J. Cent. South Univ. (2018) 25: 892-902

DOI: https://doi.org/10.1007/s11771-018-3792-x

Controlling factors of high-quality volcanic reservoirs of Yingcheng Formation in the Songnan gas field

DU Shang(杜商)¹, SHAN Xuan-long(单玄龙)¹, YI Jian(衣健)¹, LI Ji-yan(李吉焱)²

1. College of Earth Sciences, Jilin University, Changchun 130061, China;

2. School of Petrochemical Engineering, Lanzhou University of Technology, Lanzhou 730050, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2018

Abstract:

Predicting high-quality volcanic reservoirs is one of the key issues for oil and gas exploration in the Songnan gas field. Core, seismic, and measurement data were used to study the lithologies, facies, reservoir porosity, and reservoir types of the volcanic rocks in the Songnan gas field. The primary controlling factors and characteristics of the high-quality volcanic reservoirs of the Yingcheng Formation in the Songnan gas field were investigated, including the volcanic eruptive stage, edifice, edifice facies, cooling unit, lithology, facies, and diagenesis. Stages with more volatile content can form more high-quality reservoirs. The effusive rhyolite, explosive tuff, and tuff lava that formed in the crater, near-crater, and proximal facies and in the high-volatility cooling units of large acidic-lava volcanic edifices are the most favorable locations for the development of the high-quality reservoirs in the Songnan gas field. Diagenesis dissolution, which is controlled by tectonic action, can increase the size of secondary pores in reservoirs. Studying the controlling factors of the high-quality reservoirs can provide a theoretical basis for the prediction and analysis of high-quality volcanic reservoirs.

Key words:

Songnan gas field; Yingcheng Formation; high-quality volcanic reservoir; controlling factor；

Cite this article as:

DU Shang, SHAN Xuan-long, YI Jian, LI Ji-yan. Controlling factors of high-quality volcanic reservoirs of Yingcheng Formation in the Songnan gas field [J]. Journal of Central South University, 2018, 25(4): 892–902.

1 Introduction

Large numbers of volcanic reservoirs have been discovered recently in Argentina, Russia, and Japan [1–3]. In China, large volcanic reservoirs have been discovered in the Junggar, Bohai Bay, Liaohe, and Songliao basins [4, 5]. With the ongoing discovery of volcanic reservoirs, they have become an important field of oil and gas exploration. Previous researchers performed a significant amount of work on this topic and obtained theoretical results about the lithology, facies, accumulations, and distributions of different types of volcanic oil and gas reservoirs [6–8]. Previous researchers have analyzed the Songnan gas field in detail, primarily based on the volcanic sequence [9], well log analyses, volcanic facies analyses [10], accumulation characteristics, and accumulation models [11] of the reservoirs. The distribution of volcanic gas reservoirs is complex, and they are difficult to predict, but high-quality reservoirs are a key element in the formation of volcanic reservoirs. Therefore, studying volcanic reservoirs in detail is important to the exploration and exploitation of oil and gas in volcanic rocks.This study uses well, seismic, and physical property data to investigate the controlling factors and mechanisms of high-quality volcanic reservoirs and provides theoretical evidence for predicting the distribution of high-quality volcanic reservoirs.

2 Geology overview

The Songnan gas field is located in the Yaoyingtai deep structure of the Da’erhan fault and uplift zone in the central uplift of the Changling rift in the southern Songliao Basin (Figure 1). The Changling rift is controlled by the first-grade Tantu–Da’an–Fuyu and Songyuan-Changling faults and the second-grade Da’an–Qian’an fault. Secondary faults are also located in the area. Most of the faults are normal faults that grew from the basement and cut through all of the rift strata, and they developed at the same time as the rift. The Yaoyingtai deep structure is a faulted anticline that formed on a basement uplift. It strikes nearly north-south. The Changling pasture and Chaganhua rift trough are located to the east and west, respectively. The Yaoyingtai deep structure is connected to the Da’erhan structure to the south and is adjacent to the Daqingzijing and Qian’an oil fields to the north. It is a favorable area for long-term oil and gas migration.

Figure 1 Map of Songnan gas field

The Songnan gas field is generally an inherited paleohigh fault nose structure. The water ion content is different in different locations of the gas field, which indicates that the lateral connectivity between the volcanic edifices is poor. The gas-water contacts of the different volcanic edifices are different. The reservoirs inside the volcanic edifices are distributed in layers and are separated by sealing layers with poor physical properties. The gas reservoirs are also distributed in layers. The development and spatial extent of the gas reservoirs are controlled by the lithology and facies. Thus, the reservoir can be classified as an inherited paleohigh-type structural-stratigraphic composite natural gas reservoir.

