Introduction

The Tarim large igneous province (TLIP) in northwestern China was formed in the Early Permian (Fig. 1a). The TLIP includes large volumes of flood basalts and mafic-ultramafic intrusions (Mahoney and Coffin, 1997; Pirajno, 2000; Derek, 2003; Ernst and Buchan, 2003). The formation of the TLIP is related to that of the ~260-Ma Emeishan LIP in southwest China (Chung and Jahn, 1995; Xu et al. 2001; Zhou et al. 2002) and the ~250-Ma Siberian Traps in Russia (Campbell et al. 1992; Arndt et al. 1998; Reichow et al. 2009). Elucidating the process of Permian magmatism is crucial for understanding its relationship with the tectonic evolution and the Ordovician hydrothermal reservoirs of the survey area.

Fig. 1
figure 1

a The location of the Tarim Basin in China. b The distribution of the basic basalt rocks in the Tarim Basin. c The location of the SN 3-D seismic survey and distribution of igneous rocks around the survey (Yang et al. 2005). d Generalized Permian stratigraphy in the SN 3-D survey (Pu et al. 2011)

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The SN 3-D seismic survey is located in the central part of the Tarim Basin at the eastern part of the Tazhong uplift (Fig. 1b). Well SN4 in the survey has produced commercial gas from Ordovician carbonate reservoirs. These hydrothermal reservoirs are related to the faults and volcanic activity. As a result, insights into the Permian volcanic rock distribution and volcanic stages are useful for understanding Ordovician hydrothermal reservoirs. The Tarim Basin has widely distributed volcanic rocks. Yang (2005) drew a map of all the basic and acid volcanic rocks in the basin (Fig. 1c). Two sets of volcanic rocks pinch out in the eastern Tarim Basin, and it is unclear whether this hiatus resulted from erosion or the lack of deposition. Although the Permian volcanic rocks in the Tarim Basin have been intensively studied, most methods used to determine the composition, date and source of the rocks involve field investigations and geochemical experiments. In addition, Keping and Bachu have been the main study areas (Li et al. 2014; Xu et al. 2014; Zhou et al. 2009). However, the distribution, stage, lithology and pinch-out line of volcanic rocks in the SN area remain unclear. Thus, gaps exist in vital knowledge for the investigation of hydrothermal activity.

Based on drilling and 3-D seismic data, this paper analyses the lithology, distribution and stage of the volcanic rocks, the position of the craters and the pinch-out type of the volcanic rocks by the methods of 3-D seismic horizon tracking, post-stack inversion, attribute extraction, volcanic rocks logging crossplot and horizontal well tie contrast to reveal the relationships of these parameters with faults and hydrothermal activity and to obtain useful insights into the formation of hydrothermal reservoirs and hydrocarbon exploration in Shunnan area.

Geological background

The Tarim Basin is located in the south of the Xinjiang Uygur Autonomous Region in northwest China (Fig. 1a). It is the largest inland basin in China, covering approximately 530,000 km2. The basin is bounded by the Tianshan orogenic belts to the north and west and the Kunlun and Aerjin orogenic belts to the south (Fig. 1b). The Tarim Basin is composed of a Precambrian crystalline basement and a thick Palaeozoic-Mesozoic-Cenozoic sedimentary cover (Long et al. 2011). Several major periods of tectono-hydrothermal activity have been identified in the Tarim Basin, and the four most important are Neoproterozoic (774–673 Ma), Ordovician (460–484 Ma), Permian (264–282 Ma) and Cretaceous (~100 Ma). In addition, the Permian magmatism is closely related to the formation of the TLIP.

