fluorite supports pervasive hydrothermal activity in the Tahe area.

Fig. 3
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Photomicrographs of the Ordovician hydrothermal mineral assemblages, Tarim Basin. a Fluorite, S65 well, 5475.59 m, Yingshan Formation, b barite, S65 well, 5475.59 m, Yingshan Formation, c Dickite, S88 well, 6408.79 m, Penglaiba Formation, d SEM photo of figure (c), e dolomite in cavities, TS1, 7875.7 m, Lower Qiulitage Formation, Cambrian, f thin section of (e), casting section, g fine crystal dolomite and crystallized coarse dolomite, point 1 and point 2 is the electron probe test point with result shown above, S88 well, 6408.79 m, Penglaiba Formation, h the cathode luminescence of (g) with test condition of 326 µA, 16.3 kV

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Presence of saddle dolomite

The dolomitization (or recrystallization of dolomite) under high temperature resulted in the loss of original fabric, which has been reported before (Machel 2005). The dolomite can be divided into two types replacement dolomite (mostly non-planar anhedral dolomite) and primary crystals precipitated in vuggy porosity or fabric destructive (mostly euhedral to subhedral crystals). The replacement dolomite is mostly found in shallower systems associated with erosional surfaces. The pore filling or fabric-destructive dolomite is mainly found deeper and probably formed under deep burial conditions (Huang et al. 2009). In the Tahe area, the fabric-retentive replacement dolomite is found in the shallower strata of the Yijianfang and Yingshan formations. The pore-filling dolomite is found in the deep strata of the Penglaiba Formation, such as in S88 and S15 wells and Qiulitage Formation in the TS1 well. Vuggy porosity is often found in the Penglaiba Formation in the S88 and S15 wells and the Qiulitage Formation in the TS1 well (Fig. 3e, f). Fine to coarse crystalline saddle dolomite fills the pores and is often associated with fluorite (Fig. 3g, h). The saddle dolomite in the pores is generally brightly luminescent with relatively high Mn content (256 ppm) and low Fe content (70 ppm) (Fig. 3e, point 1 and Fig. 3h, Table 3). The matrix commonly has low Mn content (8 ppm) and high Fe content (202 ppm) (Fig. 3e, Table 3, point 2). Dogtooth dolomite in samples, collected 924.79 m away from unconformity, is brightly luminescent suggesting precipitation from non-meteoric or non-marine fluids, and most likely formed in deep buried conditions.

Table 3 Trace element composition of different structures of dolomite in the S88 well
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Geochemical evidence

The hydrothermal activity along faults and fractures not only left its trace through the precipitation of a variety of hydrothermal minerals, but hydrothermal fluids also reacted with the carbonate host rocks resulting in alteration of the host rock that can be traced by geochemical changes.

Fluid inclusion homogenization temperatures

The Tarim Basin had experienced several major tectonic events, including the tectonic uplift and subsidence in the Early Paleozoic (Wu et al. 2009) and rapid subsidence since Cretaceous. For most of the basin, the current burial depth is the greatest that the basin has experienced in its geological history (Chen et al. 2009). Based on the burial-thermal history of the Tahe area (Ye 1994), the Ordovician carbonates in the study areas experienced the deepest burial depth and the highest burial temperature (Wu et al. 2012). Present-day burial temperature of the limestone in the sampling location (6400 m in the S88 well) is estimated at 145 °C using a present-day geothermal gradient of 3 °C/100 m and surface temperature of 25 °C. The average measured homogenization temperatures are within this range. However, the maximum measured homogenization temperatures of fluid inclusions are as high as 193.6 °C for calcite and 158.4 °C for dolomite (Table 4), which can only be reached by hydrothermal alteration.

Table 4 Homogenization temperature of Ordovician carbonate inclusions in some wells in the Tahe region
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Stable carbon and oxygen isotope evidence

Oxygen isotope fractionation between calcite and fluid occurred during the enriching in the precipitation process (Goldstein and Jacobsen 1987). The oxygen isotopic composition of calcite and fluid was controlled by the temperature of precipitation and the fluid composition. The oxygen isotopic composition of diagenetic calcite is generally light, indicating that diagenetic fluids were either depleted, temperatures were high or a combination of both (Jay and Kevin 1989). We plot the δ18O and δ13C value of different components in the S65 and S88 wells (Fig. 4; Table 5). The results show that the calcite in cavities commonly have low δ18O value, on average less than −11 ‰ (PDB). In the S88 well, the δ18O value of cavity calcite is only −12.68 ‰ (PDB) at a depth of 6408.79 m where dickite was discovered (Fig. 4b). Although most samples are collected close to the unconformity surface, the meteoric water might cause the light oxygen isotopic composition. All the calcite inclusion had an average homogenization temperature of over 120 °C (Table 4; Fig. 5), which could exclude the influence of meteoric water.

