Relationship between volcanic rock distribution and fault combination in Anda area of Xujiaweizi fault depression

Dong Jinghai Meng Qi 'an Yang Feng Pingyin Changhai

(Exploration and Development Research Institute of Daqing Oilfield Co., Ltd., Daqing, Heilongjiang 1637 12)

Fund Project: This paper is supported by the National "973" Project (2009B219308) and "Study on Formation, Evolution and Distribution Characteristics of Volcanic Rocks in Northern Songliao Basin".

About the author: Dong Jinghai, male, engineer, mainly engaged in the exploration and deployment of volcanic oil and gas reservoirs. E-mail: dongjinghai @ petrochina.com.cn.

Abstract: Xujiaweizi fault depression is a continental volcanic-sedimentary rock basin. Each structural belt in the fault depression has a dual genetic mechanism of volcanic activity and tectonic movement, and the basement fault and its supporting secondary faults and volcanic activity control the structural framework and stratigraphic development of the fault depression. Volcanic (magmatic) activities in this area mainly occurred in the sedimentary period of Huoshiling Formation and Yingcheng Formation. Huoshiling Formation is the initial stage of the strata in the fault depression period, and Yingcheng Formation was formed in the fault depression transformation stage, with a large development scale, inheriting and transforming the early volcanic activity, and generally showing the characteristics of central type, fault type and compound type. The formation and distribution of volcanic rocks are "cross-layer", especially the transformation of the underlying strata. Therefore, it is a "traceable" method to analyze the distribution law of volcanic rocks and their relationship with faults according to the deflection characteristics of volcanic activities to the underlying strata. Based on the identification of two sets of volcanic mechanisms in Neiyingcheng Formation of the fault depression, this paper has carried out detailed structural interpretation, volcanic channel identification and volcanic facies identification in the study area during years of exploration. According to the distribution and structural position of lithofacies zones near the crater, three NW-trending volcanic trap development zones are determined, and the conclusion that volcanic rocks are controlled by different types of basement faults is drawn. According to the types and characteristics of fault combinations, there are three types of magmatic intrusion mechanisms and volcanic eruption types: (1) controlled by the low-angle Xu Xi fault, it intrudes along the secondary fault, and the volcanic activity is characterized by direct melting and penetrating the stratum, with point distribution on the plane, showing the characteristics of central eruption; (2) Controlled by the high-angle Xu Zhong fault, magma directly intruded along the fault and melted through the stratum, and distributed in a strip shape on the plane, showing the characteristics of fracture eruption; (3) Controlled by Xu Dong flower fault and branch fault, it intruded into or directly melted through the stratum along the fault, forming multi-stage eruption in the longitudinal direction and overlapping each other in the plane, showing the characteristics of compound eruption. Through careful dissection of the Anda area in Xujiaweizi fault depression, it is determined that the NW-trending echelon fault develops in the upper part of the basement fault in Anda area and communicates with the basement fault downwards, which is the upward passage of volcanic magma in the west and also reflects the characteristics of volcanic activity in the historical period. Volcanoes in the west are large in scale, mainly explosive facies, and large-scale overflow facies in the east. The influence of the combination characteristics of basement faults and secondary faults on the distribution characteristics of volcanic rocks is further expounded.

Key words: Xujiaweizi fault depression; Anda area; Fracture; Volcanic (magmatic) activity

Relationship between fault combination characteristics and volcanic rock distribution in Anda area of Xujiaweizi sag

Dong Jinghai, Meng Qi 'an, Yang, Yin Changhai

(Daqing Oilfield Exploration and Development Research Institute, Daqing, Heilongjiang 1637 12)

Abstract: Based on the prediction of fault combination characteristics and volcanic rock distribution in Xujiaweizi depression, it is pointed out that faults are the main vertical outlets of volcanic activity. According to the different types and properties of faults, three different magmatic intrusion mechanisms and volcanic eruption types are summarized, which are controlled by different basement faults. (1) They are controlled by low-angle faults in the west of Xujiaweizi sag, which are connected with secondary faults. Volcanic activity melts and penetrates the stratum, and it is point-like in plane and characterized by central eruption. (2) Controlled by the high-angle fault in the middle of Xujiaweizi sag, magma moves upward and melts and penetrates the stratum, which is distributed in a strip shape on the plane and is characterized by fracture eruption; (3) The eastern part of Xujiaweizi Depression is controlled by the flower-shaped fault and its branch faults. Magma goes up, melts and permeates the stratum, erupts in multiple stages in the longitudinal direction and overlaps with each other in the plane, which is characterized by the compound eruption of fractures. Taking Anda area in Xujiaweizi depression as an example, the influence of the combination characteristics of basement faults and branch faults on the distribution of volcanic rocks is further illustrated.

