Shu-jing Bao , Gang-yi Zhai , Zhi Zhou , Shu-fang Yu , Ke Chen , Yu-fang Wang , Hao Wang ,Yi-min Liu

a Oil and Gas Survey, China Geological Survey, Beijing 100083, China

b Unconventional Petroleum Geology Laboratory of Oil and Gas Survey, China Geological Survey, Beijing 100083, China

c China University of Geosciences(Beijing), Beijing 100083, China

ABSTRACT

There are plenty of Sinian and Cambrian potential shale gas resources in South China, which is characterized by high thermal evolution degrees, poor drilling performances and only occurs in local areas.Taking the principle “high to find low” is the key issue to achieving a breakthrough in older shale. China Geological Survey drilled in the periphery of the Proterozoic basement, i.e. the Huangling anticline, in the western Hubei, and Hannan paleocontinent in the southern Shanxi. It received high-quality gas-bearing shale with relatively low Ro in the in Lower Cambrian Niutitang formation and Sinian Doushantuo formation. Based on geological conditions of shale gas reservoirs in the Huangling anticline, this paper puts forward the new model named “Control over reservoirs by periphery of basement” about shale gas accumulation, suggesting that the shale deposited in a deepwater continental shelf in the periphery of the basement is characterized by shallow burial, a short burial time, stable tectonics, relatively low thermal evolution degrees, and shale gas reservoirs in a good condition. The shale of the Sinian-Cambrian strata deposited in deepwater continental shelves in the periphery of Chuanzhong paleo-uplift in Sichuan,Hannan paleocontinent in the southern Shanxi, Huangling anticline in western Hubei and Jiangnang-Xuefeng paleo-uplift in Hunan and Guizhou province have good shale gas exploration potential.

Keywords:

Huangling uplift

Evolution

Shale gas

Control on the accumulation and preservation

1. Introduction

Many sets of rich organic shale of marine facies in the Paleozoic have been developed in southern China (Liang X et al., 2009; Zou CN et al., 2015; Liu SG et al., 2014; Mou CL et al., 2011; Wen L et al., 2001), among which significant breakthroughs have been made in the Upper Ordovician Wufeng-Lower Silurian Longmaxi formation in Changning,Weiyuan of Sichuan, Jiaoshiba of Chongqing (Zhao WZ et al., 2016; Jin ZJ et al., 2016). Basic geological survey has revealed that the Cambrian and Sinian also have good shale gas resource potential, the rich organic shale is thick with an average thickness of nearly 100 m and widely distributed in southern China (Liang X et al., 2009; Liu SG et al., 2014;Wen L et al., 2001). Because of the old age of shale and the high degree of thermal evolution, the drilling results have been poor in most areas (Hu L et al., 2012; Huang JL et al.,2013; Han SB et al., 2013; Liang X et al., 2014). At present,more than 10000 m3/d of industrial shale gas has been drilled only in Jingyan and Jianwei areas of Sichuan (Zhao WZ et al.,2016; Xie D et al., 2015; Liu and Fu, 2016; Dong XX and Xiong L, 2016). For example, Jiaoye No.1 HF well has the production of 59000 m3/d of shale gas. Compared with the shale thermal evolution degree of the Wufeng-Longmaxi formation, which is generally 2.0%–3.0% (Zhao WZ et al.,2016), that of Cambrian and Sinian shale in most areas of southern China is more than 3.5%, which is in the overmature stage and is unfavorable for the accumulation and preservation of shale gas (Zhao WZ et al., 2016; Hu L et al.,2012; Huang JL et al., 2013; Han SB et al., 2013; Liang X et al., 2014). Zhai GY et al. (2017) put forward the shale gas accumulation model of “old uplift edge controls reservoir”,and it is believed that the shale deposits in deep water continental shelf facies at the edge of ancient uplifts are shallow, short in depth, stable in structure and relatively low in thermal evolution, which are good geological conditions for shale gas accumulation that can effectively solve the high thermal evolution problems of Cambrian and Sinian in southern China. Since 2014, China Geological Survey (CGS)have deployed a number of drilling wells in the southern margin of the Huangling uplift in western Hubei, which found the relatively low thermal maturity (Ro ＜ 3.0%) and high gas content rich organic shale in the Lower Cambrian Niutitang formation. The fracturing test gas has produced a high yield industrial airflow of more than 50000 m3/d and made a significant breakthrough in the investigation of Cambrian shale gas in southern China. In addition, this has presented good prospects for the exploration of deep-water continental shelf facies shale of the Cambrian on the peripheral edge of the ancient uplift, and it is further confirmed that old uplifts like the Huangling uplift play an important role in controlling the accumulation of shale gas of the Cambrian-Sinian.

