J. Cent. South Univ. (2018) 25: 2785-2801

DOI: https://doi.org/10.1007/s11771-018-3953-y

Reservoir characteristics and hydrocarbon accumulation of Carboniferous volcanic weathered crust of Zhongguai high area in the western Junggar Basin, China

LI Hu(李虎)^{1, 2}, TANG Hong-ming(唐洪明)^{1, 2}, QIN Qi-rong(秦启荣)^{1, 2}, FAN Cun-hui(范存辉)^{1, 2},HAN Song(韩嵩)³, YANG Cang(杨仓)⁴, ZHONG Cheng(钟城)^{1, 2}

1. State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation,Southwest Petroleum University, Chengdu 610500, China;

2. School of Geoscience and Technology, Southwest Petroleum University, Chengdu 610500, China;

3. Research Institute of Petroleum Exploration and Development, Petrochina Southwest Oil & Gasfield Company, Chengdu 610000, China;

4. CNPC International (Nile) Ltd., Beijing 100034, China

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

Abstract: Based on the drilling, cores, logs, seismic, laboratory analysis and so on, reservoir characteristics and hydrocarbon accumulation of Carboniferous volcanic weathered crust in Zhongguai high area are studied. Volcanic rocks were formed in an island arc environment. The lithology is mainly andesite and tuff; Reservoir spaces are mainly secondary pore, fracture and their combination forms, fractures have a better effect on reservoir seepage; There are four layer structures of volcanic weathered crust, weathered clay layer, strongly weathered zone, weakly weathered zone and unweathered zone and strongly weathered zone is the best, which is the main reservoir development zone; The development of reservoir is mainly affected by weathering-leaching, lithology and lithofacies, and fault (fracture); Effective reservoirs could reach to 480 m thickness (high quality reservoirs are within 240 m). Carboniferous volcanic reservoirs are distributed along three zones, which are near the fault zone, high structural part, favorable lithofacies development zone, and one plane, which is near the unconformity.

Key words: reservoir characteristics; volcanic weathered crust; fracture; hydrocarbon accumulation; Carboniferous; Zhongguai high area

Cite this article as: LI Hu, TANG Hong-ming, QIN Qi-rong, FAN Cun-hui, HAN Song, YANG Cang, ZHONG Cheng. Reservoir characteristics and hydrocarbon accumulation of Carboniferous volcanic weathered crust of Zhongguai high area in the western Junggar Basin, China [J]. Journal of Central South University, 2018, 25(11): 2785–2801. DOI: https://doi.org/10.1007/s11771-018-3953-y.

1 Introduction

The first volcanic reservoir was found in San Joaquin Basin of California, America in 1887. At present, more than 170 volcanic reservoirs have been found around the world [1, 2]. For the first time, we found volcanic reservoir in the Northwestern Junggar Basin since 1957 in China, then more volcanic reservoirs were found in 14 petroliferous basins, such as Songliao Basin, Hailar Basin, Erlian Basin, Santanghu Basin and Junggar Basin [3]. Volcanic reservoirs can be divided into two types: primary volcanic reservoirs and weathered crust reservoirs. Volcanic reservoirs discovered in China and abroad are mainly derived from the primary volcanic rocks of Meso-Cenozoic, which are mainly controlled by lithology and lithofacies. For a long time, Chinese petroleum geologists have devoted themselves to primary volcanic reservoirs in Eastern China and achieved fruitful results. They considered that primary volcanic rocks are the most important volcanic reservoirs [4–6]. However, with the exploration and development of volcanic reservoirs, petroleum geologists have paid more and more attention to weathered crust reservoirs and recognized that reservoir physical property and productivity are significantly higher than those of primary volcanic rocks, but the domestic exploration degree is still low at present. So far, volcanic weathered crust reservoirs have been found in Russia (Western Siberia, Yakutia), the United States (Texas), Cuba, Mexico, Libya, China, Kuman and Azerbaijan, and formed a large-scale oil and gas fields. In recent years, there have been great breakthroughs in the research on volcanic weathered crust reservoirs, which are mainly shown in the following aspects [7–9]: 1) The method of structural classification of volcanic weathered crust is becoming more and more mature and the research methods include mineral petrology, geochemical methods, geophysical methods, basic physical methods and basin simulation methods; 2) The study of lithology and lithofacies of volcanic rocks is carried out from the lithology identification, lithofacies division and distribution, and the research technology is becoming more mature; 3) The characteristics and controlling factors of volcanic weathered crust reservoirs are emphatically studied, and the main controlling factors are lithologic and lithofacies, weathering and leaching, fractures and so on; 4) The comprehensive evaluation and prediction of volcanic weathered crust reservoir are carried out by means of multi-disciplinary and multi-technical models, such as logging, geophysical and mathematical models. However, due to the complex reservoir types, large changes of physical properties and strong heterogeneity in volcanic weathered crust reservoirs, there are still many problems in the research process, mainly in the following aspects: 1) The lithology, lithofacies classification and naming of volcanic rocks are not standard enough; 2) Formation mechanism of volcanic reservoir and diagenesis mechanism of volcanic rocks are seldom studied; 3) Fracture identification and prediction of volcanic rock reservoir are insufficient; 4) Volcanic rock has strong shielding effect on the seismic signal, so it is difficult to use seismic data to predict the reservoir.