Key words: Xujiaweizi sag; Anda area; Fault; volcanic activity

introduce

Volcanic rocks are the surface exposure of mantle materials in geological history, and faults are the main vertical channels of volcanic activity. Volcanic rocks are likely to develop when deep faults develop, especially at the intersection of faults. The morphological study on the distribution of modern Wudalianchi crater proves that the intersection of deep faults and secondary faults controls the magma upwelling channel. For faulted basins, some scholars believe that due to the continuous extension of synsedimentary basement faults, the crust becomes thinner, and deep magma easily breaks through overlying sediments and upwelling along the faults; Some scholars believe that it is caused by magma upwelling caused by strike-slip faults. No matter what the nature of basement fault is, there is obvious volcanic disturbance in the fault explained by seismic data, that is, the understanding that the fault is a vertical channel of volcano is unanimously recognized, while the deep magma in the communication basin is controlled by large-scale basement fault, and the macro distribution law of volcanic rocks in the region is closely related to basement fault.

Geological characteristics of 1 Xujiaweizi fault depression

Figure 1 volcanic exploration results in Anda area of Xujiaweizi fault depression

Xujiaweizi fault depression is a key area for deep natural gas exploration in northern Songliao Basin. Structurally, it is a dustpan-shaped fault depression controlled by Xu Xi boundary fault, which is divided into three local depressions in the south and north by Songzhan low uplift and Fengle low uplift, namely Anda sag, Xingshan sag (including Xu Xi sag and Xu Dong sag) and Zhaozhou sag (figure 1). In recent years, remarkable achievements have been made in volcanic exploration in Anda area. After the volcanic exploration reserves were submitted in the west, the volcanic exploration in the east also made a breakthrough.

2 The relationship between Xujiaweizi fault depression and volcanic rocks

2. 1 fault combination characteristics

Xujiaweizi fault depression is a continental volcanic-sedimentary rock basin. Each structural belt in the fault depression has a dual genetic mechanism of volcanic activity and tectonic movement, and the basement fault and its supporting secondary faults and volcanic activity control the structural framework and stratigraphic development of the fault depression [1 ~ 2]. There are two sets of structural interpretation schemes for Xujiaweizi fault depression. One is Xu Xi, Songxi and Xu Dong faults; Second, there are three faults: Xu Xi, Xu Zhong and Xu Dong. The similarity between the two interpretation schemes lies in the consistent description of seismic reflection characteristics, but the difference lies in the different understanding of the nature of basement faults. In the second interpretation scheme, the previous Xu Xi and Songxi faults are interpreted as low-angle Xu Xi faults, and it is proposed that there is a deep fault with NW-SE strike at about 28km on both sides, which is named Xu Zhong fault. There are two different views on the characteristics of the central fault zone: according to the seismic reflection characteristics of the strata on the west side of the central fault zone, it is considered that there is a combination of east-dipping normal faults and west-dipping reverse faults in Xingcheng area, which is a fault bending and fault expansion structure caused by regional compressive stress; At present, it is considered that the Xu Zhong fault has a large-scale dextral strike-slip under the action of * * * yoke shear stress, forming a complex NW-trending fault structural belt.

Although there are similarities and differences between the two interpretation schemes, the description of fault distribution and seismic combination form is basically the same. For the convenience of research and the unification of related data, this paper only adopts the second interpretation scheme to understand the basement characteristics of this area. Xu Zhong fault runs through the whole middle of Xujiaweizi fault depression. On the seismic profile, the occurrence of the profile extends almost vertically to the basement. Xu Xi fault is divided into two sections by Xu Zhong fault in Xujiaweizi fault depression. The south section of Xu Xi fault generally strikes NW, with an extension length of 105km and an east dip angle of16 ~ 45. The fault distance is generally 22 17m at the top of bedrock, with a maximum of 44 15m and a minimum of 954m. The Xu Dong fault zone is composed of a series of strike-slip fault zones evolving from south to north. The east branch of strike-slip fault zone controls the deformation and displacement of Dongpo. Due to different structural positions, faults in different parts have different characteristics, but the scale of the east branch fault is generally large.

2.2 types of volcanic eruptions

Volcanic (magmatic) activities in the deep part of Xujiaweizi fault depression mainly occurred in the sedimentary period of Huoshiling Formation and Yingcheng Formation. Huoshiling Formation is the initial stage of the strata in the fault depression period, and Yingcheng Formation was formed in the transitional stage of the fault depression, with a large development scale, which inherited and reformed the early volcanic activity [3 ~ 5]. According to the analysis of drilling data and seismic profile reflection characteristics, there are three main types of volcanic eruption in Yingcheng Formation of Xujiaweizi fault depression, namely: (1) central volcanic eruption: volcanic eruption spouts out of the ground along a neck-shaped pipeline, and the eruption channel is point-shaped on the plane. The volcanic passage is located at the intersection of two or more sets of faults, and the ejecta pile up into a volcanic cone. (2) Fissured volcanic eruption: Volcanic eruption overflows the surface along a big crack in a certain direction. The crater is a fault that extends for tens of kilometers or is distributed in a beaded shape along the crater of the fault zone. (3) Compound volcanic eruption: fissure volcanic eruption and central volcanic eruption appear alternately, and the two eruption products overlap each other to form an interlayer (Figure 2).

