The upper crustal structure of the Qiangtang Basin ...

Tectonophysics 606 (2013) 171–177

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The upper crustal structure of the Qiangtang Basin revealed by seismic reflection data☆ Zhanwu Lu a,b, Rui Gao a,b,⁎, Yongtie Li c, Aimin Xue d, Qiusheng Li a,b, Haiyan Wang a,b, Chaoyang Kuang e, Xiaosong Xiong a,b a

Key laboratory of Earth Probe and Geodynamics, Ministry of land and Resources of the People's Republic of China, Beijing 100037, China Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China Exploration and Development Academy of CNPC, Beijing 100083, China d Beijing Petrosound Geoservices Ltd., Beijing 100016, China e No.6 Geophysical Prospecting Team, East China Petroleum Bureau, SINOPEC, Nanjing 210007, China b c

a r t i c l e

i n f o

Article history: Received 31 August 2012 Received in revised form 11 July 2013 Accepted 18 July 2013 Available online 27 July 2013 Keywords: Qiangtang Basin Shallow crustal structure Seismic reflection profile Central Tibet

a b s t r a c t The Qiangtang Basin, located in central Tibet, is a thick and widely developed Jurassic marine sedimentary stratum, and it is the largest marine basin on the Chinese mainland without a breakthrough in oil or gas exploration. Various forms of crustal movement related to the convergence between the Eurasian and Indian plates in the Cenozoic have played significant roles in the formation and preservation of the oil and gas resources of the Qiangtang Basin. To determine the shallow crustal structure of the Qiangtang Basin and forecast its prospects for oil and gas extraction, we reprocessed the seismic reflection data (0–6 s TWT) from 11 reflection sections acquired at different times by different groups and connected them to form a 350-km shallow seismic reflection profile across the Qiangtang Basin. This profile provides reliable data on the north to south changes of the basement and the upper crustal structures of the Qiangtang Basin. We speculated that the reflective events at 3–4 s TWT in the Qiangtang Basin represent the Paleozoic basement, which runs shallow beneath the central anticline. The location of the Proterozoic basement was determined from the discontinuous reflection events at approximately 4.5 s TWT. The data indicate that the basements of the Qiangtang Basin are deeper in the south and shallower in the north. The shallow crustal deformations (approximately 0–3 s TWT) are quite different between the north and south Qiangtang Basin. In the north Qiangtang Basin, there are strong fold deformations alternating between uplifts and depressions, while the deformations are relatively flat in the south Qiangtang Basin. Continuous arc reflections, interpreted as Paleozoic strata, were found beneath the central anticline of the Qiangtang block. A half graben on the north side of the central anticline represents a possible location for oil and gas resources. © 2013 The Authors. Published by Elsevier B.V. All rights reserved.

1. Introduction The Qiangtang Basin, located in central Tibet, is the eastern part of the Tethys tectonic domain, which is known to be rich in oil and gas reserves (Ding et al., 2013; Zhao and Li, 2000). The Qiangtang is a residual Mesozoic marine sedimentary basin in which Jurassic sediments are the most widely distributed marine sedimentary strata (Ding et al., 2011; Otto, 1997; Wang et al., 1997). Previous studies have indicated that the Qiangtang Basin is a possible location of hydrocarbon reserves (Fu et al., 2012; Guo et al., 2008; Li et al., 2010; Liu et al., 2011; Zhao et al., 2006), and it is the largest marine basin in China without a breakthrough in oil or gas exploration. ☆ This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial-No Derivative Works License, which permits non-commercial use, distribution, and reproduction in any medium, provided the original author and source are credited. ⁎ Corresponding author at: Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China. Tel.: + 86 10 68999730. E-mail address: [email protected] (R. Gao).

