“Source-Diagenesis-Accumulation” enrichment and accumulation regularity of marine shale gas in southern China

Gng-yi Zhi , Yu-fng Wng , Zhi Zhou , Guo-heng Liu Yu-ru Yng , Jun Li

a Oil and Gas Survey, China Geological Survey, Beijing 100029, China

b Unconventional oil and gas geology laboratory, China Geological Survey, Beijing 100029, China

ABSTRACT

After the breakthrough of shale gas exploration and development in the Ordovician Wufeng Formation(Fm.) and Silurian Longmaxi Fm. of Chongqing Jiaoshiba area, Changning-Weiyuan area, etc. in Sichuan basin, a series of discovery and breakthrough were obtained by China Geological Survey in the Cambrian Niutitang Fm. and Sinian Doushantuo Fm. shale of the areas with complicated structure outside Sichuan basin. Based on the understanding of the law of shale gas enrichment in Longmaxi Fm. in the basin, this paper puts forward three elements of the formation and enrichment of shale gas, which are the “Source”,the “Diagenesis” and the “Accumulation ”, after deeply studying the law shale gas enrichment and accumulation in Sinian-Cambrian reservoir of the complex structure area outside the basin. The “Source”means the sedimentary environment and petrological characteristics of organic shale. The “Diagenesis”means the basin tectonic subsidence and hydrocarbon generation and expulsion process of organic matter.The “Accumulation” means the tectonic uplift and shale gas preservation. It is proposed that the Sinian-Cambrian and Ordovician-Silurian black shale series in the middle and upper Yangtze region of southern China were both formed in the deep-water shelf environment of rift trough and foreland basin respectively.The dessert intervals were formed in the strong reduction environment under transgressive system tract.The shale lithology belongs to calcium-siliceous and charcoal-siliceous respectively. Based on the summary of structural evolution in Yangtze area, the correlation of structural burial depth with shale diagenesis and the coupling evolution of organic matter with pore structure are discussed. Combining with structural styles, the preservation conditions of shale gas are discussed. Five types of shale gas reservoir control models are further described. Two types of future exploration directions, which are reverse fault syncline and paleo-uplift margin in complex structural area outside the basin, are proposed.

Keywords:

Shale gas

Exploration and development

Enrichment and accumulation

Reservoir controlling models

Exploration direction

1. Introduction

Shale gas is a kind of natural gas that can be explored and exploited by modern technology. The accumulation process of shale gas has gone through the sedimentation of source rock rich in organic matter, the structural subsidence of basin and the hydrocarbon generation and expulsion of organic matter,the later structural uplift and transformation, and finally the accumulation by enrichment and preservation. With the largescale exploration and development of the Wufeng-Longmaxi Fm. shale gas in Fuling-Jiaoshiba, Changing-Weiyuan,Fushun-Yongchuan, and Zhaotong areas etc. in Sichuan basin,many scholars in the industry have gradually summarized and formed a series of enrichment regularities of marine shale gas in southern China, such as the regularity named “enrichment and accumulation controlled by two key factors, source rock and cap rocks” (Guo XS, 2014; Nie HK et al., 2016; Nie HK et al., 2009; Hu DF et al, 2014; Jin ZJ et al, 2014). However,there is little recognition about enrichment regularities for shale gas in Sinian-Cambrian ancient strata. a few of researchers, such as Wei GQ et al.(2015), have proposed the reservoir formation model of ancient uplift, which is different gas reservoirs appearing in different parts(Wei GQ et al.,2015; Cheng K et al., 2009; Lin T et al., 2014; Nie HK et al.,2011; Fan WF et al., 2015; Hu MY et al., 2014; Zhang ZP et al., 2015; Liang HR et al., 2016; Yu JH et al., 2016).Combined with the exploration practice of shale gas in Sinian Doushantuo Fm., Cambrian Niutitang Fm. and Silurian Longmaxi Fm. in the complex structural area of Wuling Mountain (Mt.) outside Sichuan basin in recent years, this paper summarizes the formation process of shale gas enrichment and accumulation as three elements, which are“source, diagenesis and accumulation”. “Source” refers to sedimentary environment and petrological characteristics of shale rich in organic matter; “Diagenesis” refers to the process of sedimentation, diagenesis and hydrocarbon generation,which is the process of shale gas formation during the continuous coupling evolution of organic matter hydrocarbon generation and rock pore structure with the sedimentary basin structure subsidence. “Accumulation” refers to the preservation process during tectonic uplift. “Source ” emphasizes the environmental and material basis of the formation of organic shale. “Diagenesis” emphasizes the tectonic subsidence and the formation process of shale gas; “Accumulation” emphasizes the preservation conditions of shale gas. All three are integral and indispensable. Based on Marine shale gas exploration and development research results in southern China, this paper expounds the shale gas generation and distribution as well as the tectonic sedimentary background, sedimentation and diagenesis, and uplifting and preservation conditions of Sinian-Cambrian and Ordovician in South China. On the basis, shale gas enrichment pattern was proposed to indicate the direction for further exploration.