Fig. 2 Macro eruption model of volcanic rocks in Xujiaweizi fault depression

Volcanic (magmatic) activity has a great influence on the strata and structure of Xujiaweizi fault depression, which complicates the deep strata and structure, mainly in several aspects: the influence of volcanic activity and erosion on the strata; Effects of multi-channel and multi-stage activities and lateral erosion on strata; Influence on the spatial distribution of formation sediments; Provide a certain material source for sedimentary strata; Strong transformation of fault structure. The distribution of volcanic rocks is controlled by faults, which also has a certain transformation effect on fault structures. The continuous upwelling of magma will squeeze and melt the strata on both sides of the fault, making the channel bigger, showing strong volcanic disturbance, forming a wedge-shaped volcanic channel, and at the same time, the pre-existing fault characteristics as a volcanic channel will be destroyed. Therefore, it interferes with the identification of volcanic mechanism and secondary faults [6 ~ 7].

2.3 Fault combination and volcanic activity

The formation and distribution of volcanic rocks are "cross-layer", especially the transformation of the underlying strata [8]. Therefore, it is a "traceable" method to analyze the distribution law of volcanic rocks and their relationship with faults according to the flexural characteristics of volcanic activities to the underlying strata. Based on various seismic attributes, two sets of volcanic mechanisms of Neiyingcheng Formation in the fault depression were identified and drilled in the exploration process of the study area for many years. On the basis of fine structural interpretation, crater identification and volcanic rock identification, three NW-trending volcanic trap development zones are identified according to the distribution and structural position of lithofacies zones near the crater (Figure 3).

Control mechanism of magma upwelling and distribution characteristics of volcanic rocks in basement fault zone of Xujiaweizi fault depression.

The nature of basement faults and their combination with secondary faults in Xujiaweizi fault depression lead to three different mechanisms of magma upwelling (Figure 3). First, controlled by the low-angle Xu Xi fault, it intrudes along the secondary fault, and the volcanic activity is characterized by direct melting through the stratum along the secondary fault and point-like dispersion on the plane, showing the characteristics of central eruption. It is mainly developed in Changde and An Xun, and is mainly composed of intermediate-acid volcanic rocks. The scale of volcanic eruption is relatively small, and the thickness of volcanic rocks is small. The typical well is Xu Shen 8 well. In the west of Anda, intermediate-basic volcanic rocks are mainly distributed in the late stage. It is speculated that the intensity of magmatic activity was high at that time, which led to the larger scale of volcanic rocks in Anda than in Changde. Secondly, volcanic activity is controlled by high-angle faults, which directly upwell and melt through the strata along the high-angle faults, and it is distributed in a strip shape on the plane, characterized by fracture eruption, mainly developed in Xingcheng-Fengle area, with typical wells such as Xu Shen 1, 7, 9, etc. Thirdly, controlled by Xu Dong flower-shaped faults and branch faults, volcanic activities are characterized by upward intrusion or direct melting through strata along faults, and multi-stage eruptions are formed in the longitudinal direction, which overlap each other in the plane, showing the characteristics of compound eruptions. It is mainly developed in Xu Dong and Anda in the east. In the process of fracture eruption, there is a central eruption, and the two types are superimposed together. The typical well is Xu Shen 23 well.

Case study of Anda area in Xujiaweizi fault depression

3. 1 controlled by Xu Xi fault, it has a geological structure of "west fault and east overload".

Anda sag is located in the north of Xujiaweizi fault depression, which is a small dustpan-shaped fault depression controlled by the north of Xu Xi fault, with Anda syncline in the middle and west slope in the east. Xu Xi fault and Xu Dong fault are the two most important basement fault systems in this area. Xu Xi fault is a subsidence-controlled fault, which runs through Anda area, extending over 52km, and striking NNW-NNE. The Xu Dong fault zone is a flower-like structure, with NW and NNW faults, and the whole fault zone is a dustpan geological structure of "breaking in the west and overtaking in the east".

Anda area is dominated by large-scale intermediate-basic volcanic rocks, and the eruption mode of intermediate-basic volcanic rocks determines that the volcanic mechanism of explosive phase in the crater is small and distributed in points, while the overflow phase is distributed in a large area in the whole area. At the same time, the energy of magmatic activity also has a great influence on the distribution range of volcanic rocks. At present, the main volcanic rocks in the fault depression are distributed continuously, and their development is mainly affected by the amount of volcanic eruption and the amount of strata denudation in the later period. Along the two intersecting fault zones that control volcanic rocks, the overall performance is that the volcanic rocks in the west are thick and dominated by explosive facies; The development in the east is thin, and the overflow phase is dominant (see Figure 4).