Huangling anticline has become a hot area for geological research in southern China. It not only attracts worldwide attention for the Three Gorges Water Conservancy Project,but also attracts the attention of Chinese geologists as typical area of the basement and Early Paleozoic strata of the Yangtze Craton where some standard Sinian-Early Paleozoic profiles are built (Chen MS and Zhang SH, 1998; Wang KW et al., 2015). As a typical sedimentary area and basement outcropping area of the Yangtze craton (Chen HM et al.,2002; Meng FS et al., 2003; Li SB et al., 2008), the evolution of the Yangtze platform, the oldest rock age in Southern China, is recorded here (Gao S and Zhang BR, 1990; Ma DQ et al., 1997; Li ZC and Fang X, 1998; Gao S et al., 2001;Zhao FQ et al., 2006; Zhou ZY et al., 2007). The Huangling anticline is located at the special structural position where the NWW-trending Dabashan arcuate structural belt intersects with the NE-trending Bamianshan arcuate structural belt (Ge XH et al., 2010). In addition, this area is a blank area of oil and gas rights. The discovery, breakthrough and later demonstration of this area will promote the understanding of the preservation conditions of shale gas in complex tectonic areas in southern China. The formation of technical theory can also promote the progress of exploration and development of shale gas in southern China. Taking the Huangling uplift in western Hubei as an example, this paper analyzes its structural evolutionary history, combined with the geological conditions of shale gas accumulation in the Lower Cambrian Niutitang formation, the control effect of the Huangling uplift on shale gas accumulation and the preservation of the Niutitang formation have been analyzed, to provide references for shale gas exploration in the future.

2. Regional geology

The tectonic position of the Huangling uplift is located at the north margin of the middle Yangtze Craton, south of the Jiangnan orogenic belt, north of the Qinling-Dabie orogenic belt, and it is located in a relatively stable area between the two orogenic belts (Xu DL et al., 2013). The base of the Huangling uplift has a double deck structure with an overlaying layer. The base is exposed to the anticlinal core,which consists of metamorphic rocks of the Kongling group of Late Archean and Early Proterozoic and new Proterozoic Huangling granite (Xu DL et al., 2013; Zhang HD et al.,1986). The sedimentary cover consists of the Nanhua-Triassic marine deposits and a small amount of continental deposits since the Jurassic, its main body is a set of clastic carbonate rocks, which forms a “ring” distribution around the base from old to new (Fig. 1). The Lower Cambrian Niutitang formation consists of a set of black or black gray siliceous rocks and carbonaceous shale with a horizontal bedding development,and the Doushantuo formation of the Sinian is composed of a set of black and gray muddy dolomite with black thin layer shale, usually with a striated layer structure.

Fig. 1.Geological map of Huangling uplift and its peripheral area.