2. Source-formation environment and petrologic characteristics of organic-rich shale

2.1. The tectonic background and sedimentary environment of the formation of organic shale

2.1.1. Sinian-Cambrian shale formed in the sedimentary environment of rifted trough

Influenced by the large-scale rifting in the Nanhua period,the upper Yangtze block formed an obvious structural pattern of “platform-trough-basin” and developed two nearly North-South trending rifting trough, the mid-Sichuan trough and the west of Hunan and Hubei trough. The development of Sinian and Cambrian shale is controlled by rifting trough. The Cambrian Qiongzhusi Fm. developed in the west trough of Sichuan basin is a hydrocarbon source rock of large gas fields,such as Moxi, Gao shiti, etc. (Mou CL et al., 2011; Wen L et al., 2001; Hu L et al., 2012; Huang JL et al., 2012; Xie D et al., 2015; Liu B et al., 2016; Dong XX et al., 2016). Two sets of black shale, namely Sinian Doushantuo Fm. and Cambrian Niutitang Fm. (Mou CL et al., 2011; Wen L et al., 2001; Hu L et al., 2012; Wang YF et al., 2017; Chen XH et al., 1999;Chen XH et al., 2015; Chen XH et al., 2016; Chen XH et al.,2018) have been deposited in the Xiangexi trough in the east,which are important strata for shale gas exploration breakthroughs in recent years. Shale gas survey in western Hubei has confirmed that the Sinian Doushantuo Fm. and Cambrian Niutang Fm. are mainly developed in a zonal rifted trough with a length of about 400 km and a width of about 160 km. The eastern and western parts of the rifted trough belong to the Yangtze block, which is adjacent to the continental margin in the north and the southern ocean-basin in the south. Because the south part of the rift trough in western Hubei is close to the South China ocean basin, it is relatively obvious to be affected by the hydrothermal fluid of ocean volcanic eruption. The siliceous content in the shale is relatively high, generally about 80%. However, due to far away from the ocean in the middle and north parts of the rift trough in western Hubei, the sedimentation of shale is mainly affected by biological action, so the siliceous in shale is mainly biogenic. Biogenic siliceous shale is the main sedimentation in the rift trough of central Sichuan (Fig. 1 and Fig. 2).

Fig. 1.Lithofacies paleogeography map of the Cambrian Niutitang Fm. in the Upper Yangtze platform.

Fig. 2.Stratigraphic correlation for Cambrian Niutitang Fm. in ocean trough of west of Hunanand Hubei, China.

