J. Cent. South Univ. (2016) 23: 1459-1467

DOI: 10.1007/s11771-016-3198-6

Seismic volcanostratigraphy of large, extrusive complexes in continental rift basins of Northeast China: Analysis of general bedding patterns in volcanostratigraphy and their seismic reflection configurations

YI Jian(衣健)¹, WANG Pu-jun(王璞珺)¹, GAO You-feng(高有峰)²,

CHEN Chong-yang(陈崇阳)¹, ZHAO Ran-lei(赵然磊)¹

1. Earth Science College, Jilin University, Changchun130061, China;

2. Paleontology and Stratigraphy Research Center, Jilin University, Changchun 130061, China

Central South University Press and Springer-Verlag Berlin Heidelberg 2016

Abstract: The aim of this work is to establish volcanic seismic reflection configuration models in the rift basins of Northeast China from a new perspective, the volcanostratigraphic structure. Accordingly, the volcanostratigraphic structure of an outcrop near the Hailaier Rift Basin was analyzed to understand the characteristics and causal factors of physical boundaries. Further, 3D seismic reflection data and analysis of deep boreholes in the Songliao Rift Basin were used to establish the relationship between volcanic seismic reflection configurations and volcanostratigraphic structures. These studies suggested that in volcanic successions, physical boundaries coincide with volcanic boundaries, and their distributions are controlled by the stacking patterns of volcanic units. Therefore, volcanic seismic reflection configurations can be interpreted in terms of the stacking patterns of volcanic units. These are also referred to as general bedding patterns in volcanostratigraphy. Furthermore, four typical seismic reflection configurations were identified, namely, the chaotic, the parallel continuous, the hummocky, the multi-mound superimposed and the composite. The corresponding interpretation models comprised single massive unit, vertical, intersectional, lateral multi-mound, and composite stacking patterns. The hummocky and composite reflection configurations with intersectional and composite stacking patterns are the most favorable for the exploration of volcanic reservoirs in rift basins.

Key words: seismic volcanostratigraphy; volcanostratigraphic structure; seismic refection configuration; stacking patterns; volcanic reservoirs

1 Introduction

Volcanostratigraphy in the rift basins of NE China has recently become a focus of geological research, in view of the discovery of significant volcanic gas reservoirs in this region [1]. The average depth of the volcanic reservoirs in these rift basins is greater than 2500 m, but deep boreholes are scarce [2]. Therefore, a study on seismic volcanostratigraphy is particularly important in volcanic gas reservoir exploration and further the study of ancient volcanism.

The field of seismic stratigraphy was introduced by VAIL et al [3] to interpret the succession and facies of continental clastic deposition. To facilitate volcanic reservoir exploration in rift margin basins on both sides of the Atlantic Ocean, PLANKE [4], CALVS [5] and JACKSON [6] conducted a study on seismic volcanostratigraphy. This involved the construction of a subset of data on seismic stratigraphy, including six volcanic seismic facies units; these can be widely applied in rift margin basins [4]. Subsequent studies on seismic volcanostratigraphy were conducted extensively around the world in the interpretation of volcanic frameworks [7], volcanic facies [8] and volcano growth mechanisms [9]. In the continental rift basins of NE China, several series of seismic volcanic facies units have been identified [10]. Unfortunately, these seismic units are interpreted in light of their lithology and sub-facies associations, which are extremely random due to the presence of infinite combinations of the two. An additional complication is the absence of geophysical means to study them. In other words, this analysis cannot be applied uniformly to different areas with different geologies. In the seismic interpretation of deposit stratigraphy, the seismic reflection configurations are interpreted as general bedding patterns, which have clear geophysical origins; this analysis can be widely applied to most types of basins [3]. In comparison with the seismic interpretation of deposit stratigraphy, the focus of our study was to determine what the general bedding patterns in volcanostratigraphy are in the rift basins of NE China.

In this work, the Songliao and Hailaer rift basins were selected as representative areas to study volcanic seismic configurations. The Songliao Basin contains an unusually large number of boreholes and 3D seismic subjects, while the Hailaer Basin contains a well-exposed, highly continuous basalt outcrop that can reveal its volcanostratigraphic structure more completely than in other basins. The fundamental relationship between the seismic reflection configurations and the volcanostratigraphic structures were investigated using analysis of outcrops, boreholes and high-quality 3D seismic data.

2 Geological setting

The Songliao and Hailaer basins are situated in northeastern China between the Xilamulun belt and the Mongolia–Okhotsk belt [11] (Fig. 1(a)). Both are continental rift basins formed during the Jurassic to Early Cretaceous; this was followed by extension and thermal subsidence associated with both the closure of the Mongolia–Okhotsk Ocean and the subduction of the Pacific plate [12]. In this period, a series of independent fault depressions formed in the two basins [13-14]. Volcanostratigraphy filled in these fault depressions independently (Fig. 1(b)). In the Songliao Basin, there were three volcanic cycles, namely, the Upper Jurassic Huoshiling Formation, the Lower Cretaceous First Member, and the Third Member of the Yingcheng Formation (Fig. 1(c)) [15]. In the Hailaer Basin, the basalt outcrop studied here developed in the middle Jurassic Tamulangou Formation in the Xinganling Group [16]. It was uplifted and exposed at the margin of the Hailaer Basin adjacent to Hulun Lake in Manchuria (Fig. 1(a)).