Due to the control of geotectonic evolution, volcanic weathered crust reservoirs are mainly distributed in Western China, and Carboniferous volcanic rocks in the Northern Junggar Basin have become important exploration targets in recent years. After 2005, Chinese scholars have made a deep research on Carboniferous volcanic rocks in Northern Xinjiang, and a series of new knowledge and exploration results completely changed the viewpoint that “The Carboniferous was considered to be with no hydrocarbon generation and accumulation conditions” [10], expanded the exploration field and guided the discovery of Kelameili Gasfield, Niudong Oilfield. Carboniferous volcanic reservoirs in Zhongguai high area are typical volcanic weathered crust reservoirs and primary volcanic reservoirs are less developed. Since the exploration in the 1950s, a number of volcanic weathered crust reservoirs have been discovered, such as Karamay Oilfield, Hongshanzui Oilfield and Jinlong Oilfield, with a total proved oil geological reserves of more than 1×10⁸ t. Among them, Jinlong Oilfield, which has been found in recent years, has achieved significant breakthrough in the testing of multiple Wells (JL6, JL10, JL101, etc.), all of which fully proved that the exploration potential of Carboniferous volcanic rocks is huge in this area. At the same time, many domestic scholars have carried out a lot of research on volcanic weathered crust reservoir of Zhongguai high area. It mainly focuses on the identification of lithology and lithofacies, fracture characteristics and prediction, reservoir characteristics and control factors [11–14], but the research level is low on the whole. Also, the formation condition and distribution of high quality volcanic reservoirs are quite different in different regions, especially in Zhongguai high area, which has the characteristics of multi-cycle, fracture development and complex formation mechanism. Predecessors have not researched systematically on the reservoir characteristics, which resulted in less huge breakthroughs for years and has seriously hindered the exploration and development of oil and gas in this area. Therefore, based on previous and latest study results of Carboniferous volcanic rocks in Northern Xinjiang, oil and gas geological conditions of volcanic weathered crust reservoirs are discussed in Zhongguai high area.