2.1.2. Ordovician-Silurian system formed in the sedimentary environment of restricted basin deep water shelf

Since the Middle Ordovician the tectonic dynamic environment in the middle and upper Yangtze region has been transformed from the early tensile rifting state to the compressive environment under the influence of Cathaysian block extrusion. The Cathaysian block was extruded and uplifted to form the ancient land, which provided sufficient material source to the residual ocean basin in south China.The southern edge of the Yangtze plate is subject to depression and deflection, and foreland basin is formed from Middle and Late Ordovician to Early Silurian (Fig. 3). Deep water continental shelf facies are developed in southwest Sichuan, southeast Sichuan, northeast Sichuan and west hunan and Hubei. During the Late Ordovician to early Silurian, it accepted the sedimentation of the basin developed during the foreland basin evolution, and formed the graptolite-rich siliceous shale and siliceous rock of Wufeng Fm., as well as the graptolite shale of Longmaxi Fm. (Liang DG et al., 2009;Zou CN et al., 2015; Liu SG et al., 2014; Li A et al., 2016;Wang YM et al., 2016; Zhang HR, 2016; Zhang XM et al.,2015; Wang HY et al., 2015; Wang SJ et al., 2009). In the middle and late stage of Early Silurian, foreland basin entered the period of rapid filling of clastic materials, and deposited mud shale and siltstone up to kilometers thick. In the late stage of Early Silurian, the Yangtze was basically lifted to the land as a whole and nearly accepted no sedimentation. The evolution of foreland basin also ended accordingly.

Fig. 3.Lithofacies paleogeographic map of the Silurian Longmaxi Fm. in the Upper Yangtze platform.

2.2. Lithofacies and petrologic characteristics of high-quality shale

2.2.1. High quality shale sweet spots all developed in the transgressive system

According to the sequence stratigraphic correlation of single-well sedimentary strata, the high-quality shale dessert section is all located in the transgressive system for both Sinian-Cambrian rifted trough deposits and Ordovician-Silurian deep-water continental shelf deposits. Doushantuo Fm. shale in well Eyangye 1 is composed of three three-stage sequences, of which the second steep section is the sweet section with a thickness of roughly 90–110 m. The lower transgressive system mainly develops black shale, and a large number of pyrite laminates and phosphorous nodules are developed, indicating the deep-water body and the reductive sedimentary environment. The dessert formation of Niutitang Fm. is about 40–70 m thick, and mainly composed of black shale, marl and limestone mudstone interbeds with many pyrite laminates and phosphorous nodules, which means the deep-water body and reductive sedimentary environment in this period, indicating the transgressive system. In the Jiaoye1 well of Jiaoyanba area, the Ordovician Wufeng Fm. and the Long-1 member of the Silurian Longmaxi Fm., as the dessert intervals, are also in the transgressive system. The lithology is carbonaceous siliceous shale, rich in graptolites, foraminifera and radiolarians (Fig. 4).

Fig. 4.Sequence strata of the Cambrian Niutitang formation and the Wufeng-Longmaxi formationshale of Eyangye 1 well andJiaoye 1well.

2.2.2. High-quality shale formed in the weak oxidation and deoxidization environment

The dessert section of the Cambrian Niutitang Fm. is mainly developed in the trough basin of rifting trough and the anoxic reduction environment of the slope belt, some parts of which were affected by hydrothermal action. The Niu2 member of the Niutitang Fm. in western Hubei, as the dessert interval, is interbedded by marl and limestone mudstone, and collophanite at the bottom. The ratio of V/Cr, Ni/Co, Ni/V and V/(V+Ni) is a relatively reliable indicator of paleo oxidation-reduction geochemistry. Through the V/Cr comparison between the Niutitang Fm. and the Doushantuo Fm. shale, it is found that the Niutitang Fm. developed in a reduction environment (Fig. 5a), while the Doushantuo Fm.was in relatively weak oxidation environment (Fig. 5b). The high-quality shale sweet spot section of the Ordovician Wufeng Fm. and Silurian Longmaxi Fm. was mainly developed in the strong reduction environment of the deep water continental shelf belt, and a large number of carbonaceous siliceous shale containing bone needles,radiolarians and graptolites were developed (Fig. 4).