According to the increasingly rich set of seismic and drilling data, large volumes of both mafic and felsic igneous rocks were emplaced in the Tarim block during the Permian. The mafic series are dominated by basalt, andesitic basalt, diabase and ultramafic rocks, whereas the felsic bodies are dominated by syenite, rhyolite and dacite (Yang et al. 1997; Chen et al. 2006; Li et al. 2014; Wu et al. 2012a; Yang et al. 1996; Chen et al. 2010; Xu et al. 2014). The Tarim LIP consists predominantly of flood basalts that occupy a large area in the western and southwestern parts of the Tarim Basin. The Permian flood basalts in the Tarim Basin thicken from southwest to northeast, and the two most important areas of exposure are the Keping and Damusi regions. The full extent of volcanic rocks in the basin remains unknown because of the deep Palaeozoic to Cenozoic sedimentary cover, and some parts of the volcanic rocks occur in the subsurface (covered by the Taklamagan desert) (Xu et al. 2014). However, the geophysical and borehole data suggest that the Permian basalts extend over an area of ca. 250,000 km2 in the Tarim Basin (Li et al. 2011). In addition, drill-hole data indicate that the basalts range from several metres to several hundreds of metres thick, with an estimated average thickness of ~300 m, suggesting a volume of more than 75,000 km3 (Yu et al. 2011). According to the latest zircon U–Pb age results, the basalts in the Tarim Basin formed between 285 and 290 Ma (Tian et al. 2010; Yu et al. 2011). Furthermore, the Tabei uplift, the northern depression and the western region of the basin developed acid rocks, such as rhyolite and syenite. The area of rhyolite is approximately 0.46 × 105 km2 (Pan et al. 2014), smaller than that of the basalt in the Tarim Basin. Based on the Ar40/Ar39 and SHRIMP test data, the acid rocks formed mainly between 274 and 284 Ma. Only in some northern parts of the basin do both basalt and rhyolite exist (Yang et al. 2006; Huang et al. 2012).

The SN 3-D seismic survey is located in the central part of the Tarim Basin and the eastern part of the Tazhong uplift (Fig. 1c). It is adjacent to the Manjiaer depression in the north and the Tangguzibasi depression in the south. The Permian System in the Tarim Basin consists of the Nanzha Formation, the Kupkuciman Formation, the Kaipeleicike Formation and the Shajingzi Formation, from bottom to top (Xinjiang Bureau of Geology and Mineral Resources, 1993). In addition, the Kupkuciman Formation primarily comprises a cycle from basic to acid volcanic rocks or from basic volcanic rocks to terrestrial clastic rocks (Pu et al. 2011). Based on drilling data, the lowermost Permian Nanzha formation is approximately 200 m thick. This formation mainly consists of grey mudstone and clastic rocks (Fig. 1d). The Kupukuziman Formation within the survey is approximately 100 m thick and contains one basalt layer in the lower part. This basalt layer differentiates the Kupukuziman Formation and the Nanzha Formation. Furthermore, alternating reddish mudstone and siltstone developed beyond the flood basalt (Fig. 1d). The Kupukuziman Formation is characterized by a natural positive-gamma cycle. The Kaipeleicike Formation within the survey area is also approximately 200 m thick and consists of mudstone and glutenite, with thick mudstone on the top part (Fig. 1d). The upper Shajingzi Formation has been eroded. The high-velocity volcanic rocks usually alternate vertically with the low-velocity terrestrial clastic rocks. Additionally, volcanic rocks display a very high-amplitude reflection on seismic sections and are therefore easily recognized and traced to determine their distribution based on integrated interpretation of 3-D seismic and well data (Pu et al. 2012).

Database

The data used in this study consist of a poststack 3D seismic volume and wireline logs. The 3-D seismic survey covers an area of approximately 1195 km2. There are 2700 lines and 2200 traces in this 3-D seismic volume with 25 and 25 m bin spacing, and the sampling interval is 2 ms. The dominant frequency is approximately 35 Hz. The P-wave velocity of volcanic rock is approximately 5800 m/s and of the overlying and underlying clasticstone are approximately 3500 m/s. The horizon of Permian volcanic rock (T5 1) was picked for attribute extraction. The T5 1 horizon is shown as a continuous peak in entire seismic survey.

Since 2013, exploration wells SN1, SN4 and SN5 have been drilled within the survey area, and exploration wells SN2, SN3, GL1 and GL2 have been drilled near the survey area. The wells TZ45, Z13 and AD1 are distributed in the Tazhong area. Various combinations of natural gamma-ray, resistivity, density and acoustic logs are available for these wells. The Permian volcanic rocks and some Ordovician carbonate hydrothermal reservoirs were drilled in wells SN1, SN4 and SN5. Records of the gas production from the well SN4 indicate that study area produced commercial gas from the Ordovician carbonate reservoirs. No volcanic rocks were drilled in wells SN2 and SN3 adjacent to the survey area.