3 Basic characteristics of high-quality reservoirs

The Songnan gas field reservoirs are primarily located in a sequence of volcanic rocks in the Cretaceous Yingcheng Formation, which include volcanic lava, pyroclastic rock, pyroclastic lava, and tuffite. The high-quality volcanic reservoirs are primarily composed of vesicular rhyolite and pyromeride volcanic lavas and rhyolitic tuff pyroclastic rocks, which include volcanic conduit, explosive, effusive, extrusive, and sedimentary facies. The top and middle subfacies of the effusive facies and the pyroclastic flow subfacies of the explosive facies are the predominant facies of the high-quality reservoirs.

The reservoir porosity of the Yingcheng Formation in the Songnan gas field is divided into three categories: primary pores, secondary pores, and fractures [12]. These types of reservoir porosity often occur in different combinations and can be summarized as three types, lava reservoirs, volcanic pyroclastic rock reservoirs, and pyroclastic lava reservoirs, which can be further divided into five subtypes [13] (Table 1). The high-quality reservoirs mainly developed in the primary pore-microfracture subtype, the secondary pore-microfracture subtype, and the primary pore-structural fracture subtype of the volcanic lava reservoir type. The main reservoir porosity includes primary vesicles, inter-flow pores and fractures, several types of dissolution pores, devitrification pores, and microfractures.

4 Controlling factors of high-quality reservoirs

By identifying the stages, volcanic edifices and facies, and cooling units of the volcanic rocks of the Yingcheng Formation in the Songnan gas field (Figures 2(a),(b), Table 2) and analyzing the physical properties of the reservoirs, the controlling factors of the high-quality volcanic reservoirs were identified, which include volcanic eruptive stages, edifices and facies, cooling units, the development of primary pores, and dissolution effects.

Table 1 Volcanic reservoir types of Yingcheng Formation in Songnan gas field

4.1 Volcanic eruptive stages

Generally, volcanic eruption is characterized by multiple cycles and stages. The eruption cycle is presented as the evolution of magmatic properties from basic to acidic. In different magmatic evolution periods (cycles), volcanic eruptions are characterized by intermittent eruptions, that is, one eruptive cycle consists of many eruption stages. The volcanic rocks of the Yingcheng Formation in the Songnan gas field are predominantly acidic; therefore, these rocks should belong to one acidic eruption cycle from the perspective of magma evolution. In addition, the drilling and seismic section features reveal that several long eruption discontinuities (weathering crust) were developed in this eruption cycle. According to these eruption discontinuities, the eruption cycle would be further divided into four stages. There are some differences of magmatic volatilization in these stages, resulting in differences of primary vesicles and reservoir physical properties in different stages. Among the differences, the average porosity and permeability (5.019% and 1.162 mD, respectively) of high- quality reservoirs in stage three are both higher than those (4.211% and 0.737 mD, respectively) in stage four. Because of the larger content of magmatic volatilization and more primary vesicles in stage three, the reservoir’s physical properties in stage three are better than those in stage four.Based on the above analysis, high-quality reservoirs are shown to be more conductive as a result of being formed in the eruption stage, with the primary vesicles caused by the magma having more volatile content.

Figure 2 Seismic section and interpretation of volcanic edifices, facies, cooling units, gas reservoirs of Yingcheng formation in Songnan gas field:

4.2 Volcanic edifices and facies

A volcanic edifice is composed of the volcanic materials that formed from one or multiple interconnected volcanic conduits over a certain time period [14]. The spatial distribution of volcanic rocks inside a volcanic edifice is controlled by the eruption of magma. The volcanic rocks at different distances from the volcanic conduits have significantly different lithologies, facies, thicknesses, slopes, and physical properties. Therefore, different volcanic edifice facies can be divided into rings based on the distance between the volcanic rocks and the volcanic conduits [15].