Fig. 5.V/Cr statistical results of the Niutitang Fm. and Doushantuo Fm. shale.

2.2.3. Petrologic characteristics of different types of highquality shale

It is found that the Wufeng-Longmaxi Fm. shale in Anye1 well is similar to the Barnett shale in the United States, both of which are dominated by siliceous materials. Similar to the Haynesville shale, the Niu2 member of Niutitang Fm. and Dou2 member of Doushantuo Fm. in well Eyangye 1 are both rich in carbonate. The Doushantuo Fm. has a higher content of carbonate than the Niutitang Fm. (Fig. 6).

Fig. 6.The triangle chart of mineral compositions for different types of shale rocks from different regions.

3. Diagenesis-tectonic subsidence and hydrocarbon generation of organic shale

3.1. Relationship between tectonic subsidence and thermal evolution

Organic matter deposited on the bottom of the basin. With continuous tectonic subsidence, the temperature and pressure rise gradually caused by the overlying sediments. The organic matter has experienced the stage of biochemistry (1500–2500 m depth), the stage of oil generation induced by thermal catalysis (3000–4000 m depth), the stage of raw moisture by thermal cracking (buried depth deeper than 4000 m) and the stage of gas generation under high temperature (6000–7000 m depth).Therefore, the maximum burial depth of shale is the main controlling factor of organic matter thermal evolution and gas generation. The thermal evolution degree of Sinian-Cambrian shale is generally high (Zhang L et al., 2007; Rao S et al., 2013). By comparing the maximum burial depth and thermal evolution degree between the Cambrian and Silurian shale, it can be said that the burial degree of lower Cambrian shale is generally higher than that of lower Silurian shale,indicating that the evolution degree of Cambrian shale is generally too high. This can also be confirmed by differences in the composition of clay minerals.

In the process of diagenesis, the types of clay minerals in different diagenetic stages are also different. The clay minerals of Longmaxi shale are mainlyillite/smectite mixed layer, which reflects the characteristics of early stage of middle diagenesis. Shale clay minerals of Niutitang Fm. are mainly illite, reflecting the characteristics of late diagenetic stage, while those of Doushantuo Fm. are mainly chlorite,reflecting the characteristics of middle diagenetic stage and late diagenetic stage (Fig. 7). Clay minerals also reflect the diagenetic stage.

Fig. 7.Clay minerals characteristics of different types of shale.a-Longmaxi Fm.; b-Niutitang Fm.; c-Doushantuo Fm.

3.2. The coupling relationship between thermal evolution degree and pore structure

Zou CN et al. (2015) found that in the process of sedimentary and diagenesis of organic shale, the porosity of shale showed the characteristics of firstly increasing and then decreasing with the increase of thermal evolution degree of organic matter. It is found through data statistics that there is a significant positive correlation between total organic carbon content (TOC) and porosity in shale. However, with the increase of thermal evolution degree, the pore structure in shale will also change. The higher the thermal evolution degree is, the smaller the porosity of shale will be. According to the statistics of different typepores in shale, it is found that the pores in Longmaxi Fm. shale are dominant by organic matter pores, and pores in the interstitial organic matter account for the largest proportion. In the Niutitang Fm. and Doushantuo Fm., pores in organic matter and in the matrix are nearly the same content (Fig. 8) and pores in the interstitial organic matter and pores between the authigenic(recrystallized) minerals account for about 50% respectively.The organic matter pores in Niutitang Fm. are smaller with an average pore diameter of about 15 nm, while those in Longmaxi Fm. are mainly 50 nm.

Fig. 8.Comparison of pore types in different types of shale.