2 Geological background

The study area is located in the northwestern border of Junggar Basin, about 2200 km²; Primary tectonic unit is located in the western uplift; The northern and western boundaries are Ke-Wu fault zone and Hongche fault zone respectively; the southern and eastern boundaries are Central Depression and Mahu Depression respectively (Figure 1). Ke-Wu fault zone is a N-E alignment thrust fault; Hongche fault zone is a S-N alignment thrust fault. The study area is located in the footwall of thrust fault [15]. There are multiple tectonic movements after Carboniferous volcanic rocks formed, mainly representing that it began to uplift at the middle-late Hercynian tectonic movement, inherited and developed during the period of Indosinian and Yanshanian movement, finally formed during the Himalaya movement, and experienced a entire cycle of tectonic movement from strong to weak till died out. And the influence of Carboniferous tectonic movement is especially important, which is a key period for the whole basin transited from an active basin-mountain system to a more stable basin system [16]. According to drilling and field geological outcrop investigation, the strata of the area are mainly composed of Neogene (N), Paleogene (E), Cretaceous (K), Jurassic (J), Triassic (T), Permian (P) and Carboniferous (C). The thickness of Carboniferous in this area is 1300– 2000 m, and lithology is mainly of intermediate- basic eruptive rocks, some acid eruptive rocks and metamorphic rocks. From Carboniferous to late Jurassic, it was always in the high structural part; The basement exposed to surface for a long-term after Carboniferous volcanic rocks formed, and suffered great weathering-leaching. Therefore, volcanic weathered crust is well developed [17]. Zhongguai high area is a gentle nose-shaped paleohigh formed during the Carboniferous- Early Permian (P₁) period; the structural morphology is monoclinal and inclined from NW to SE, with steep south wing and gentle north wing. The top stratum of sedimentary cap of Carboniferous volcanic rocks is thin or partially denuded and wing strata are thick (Figure 1) [18]. There is a serious erosion phenomenon at the bottom strata of Jiamuhe Formation (P₁j) in the fault zone and the high structure part; In the late deposition of Jiamu Formation (P₁j), the strata of Fengcheng Formation (P₁f) and Xiazijie Formation (P₂x) were mostly missing, resulting in the erosion of the top Jiamuhe Formation (P₁j); During the deposition of lower Wuerhe Formation (P₁j), the sedimentary range of Zhongguai high area is extended, Stratigraphic deletion in the upper plate of K-W fault and X-H fault, which made angular unconformity with Triassic Karamay Formation (T₂k¹). In addition, there is also angular unconformity between the upper Triassic Baijiantan Formation (T₃b) and the lower Jurassic Badaowan Formation (J₁b), the upper Jurassic Qigu Formation (J₃q) and the lower Jurassic Tugulu Formation (K₁tg) [19, 20].

3 Types and characteristics of volcanic weathered crust reservoir

3.1 Lithologic characteristics

According to the statistic data of 310 slices from 17 wells (Figure 1) in Zhongguai high area, volcanic breccias are mainly andesite and tuff, secondly is volcanic breccia and basalt. Andesite is composed of dark gray, brown-gray and brown-gray and it is mainly porphyry structure, interlaced structure, with stomata or amygdaloid structure (Figure 2(a)). Tuff is mainly dark gray, gray-green, gray-white, gray-brown, and its structure is composed of debris, breccia and volcanic ash, with block structure, and there are crystal-vitric tuff, melted tuff and volcanic breccia tuff in the area (Figures 2(b), (c)). Volcanic breccia is mainly brick red, dark gray, gray-green, composed of volcanic breccia structure, with massive structure (Figures 2(d), (e)). The components of volcano breccia are more complex and varied, including basaltic, andesitic and rhyolitic etc; Basalt is mainly gray, brown gray and brown gray, and its structure is composed of intergranular and intergranular structure, with amygdaloid structure and matrix glass intertexture structure (Figures 2(f), (g)). Tuff accounts for 34% of the total volcanic rock thickness; Andesite accounts for 28%; Volcanic breccia accounts for 19%; Basalt accounts for 10% and granite accounts for 9%. There are a few intrusive rocks developed in some local wells and lithology is mainly acidic granite (Figure 2(h)).

Figure 1 Location of study area and top tectonic map of Carboniferous

Total Alk–SiO₂ (TAS) diagram shows (Figure 3(a)) that the samples are in the subalkalic range; only 1 sample fell in basaltic andesite; 2 samples fell in andesite range vicinity; The remaining samples fell in dacite and rhyolite range. SiO₂–K₂O diagram shows (Figure 3(b)) that the samples fell in low-potassium tholeiite calc-alkalic series; all of the granite samples fell in calc-alkalic series; most of the remaining samples fell in low-potassium tholeiite series, which have the characteristics of the same term of magmatic evolution; The product (granite) of late magmatic evolution is potassium-rich relatively. Rittmann index δ((Na₂O+K₂O)²/(SiO₂–43))=0.05–2.79; δ value of the 15 samples is less than 1.8, belonging to calc series; δ value of the other 7 samples is in the range of 1.8 to 3.3, belonging to calc-alkalic series.