Table 2 Volcanic edifice characteristics of volcanic rocks of Yingcheng Formation in Songnan gas field

The volcanic rocks of the Yingcheng Formation in the Songnan gas field were divided into four stages using the regional structural unconformities as stage boundaries. Using the stage boundaries as the time periods of the volcanic edifices and the regional unconformities and hiatuses in sedimentation, seven volcanic edifices were identified. The lithologies, facies, physical properties, and distribution [16] of the volcanic edifices (Table 3) indicate that the largest edifice is VE-4, which is a multi-cone lava volcanic edifice that has the highest mean reservoir porosity and permeability. The reservoir is mainly a primary pore-microfracture type reservoir. The reservoir porosity of VE-4 is mostly composed of primary vesicles, dissolution pores, and microfractures, and it contains high-quality reservoirs. The second largest edifice is VE-6, which is a small single-cone lava volcanic edifice. Although it is small, the main reservoir lithology is lava, which can form good reservoirs. However, the high-quality reservoirs in this edifice are smaller than those of the large volcanic edifice (VE-4). VE-7, which is a no-cone pyroclastic rock volcanic edifice, is relatively large. However, it is mainly formed by pyroclastic rock. The reservoirs are mostly secondary pore- microfracture and structural reservoirs, and they have poor physical properties. The smallest edifice is VE-5, which is a no-cone pyroclastic volcanic edifice. The scale and physical properties are poor, and it generally did not develop high-quality reservoirs.

Table 3 Mean porosity and permeability of volcanic edifices of Yingcheng Formation in Songnan gas field

The physical properties of the reservoirs in different facies within the same edifice are also quite different (Table 4). The crater and near-crater facies are mostly composed of rhyolite and tuff and have the highest porosity, permeability, and fracture density. Well YS1 and other wells drilled into it all found large numbers of high-quality reservoirs. The physical properties of the reservoirs in the proximal and distal facies are worse, and the proportion of high-quality reservoirs is also lower.

Therefore, different volcanic edifices can develop reservoirs of different scales, types, and qualities. The edifice is thus one of the main controlling factors of high-quality volcanic reservoirs. For volcanic edifices of the same size, the acidic lava edifices are mostly rhyolite reservoirs with primary pores, whereas edifices with acidic pyroclastic rocks are mostly tuff reservoirs with secondary pores. The proportion of high-quality reservoirs in the former is greater than in the latter (e.g., VE-4 contains better reservoirs than VE-7). For volcanic edifices of the same lithology, large volcanic edifices are widely distributed and contain more high-quality reservoirs than small volcanic edifices (e.g., VE-4 is better than VE-7, and VE-7 is better than VE-5).

The conditions of high-quality reservoirs within volcanic edifices are controlled by the deeper facies.The proportion of high-quality reservoirs within volcanic edifices decreases from the crater and near-crater facies to proximal facies and then to distal facies. The facies near volcanic conduits are mostly effusive rhyolite with primary pores, and they are generally thick. In addition, the fracture density is high, which favors pore connections. The facies far from the conduits are mostly explosive tuff with secondary pores. The volcanic sedimentary facies farther away contain poor primary pores, have few fractures, and are relatively thin; thus, they generally do not form high-quality reservoirs.

Table 4 Characteristic physical properties of volcanic edifice facies of Yingcheng Formation in Songnan gas field

4.3 Cooling units

A volcanic edifice consists of multiple eruption pulses. When the time interval between two pulses is sufficient to allow the volcanic rocks that erupted during the first pulse to cool to the ground temperature, a cold-hot interface forms between the two eruption units. A cooling unit is located between two adjacent cold-hot interfaces.

Several dozen cooling units were identified in the volcanic edifices of the Yingcheng Formation in the Songnan gas field (Figure 2(b)). The lithologies and physical properties of the cooling units vary. Based on the porosity data of the 10 cooling units that were interpreted from well logs (Figure 3),cooling units ⑩ and ⑦ in VE-4 have the best reservoir properties. The primary pores are large and abundant (Figure 4), and they form the main development horizons of high-quality reservoirs. Cooling unit ⑨ of VE-4, cooling unit ① of VE-6, and the two cooling units in VE-7 and VE-5 all formed smaller, high-quality reservoirs. The other cooling units did not form large-scale high-quality reservoirs.

Figure 3 Mean porosity and permeability of different cooling units in Yingcheng Formation in Songnan gas field

Figure 4 Primary pores in a core and a microscope image of cooling unit ⑦ in Yingcheng Formation in Songnan gas field:

Therefore, the cooling units are one of the main controlling factors in the development of high-quality reservoirs in the Songnan gas field. Because the material composition, cooling time, and distribution of each cooling unit are different, the volatile components and cooling processes will be different, which will cause different levels of porosity and fracture development in the reservoirs. Cooling units with higher volatiles contents more easily form reservoirs with abundant primary pores, and thus, the probability of forming high-quality reservoirs is higher.