3.3. The influence of ancient uplift structure on thermal evolution

In view of the generally high thermal evolution degree for the upper Yangtze region Sinian-Cambrian shale, and on the basis of thorough dissection of Huangling anticline ancient uplift (Xu DL et al., 2013; Ge XH et al., 2010; Xiong CY et al., 2004; Zhang HD et al., 1986), Zhai GY et al. (2017) put forward the basic recognition described as “the ancient uplift structure was relatively stable because of the rigid basement,and experienced shorter buried time and relatively shallower buried depth, and would display relatively lower thermal evolution degree in consequence, which means beneficial for shale gas enrichment and accumulation” (Zhai GY et al.,2017). Recent researches have proved that the shale evolution degree of Sinian Doushantuo Fm. and Cambrian Niutitang Fm. at the edge of the Sichuan basin is relatively low. The uplifts surrounding the Sichuan basin, such as Hanan,Shenlongjia, Huangling, Xuefeng Mt. and Qiannan uplift,formed an ancient uplift and low-evolution island chain. The shale of the lower Cambrian Niutitang Fm. at the periphery of the ancient uplift has a relatively short burial time (no more than 100 Ma for burial depth exceeding 6000 m), relatively shallow burial depth (maximum burial depthless than 7000 m)and low thermal evolution (Fig. 9), which is in favor of shale gas enrichment. This point has been fully confirmed by the exploration and discovery of Zhendi1 well in south part of Shanxi, Hongdi1 well in Shennongjia district, Eyangye1 well in Hubei and Qianhuangdi1 well in Guizhou.

Fig. 9.Burial thermal evolution history of different ancient uplifts.

4. Accumulation-rich integration reservoir of tectonic uplift and preservation

Shale gas is formed 7000–8000 m deep under the earth's crust and can only be preserved as a gas reservoir after superimposed transformation of geological tectonic movement. Shale gas preservation is affected by geological factors such as stratigraphic fold, fault, tectonic pattern, uplift and denudation.

4.1. Uplift denudation and shale gas preservation conditions

It is generally believed that the thicker the overlying strata are, the denser the strata are, and the better the preservation conditions of shale gas will be. Since the Indosinian movement, a series of thrust-nappe structures have been formed in the middle and upper Yangtze region from Xuefeng Mt. to the west, which are manifested as early uplift in the east, high denudation, late uplift in the west, and low denudation (Fig. 10). As a result, the shale in the east stopped generating gas too early, suffered from uplift and denudation for a long time, experienced longer time of dissipation and seepage loss. Hence, lost gas volume was larger, and gas content was worse. In the western Sichuan basin, the overlying strata have a small amount of denudation because of late uplift time. With the addition of the protection of the gypsum rocks strata, the good preservation conditions of shale gas well explain the reason the high-yielding gas fields in the Sichuan basin.

Fig. 10.Statistics of uplift and denudation in different regions of west Xuefeng Mt.

4.2. Structural patterns and shale gas preservation conditions

Due to the strong thrust-nappe movement from east to west in the Indosinian epoch, a series of grooved, slotted and slotted folds were formed from the Xuefeng uplift to the west.Qiyueshan fault zone in the west and Sichuan basin are mainly divided blocking or barrier anticline. Two target strata of Sinian-Cambrian and Ordovician-Silurian are covered with thick overlying strata, indicating good preservation conditions for shale gas. The Wuling Mt. fold belt in the east is dominated by residual syncline, with two wings exposed to the surface. Because the permeability in the horizontal direction of shale is much larger than that is in the vertical direction. Hence, lateral dispersion of shale gas lead to bad preservation condition. However, there are some over-thrust faults and the footwall can be well preserved. The exploration practice of Anchang syncline in Guizhou has proved that the footwall of the thrust fault can form over pressure because of the blocking by thrust fault in the residual syncline area.Hence, the footwall of the thrust fault can be the favorable area for high-yielding gas accumulation. Therefore, in the complex structural areas outside the basin, areas with relatively good structural preservation are the key to evaluate the dessert of shale gas.