Figure 2 Lithology and lithofacies of volcanic rocks:

Figure 3 Types and characteristics of reservoir space [21]:

The chemical analysis of main and trace elements of 6 wells in this area shows that (Tables 1 and 2) [22]: main element content of the sample is much different. SiO₂ content ranges from 53.99% to 76.04%, showing that the samples are mainly neutral volcanic rocks and acidic volcanic rocks; Na₂O+K₂O content ranges from 1% to 8.12% and the value of K₂O/Na₂O is 0.01 to 2.91, which is alkali-rich; CaO content is 0.34% to 15.56%, which is calcium-rich. Trace elements are deficient of Rb, Nb, Ta, Sr, P and Ti, which is U-rich, suggesting that it is arc volcanic rocks [23, 24]. Combined with overlying mudstone and sandstone of fan delta facies, Carboniferous volcanic rocks were formed in an island arc environment.

3.2 Structure of volcanic weathered crust

According to the classification results of Carboniferous volcanic weathered crust in Northern Xinjiang by ZOU et al [3], and HOU et al [25], combined with the 1125 m core description of 10 wells (597, 589, 581, JL5, JL6, JL10, JL061, H056A, G6, K021), conventional logging response characteristics of 7 wells, element measurements and physical characteristics of 6 wells in Zhongguai high area. Vertical structure of weathered crust can be divided into weathered clay layer, strong weathered zone, weak weathered zone and unweathered zone (Figure 4). Different weathered crust structures have different characteristics (Table 3). Weathered clay layer is located in the uppermost layer, which is weathered fine residuum; It is affected by lithology, climate, exposure time obviously; Its development degree is different in different tectonic position; Logging response characteristics present low resistivity; Deep-shallow resistivity curve is close to the “box” shape and without any amplitude differences, low density, high acoustic travel time; Detrition is much serious in strong weathered zone; Dissolved pores and structure fractures are much developed; Reservoir spaces are diversified; Porosity and permeability are much better than others; strong weathered zone is the main reservoir space; Porosity and permeability of weak weathered zone decreased sharply compared to strong weathered zone; There is a transition zone at the top of weak weathered zone, but there are still effective reservoirs;Unweathered zone is located at the bottom of weak weathered zone and almost unaffected. Lithology is dense and solid, porosity and permeability are the worst. Through statistics of 1542 porosity samples of different structures from 9 wells, the average porosities of weathered clay layer, strong weathered zone, weak weathered zone and unweathered zone were 3.5%, 13.6%, 7.2%, 4.9%, respectively. Refer to the lower limit of effective porosity (6.4%) in northern Xinjiang [26], strong weathered zone and part of weak weathered zone (the top part and fracture affected area) are effective reservoirs. Formation testing results also proved that effective reservoirs are mainly distributed in strong weathered zone.

Table 1 Test data of main elements

Table 2 Test data of trace elements

Figure 4 Vertical structure of volcanic weathered crust (597, 1: 2000)

Table 3 Characteristics of lcanic weathered crust structure

3.3 Types and characteristics of reservoir space

3.3.1 Types and characteristics of pores

The development of volcanic reservoir space is related not only to the characteristics of volcanic rocks, but also to the later external environment. According to 1125 m cores, 310 slices and more than 500 pieces of image data of 17 wells (Figure 1) in this area, combined with FMI imaging logging data interpretation, reservoir space types are divided into primary type and secondary type, but often their combination forms, the main forms are dissolved pore-shrinkage fracture-intergranular pore, stomata-dissolved pore-dissolved fracture, dissolved pore-fractured stomata-weathered fracture [27, 28].

The primary reservoir space includes stomata (primary and residual pores), intergranular pores and primary fractures. Primary pores are the main reservoir space in this area and are the most developed in the effusive facies basalt and andesite. The maximum pore diameter is more than 8 mm; The average is 0.5–3 mm; Stomata can account for more than 30% of the total rock volume and usually account for 10%–15%, which is often isolated, disordered, distributed lumpy, but connectivity is poor (Figure 5(a)). If the stomata pores were filled with calcite, zeolite and chlorite in the later stage, then residual pores formed (Figure 5(b)) and pores diameter is fine, most of which are less than 0.1 mm. Intercrystal pores developed among phenocrysts of the effusive rocks (Figure 5(c)); pore diameter is smaller, usually less than 0.2 mm, irregularly in shape. Shrinkage pores are volcanic glasses or components filled in the internal rock space due to the expansion and contraction caused by temperature variation of condensation diagenesis of volcanic rock (Figure 5(d)).