Fig. 4
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Scatter plot of the oxygen isotopes of different components in the S65 and S88 wells

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Fig. 5
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The fluid inclusion homogenization temperature with carbon and oxygen isotope composition plot for Ordovician carbonates. Isoline is the oxygen isotope of fluids (Zhang 1985). The δ18O value of seawater and meteoric water is that of modern seawater (Ren et al. 2000)

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Table 5 The isotope composition of different components of Ordovician carbonate in the S65 and S88 well of the Tahe oilfield
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The fluid inclusion homogenization temperatures constrain precipitation temperatures and provide the means to identify the composition and source of diagenetic fluids (Fig. 5). The isotopic composition and fluid inclusion temperatures of Ordovician carbonate require that the δ18O value of diagenetic fluid should be enriched with values well above those of meteoric fluids and even seawater. While precise salinity measurements are lacking for the fluid inclusion, the limited data suggest salinity certainly higher than that of modern seawater (Ren et al. 2000). Therefore, we conclude that the calcite in Ordovician carbonates was formed under high temperature and high salinity fluids. The source of these fluids is either seawater modified through rock–water interactions with magmatic rocks or was magmatic or metamorphic fluids.

Rare earth elements

Rare earth elements (REEs) have been widely applied for the identification of the source and the study of geochemical behavior of geological bodies and oil and gas reservoirs (Condie 1991). REEs are a powerful tool to track fluid activities in deep zone. Previous studies (Lv et al. 2007; Ding et al. 2000; Zhu et al. 2013; Liu and Cai 2009) revealed that a positive Eu anomaly could be used as an indicator of high temperature fluids. In this study, the standard distribution pattern of the REEs of dolomite was mapped and some dolomite samples had a positive Eu anomaly (Fig. 6), indicating that the dolomite reservoirs of the Yingshan Formation in the Tahe area were affected by hydrothermal fluids. However, in most dolomite samples, there was a negative Eu anomaly suggesting that although the dolomite was obviously related to burial dolomitization, the fluids were not of hydrothermal origin.

Fig. 6
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REE distribution pattern of the dolomite reservoir of Tahe Oilfield, Tarim Basin

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87Sr/86Sr isotopes

The 87Sr/86Sr ratio of Ordovician seawater was between 0.707854 and 0.709132 (Huang et al. 2004a, b). The 87Sr/86Sr ratios of dolomite, calcite in cavities, sparry grainstones and micrite in Ordovician carbonate of the Tarim Basin are all higher than that of contemporary seawater. The strontium isotope ratios in calcite in cavities have notably higher 87Sr/86Sr ratios than contemporary seawater (Fig. 7, Table 5). The 87Sr of continental igneous rocks is generally elevated and are from continental igneous rocks (Zhu et al. 2010); thus, the 87Sr/86Sr ratio of continental hydrothermal fluid is commonly high. These high values suggest that the diagenetic water was influenced by hydrothermal fluid.

Fig. 7
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Histograph of 87Sr/86Sr value in different components in the Tahe oilfield, Tarim Basin

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Discussions

A wide range of evidence demonstrates that deep hydrothermal fluids affected the Tarim Basin. Numerous researchers have studied the alteration and dissolution of reservoirs by hydrothermal fluids (e.g., Jin et al. 2006; Lv et al. 2007; Pan et al. 2009; Zhang et al. 2006). There are, however, few studies on the dissolution mechanism. During fluid migration, the retrograde solubility of carbonate rocks results in increased solubility as the fluid cools down and is a most effective way to enhance the dissolving capacity of carbonate minerals under deep burial conditions (Huang et al. 2010a, b). Previous studies were reviewed and data generated to address the dissolution mechanism of carbonate rocks. It was considered that carbonate rocks would dissolve under the three conditions discussed below.

Development of fault system provides the migration path for hydrothermal fluids

The saddle dolomite with high temperature features and most hydrothermal minerals such as dickite, discovered in the S88, T708, S112-1 and S76 wells, fluorite (with fluid inclusion homogenization temperatures >200 °C) in wells S85, S65, S86 and S76, and barite in well S65 are all found in the western part of the Tahe area. In this area, faults and fractures are well developed (Fig. 8). This association suggests that the migration of hydrothermal fluids and attendant mineralization might have been related to the F1 fault and associated minor faults. Hydrothermal dissolution is most prominent along the F1 and associated faults (Fig. 8).