From 2014 to 2017, the CGS successively deployed four geological survey wells for the Lower Cambrian Niutitang formation and Sinian Doushantuo formation in the southern margin of the Huangling uplift, which are Zidi No.1 well, Zidi No.2 well, Yidi No.2 well, and Wudi No.1 well. There are also four shale gas exploration wells in addition, which are E’Yiye No.1 HF well, E’Yangye No.1 HF well, E’Yican No.1 well and E’Yican No.2 well. In addition to the Wudi No.1 well, other wells have been drilled to obtain the rich organic shale with a thick layer, relatively low maturity and high gas content. Among them, the high-yielding industrial airflow of 60200 m3/d has been obtained by fracturing into 26 sections the 1875 m horizontal well in E’Yiye No.1 HF well, and 78400 m3/d in 33 sections of the horizontal layer of 1836 m in the E’Yangye No.1 HF well. These results have shown the superior shale gas accumulation geological conditions of the Lower Cambrian Niutitang formation and the Sinian Doushantuo formation in the southern margin of the Huangling uplift.

3. Structural evolution characteristics of the Huangling uplift

The structural evolution of the Huangling uplift has been the focus of research by many geologists, but there is no unified view among domestic scholars as for its main formative period. However, the study on the structural evolution of the Phanerozoon is more advanced. According to the residual stratum thickness and the lithologic distribution of the surrounding area, Zhang HD et al. (1986) have discussed the structural development of the Huangling uplift during the Indosinian and Early Yanshan, which shows that the geological evolution of the Huangling uplift has been in uplift for a long time.

3.1. Pre-Indosinian movement

Before the Indosinian movement, the Huangling uplift was located in the uplift zone from Kaixian to Huangling in the east-west direction of metamorphic basement. It had been an inherited uplift, and its structural development can be divided into four stages: (1) Early Caledonian slope stage:The residual stratum thickness of Cambrian and Ordovician in the margin of the Huangling uplift was generally thick in the South and thin in the North, which reflected that the Early Caledonian study area including the Huangling uplift area was an ancient slope high in the North and low in the South. (2)Late Caledonian uplift: Since the Silurian, the Kaixian-Huangling uplift had further developed toward the West. By the end of the Silurian, an East-West uplift with a range of more than 1000 m had formed. (3) Early Paleoslope stage of the Hercynian: At the end of the Devonian, the top surface layer of Silurian was an ancient slope high in the North and low in the South, which continued to the end of the Carboniferous. (4) The paleodepression stage in the Late Hercynian: By the end of Permian, the upper paleostructure of the Carboniferous formed the Xiangxi centering ancient depression and spread to the Huangling uplift (Zhang HD et al., 1986).

3.2. Indosinian movement

The Indosinian movement from the Middle–Late Triassic to the pre-Jurassic completely ended the ancient depression state of the Huangling uplift in the Permian. After entering the Triassic, relative to the Zigui and Dangyang regions, the Huangling uplift began to swell, leading to the thinning of the Middle and Lower Triassic layers in the Huangling uplift.Along with the swelling of the Huangling uplift, two major faults developed on both sides: The Xiannv mountain fault on the west side formed the boundary between the Huangling uplift and the Zigui basin, and the eastern Yuan'an fault formed the boundary between the Huangling uplift and the Dangyang synclinorium. The relative height of the Huangling uplift in the Indosinian was about 1300 m, and it had been exposed to water, becoming one of the source areas of the Shazhenxi formation in the Zigui basin.

3.3. Yanshan movement

The Indosinian movement changed the topography of the Huangling uplift from the east to the west direction in the Paleozoic from the south to north and became one of the main sources of the Upper Triassic in the Zigui basin. The Jurassic strata were deposited by the Early Yanshan movement in the Huangling uplift and there was a gradual descent on its east and west sides. At that time, the Huangling uplift was a low uplift in the middle of the lake basin. The Huangling uplift and its two sides gradually decreased in the Late Jurassic, and the Xiangxi group was deposited in the Zigui and Dangyang regions, which was in conformity with the lower overlying Triassic layers. From the distribution of the Xiangxi group in Zigui basin, the general trend was thick in the West and thin in the East, which indicated that the Huangling uplift was still the denudation area on the water surface at that time, but the water area was obviously larger than that of Shazhexi, and the lake shoreline had been significantly shifted to the east.During the Middle Jurassic, the Huangling uplift and its surrounding area continued to decline. The Zigui basin and the lake water in the Dangyang synclinal were connected, and the Huangling uplift was an underwater uplift at this time. In the second stage of the Yanshan movement, the Huangling uplift entered a new period of large-scale uplifting. The precretaceous layers in the Zigui and Dangyang areas began to fold, forming the present north-south axial Huangling uplift and two synclinoriums (Zhang HD et al., 1986).