The primary fractures are mainly condensation-shrinkage fracture and gravel fractures formed in the process of diagenesis. Condensation-shrinkage fractures are formed by the temperature variation in the magma condensation process (Figure 5(e)). Gravel fractures are distributed among breccias or in the breccia border and are controlled by volcanic facies obviously (Figure 5(f)).

The secondary reservoir spaces mainly include secondary pores and fractures, which are mainly composed of phenocrystal dissolved pores, matrix dissolved pores, intergranular dissolved pores, intragranular dissolved pores and grain pattern pores. Phenocrystal dissolved pores are developed in basalt and andesite, which are formed from the dissolution of feldspar phenocrysts, present honeycomb and sieve porous (Figure 5(g)). Matrix dissolved pores are very common in andesite and basalt of effusive facies, which are the favorable volcanic reservoir spaces and the matrix is composed of hyaline and micritic feldspar; Micritic feldspar can be easily argillation dissolved and then form matrix dissolved pores (Figure 5(h)). Intragranular dissolved pores are distributed in andesite of effusive facies, volcanic breccias and tuff of eruptive facies, irregularly in shape and often in an embayed shape (Figure 5(i)), which are formed from tuff, volcanic breccia and volcanic crystal after the dissolution diagenesis. Intergranular dissolved pores are developed in andesite and volcanic breccia, which are mainly formed from intergranular matrix after leaching of atmospheric water or fresh water; The shape is elongated or tuberose, and pore diameter is 0.15–0.5 mm (Figure 5(j)). Grain pattern pores are distributed in volcanoclastic rocks, mainly are feldspar moldic pores, which are the original components of dissolved volcanic rock but still retain the original appearance of the components (Figure 5(k)). The secondary fractures include structural fractures, weathered fractures and dissolved fractures. Structural fractures are formed under the tectonic stress in the later stage, surface smooth, attitude stable, extended farther, and have obvious directivities (Figure 5(l)), which are the main fracture type in this area and have an important effect on improving the reservoir seepage ability. Weathered fractures are mainly developed near the unconformity surface. Weathering-leaching caused the shape extremely irregular, extended shorter, often half filled or filled with mauve ferruginous or argilaceous, which made few contributions to oil and gas accumulation.

Figure 5 Types and characteristics of reservoir space:

3.3.2 Types and characteristics of fractures

The fracture is not only an effective reservoir space of volcanic reservoir, but also an important seepage channel. According to the cause of fracture in the area, it can be divided into five types: structural fracture, diagenetic fracture, weathered fracture, dissolved fracture and drilling induced fracture [29]. According to its attitude, it can be divided into horizontal fracture, oblique fracture, reticulatus fracture and vertical fracture (Figure 6). Oblique fractures are much developed in the area and reticulatus fractures are mainly structural fractures [30]. Diagenetic fractures and low angle fractures can also be combined with structural fractures. Reticulatus fractures are mainly developed near the fault zone.

The development of different fracture types is different and structural fractures related to the tectonic stress are more developed in this area.

Figure 6 FMI logging characteristics of fracture with different attitudes in volcanic rocks:

Other types are less developed and reservoir stimulation is limited; structural fracture is mainly controlled by fault [31]. Horizontal permeability is the main factor of well production; According to the fracture attitude, seepage abilities from high to low are reticulatus fracture, oblique fracture, horizontal fracture and vertical fracture. Different types of fractures crisscross formed reticulatus fractures in the area, which have an important effect on improving reservoir seepage ability [32]. According to the fracture angle, it can be divided into horizontal fracture (less than 10°), low angle fracture (10°–30°), oblique fracture (30°–60°), high angle fracture (60°–80°), vertical fracture (more than 80°), low angle fracture and oblique fracture extended farther, and mostly in the form of reticulatus fractures in the area. The statistics of fracture filling degree in this area shows that filling degree of vertical fracture and horizontal fracture is the highest and about 90% is fully or half filled. Followed by low angle fracture, more than 80% is fully or half filled. High angle fracture and oblique fracture are relatively lower and about 60% is not filled or half filled (Figure 7).