Study of the Permian volcanic rocks

Identification of the volcanic rocks lithology

Each type of volcanic rock exhibits a distinctive log response characteristic. The Permian volcanic rocks encountered in wells SN1, SN4 and SN5 within the SN survey are identified according to the well logging curves and ditch cutting data because cores are lacking. The volcanic rock type in the area is simple and mainly comprises flood basalt and tuffite generated from Permian magmatic activity. The basic basaltic magma effusived from the craters and formed the flood basalt. The great distribution extent of flood basalt is because of the lava’s fluidity. The tuffites which are thin-bed interbed with different lava flow are located beyond the flood basalt. The nature gamma values of tuffites within the study survey are between 50~60 API. According to Zhang et al. (2014), the GR value of basic flow deposition tuffite is always between 40.84 and 64.48 API and of the fallout is always between 68.43 and 112.4 API. Moreover, the tuffite within study survey distributed in a narrow scope and near the volcanic craters, so they should belong to basaltic flow deposition. At the early period of flood basalt activity, the basaltic magma met the water-bearing layer underground when they upwelled and then intensive interaction occurred. This phenomenon has a certain relationship with the formation of the tuffite within study survey (Peate et al. 2003; Peate et al. 2005). No intrusive rocks are found in this area. The depth of flood basalt in well SN4 is 2841–2861 m, and its thickness is approximately 20 m. It is characterized by straight and low natural gamma (smaller than 50 API), a low acoustic time (50–70 μs/ft) and high resistivity (2–1000 Ωm) (Fig. 2). The flood basalt data from wells SN1, SN5 and SN4 show wide variations in their acoustic time, density, natural gamma and resistivity logs (Fig. 2). The resistivity and density are low (2 Ωm and 2.4 g/cm3) at the flow top, gradually increasing to a maximum (1000 Ωm and 2.7 g/cm3) in the flow core and then decreasing sharply at the flow base. In contrast, the natural gamma and acoustic times are high (47 API and 75 μs/ft) at the flow top, gradually decreasing to a minimum (33 API and 55 μs/ft) in the flow core and then increasing sharply at the flow base. This cyclical pattern in the wireline log data is often observed in flood basalt facies (Planke 1994; Bücker et al. 1998) and is caused by the vertical variations in vesicularity and fracturing that characterize flow lobes of flood basalt (Nelson et al. 2009). On the top part, many vesicles and fractures reduce the sonic velocity and density. In addition, the top often shows a greater degree of alteration than other parts, resulting in a high gamma ray value. Because the flow core has low degrees of fracturing and vesicularity, the velocity and density are high. The Permian flood basalts in the three wells have natural gamma log values of approximately 37 API, but their average acoustic time values increase from west to east: well SN1, 50 μs/m; well SN5, 55 μs/m; and well SN4, 70 μs/m. In addition, the resistivity decreases to a certain extent. This phenomenon probably indicates that the eastern parts suffered severe alteration and secondary mineralization.

Fig. 2
figure 2

The Permian stratigraphic cross-sections of wells SN1, SN5 and SN4 in the SN3-D survey. The Permian Kupkuciman Formation developed one stage of basalt, whereas the Kaipeleicike Formation is composed of terrigenous clastic rock only. See Fig. 1 for the well locations

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The cutting logging indicates that tuffite developed in wells SN4 and SN5 in the study area. These tuffites are similar to the core tuff from the adjacent area on the crossplot of volcanic logging (Fig. 3). The tuffites are characterized by high acoustic time (95 μs/ft) and low natural gamma values (45 API). However, it is very difficult to determine the source and components of the tuffites using only ditch cutting information. In addition, because some terrigenous materials mixed into the tuffite during the diagenetic process, the tuffite and terrigenous clastic rocks usually have similar or approximately identical log values. Therefore, because of the lack of core data, cuttings and logs are used to infer that the basalt is dominant in the area, with the occasional appearance of very thin tuffite.

Fig. 3
figure 3

The logging crossplot of volcanic rocks in the Tahe and SN areas (modified by Luo et al. 2006). Some of the samples within the 3-D survey exhibit low natural gamma (approximately 40 API) and low AC (57–65 μs/ft) and are basalt. In contrast, others within the 3-D survey exhibit high natural gamma (57–60 API) and high AC (78–92 μs/ft) and are tuff

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Volcanic stage and horizon

The Shunnan area Permian volcanic rocks are all categorized as effusive rocks. The magma upwelled along the volcanic feeders and condensed to rocks after erupting to form the craters. Thus, the areas where volcanic rocks are distributed have developed volcanic feeders, and the underlying stratigraphy experienced hydrothermal activity. Such hydrothermal activity has a vital influence on the formation of Ordovician dolomite hydrothermal reservoirs. Therefore, the location of the volcanic rocks and the lacuna type of the volcanic rocks (lack of deposition or erosion) both warrant further study.

Previous research studies have shown that volcanic rocks are mostly developed in the Permian Kupkuciman Formation and the Kaipeleicike Formation in the Tarim Basin (Pu et al. 2011). Based on the integrated interpretation of wireline logs, cuttings and seismic data, the wells in the SN survey contain Permian Kupkuciman volcanic rocks that were deposited in a restricted area. The Kaipeleicike Formation in the 3-D survey was subjected to incomplete denudation, and no volcanic rocks were drilled (Fig. 4).