Structurally, the Qiangtang Basin is bordered by the Jinsha suture zone to the north and the Bangong–Nujiang suture zone to the south, and the region consists of the north Qiangtang depression, a central uplift and the south Qiangtang depression (Wang et al., 2008; Zhao and Li, 2000). The Mesozoic and Cenozoic convergences of the Eurasian and Indian plates created various forms of crustal movement in the basin (Guynn et al., 2006; Pullen et al., 2008; Shi et al., 2004; Zhai et al., 2011), exerting a significant influence on the formation and preservation of the area's oil and gas resources. To determine the subsurface structure of the Qiangtang Basin and determine its prospects for oil and gas extraction, China's petroleum industry conducted seismic exploration tests in the 1990s (Zhao et al., 2001). Since 2004, the Ministry of Land and Resources of China, in its strategic assessment survey of the oil and gas in the Qinghai-Tibet Plateau, has conducted petroleum seismic reflection profiling experiments in the Qiangtang Basin, which have yielded a new understanding of the subsurface structure of the area (Lu et al., 2006a, 2006b; Lu et al., 2009). In 2009, the SinoProbe Project conducted a north–south seismic reflection profile in the Qiangtang Basin, and most of the lines obtained

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Z. Lu et al. / Tectonophysics 606 (2013) 171–177

from this study coincided with the previously completed petroleum seismic reflection profiles. In comparison with previous experiments, however, the new probe used deeper shot depths, larger charge sizes, longer recording times and higher folds. After splicing and reprocessing these new data along with the original oil seismic reflection data, we obtained a complete north–south profile across the Qiangtang Basin that provides valuable data on the upper crustal structure and its variations from north to south along the region. 2. Location of the profile and data acquisition The seismic reflection data used in this study, consisting of 11 separate sections, were acquired by SinoProbe in 2009, the Ministry of Land

and Resources in 2007–2008, and an oil company in 1996–1997. Only the first 6 s of the SinoProbe section data were integrated with the earlier datasets. Shown in Fig. 1 are the 11 sections, which trend along E88.5° nearly north–south, and Table 1 displays the basic acquisition parameters of these sections. The SinoProbe profile consists of 11 sections (1 and 2 are completely new, while the rest coincide with previously obtained sections). However, the data for the SinoProbe sections were taken over comparatively longer periods (30 s), as they were meant to explore the structure of the area at greater depth, while the oil industry exploration focused on the near-surface structure and only recorded 6 s of data for each section. The SinoProbe sections were obtained using explosive sources with variable shot sizes. In the two southernmost sections (1 and 2), small shots

Fig. 1. Map showing BNS-QT seismic reflection profile locations in the Qiangtang Basin with an inset showing research area locations (1–11 are section numbers; base map is the terrain map downloaded from http://www.globalmapper.com).


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Table 1 Acquisition parameters corresponding to the sections shown in Fig. 1. Number in Fig. 1

Spread

Shot depth/charge size

Shot interval/group interval (m)

Fold

Acquisition time

1

17975-225-50-225-17975

2009

17975-225-50-225-17975

72

2009

3

4980-220-40-220-4980 17975-225-50-225-17975 9580-20-40-20-9580 17975-225-50-225-17975 23980-20-40-220(20)-20-23980 17975-225-50-225-17975 4980-220-40-220-4980 17975-225-50-225-17975 6070-125-50-125-6075 17975-225-50-225-17975

250/50 (small shot) 1000/50 (middle shot) 250/50 (small shot) 1000/50 (middle Shot) 80/40 500/50 120/40 500/50 100/20; 160/40 500/50 80/40 500/50 100/50 500/50

72

2

50 m/100 kg (small shot) 2 × 50 m/200 kg (middle shot) 50 m/100 kg (small shot) 2 × 50 m/200 kg (middle shot) 12 m/6 kg 2 × 50 m/200 kg 18 m/18 kg 2 × 50 m/200 kg 18 m/18 kg 2 × 50 m/200 kg 12 m/6 kg 2 × 50 m/200 kg Vibrators 2 × 50 m/200 kg