Select 1486 porosity and permeability test samples from 9 wells (597, 589, 581, JL6, JL10, JL061, H056A, G6, K021) in this area. With the porosity increasing, the permeability variation regularity is not strong and correlation coefficient is only 0.4257. A few samples have an obvious phenomenon of low porosity and high permeability, which is directly connected with the development of fracture. Fractures have an obvious effect on improving the percolation ability of the reservoir (Figure 8).

Figure 7 Filling degree of different angle fractures

Figure 8 Relationship between reservoir porosity and permeability in volcanic reservoir

4 Controlling factors of volcanic weathered crust reservoir

The main controlling factors of volcanic reservoir are key problems in oil and gas exploration. Primary volcanic rock is mainly controlled by lithology and lithofacies; but volcanic weathered crust reservoir is more complex, mainly controlled by the structure of weathered crust, weathering-leaching, fault (fracture), lithology and lithofacies, etc.

4.1 Weathering-leaching

The intermediate-basic volcanic rock is mainly developed in western China; only undergo a long- term weathering-leaching can it will become favorable reservoir. Carboniferous volcanic rocks have suffered strong tectonic compression cataclasis and long-term weathering-leaching. Weathering-leaching is extremely important to improve the weathered crust reservoir.

4.1.1 Palaeogeomorphology

The palaeogeomorphology is the result of combined effects of weathered crust karstification and various geological processes. Different palaeogeomorphologies play different controlling role on weathering-leaching, and also affect the distribution of reservoirs [33, 34]. According to tectonic evolution, Carboniferous palaeostructure and volcanic facies model in the study area, palaeogeomorphology can be divided into monadnock, gentle slope, steep slope, denudation, depression and fluting unit (Figure 9). Monadnock unit is located in the high part of paleogeomorphology, near the main fault. Weathering-leaching is the strongest, fracture is more developed relatively and provided favorable conditions for weathering-leaching and groundwater denudation, which is conducive to the forming of effective reservoirs. For example, formation testing of well K021, JL10 and H019 is 66.38 t/d, 20.16 t/d and 105.96 t/d, respectively. Gentle slope unit is located in the relatively high part of paleogeomorphology, near the gentle slope zone of the fault and weathering-leaching and denudation is strong. For example, formation testing of well 574, JL6 is 14.66 t/d and 5.3 t/d respectively, which is slightly worse than that of monadnock. Steep slope unit is located in the relatively lower altitude part of paleogeomorphology and weathering-leaching degree is weak; However, some still can form favorable reservoirs (formation testing of well G6 is 5.2 m³/d). Denudation, depression and fluting unit are in the lowest position, weathering-leaching degree is weak and it is difficult to form effective reservoir.

4.1.2 Longitudinal development of weathered crust

Because of long-term exposure, volcanic rock suffered epidiagenesis and leaching dissolution of atmospheric water. Dissolution pore, hole and fracture are developed near weathered crust, which improved the reservoir physical property effectively [35]. The closer to the top of weathered crust it is, the more the dissolution pore, hole and fracture developed.

Through the statistical relationship about 1542 porosity samples of 9 wells (597, 589, 581, JL6, JL10, JL061, H056A, G6, K021) and the distance to the top of weathered crust (Figure 10), the samples with porosity greater than 15% are located within 240 m under weathered surface, and 6% to 15% are mainly distributed in the range of 240 to 480 m. With the distance to the top Carboniferous increasing, the porosity decreased sharply. Within 480 m of the top weathered crust, it can form effective reservoir (within 240 m is the best) and the maximum porosity can reach 23.1%. Because of the fault (fracture), surface water along with fault infiltrate and dissolute reservoir, porosity can reach 20.7% and the distance to the top weathered crust of effective reservoir can be extended to 1000 m.

Oil and gas are closely related to the weathering-leaching of unconformity surface. The formation testing results of well 573, 563, 574, H019, H56A, K021 and JL10 are favorable, and testing depths are all near the top of weathered crust, and basically located in strong weathered zone. Thus, weathering-leaching has an important effect on reservoir reconstruction, migration and accumulation of oil and gas. Statistical analysis shows that the distribution of favorable reservoirs is mainly controlled by the distance to the top weathered crust, ranging from 20 to 240 m. High production wells are enriched where it is the favorable reservoir development zone. Oil and gas are mostly enriched in the range of 20–130 m (Figure 11).