4. Discussion on the controlling effect of the uplift evolution on the accumulation and preservation of shale gas

Throughout the history of the structural evolution of the Huangling uplift, it was found that its evolution was in an uplifting state for a long time. In the Early Proterozoic, the Zhongtiao orogeny caused the uplifting of the Huangling uplift and the surrounding areas. In the Paleozoic, it was a southwest ancient slope. It uplifted again in the Late Triassic,although it declined relatively steadily in the Late Jurassic to Early Cretaceous, it remained in the ancient uplift state.Finally, another large uplift in the Late Cretaceous laid the present tectonic framework. This kind of uplift was mainly inherited by the transformations of the uplift, the slope, the depression, resulting in a breakthrough in the exploration of Cambrian shale gas in the peripheral area of the Huangling uplift. The controlling effect of the structural evolution of the Huangling uplift on its peripheral Cambrian shale gas accumulation and preservation was mainly reflected in the four aspects following.

4.1. Rich organic shale

Although the Huangling uplift had been in uplift for a long time in its geological evolution, it was characterized by an ancient slope that leaned southwest in the Paleozoic.Influenced by this paleogeographical pattern, rich organic shale with a thick layer of the Lower Cambrian Niutitang formation was deposited in both the Huangling uplift and its surrounding areas. The field geological survey and the drilling data revealed that the Niutitang formation in the peripheral margin of the Huangling uplift belonged to the shallow water continental shelf to the deep-water continental shelf facies deposits of clastic rocks. The thickness of the formation was generally 150–240 m (Fig. 2), and 141 m of black shale of Niutitang formation drilled in E’Yangye No.1 well, 99.5 m in Zidi No.1 well, 50.64 m in Zidi No.2 well, 46 m in Yidi No.2 well, and 9 m in E’Yican No.1 well. In addition, black shale had distribution characteristics of thick in the West and thin in the East. In terms of lithology, grayish-black shale in the West was developed in the Niutitang formation. For example, the Niutitang formation was mainly black shale in the Zidi No.1 well, Zidi No.2 well and E’Yangye No.1 well, which reflected a set of deep-water facies deposits. In the East, gray to dark gray muddy dolomite or limestone were mainly developed.For example, the Niutitang formation in the E’Yican No.1 well was mainly composed of dark gray muddy limestone,which reflected a set of shallow water deposits. According to the stratigraphic thickness and lithology of the Niutitang formation of the Cambrian in the 5 wells, the distribution of black shale was obviously influenced by the paleogeographical pattern of the Paleozoic slope that leaned southwest.

Fig. 2.Cross-well profile of the Cambrian in the peripheral areas of the Huangling uplift.

4.2. The maximum buried depth of shale

The Huangling uplift had been in the uplift state for a long time, so that the Sinian and Cambrian organic-rich shale in the surrounding areas had not been deeply buried. According to the simulation analysis of hydrocarbon generation history of Sinian and Cambrian shale in the E’Yangye No.1 well, the maximum buried depth of Sinian and Cambrian strata was no more than 7000 m (Fig. 3). At present, the buried depth is between 0–4800 m, which preserved the organic holes and secondary dissolution holes formed during the burial evolution of shale. According to the shale scanning electron microscopy of the Zidi No.1 well, the secondary dissolution pores of shale were highly developed and honeycombed (Fig.4), which was good for the accumulation and preservation of shale gas.