3 Terminology

The following terms are primarily used in this work: 1) Seismic volcanostratigraphy: the study of the nature and geological history of volcanic rocks and their emplacement environment, as extrapolated from seismic data [4]. 2) Eruptive unconformity boundaries: the most common type of volcanic boundary, which formed during the short intervals of eruptions [17]. 3) Volcanic unit: a separate cooling unit possessing an upper portion that cooled significantly and solidified before another layer was superimposed on it [18]. 4) Vesicle zones: the layers with the greatest concentrations of pores in volcanic units [19]. 5) Physical boundaries: the boundaries in rocks that separate zones with different physical properties. These physical parameters include porosity, permeability, density, velocity, and impedance [20].

4 Dataset

High-quality time-migrated 3D seismic reflection data spanning a total of 6297 km² in the Changling and Xujiaweizi Fault Depressions of the Songliao Basin were used in this study (Fig. 1(a)). The seismic data are zero-phase with negative polarity and were interpreted using Open Works (Landmark Corporation, L.A.). The dominant frequency in the host volcanic succession is 32 Hz, and the interval velocity is 5500 ms^-1. This suggests that volcanic rock >42 m in thickness can be resolved and that their tops and bases will generate discrete reflections. Volcanic rocks <42 m cannot be resolved completely, but several layers of volcanic rocks can be expressed as a single seismic reflection event (the limits of separation were calculated by BROWN [21]. Four seismic sections were selected from the two Fault Depressions, and the boreholes D1, Y1, Y2, Y7, Y101 and Y102 were used for studying the lithology, volcanic boundaries and volcanic units of volcanostratigraphy in these seismic sections (Figs. 1(a) and (d)).

5 Seismic expression of volcanoes

In this study, numerous volcanic seismic reflection configurations drilled by deep boreholes were analyzed. Without considering the geometrical shape of the envelope, primarily five typical volcanic seismic reflection configurations were found (Fig. 2).

6 Interpretations of volcanic reflection configurations

6.1 Volcanostratigraphic structure

Seismic reflection configurations are controlled by physical boundaries in rock [3]. Therefore, understanding the factors that determine the distribution of physical boundaries in volcanic rocks is crucial in the interpretation of volcanic seismology. To this end, it is necessary to physically study volcanostratigraphic structures, based on outcrop analysis. Accordingly, we focused on a basalt outcrop adjacent to Hulun Lake in Machchuria (Fig. 3); this outcrop reveals relatively complete volcanostratigraphic structure, enabling ease of analysis.

The observations indicated that the large basalt lava flow characterizing the outcrop under study is composed of several relatively small lava flow units (Figs. 3(a₁) and (b₁)), which are separated from one another by eruptive unconformity boundaries (Figs. 3(a₂) and (b₂)). These lava flow units are stacked together in different patterns: 1) The vertical stacking pattern formed by large, tabular, vertically stacked lava flow units (Fig. 3(a₁)); 2) The intersectional stacking pattern comprising numerous,intersectional stacked small braided lava flow units (Fig. 3(b₁)). To summarize, volcanostratigraphic structures include three elements: volcanic boundaries, volcanic units, and stacking patterns.

Fig. 1 Geological background of study area and distribution of seismic and borehole data used in this study:

6.2 Impedance boundary distribution

In the studied outcrop, it is found that vesicle zones developed in the upper layers (upper vesicle zones) and base layers (base vesicle zones) in individual lava flow units (Figs. 3(a₃) and (b₃)). Lava flow units stacked in vertical or intersectional patterns revealed two distribution patterns of high-porosity zones coinciding with eruptive unconformity boundaries: 1) several vertically separated, continuous, high-porosity zones(Fig. 3(a₄)); (2) numerous intersectional, discontinuous, high-porosity zones (Fig. 3(b₄)). Due to the fact that high-porosity zones in volcanic rocks always display low density and seismic wave velocity (Table 1), the former can form a stable vertical impedance interface and the latter can form an unstable intersectional impedance interface. Therefore, the distribution of impedance interfaces in the volcanostratigraphy of the studied outcrop coincides with the volcanic boundaries, and that these distributions are determined by the stacking patterns of volcanic units.

Fig. 2 Typical seismic reflection configurations of volcanostratigraphy in Songliao Basin:

6.3 General bedding patterns in volcanostratigraphy

In deposit stratigraphy, the reflection configurations can be interpreted as general bedding patterns, and can be used as a basis for analyzing deposition processes, erosion, and paleotopography. This is because the general bedding patterns in deposit stratigraphy determine the seismic reflection configurations by controlling the distribution of general physical boundaries [3]. In volcanostratigraphy, because the distribution of impedance interfaces is determined by the stacking patterns of volcanic units, the shape, overlap and termination of seismic events are the reflections of these stacking patterns. Therefore, volcanic seismic reflection configurations can be interpreted directly in terms of the stacking patterns of volcanic units; this is commonly referred to as the general bedding patterns of volcanostratigraphy.