4.4 Degree of primary pore development

The high-porosity high-permeability cooling units of VE-4 and the crater and near-crater facies of the Yingcheng Formation volcanic rocks in the Songnan gas field are all mostly rhyolite of the top and middle subfacies of the effusive facies. They have the highest degree of primary pore development and form primary pore-microfracture reservoirs. They contain the highest-quality and largest reservoirs and have the best reservoir properties. The secondary pore reservoirs and structural fracture reservoirs that formed in the other lithologies and facies have poor reservoir properties because most of the reservoir porosity is composed of secondary pores and there are few primary pores.

The primary reason for the control of the volcanic edifices, facies, and cooling units on the high-quality reservoirs is the combination of lithology and facies [17–19]. The fundamental factor is the degree of pore development of the reservoir rocks, which directly controls the differences in the reservoir porosity, oil and gas migration channels, and permeability. The degree of primary pore development is the primary factor that affects the high-quality reservoirs in the Songnan gas field. With increasing degree of primary pore development, the probability of developing a high-quality reservoir increases.

4.5 Dissolution effect

Magma diagenesis generally involves late- stage changes that can significantly change the rock’s pore and fracture characteristics and thus change the reservoir properties [20].

Secondary pores that formed by dissolution are commonly observed in the high-quality volcanic reservoirs in the Songnan gas field. For example, the feldspar crystal clasts in the rhyolite in well YS101 formed abundant intragranular dissolution pores due to late stage dissolution (Figure 5) that became very favorable reservoir porosity. Abundant amphibole and calcite dissolution pores were also found in many wells.

Figure 5 Intragranular dissolution pores in rhyolitic volcanic rocks of Yingcheng Formation in Songnan gas field:

Dissolution can produce large numbers of secondary pores and increase the reservoir porosity and pore connectivity [21]. Thus, dissolution is a favorable diagenetic effect that can greatly improve reservoirs and is an important controlling factor of high-quality reservoirs.

The dissolution of volcanic reservoirs in the Songnan gas field is controlled by tectonic action.The Songnan gas field is located in the Da’erhan fault and uplift zone, which has experienced an extended period of weathering and erosion. Intense dissolution has formed abundant dissolution pores in a thick weathering crust. The Chaganhua rift trough is located at the depression area to the east and is not affected by weathering and erosion. The thickness of the weathering crust is so small that dissolution pores cannot be easily formed (Figure 6).

5 Prediction of high-quality reservoirs

Based on an analysis of the controlling factors of high-quality reservoirs, the high-quality reservoirs and gas accumulations in the volcanic rocks of the Yingcheng Formation in the Songnan gas field should be located in cooling units ⑩, ⑦, and ⑨ of VE-4, cooling unit ① of VE-6, and cooling unit ② of VE-7. The analysis of the volcanic gas reservoirs in the Songnan gas field showed that these horizons all contain high-quality gas reservoirs. In particular, cooling unit ⑩ of VE-4 is the best gas reservoir in the Songnan gas field (Figure 2(c)).

Figure 6 Contour map of weathering crust thickness for volcanic rocks of Yingcheng Formation in Songnan gas field and surrounding area

Based on the vertical and horizontal distributions of the volcanic edifice facies of the Yingcheng Formation in the Songnan gas field (Figures 2(b), 7 and 8), the highest quality reservoirs should be located near wells YS1, YS101, YP1, YP3, and YP9. This region is mainly composed of crater and near-crater facies of VE-4 and VE-7, and several locations contain proximal facies. This region mainly contains rhyolite with abundant primary pores.

Drill stem test data of the wells (Table 5) indicate that wells YS1, YS101, YP1, YP3, and YP9 all have ideal open flow capacities of natural gas. They are located in a region with numerous high-quality reservoirs, which also has the best natural gas production in the Songnan gas field.