4.3. Relatively low geothermal gradient

According to the distribution characteristics of global heat flux, the heat flux value was closely related to the geotectonic background. Generally, this value was high in the structural units with strong activity, and it was low in the stable structural units. As a rigid Proterozoic basement, the Huangling uplift had a stable structure. The Fissile Track test data of apatite revealed that the sedimentary cover of the Huangling uplift and its surrounding area underwent a unidirectional cooling process, it had risen to 210℃ in the Middle-Late Triassic, Middle Jurassic and Early Cretaceous,and it did not experience mid to high temperature heating after that. Now, the measured geothermal gradient of Huangling uplift and its peripheral area was only 2.17℃/100 m.Moreover, due to the short duration of deep burial, the middle and high temperature baking times of rich organic shale was relatively short, and the thermal evolution degree of shale was relatively low. For example, the Sinian buried time of the E’Yangye No.1 well with more than 5000 m in depth was about 85 Ma long. The average thermal maturity of shale in the Niutitang formation was 2.35%, and that of shale in the Doushantuo formation was 2.83%. The thermal maturity of shale in the Niutitang formation in Zidi No.1 well ranged from 1.43% to 1.79%, with an average of 1.62% (Fig. 4),which was good for the accumulation of shale gas. According to the analysis of surrounding geological outcrops, the thermal evolution degree of shale organic matter gradually increased further away from the core of the Huangling uplift (Fig. 5).For example, the shale thermal maturity of the Niutitang formation in the Wudi No.1 well was 3.5% on average,indicating that the Huangling uplift had obvious control on the thermal evolution degree of shale.

Fig. 3.The buried history of Sinian and Cambrian in E’Yangye No.1 well.

Fig. 6.2D seismic profile of the E’Yangye No.1 well in the area of the Huangling uplift. A-B is the section position.

4.4. Relatively stable structure

As a rigid Proterozoic basement, the Huangling uplift had a very good protective effect on the late structure. According to the analysis of 2D seismic data in the surrounding areas of the Huangling uplift, the late structures were mostly thinskinned structures (Fig. 6), and the basement involved structure was underdeveloped. The faults formed since the Indo-Yanshanian had little influence on the structural characteristics of the Cambrian-Sinian. At present, the Sinian and the Middle and Lower Cambrian in the southern margin of the Huangling uplift were monoclinal structures that leaned southward. The fault was underdeveloped and small in size,and this was very beneficial to the preservation of Cambrian and Sinian shale gas. According to the existing shale gas drilling display and the analysis data of shale gas content in the southern margin of the Huangling uplift, the correlation between TOC and gas content was good. High TOC corresponded to high gas content, which proved that the late tectonic activity had little influence on the early reconstruction of shale gas reservoirs (Zhang HR et al., 2016).

5. Conclusions

(i) The geological evolution of the Huangling uplift was mainly inherited by the transformation of the uplift, slope,depression and their mutual transformation. In Early Proterozoic, the Zhongtiao orogeny caused the uplifting of the Huangling uplift and the surrounding areas. In the Paleozoic,it was a southwest ancient slope. It uplifted again in the Late Triassic, although it declined relatively steadily in the Late Jurassic to Early Cretaceous, it remained in the ancient uplift state. Finally, another large uplifting in the Late Cretaceous laid the present tectonic framework.

(ii) The Huangling uplift in the Paleozoic was an ancient slope that leaned southwest, and its ancient geographical pattern controlled the spatial distribution of black distribution in the Lower Cambrian Niutitang formation. The structure pattern of its long-term uplift caused the black shale of the Niutitang formation in the peripheral area to have a buried depth that was shallow, a short-buried time, a stable structure,a relatively low thermal evolution degree, providing good shale gas accumulation conditions. The exploration breakthrough of the E’Yiye No.1 HF well and the E’Yangye No.1 HF well further proved that the Huangling uplift and other old uplifts provide great possibilities for the gas accumulation and preservation of Cambrian shale.

Acknowledgment

The authors are grateful to Pro. Zhang Junfeng, Hao Ziguo, Dr. Yang Yan and anonymous reviewers for their valuable suggestions and comments on the manuscript. This work was funded by the Project of China Geological Survey(12120121000172121) “Strategic Investigation of the Expansion Area of the West Hubei Shale Gas Demonstration Base”.