Fig. 8
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Map distribution of the Ordovician fault system in the Tahe area and the hydrothermal minerals discovered in the wells

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Dissolution by hydrothermal acidic fluids

Deep acidic fluids containing large quantities of CO2 and HF migrating through the faults are believed to be responsible for carbonate rock dissolution at shallower depth. The dissolution of carbonate rocks under deep burial conditions is believed to be associated with thermochemical sulfate reduction that could generate a large quantity of CO2 and H2S. The H2S, when dissolved in water, produced H+ and HS−, resulting in enhanced dissolution capacity for carbonate rocks (Zhao et al. 2007, 2013).

The dissolution of grainstones and calcite in pores by strong corrosive media, such as organic acids, hydrothermal brines, and hydrogen sulfide, generally generated the matrix porosity at various scales. The matrix vuggy porosity formed due to burial dissolution was an important complement to original fractures and cavities, which significantly increased the matrix porosity of surrounding rocks. Fault-related vuggy porosity was spatially limited and predominantly occurred in the grainstones along faults and fractures.

Enhanced dissolution with temperature decrease

Hydrothermal fluids are here defined as the fluids that had temperature 5 °C or higher than surrounding rocks (Shen et al. 2010). The solubility of CO2 and H2S in water increases with decreasing temperature (Huang et al. 2008). Cooling of hydrothermal fluids and increased solubility of CO2 and H2S most likely occurred during migration of hydrothermal fluids as they ascended through faults. Therefore, when the hydrothermal fluids containing H2S and/or CO2 (mostly derived from thermochemical sulfate reduction (TSR) in relatively deep strata) migrated upward along faults (Fig. 9), original calcite or dolomite-saturated fluids would become unsaturated, resulting in the dissolution of calcite or dolomite in shallow formation. Thus, high-quality carbonate reservoirs should be better developed along the faults that acted as hydrothermal fluid migration pathways. However, the premise for fluid lateral migration is that the rocks with original high primary porosity and permeability, i.e., grainstones (dolomite) or rudstones/framestones (dolomite), were the facies where secondary vuggy porosity would further develop during the diagenetic process.

Fig. 9
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Schematic diagram of the carbonate dissolution and secondary pore formation mechanism resulting from the upward and lateral migration of hydrothermal fluids containing H2S and CO2 (modified from Huang et al. 2008)

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Hydrothermal dissolution formed a series of cavities with varying sizes (Fig. 3e–h), along faults, which were generally filled or half filled by hydrothermal minerals, high temperature millimeter-scale calcite and saddle dolomite.

A large amount of shrinkage pores are formed during the formation of hydrothermal minerals

Some hydrothermal minerals could generate a large amount of shrinkage porosity, enhancing reservoir space. The occurrence of dickite played a constructive role in increasing reservoir space. Measurements of dickite and kaolinite volumes by Bish and Von (1989) and Joswig and Drits (1986) indicate that during the transformation of kaolinite into dickite, there is a 0.8 % volume reduction. If we consider the primary intercrystalline porosity of kaolinite, then the resulting intercrystalline porosity of dickite would be over 1 %, greater than that of kaolinite. This alone could account for the pervasive intercrystalline porosity associated with dickite. Furthermore, the precipitation of primary dickite from acidic fluids is generally associated with vuggy porosity in carbonate rocks. This phenomenon is also observed in the Ordovician strata in the Tahe area (Fig. 3a, d); therefore, dickite might be an indicative mineral for reservoir development or enhancement by hydrothermal fluids.

The occurrence of fluorite can also significantly improve reservoir quality. During the fluoritization of CaCO3 during metasomatism, pore space relative to that of calcite would increase by 26.4 % (Liu and Cai 2009).

Conclusions

  1. 1.

    The analysis of mineralogical and geochemical data indicates that fluorite with high inclusion homogenization temperatures, dickite, barite and high temperature dolomite (mainly in the Penglaiba Formation) occured in the strata far away from the unconformity surface and the other Ordovician strata, indicating that deep hydrothermal fluids migrated through the Ordovician strata of the Tahe area, Tarim Basin.

  2. 2.

    The wells with hydrothermal minerals are mainly distributed in the west of the basin, where faults and fractures are well developed. When hot hydrothermal fluids ascended along faults, their temperature decreased making them more acidic and aggressive to carbonate rocks.

  3. 3.

    The evolution of hydrothermal fluids in the carbonate reservoir resulted in a large amount of fabric-selective and vuggy porosity formation. In addition, the hydrothermal minerals, such as dolomite, fluorite and dickite, generated a large amount of shrinkage pores that contributed to increased reservoir space.

  4. 4.

    The dissolution of carbonate rocks caused by hydrothermal fluids generally occurred in the vicinity of fluid upward migration pathways (faults or other channels); thus, where these structures can be identified by seismic surveys or other prospecting method, we can predict that a good reservoir would be developed in the area.