Figure 9 Paleomorphology of Carboniferous

Figure 10 Relationship between reservoir and distance to top Carboniferous

4.2 Lithology and lithofacies

In the same hypergene environment, rock mechanics parameters, primary physical property and soluble minerals are different because different lithology and lithofacies present stronger differential weathering and the intensity of weathering degree affects reservoir physical property directly [36].

According to the statistics of more than 800 physical property data of main lithology (lithofacies) in the study area (Figure 12), it shows that without weathering, only volcanic breccia is effective reservoir with the average porosity 8.6%; Eruption facies can form effective reservoir with the average porosity 8.2%. After weak weathering, volcanic breccia, andesite and tuff can form effective reservoirs with the average porosities 9.4%, 7.3%, 6.5% respectively. Eruption facies, volcanic conduit facies, effusive facies, extrusive facies can form effective reservoirs with the average porosities 9.7%, 8.6%, 7.8%, 7.5% respectively. After strong weathering, only granite can’t form effective reservoir (the average porosity is 3.8%); The porosity of other lithology is different and volcanic breccia, andesite, tuff and basalt are 12.4%, 10.3% 9.2% and 7.6% respectively. Also, except volcanic sedimentary facies can’t form effective reservoir (the average porosity is 4.9%), the rest can form effective reservoirs with the average porosity of eruption facies, volcanic conduit facies, effusive facies, extrusive facies 11.5%, 10.6%, 8.9%, 6.8% respectively. Obviously, with the increase of weathering degree, reservoir porosity increased. Weathering can improve reservoir physical properties, and physical properties of volcanic braccia of eruption facies and andesite of effusive facies are the best in Zhongguai high area.

Figure 11 Relationship between oil gas show and distance to top Carboniferous

4.3 Fault (fracture)

After Carboniferous volcanic rock formed, there were different tectonic movements such as Hercynian period, Indosinian period, Yanshanian period, so fault and fracture are much developed in this area. The development of fracture is generally controlled by faults and partial tectonic stress vicinity of fault affects the development and distribution of fractures [37].

Through statistics of the relationship between fracture density and the distance to main controlling fault, high fracture density mainly located in the main controlling fault within about 500 m. The closer the distance is, the more the fractures developed. Beyond 500 m from the main controlling fault, the fracture density decreased obviously (Figure 13(a)). Moreover, there is a direct relationship between volcanic lithology and fracture development [38]. Fracture density of volcanic breccia, andesite, tuff, basalt and granite are 1.7 m^–1, 1.5 m^–1, 0.8 m^–1, 0.5 m^–1, 0.3 m^–1 respectively. The maximum fracture density is volcanic breccia and andesite, which has a direct relationship with rock mechanic strength decreasing due to a long-term weathering-leaching(Figure 13(b)).

Figure 12 Average porosities of volcanic rocks of different lithologies (a), lithofacies (b) for weathered degrees

Figure 13 Influencing factors of fracture development:

The development of fracture has an obvious controlling effect on weathered crust reservoir. The fracture development in the study area is divided into three levels. Class I is mainly located in the middle part, near well K021-H56A, well H019- G6-JL11-JL10-JL061-JL6, well G10, well 563-591, well 573-JL3, well 593-572-596, well 574-K75- K85, well K007-K010-K79-K301 area and southern area of well JL2; class II and class III distribute outside the class I development area. Well test results are generally better in the class I fracture developed zone.

5 Hydrocarbon accumulation of volcanic weathered crust reservoir

There are several hydrocarbon accumulation characteristics in the area catching oil and gas in the vicinity of source rocks. Weathering-leaching and diagenesis improve reservoir performance, hydrocarbon accumulation with fault and unconformity, matching and accumulation with favourable structure and volcanic rocks [23, 24, 39]. Volcanic rock facies are mainly intermediate-basic eruption facies and effusion facies. After volcanic rock formed in Carboniferous period, there were strong uplift, fold and denudation. Unconformity formed by weathering-leaching and its secondary faults improved the reservoir performance; Lacustrine mudstone of upper Wuerhe Formation and Carboniferous Formation formed favorable reservoir-seal assemblage; Main faults provided paths for oil and gas migration, formed banded and blocklike hydrocarbon accumulation zone along both sides of the main faults. Thus, it formed different scales of volcanic weathered crust reservoirs (Figure 14).