Fig. 4 Seismic reflection profile of Lianjing volcanic rock in central Anda (east-west direction)

3.2 The NW-trending echelon fault communicates downward with the basement fault, which is the channel for the upwelling of volcanic magma.

Using coherent slices to explain the internal fault system of Shahezi Formation in Anda area, it is found that two groups of NW-trending yanxing faults intersect with nearly N-S faults and vertically communicate with basement faults, forming fault channels of magma upwelling. On the plane, magma generally develops at the intersection of faults, forming two nearly north-south magmatic active zones. Due to the upwelling and melting of volcanic magma through the overlying strata, its migration channel shows disturbance to the underlying sedimentary strata, which makes the fault have discontinuous profile reflection characteristics in seismic reflection profile, but it shows strong discontinuity in slice (Figure 5).

In addition, there is an S-shaped secondary fault in the middle of Anda area, which is consistent with the deep regional tectonic trend. The longest fault 1 1km is developed between 1 well depth and 6 well depth. Seismic profile and coherence slice are clear, but the fault distance is not large, generally 10 ~ 100 m, which belongs to late fault.

According to the seismic response characteristics of volcanic rocks, based on the seismic attribute analysis technology, the top-bottom interface of volcanic rocks is calibrated by using contiguous three-dimensional seismic interpretation data, and the distribution of volcanic rocks in Yingcheng Formation in Anda Depression is predicted. The top is buried shallowly, and the elevation in the deepest part is about -3200m. The coherent slices extracted along the third member of Yingcheng Formation show that the volcanic traps near the crater are mainly concentrated in the western fault zone, and only sporadically distributed in the east, reflecting that two groups of faults control the eruption intensity and formation scale of volcanic rocks.

Combining the magmatic intrusion channel fault of Shahezi Formation and the identified volcanic trap of Yingcheng Formation, it shows that the volcanic rocks in Anda Depression are mainly controlled by two intersecting fault zones. From the seismic profile, there are many craters in the west, with large scale and chaotic seismic reflection characteristics. There are few craters in the east, with small scale and continuous seismic reflection characteristics; Judging from the current thickness, the thickest part of volcanic deposits mainly develops along a series of volcanic institutions in the west, becoming thinner to the flank, thicker in the west as a whole and gradually thinner to the east, and volcanic activity has the characteristics of being strong in the west and weak in the east (Figure 6).

Therefore, different from the sedimentary characteristics of normal sedimentary rocks, volcanic rocks are formed from the deep and have the form of bottom-up eruption. The combination of basement faults and secondary faults on it plays an important role in the distribution of volcanic rocks, and the secondary faults that control volcanic eruption are mainly concentrated in the underlying sedimentary strata. The superposition analysis of basement faults and volcanic traps shows that there is a vertical one-to-one correspondence between the volcanic belt on the west side of Anda Depression and the Xu Xi fault, and between the volcanic belt on the east side and the Xu Dong fault, indicating that the activities of the two basement faults are closely related to the development of volcanic rocks and the formation of volcanic uplift belts in this area.

Fig. 5 Coherent slices in Shahezi Formation (T4 1, 10ms) reflect the upwelling channel of volcanic activity in the underlying stratum.

Fig. 6 coherence slice (T4 10 ms) of Yingcheng Formation reflects the distribution of volcanic mechanism of Yingcheng Formation.

4 conclusion

(1) Through the analysis of the characteristics of fault combination and the distribution relationship of volcanic rocks in Anda Depression, it is found that the volcanic rocks in Xujiaweizi fault depression are controlled by three different types of basement fault combination, forming different types of magma upwelling patterns, and showing the characteristics of central, fault and compound volcanic eruption.

(2) The distribution of volcanic rocks is controlled by the basement fault and the secondary fault communicating with it, and it also plays a certain role in reforming the fault structure, showing strong volcanic disturbance to the underlying strata, which not only forms volcanic channels, but also destroys the seismic reflection characteristics of the secondary fault. Therefore, strengthening the fault identification of the underlying strata of Yingcheng Formation, especially the fault relationship with Shahezi Formation, is the main direction to predict the distribution of volcanic rocks.

(3) The NW-trending echelon fault developed in the upper part of the basement fault in Anda area and communicated with the basement fault downward, which is the upward passage of volcanic magma in the west, and also reflects the characteristics of volcanic activity in the historical period. Volcanoes in the west are large in scale, mainly explosive facies, and large-scale overflow facies in the east.

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