Fig. 4
figure 4

The Carboniferous-Permian stratigraphic cross-sections of wells Z13-TZ45-SN1-SN5–SN4-GL1 from the centre to the east of the basin; see Fig. 1c for the well locations

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The lowermost Permian Nanzha Formation is lithologically stable within the whole basin and is primarily composed of dark-coloured marls and light-coloured chalks with locally interbedded silt to fine sandstone (Guo et al. 2011). The Nanzha Formation is easily identified by the underlying disconformity contact with the Carboniferous Kalashayi Formation limestone and is overlain by Kupkuciman Formation volcanic rocks (Fig. 4). Based on the existence of the Nanzha Formation and volcanic rocks, the cycle from low natural gamma volcanic rock to high natural gamma terrigenous silicic clastic rock situated above the Nanzha Formation in wells SN1, SN5 and SN4 should belong to the Kupkuciman Formation. No volcanic rocks were drilled in the Kaipeleicike Formation, and the uppermost Permian Shajingzi Formation was eroded out completely from wells SN1, SN5 and SN4 (Fig. 4). The above stratigraphic correlation confirms that no volcanic rock of the Kaipeleicike Formation occurs in the SN1 3-D survey; thus, the volcanic rock observed here should belong to the Kupkuciman Formation. No volcanic rock in the Permian Kupkuciman Formation is contained in the wells outside of the SN1 3-D survey (e.g. GL1, GL2, SN2 and SN3); however, equivalent strata exist, with lithologies changed to sandstone or sandy conglomerate. Therefore, the pinch-out of volcanic rocks in the SN1 3-D survey likely resulted from the lack of deposition. Compared with the wells in the Tazhong and Tahe regions, the Carboniferous System in wells SN1 and SN4 contains the Standard Limestone section and the Bioclastic Limestone section; similar findings were noted in well TZ45 (Fig. 4). After the Permian Period, intense uplift occurred in the Tadong and Bachu regions, and the remaining Permian System thickness decreases eastward. The Permian Kaipeleicike Formation around the SN1 3-D survey is more likely to be eroded to varying degrees. However, it is free of volcanic rock.

Distribution of volcanic rock and changes in lithofacies

The volcanic rock typically shows high-amplitude reflections on the seismic sections (Pu et al. 2012). The distribution range and thickness change of volcanic rocks can be determined by the integrated interpretation of seismic attributes and inversion. The high-amplitude reflection at the volcanic horizon (T5 1) is caused by the large wave impedance contrast between volcanic rock with high wave impedance and sandy mudstone with low wave impedance. Therefore, high-amplitude areas on the amplitude map typically correspond to the distribution of volcanic rocks. The maximum wave crest amplitude attribute (Fig. 5) extracted along the T5 1 ± 15-ms time window indicates that the volcanic rocks in the 3-D survey show a distribution with two branches from south to north, with the larger branch in the NNW direction (70 km long and 30 km wide) and the smaller branch in the NNE direction (20 km long and 2–5 km wide). The volcanic rock margin shows a branching zigzag pinch-out line in the downdip direction to the north and northwest but a straight pinch-out line in the higher south, suggesting a steep southern updip ancient landform with completely obstructed magma and a gentle smooth northern downdip slope on which lava flowed forward until terminated.

Fig. 5
figure 5

The maximum peak amplitude map of the volcanic rock horizon (T5 1 ± 15 ms) in the SN 3-D survey. The volcanic rocks show a distribution with two branches from south to north, and the high amplitude means thick volcanic rocks. See Fig. 1c for the location of the survey

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Wells SN2, SN3 and GL1 are located outside the depositional range of the volcanic rocks, and thus, the absence of volcanic rock in these wells is attributable to a lack of deposition rather than erosion. The spatial distribution of volcanic rock and lateral changes in lithology and thickness can be interpreted from well-constrained seismic inversion data on wave impedance and natural gamma values. Indeed, the wave impedance and natural gamma inversion profiles show that volcanic rocks are thicker in well SN1, thinner in well SN4 and thinnest in well SN5 (Fig. 6). The wave impedance within the range of volcanic rocks is uneven in distribution. Some minor uneven variations in the wave impedance of basalt between wells SN1 and SN5 suggest a change in the lithology or pore gas content. A decreasing trend of wave impedance exists from west to east, whereas the natural gamma values remain nearly unchanged.