Carboniferous volcanic reservoirs are distributed along three zones, which are near the fault zone, high structural part, favorable lithofacies development zone, and one plane, which is near the unconformity. The enrichment of oil and gas in plane is closely related to fault zone, unconformities (palaeogeomorphology) and lithofacies distribution. Near the main controlling fault zone, the high part of palaeogeomorphology and slope zone near the fault, eruption facies and effusion facies development zone are the enrichment zones. Oil and gas distribution is mainly controlled by the Carboniferous unconformity in vertical, the closer to the unconformity it is and the higher the degree of weathering-leaching, the better the reservoir property of volcanic rock is. With the increase of the distance to the unconformity, porosity and permeability decreased obviously.

6 Conclusions

1) Carboniferous volcanic rocks in Zhongguai high area formed in inner continental island arc environment, which are low-potassium tholeiite calc-alkali serie rocks; Volcanic weathered crust structure can be divided into 4 layers: weathered clay layer, strong weathered zone, weak weathered zone and unweathered zone. Physical property of strong weathered zone is the best and the porosity can reach 13.6%.

2) Effective reservoir spaces are mainly secondary pores and fractures. The main combination forms are dissolved pore-shrinkage fracture-intergranular pore, stomata-dissolved pore-dissolved fracture, dissolved pore-fractured stomata-weathered fracture. Fracrures are much developed and have a favorable effect on reservoir seepage ability that are controlled by fault and lithology (lithofacies).

Figure 14 Reservoir forming model of Carboniferous volcanic weathered crust:

3) The development of reservoir is affected by weathering-leaching, lithology and lithofacies, and fault(fracture). Monadnock, gentle slope and other parts near the fault (fracture) are the main effective reservoirs; Effective reservoir is up to 480 m of the top Carboniferous, and the best reservoir physical property is within 240 m. Weathering has an important effect on improving the reservoir physical property, volcanic breccia of explosive facies and andesite of effusive facies are the best in the study area. Carboniferous volcanic reservoirs are distributed along three zones, which are near the fault zone, high structural part, favorable lithofacies development zone, and one plane, which is near the unconformity.

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(Edited by YANG Hua)

中文导读

准噶尔盆地西北缘中拐地区石炭系火山岩风化壳储层特征与油气成藏

摘要：综合利用钻井、岩心、测井、地震、分析化验等资料，对准噶尔盆地西北缘中拐地区石炭系火山岩风化壳储层特征及油气成藏开展综合研究。研究表明：研究区石炭系火山岩形成于岛弧环境；岩性以安山岩、凝灰岩为主；有效储集空间以次生孔隙、裂缝以及它们的组合形式为主，裂缝的发育改善了储层渗流能力；石炭系火山岩风化壳结构分为风化黏土层、强风化带、弱风化带及致密未风化带4层结构，其中强风化带储层物性最好，为主要的储层发育区；风化壳储层主要受风化淋滤作用、岩性岩相、构造位置及断裂（裂缝）等因素的控制，有效储层厚度分布在深度480 m以内（优质储层在240 m以内）。石炭系储层的富集规律为沿三带（靠近断裂带、构造高部位地带、有利岩相发育带）、一面（不整合面附近）分布。

关键词：储层特征；火山岩风化壳；裂缝；富集规律；石炭系；中拐地区

Foundation item: Project(51674211) supported by the National Natural Science Foundation of China; Project(51534006) supported by the Key Projects of the Natural Science Foundation, China; Project(2017ZX05036003-003) supported by the National Science and Technology Project of China

Received date: 2017-07-30; Accepted date: 2018-03-08

Corresponding author: TANG Hong-ming, PhD, Professor; Tel: +86-17302868501; E-mail: swpithm@vip.163.com; ORCID: 0000- 0003-1153-1467