Fig. 3 Structure of basalt lava flows near Hulun Lake, China:

Table 1 Petrophysical property in different vesicle zones in basalt

6.4 Five typical stacking patterns

Volcanic boundaries and units can be identified in boreholes by examining data logs, and also can be identified in seismic sections by the lateral terminations of seismic events [4]. Through the well–seismic comparison, the five typical volcanic seismic reflection configurations discussed previously (Fig. 2) were interpreted in terms of volcanic units and their stacking patterns (Fig. 4): 1) The vertical stacking pattern, formed by several tabular basalt lava flow units (Fig. 4(a)),corresponds to parallel continuous reflection configuration (Fig. 2(a)); 2) the intersectional stacking pattern, formed by numerous braided lava flow units (Fig. 4(a)), corresponds to hummocky or tadpole-shape reflection configuration (Fig. 2(a)); 3) The single massive unit stacking pattern, formed by one massive rhyolite lava dome (Fig. 4(b)), corresponds to Chaotic massive reflection configuration (Fig. 2(b)); 4) the lateral multi-mound stacking pattern, formed by several mound-like pyroclastic domes (Fig. 4(c)), corresponds to multi-mound superimposed reflection configuration (Fig. 2(c)); 5) The composite stacking pattern, comprising vertical, single massive unit and lateral multi-mound stacking patterns (Fig. 4(d)), corresponds to the composite reflection configuration (Fig. 2(d)).

Fig. 4 Five typical stacking patterns of volcanic units and distributions of vesicles and fissures:

7 Discussion

7.1 Interpretation models

Based on the analyses above and other comparisons of well and seismic datasets in the Songliao Basin, the volcanic seismic reflection configuration models and their interpretation schemes are summarized in Fig. 5. Because these interpretation models were established based on the relationship between the volcanostratigraphic structures and their physical boundaries, they have a solid geophysical foundation and can therefore be applied more universally than the models derived from the interpretation of lithology and sub-facies associations. Furthermore, the volcanic units and their stacking patterns can reflect the underlying volcanism and emplacement environment [22-23]. The latter parameters can be indirectly analyzed by the aforementioned interpretation models (Fig. 5).

7.2 Reservoir significance

Volcanic reservoir depends on both primary and secondary pores [16]. Both types of pores are closely related to volcanostratigraphic structures and develop near volcanic boundaries by volatile differentiation and by the influence of weathering and fluid processes [24-25]. Therefore, different stacking patterns form different distribution patterns of vesicle zones (Fig. 4) and correspondingly, different distribution patterns for favorable oil and gas reservoirs (Fig. 5). Furthermore, porosity data in this area indicate that a greater number of high-porosity reservoirs develop in the intersectional and composite stacking patterns, due to the presence of relatively small units and multiple favorable reservoirs (Fig. 6). Accordingly, volcanic seismic reflection interpretation models can be used to predict the presence of volcanic reservoirs in the rift basins that lack deep wells.

Fig. 5 Volcanic seismic reflection configuration models and their interpretation schemes (Favourable reservoirs have high porosity (>8%), and corresponding high permeability due to common development fracture in the rift basins of NE China)

Fig. 6 Logging porosity under different stacking patterns:

8 Conclusions

1) The impedance interfaces in the volcanostratigraphy coincide with the volcanic boundaries, and their distributions are determined by the stacking patterns of volcanic units. Therefore, volcanic seismic reflection configurations can be interpreted to stacking patterns of volcanic units—the general bedding patterns in volcanostratigraphy.

2) Five typical volcanic seismic reflection configurations and stacking pattern interpretation models in the rift basins of NE China are established. These models based on volcanostratigraphic structure have a solid geophysical foundation and can therefore be applied more universally.

3) Different stacking patterns have different favorable reservoir distribution patterns and porosity features. High-porosity reservoirs develop in the intersectional and composite stacking patterns, which possess relatively small volcanic units and multiple favorable reservoirs. The volcanic seismic reflection interpretation models presented here can be used to interpret volcanostratigraphic structure, volcanism and predict the existence of volcanic reservoirs in rift basins.

9 Acknowledgements

Thanks to Yao Rui-shi, Qu Xue-jiao for their contributions toward field work, core and thin section descriptions, and data processing.

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(Edited by DENG Lü-xiang)

Foundation item: Projects(41472304, 41430322) supported by the National Natural Science Foundation of China; Project(2012CB822002) supported by National Major State Basic Research Program of China

Received date: 2015-03-11; Accepted date: 2015-07-22

Corresponding author: YI Jian, PhD; Tel: +86-431-88502620; E-mail: yijian_x@yahoo.com