60 36 80 36 60–96 36 60 36 60 36

1997 2009 2008 2009 2007 2009 1997 2009 1996 2009

4 5 6–10 11

(50 kg of explosives) at 30 m depth were spaced at 250 m apart, augmented by larger shots (200 kg) at 50 m depth spaced horizontally at 1 km apart. In the remaining sections (3 to 11), only 200 kg shots spaced at 500 m apart were used because of the previously collected petroleum seismic reflection sections. A linear array of receivers was used, with a group interval of 50 m. The data were acquired on 720 channels of a Sercel 408 XL. In addition, a large (1000 kg) shot (a combination of 10 wells, each at a depth of 50 m and containing a 100 kg shot) was placed every 50 km and measured using 960-channel receivers spaced at intervals of 50 m. Each of these large shots covered a horizontal data collection distance of 48 km, with the shot centered in the middle. Sections 1 and 2, as previously discussed, have 72-fold coverage from entirely new data. The other 9 sections have 96-fold coverage after the SinoProbe and existing data are combined. The complete data set creates a seismic reflection profile, approximately 350 km in length, across the Bangong–Nujiang suture zone, the south Qiangtang Basin, the central anticline, the north Qiangtang Basin and beyond the southern boundary of the Jinsha suture zone (referred to as the BNS-QT profile). This integrated dataset provides a foundation for the study of the basement of the Qiangtang Basin and its upper crustal structure and the north–south variation.

3. Data processing In the profiles of the study, the complicated and changeable geological structure of the region, its rugged topography and wind noise are the main factors affecting the quality of the processed data. A number of commonly used processing methods, including data splicing, tomography static, noise attenuation, amplitude compensation, pre-stack deconvolution, velocity analysis, residual statics, and dip moveout

Table 2 Basic steps for the reprocessing of the reflection data for the Qiangtang Basin. Read 6 s SEG-Y data Apply geometry Data splicing Trace editing Tomography static: datum level = 5000 m; replacement velocity = 5000 m/s Noise attenuation Surface consistent amplitude compensation Surface consistence deconvolution: prediction step = 24 ms; operator length = 200 ms; white noise coefficient = 0.1% Velocity analysis: CDP increment = 20 Residual static correction Dip moveout (DMO) correction CDP stack Post-stack noise suppression Data output

correction, were used to generate a long north–south seismic reflection profile for the Qiangtang Basin. Table 2 shows the basic protocol for the re-processing of the reflection data from the Qiangtang Basin.

4. Basement structure and south–north change in the Qiangtang Basin 4.1. Basement structure of the Qiangtang Basin The basement structure of a petroliferous basin affects its tectonic framework and restricts the distribution pattern, thickness variation and the macroscopic oil-controlling structure of oil and gas deposits. Because the subsurface structure of the Qiangtang Basin was strongly affected during the geological formation of Tibet, a determination of whether the basin has a rigid basement is essential to the evaluation of the preservation conditions of the basin's oil and gas reserves. The nature of the basement of the Qiangtang Basin is an issue of long-standing controversy. Wang et al. (1997) hypothesized that the basement was composed of a pre-Devonian metamorphic complex. Overlying this complex are marine strata of the Middle Devonian to Jurassic ages, which include the main target horizons for hydrocarbon exploration. Kapp et al. (2005) inferred that the Paleozoic basement of the Qiangtang Basin was nonconformably overlain by Carboniferous– Permian shallow-marine strata, while Wang and Wang (2001) concluded that the Qiangtang Basin has a double basement: an early Proterozoic crystalline basement and a late Proterozoic folded basement. The two basements have completely different rock mixes, physical properties, metamorphism, and deformation characteristics. Cambrian and older orthogeneses in the Amdo area along the Bangong suture have been suggested as representative of exhumed portions of the southern Qiangtang terrane basement (Guynn et al., 2006). Li et al. (2006) reported that no reliable isotopic evidence exists for either the crystalline or the old basement of the Qiangtang region. However, Tan et al. (2009) concluded that the Qiangtang Basin had a Precambrian crystalline basement, formed in the period from approximately 1666–1780 Ma, as based on zircon SHRIMP ages. Images of the seismic reflection profiles provide a solid foundation for the study of the base structure of the Qiangtang Basin. Lu et al. (2006a) had previously interpreted the characteristics of the region's basement structure using a seismic reflection profile collected in 2004. This section is close to the INDEPTH-III profiles (Fig. 2). Two strong reflection layers were marked on the profile: strong reflection A appeared between 2.1 and 3.0 s TWT, while strong reflection B was detected between 4.2 and 5.5 s TWT. The amplitude analysis shows that the tops of these two strong reflections correspond to significant energy enhancements. As these two reflections significantly differ between the reflections of the upper and under strata, they could be interpreted as an interface.