Fig. 6
figure 6

The well-tied wave impedance (a) and natural gamma ray (b) inversion profiles of wells SN1, SN5, and SN4; see Fig. 1c for the location of wells. The volcanic rocks are thicker in well SN1, thinner in well SN4 and thinnest in well SN5

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The wave impedance values of volcanic rocks near wells SN4 and SN5 are much lower than those near well SN1 (Fig. 7a). In contrast, the natural gamma values of volcanic rocks near wells SN4 and SN5 are slightly higher than those near well SN1 (Fig. 7b). The higher wave impedance and lower gamma ray values in the west near well SN1 indicate pure basalt rock. The high gamma values and low wave impedance in the east near wells SN4 and SN5 indicate that the volcanic rocks contain more tuff or pores. The processed waveform classification of the volcanic horizon (T5 1 ± 15 ms) is used to divide the waveforms of volcanic rocks into ten types in terms of lava thickness, natural gamma values, wave impedance and density. Different colours indicate different facies. Combining the inversion results (Figs. 7a, b) and the waveform classification reveals three facies zones in the Permian volcanic rock in the SN 3-D survey. These three seismic facies zones are a compact basalt facies zone with maximum wave impedance (40,000–46,000 f/s*cm3/g) and minimum natural gamma (38–43 API), a pore-bearing basalt facies zone with medium wave impedance (37,000–41,000 f/s*cm3/g) and natural gamma (45–50 API) and a tuffaceous basalt facies zone with lower wave impedance (30,000–33,000 f/s*cm3/g) and higher natural gamma (50–55 API) (Fig. 7c). The flood basalt thickness of the compact basalt facies zone is the greatest (exceeding 40 m) because of its low topography. Thus, the natural gamma value here is low, which is in good agreement with the inversion results. Wells SN4 and SN5 are located in the tuffaceous basalt facies zone, and both developed thin sedimentary tuff beyond the flood basalt layer (Fig. 2), which resulted in their high natural gamma values. The pore-bearing basalt facies zone is located at the margin of the volcanic rocks and has experienced alteration, which has been shown to increase the gamma values in basaltic rocks (Planke et al. 1999 ).

Fig. 7
figure 7

a The average inversion impedance map at the T5 1horizon of volcanic rocks. b The average inversion gamma map at the T5 1 horizon of volcanic rocks. c The waveform classification map of the reflection of volcanic rocks in the time window T5 1 ± 15 ms. d The thickness map of the volcanic rocks. e The time thickness map of the Lower Permian reflection from horizon T5 1 to T5 4. f The depth contour map of the top volcanic rocks

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The volcanic rock thickness map is drawn according to the relationship among the tuning amplitude, the thickness of volcanic rock and the wave impedance inversion result (Fig. 7d). It clearly shows the two largest thickness centres with maximum thicknesses of approximately 40 m. The thicker area in the northwest corresponds to the high inversion wave impedance of volcanic rocks. Based on previous research on volcanic rocks in the Tahe field (Pu et al. 2012) and the south rifts of Songliao Basin (Wang et al. 2003), the flood basalt is thicker in the ancient lower topography because of lava downdip flow and accumulation. Therefore, the considerable thickness of the flood basalts in the northwestern SN 3-D survey was caused by the low palaeogeomorphology in this direction.

The T5 4-T5 1 stratigraphic thickness map approximates the depositional thickness of the lower Permian Nanzha Formation to the Kupkuciman Formation. Because the Kupkuciman Formation was not subjected to later denudation, the thickness map reflects the overall ancient structural or topographic background when the volcanic rocks were being deposited. It shows a north-dipping ancient structure (Fig. 7e) and lava flowing from south to north after eruption. This ancient structural feature is consistent with the current structure of the top volcanic rocks (Fig. 7f).

Crater characteristics and distribution

Previous observations of volcanic craters have always been based on field work and laboratory studies, and analysing the geochemistry and petrography has played an important role. However, with the improvement of 3-D seismic data, seismic profiles can reveal the location, external form and internal structural characteristics of many craters (Joppen and White, 1990; Hansen, 2006; Magee et al. 2013; Hansen and Cartwright 2006). In addition, the seismic characteristics can also provide valuable evidence for studying the magma’s intrusion and eruption mechanisms, the factors that control magmatism and several other important issues.

The volcanic craters in the Tazhong area have arch reflections, weak amplitude coherence anomalies, increased thickness and are rich in volcanic explosion facies (Pu et al. 2012). These craters have a direct relationship with magmatism. The basaltic magma upwelled along the volcanic feeders and erupted from these craters. The feeders cannot be seen on the seismic profile because their diameters are always less than 100 m in the Tazhong area. The craters consistently developed around the intersections of multi-set faults. The crater diameters are typically 200–500 m and are sometimes unclear in the seismic profiles.