Figure 7 Facies distribution of stage four volcanic edifice of Yingcheng Formation in Songnan gas field

Figure 8 Facies distribution of stage three volcanic edifice of Yingcheng Formation in Songnan gas field

Table 5 Single well open flow capacity of gas reservoirs in volcanic rocks of Yingcheng Formation in Songnan gas field

6 Conclusions

1) The Songnan gas field is an inherited paleohigh-type structural-stratigraphic composite natural gas reservoir. The main types of high-quality volcanic reservoirs of the Yingcheng Formation include primary pore-microfracture, secondary pore-microfracture, and primary pore- structural fracture reservoirs. The main types of reservoir porosity include primary vesicles, inter-flow pores and fractures, dissolution pores, devitrification pores, and microfractures.

2) Several factors control the distribution of the high-quality volcanic reservoirs. ① Volcanic eruptive stage: the high-quality reservoirs are more conductive as a result of being formed in the eruption stages, with the primary vesicles caused by the magma having more volatile content. ② Volcanic edifices and facies: more high-quality reservoirs are located in the volcanic edifices with acidic lava than in those with acidic pyroclastic rocks, and more high-quality reservoirs are located in large volcanic edifices than in small volcanic edifices. In addition, the proportions of high-quality reservoirs decrease from the crater and near-crater facies to the proximal facies and then to the distal facies. ③ Cooling unit: the cooling units with high volatile contents more easily form reservoirs with abundant primary pores, thus they have a high probability of forming high-quality reservoirs. ④ Degree of primary pore development: with an increasing degree of primary pore development, the probability of forming a high-quality reservoir increases. ⑤ Dissolution effect: dissolution formed in fault and uplift zones can produce large numbers of secondary pores and increase the reservoir porosity and pore connectivity.

3) Cooling units ⑩, ⑦, and ⑨ of VE-4 and other regions were predicted to develop high-quality gas reservoirs.The region with high-quality reservoirs near wells YS1, YS101, YP1, YP3, and YP9 is a region of good production in the Songnan gas field.

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(Edited by HE Yun-bin)

中文导读

松南气田营城组火山岩优质储层的控制因素

摘要：营城组火山岩是松南气田深层重要的天然气储层，优质储层预测是本区油气勘探的关键问题之一。充分利用岩心、地震和测试数据等资料，对松南气田火山岩岩性、岩相、储集空间和储层类型等进行了研究，得到了松南气田营城组火山岩优质储层的主要控制因素及其机理。控制因素包括火山机构、火山机构相带、冷凝单元、岩性和岩相及溶解作用。大型酸性熔岩火山机构的火山口―近火山口和近源相带及高挥发分含量的冷凝单元内发育的喷溢相流纹岩和爆发相凝灰岩、凝灰熔岩是松南气田优质储层最有利的发育部位。溶解作用可增加储层次生孔隙，是非常有利的改造作用。通过对松南气田优质储层控制因素的研究，可为火山岩优质储层预测和成因分析提供理论依据。

关键词：松南气田；营城组；优质火山岩储层；控制因素

Foundation item: Project(2009CB219306) supported by the National Basic Research Program of China; Project supported by the Key-Lab for Evolution of Past Lift and Environment in Northeast Asia, Ministry of Education, China; Project supported by the third-phase Project 211 at Jilin University, China; Project supported by the Basic Research Fund of the Ministry of Education in 2009 (Innovation Team Development Program, Jilin University)

Received date: 2016-07-15; Accepted date: 2017-05-10

Corresponding author: SHAN Xuan-long, PhD, Professor; Tel: +86–13943133050; E-mail: shanxl@jlu.edu.cn; ORCID: 0000-0003- 0629-2713

Abstract: Predicting high-quality volcanic reservoirs is one of the key issues for oil and gas exploration in the Songnan gas field. Core, seismic, and measurement data were used to study the lithologies, facies, reservoir porosity, and reservoir types of the volcanic rocks in the Songnan gas field. The primary controlling factors and characteristics of the high-quality volcanic reservoirs of the Yingcheng Formation in the Songnan gas field were investigated, including the volcanic eruptive stage, edifice, edifice facies, cooling unit, lithology, facies, and diagenesis. Stages with more volatile content can form more high-quality reservoirs. The effusive rhyolite, explosive tuff, and tuff lava that formed in the crater, near-crater, and proximal facies and in the high-volatility cooling units of large acidic-lava volcanic edifices are the most favorable locations for the development of the high-quality reservoirs in the Songnan gas field. Diagenesis dissolution, which is controlled by tectonic action, can increase the size of secondary pores in reservoirs. Studying the controlling factors of the high-quality reservoirs can provide a theoretical basis for the prediction and analysis of high-quality volcanic reservoirs.