174


}

Fig. 2. Analysis of the reflection characters and amplitude curve of the basement in the south Qiangtang Basin (the left map is the stacked profile; the right map is the amplitude curve of the profile; A is hypothesized to be the Paleozoic basement; B is hypothesized to be the Proterozoic basement).

Fig. 3. Geology map and CDP locations along the seismic reflection profile in the Qiangtang Basin (after Kapp et al., 2005).


Fig. 4. Interpretation of the Qiangtang seismic reflection profile. Fig. 4a represents the short-axis reflections in the south Qiangtang Basin; Fig. 4b–c represents the large-scale depression structures in the north Qiangtang Basin.

175

176


From the overall profile, the shallow strata deformation is seen to be concentrated mainly in the top stratum above strong reflection A, and there is less deformation underneath. Considering the stratum and the geology of the Qiangtang Basin, the many uplifts observed above strong reflection A may represent the Mesozoic and Paleozoic strata undergoing strong and squeeze folding in the plateau. Haines et al. (2003) hypothesized that ~3.5 km of Phanerozoic sediment is present in the southern Qiangtang block. A distinct interface does exist between the sedimentary cover and the Qiangtang basement. The Vp model of these authors indicates a transition from a velocity of b 5 km/s to a velocity of 5.8 km/s at a depth of approximately 3 km. We infer that the 3.5 km Phanerozoic sediment observed in the wide-angle profile coincides with the reflection event at ~3 s TWT recorded in the seismic reflection profile. We therefore hypothesize that strong reflection A is itself a consequence of tectonic deformation and should be considered the bottom interface of the Mesozoic and Paleozoic strata in the Qiangtang Basin, or its Paleozoic basement. The top of strong reflective interface B is located at 4.2 s TWT with the continuous phase axis, as indicated by energy concentration, tilting slightly southward; its greatest depth is near 5 s TWT, which could be interpreted as the interface of the Paleozoic and elder strata from which the Proterozoic basement of the Qiangtang Basin grew. The reflection event at ~5 s TWT, recorded using a velocity of 6.0 km/s, indicates that the pluton extends to at least 15 km in depth (Haines et al., 2003). The Paleozoic basement and the Proterozoic basement displayed in Fig. 2 are in agreement with the fold and crystalline basements of the Qiangtang Basin reported by recent research (Li et al., 2013). Based on the reflection characteristics of the basement, we analyzed the spliced and reprocessed BNS-QT profile. We plot the common depth point (CDP) locations on the geological map (Fig. 3), which can be used to understand the structure and strata information of the reflection profile. Fig. 4 shows the interpreted image of the upper crust of the Qiangtang Basin. The orange and red dotted lines indicate the reflections of the Paleozoic and Proterozoic basements of the Qiangtang Basin. The Paleozoic basement was shallow in the central part of the basin and deep at both the south and north ends. Beneath the Lhasa block and the south Qiangtang Basin, the basement was detected at 3.5 s TWT. The basement deepened to 4 s TWT at the southern edge of the central uplift, then became shallower, being detected at 2 s TWT in the central uplift. In the north Qiangtang Basin, the basin continued to deepen to approximately 3 s TWT, with little variation. The Proterozoic basement was distributed discontinuously, appearing at 5.0 s TWT in the southernmost Qiangtang Basin, 3.5 s TWT below the central anticline, and approximately 4.5 s TWT in the north Qiangtang Basin. 4.2. Changes in the shallow crustal structure of the Qiangtang Basin The results of the spliced and reprocessed seismic data show differences in the shallow structure of the Qiangtang Basin from the north to the south. The shallow structure covering the Paleozoic basement varies significantly in its styles and magnitudes of structural deformation from the south to the north of the Qiangtang Basin. In the north Qiangtang Basin, many large-scale depression and uplift structures and weak surface deformations were observed, as well as more continuous sedimentary strata (see Fig. 4b and c). The southern Qiangtang Basin bears little similarity to the northern region, containing more short-axis reflectors of poor continuity without a significant number of depression constructs (see Fig. 4a). The Mesozoic strata above the Paleozoic basement had strong reflective energy and good continuity of the reflective phase, most of which could be continuously tracked. The abundant reflections of the data reveal changeable tectonic styles, clear uplifts, depressions, fractures and other structural elements. Under the Paleozoic basement, the reflections became weak, with only sporadic continuous tracking of the reflection axis. We hypothesized that a tectonic slippage occurred between the Paleozoic basement and