The craters in the 3-D area are clearly shown in both volcanic horizon slices of the coherence cube (Fig. 8a, b) and the seismic profile (Fig. 8c). The craters on the horizon (T5 1–5 ms + 20 ms) slice of the coherence cube of the research area are characterized by circular low coherence, with the crater diameters ranging from 100 to 1000 m (mainly 200 to 500 m). The craters in the south to the midpoint between wells SN4 and SN5 (Fig. 8b) and around the NNE fault zone of well SN4 are located in a rectangular area (5 km wide and 20 km long along the northeast direction); the distance between craters ranges from 1 to 10 km (typically 2 to 4 km) for approximately 18 craters, among which the 9 larger craters are remarkable. The craters are similar to the modern basalt craters seen in Wudalianchi, Australia (Blaikie et al. 2014) and Japan (Kenji et al. 2014) in terms of their scale.

Fig. 8
figure 8

a A coherent slice map along the T5 1 horizon within the SN 3-D survey, where the small white dots in the blue rectangle are craters. See Fig. 1c for the location of survey. b Enlarged view of the blue rectangle, with the volcanic craters shown as small white dots. c The seismic section across the crater, showing the bending and decrease in the amplitude at the craters of the T5 1 volcanic reflection event

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The T5 1 event of the crater position on the seismic profile shows weak amplitude and fall-down and subsequent fill-up at the location of the fall. The overlying horizon T5 4 and underlying horizon T5 0 do not change in their attitude. The fall-down of the volcanic rock surface is attributable to the condensation and contraction of magma at the crater and local subsidence. The fall-down event indicates the collapse of the volcanic rocks, with the overlying deposits subsequently filling and levelling the collapse. The attitudes of the formations overlying and underlying the crater remain horizontal. These features typically form craters with basic magma, whereas the acidic magma craters observed in the Tahe field and the Songliao basin readily take on mound-shaped appearances because of their high magma viscosity, weak mobility and palaeo-topography.

Relationship between volcanic rock and faults

Faults are likely the channels for volcano formation and magmatism (England et al. 1987; Wu et al. 2012b). The Tarim Basin experienced multi-stage tectonic movements and includes hundreds of faults with different locations, stages, properties and dimensions (Coleman et al. 1989; Allen et al. 1999; Ren et al. 2011). The faults in the Tarim Basin are mainly NW, NE and E-W trending (Yan et al. 2014). There are three sets of vertical faults within the 3-D survey: NNE, NEE and NWW. The NWW set of faults and NEE set of faults are shown in Figs. 9a, b, respectively. The dip angles of these strike-slip faults are close to vertical, and the displacements are small. The flower structure can be seen in profiles, and the faults are characterized by an en echelon distribution in a planar view (Fig. 9c). Detailed examination of the 3-D seismic profile reveals that the strata thicknesses at the vertical fault zone differ from those of the bilateral fault sides at the same period. Moreover, the horizons of the fault zone pull down as their thicknesses increase but pull up as their thicknesses decrease. The former are identified as synsedimentary tension-shear faults, and the latter are classified as synsedimentary compression-shear faults. Based on the insight into the fault properties and active phases presented above, we conclude that in the 3-D survey, the NEE and NWW faults are conjugate and syngenetic transtensional faults from the early Caledonian period. From the Middle Cambrian period, the NWW faults subsequently stopped their activity. After the standstill episode in the Late Cambrian period to the Early Ordovician period, however, the NEE faults initiated transtensional action in the initial depositional stage of the Middle Ordovician Lianglitage Formation and then became quiet at the end of deposition of this formation. NNE faults resumed their syngenetic and compressive actions from the Late Ordovician Sangtamu Formation to the Early Silurian Kepingtage Formation and then exhibited transtensional movement during the deposition of the Middle Silurian Tataaiertage Formation. During the Devonian-Carboniferous period, the NEE faults remained dormant, but in the Permian System, they were revived by the upward movement of magma. Briefly, the NNE faults are the latest, the NEE faults are earlier and NWW are the earliest (Table 1).