4.3. Typical regional shale gas enrichment and control reservoir model

Based on the summary of typical marine shale gas accumulation patterns in Sichuan basin and its surrounding areas, Zhai GY et al. (2017) classified them into five accumulation control models based on structural patterns(Zhai GY et al., 2017; Zhai GY et al., 2018), namely, basic anticline, inverse fault anticline, basic syncline, inverse fault syncline and ancient uplift edge (Fig. 11). Anticline reservoir control mode is mainly distributed in Sichuan basin, reverse fault syncline mode is mainly distributed in Wuling Mt. fold belt, and ancient uplift edge reservoir control mode is mainly distributed in the periphery of Sichuan basin. From the perspective of “diagenesis”, in the case of generally high thermal evolution degree in the Simian-Cambrian shale, the ancient uplift and low-evolution island chain composed of Hannan uplift, Shenlongjia anticline, Huangling anticline,Xuefeng ancient land and Qiannan ancient uplift displays relatively shorter buried a time, relatively shallower burial depth, and lower thermal evolution degree because of the ancient uplift tectonic. This is also the direction of exploration for favorable areas of Sinian-Cambrian shale gas, which has been fully confirmed by exploration practices in recent years.From the perspective of “accumulation”, in the case of being generally subjected to tectonic uplift and denudation for Ordovician and Silurian shale in the complex structural areas around the Sichuan basin, sealing faults formed by thrustnappe movement are the new direction of shale gas exploration in these areas, which has been confirmed by the breakthrough of shale gas exploration in Anchang synclinal,Guizhou.

Fig. 11.Shale gas accumulation patterns in typical areas of Sichuan basin and periphery. a-Jiaoshiba; b-Weiyuan; c-Pengshui; d-Anchang;e-Huang anticline.

5. Conclusions

(i) Based on a systematic summary on sedimentary,diagenetic evolution and structural preservation characteristics of Sinian, Cambrian, Ordovician, Silurian shale in southern China, this paper puts forward three elements about shale gas enrichment and accumulation, namely “source”, “diagenesis”and “accumulation”. “Source” means the environmental and material basis of the formation of organic shale; “Diagenesis”emphasizes the tectonic subsidence and the formation process of shale gas; “Accumulation ” emphasizes the preservation conditions of shale gas. All three are integral and indispensable in evaluating and optimizing favorable areas for shale gas exploration and development.

(ii) Two sets of black rock in Sinian-Cambrian and Ordovician-Silurian in the middle and upper Yangtze region were both formed in the deep-water continental shelf environment of rifted trough and foreland basin, respectively.The lithology belongs to calcic-siliceous and carbonaceous siliceous shale, respectively. However, the sweet spot layers are both in a strong reduction environment in the transgressive system, which is characterized by high gamma, high silicon content and high pyrite content. The composition of Sinian-Cambrian shale is more similar to that of the Haynesville shale in North America. So, it is not appropriate to emphasis too much on the deep-water shelf sedimentary environment in exploration practice.

(iii) Most areas in southern China have experienced multiple periods of complex tectonic movements, and preservation conditions are the key to finding favorable areas for shale gas. For the exploration of Ordovician and Silurian shale gas, more attention should be put on the synclinal structures with certain preservation conditions in the complex structural area outside Sichuan basin, when increasing the exploration and development inner Sichuan basin. In view of the ancient shale gas layers such as Sinian-Cambrian shale,the exploration should focus on the marginal areas of Hannan uplift, Shenlongjia anticline, Huangling anticline, Xuefeng ancient land and Qiannan ancient uplift.

Acknowledgment

This article is under the support of National Science and technology major projects of “Shale gas resource evaluation method and exploration technology research” (2016ZX05034),projects of Ministry of Natural Resources “Basic geological survey project for shale gas in southern China ”(121201229000160021) and “Strategic investigation of Zigui-Changyang shale gas favorable area in Hubei” (DD20179623),Sino US inter-governmental cooperation projects of “Sino US collaborative research project on Carboniferous Permian shale reservoir evaluation technology” (2017YFE0106300).