the overlying strata in the upper crust of the Qiangtang Basin, which resulted in the inconsistency of the degree of deformation observed between the upper and lower strata. On the northern margin of the central anticline, a large-scale deep graben was observed with a horizontal span of 20 km and up to 2.8 s TWT, nearly 8 km in depth (see Fig. 4b); the feature contained a steep fault at its north side and a slope at its south side. The reflection stratigraphic architecture in this depression was very clear. Closer to the source area, coarse clastic material could have easily deposited on the south side of the slope, possibly serving as a reservoir. Fine-grained material can deposit at the deepest point of such features and serve as hydrocarbon source beds. In this region, the salt deposits developed in the Middle Jurassic, Upper Jurassic and the Paleogene are good caps for hydrocarbon deposits. Therefore, this half-graben structure may be a favorable site for hydrocarbon accumulation. The largest-scale depression observed along the study profile is located in the north Qiangtang Basin (Fig. 4c). This structure spans from CDP 12001 to CDP 13001, a distance of approximately 25 km. The bottom reflection of this structure was recorded at approximately 2.5 s TWT, equivalent to a depth of 6.1 km (using a mean velocity of 4.9 km/s) (Zhao et al., 2006). Zhao et al. (2001) concluded that the bottom reflection of this depression represented the upper Triassic Xiaochaka Group, a truncated unconformity interface, hypothesizing that it may be a marker interface widely distributed across the basin. Above this group, Jurassic strata, including the Nadigangri, Quemocuo and Buqu groups, are prevalent in the depression. The presence of reflective layers parallel to each other, the lack of obvious folding deformation, and the relatively undeveloped faults of the depression indicate a less severe transformation intensity during the Cenozoic in this region. 5. Conclusions Based on the BNS-QT profile, we hypothesized that Paleozoic and Proterozoic basements exist in the Qiangtang Basin. The Paleozoic basement provides a series of continuous reflection strata, and most of the shallow crustal deformation occurs above it. The basement is shallow in the central anticline and deeper in the south and north sides of the basin. The Proterozoic basement is distributed intermittently at 5.0 s TWT in the south Qiangtang Basin, 3.5 s TWT beneath the central anticline and approximately 4.5 s TWT across the north Qiangtang Basin. In the north Qiangtang Basin, many large-scale depressions and uplift structures are observed above the Paleozoic strata, while more flat reflections with poor continuity and a short axis are recorded in the south Qiangtang Basin. A large-scale half graben on the northern margin of the central anticline may be a favorable site for hydrocarbon accumulation. Acknowledgments This study was financially supported by the Ministry of Land and Resources of China (Nos. SinoProbe-02-01, 201011040; SinoProbe-0206, 201011045) and the National Natural Science Foundation of China (Nos. 40830316 and 41274096). We would like to express our special thanks to our reviewers for their constructive comments, which helped us to improve the manuscript. References Ding, W.L., Wan, H., Su, A.G., He, Z.H., 2011. Characteristics of Triassic marine source rocks and prediction of favorable source kitchens in Qiangtang Basin, Tibet. Energy Exploration & Exploitation 29, 143–160. Ding, W.L., Wan, H., Zhang, Y.Q., Han, G.Z., 2013. Characteristics of the Middle Jurassic marine source rocks and prediction of favorable source rock kitchens in the Qiangtang Basin of Tibet. Journal of Asian Earth Sciences 66, 63–72. Fu, X.G., Wang, J., Zeng, Y.H., Tan, F.W., Feng, X.L., 2012. Trace elements and their behaviour during the combustion of marine oil shale from Changliang Mountain, northern Tibet, China. Environmental Earth Sciences. http://dx.doi.org/10.1007/s12665-0122199-5.

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