Fig. 9
figure 9

The seismic responses of three sets of faults in the SN3-D survey: NNE, NEE and NWW trending. a The profile of NWW fault ( A–A’). b The profile of NEE fault ( B–B′). c The map of the three sets of faults in the SN3-D survey

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Table 1 The elements of three groups of differently trending faults in the SN 3-D survey
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The planar relationship between the three sets of faults and the craters is shown in Fig. 10. The craters are primarily located near the intersections between the NWW fault F7 and the NNE fault F3. The volcanic rocks on both sides of the NNE fault F3 are the thickest (up to 40 m). The NWW fault F7 furcates into three small ones close to the craters from an individual fault in the centre of the study area far away from the craters, showing an apparent increase in the fault density or a decrease in the fault interval from 10 to 2–3 km near the craters. The older Caledonian faults were the passageways for magma flowing from the deep strata to the shallow strata, and the newer Hercynian faults allowed lava to erupt through the surface from underground. This spatial distribution of craters accounts for the joint contribution to the intensity and intersection of multiple set of faults instead of only the newer-stage Permian faults when the volcanoes were erupting.

Fig. 10
figure 10

The relationships among the Ordovician high-amplitude anomalies and faults, the volcanic craters and the thicknesses of volcanic rocks in the 3-D survey. See Fig.1.c for the location of survey. The craters developed on both sides of the NNE faults near well SN4, and their positions are related to the intersections of the dense early NEE and NWW faults and the late NNE faults

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Relationship between volcanic rock and hydrothermal activity

The outcrop and volcanic dating analysis shows that four major times of magma activity and geological thermal events exist in the Tarim Basin—the Sinian-Cambrian period (774 ± 0.18 ~ 673.1 ± 55.4 Ma), the Ordovician period (460.2 ± 2.2 ~ 484.5 ± 2.2 Ma), the Permian period (26 ~ 282 Ma) and the Cretaceous period (100.9 ± 3.4 Ma)—of which the Permian magma activity is the most intense and the most extensive in distribution (Chen et al. 1997). The generation of hydrothermal fluid should be closely related to volcanic magma, and the magmatic fluid and hydrothermal fluid reworked by magma in adjacent strata dissolved the Ordovician carbonate rocks when migrating along the faults and fissures (Jin et al. 2006).

The Tarim Basin Cambrian-to-Ordovician strata form a marine sedimentary system that mainly comprises carbonate rocks. The Tazhong area developed the Middle Cambrian Shayilike Formation (∈2S) and Avatager Formation (∈2a), the Upper Cambrian Qiulitag Formation (∈3ql), the Lower Ordovician Penglaiba Formation (O1p), the Lower-Middle Ordovician Yingshan Formation (O1~2y), the Middle Ordovician Yijianfang Formation (O2yj) and the Upper Ordovician Qiacrbake Formation (O3q), Lianglitake Formation (O3l) and Sangtamu Formation (O3S) (Fig. 11). Contacts between these formations are conformable or paraconformities (Zhang and Gao 1992). The Cambrian and Penglaiba Formations mainly comprise dolomite rocks and are interbedded with thin limestone and calcitic dolomite. The lower Yingshan Formation is primarily composed of calcitic dolomite, and the upper Yingshan Formation mainly consists of limestone. The dolomitic component exhibits an obvious decrease from bottom to top (Zhao et al. 2012). The Yingshan Formation developed large amounts of hydrothermal dolomite and thermally dissolved pores, which occurred along the faults and paraconformity planes. This set of hydrothermal dolomite reservoirs is one of the most significant gas reservoirs within the Tarim Basin.

Fig. 11
figure 11

The Cambrian-to-Ordovician stratigraphic column of Shunnan area in Tarim Basin (modified by Zhao, 2012)

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There is no unconformity or disconformity between the Ordovician carbonate rocks and the overlying Qiaerbake Formation and Lianglitage Formation. No hiatus occurs within the Ordovician Penglaibaba Formation, Yingshan Formation and the Yijianfang Formation. Thus, the talpatate karst of the Ordovician carbonate rocks is not the main dissolution type. The burial dissolution and organic acid dissolution are related to hydrocarbon generation, and the entrances into the reservoir exhibit limited dimensions and strength. On the seismic sections, therefore, the many karst-related high-amplitude reflections (narrow vertical bead reflections) observed in the weak amplitude background of the thick Ordovician carbonate rocks did not originate from buried karst or talpatate karst. The Ordovician limestone reservoir that produced oil and gas and displays many dissolved vugs on the cores in the well SN4 should have formed principally via hydrothermal dissolution.

Hydrothermal fluid carries abundant volatile substances, such as organic acids, CO2, H2S and SO2. These substances entered into reservoirs along faults, fissures, bedding, unconformities and other pores, forming a large number of tubular and tabular caves or vugs through karst alteration of the surrounding limestone. These cave-related reservoirs show exceptionally strong vertical bead reflections or horizontal anomalies on their seismic sections (Fig. 12a, b). The tubular caves appear as dots on the plane and cover areas smaller than 0.3 km2, whereas the tabular caves appear as flakes on the plane and cover areas exceeding 0.3 km2 (Fig. 12c). The analysis shows that the tubular anomalies are distributed widely throughout the 3-D survey and that the high-amplitude tabular anomalies are primarily distributed in the eastern and southern parts of the survey. We also analysed the Reconnaissance Map Series (RMS) maps extracted from the Yijianfang Formation (O2yj, T7 4) to the bottom of the Cambrian system (T8 0 + 160 ms) with an interval of 80 ms. The high-amplitude anomaly statistics (Table 2) show that the distribution of high-amplitude tubular anomalies is concentrated in the Penglaiba Formation (horizons T8 0-T7 8), accounting for 85 % of all tubular anomalies, whereas the tabular anomalies are mainly distributed in the lower Penglaiba Formation and the Cambrian system (under T7 9), accounting for 71.9 % of all tabular anomalies. Why are the tubular caves more likely to develop in the Penglaiba Formation than in the Yingshan Formation and the Yijianfang Formation? One reason is that the carbonate rocks in the Early Ordovician Penglaiba Formation were more likely to be subjected to hydrothermal processes than those in the later Yingshan Formation and Yijianfang Formation because the Penglaiba Formation experienced one more magma and hydrothermal fluid reworking event in the Early Ordovician period than the other, later deposited carbonate rocks of the Yingshan Formation and the Yijianfang Formation.

Fig. 12
figure 12

a The profile shape of the tubular high-amplitude anomalies. b The profile shape of the tabular high-amplitude anomalies. c The planar graph of the T7 8 + 80 ms average reflection intensity amplitude. See Fig. 1c for the location of survey

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Table 2 Ordovician Yijianfang Formation andCambrian system high-amplitude anomaly-type statistics in the SN 3-D area
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Although no correlation between the distribution range of the tubular high amplitude and the craters and faults was observed, the tabular karst caves and craters have similar development and distribution patterns: they are all concentrated in the dense old fault zone or the intersections between new and old faults (Fig. 13). The thickness of the high amplitude relevant to the karst is increased in the intersection between the new and old fault zones. Similar to the volcanic activity, the distribution of these flaky karst caves is also concentrated in the dense fault zone and the intersections between new and old faults. NEE and NWW old faults broke and dislocated rock at depth and reduced their strength, thereby allowing the magma and hydrothermal fluid to upwell and dissolve the surrounding carbonate rocks.

Fig. 13
figure 13

The relationship between the high-amplitude anomalies of the Ordovician carbonate rocks and the locations of volcanic craters, showing similar development and distribution patterns. See Fig. 1c for the location of survey

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Conclusions

  1. 1.

    Permian volcanic rock in the SN3-D survey is primarily composed of basalt and tuffite rocks, which constitute the lower Permian Kupkuciman Formation. Above this, the Kaipeleicike Formation consists of terrigenous clastic rock, and the Shajingzi Formation is denuded. The absence of volcanic rock near wells SN2 and GL1 is attributed to lack of deposition. The Permian volcanic magma in the survey erupted and flowed northwestward along a furcating low ravine under the ancient northwest dip topographic background, creating a northward branching lobe of volcanic rocks. The lobe shows a dendritic pinch-out line in the down-dip north boundary and a straight pinch-out line in the steep up-dip south boundary.

  2. 2.

    Three sets of faults developed in the 3-D survey, namely NNE, NEE and NWW trending, with craters developed on both sides of the NNE fault near well SN4. The crater positions are related to the intersections of the dense early (Caledonian period) NEE and NWW faults and the late (Hercynian period) NNE faults. The older Caledonian faults allowed magma to flow from deep to shallow strata, and the newer Hercynian faults provided a path for the Permian magma to erupt through the surface from the shallow strata.

  3. 3.

    The Ordovician limestone reservoir can be primarily attributed to hydrothermal dissolution in the 3-D survey. A large number of tubular and tabular high-amplitude anomalies are evident in the Ordovician carbonate rock reflection profiles. These anomalies resulted from the hydrothermal dissolution that occurred in the Penglaiba Formation and its surrounding carbonate rocks. The distribution of Ordovician tabular high-amplitude anomalies that are possibly related to the hydrothermal dissolution of carbonate rocks is similar to that of the Permian craters. Both of these structures were affected by the intensities and intersections of multiple sets of faults.