Sedimentary lithofacies characteristics and sweet-spot interval characterization of the Sinian Doushantuo Formation in Upper Yangtze Platform, South China

Yu-fng Wng, Gng-yi Zhi, Yong-ho Lu, Yi-qun M, Jun Li,, Guo-heng Liu,Yun-xio Zhng,

a Oil and Gas Survey, China Geological Survey, Ministry of Natural Resources, Beijing 100083, China

b Unconventional Oil and Gas Geology Laboratory, China Geological Survey, Ministry of Natural Resources, Beijing 100083, China

c China University of Geosciences, Wuhan 430074, China

Keywords:

Shale gas

Sequence correlation

Sweet-spot interval

Doushantuo Formation (Late Proterozoic)

Upper Yangtze Platform

Oil and gas exploration engineering

Hubei Province

China

A B S T R A C T

The Sinian Doushantuo Formation is the oldest shale gas reservoir discovered in the world, which contains good shale gas shows as the Eyangye-1 and Zidi-1 wells in the Yichang Region of western Hubei province, China.The shales in the Doushantuo Formation feature considerable thickness (135 m), high gas content (4.83 m3/t) and high fragile mineral content (up to 75%).Due to the influences of the sedimentary environment, the shale reservoirs here have high dolomitic content (54%) and abruptly changes in vertical lithofacies.Moreover, the characteristics of the shale sweet-spot differ significantly from that in the Wufeng Formation and Longmaxi Formation in southeast Sichuan Basin.A high-resolution sequence stratigraphic correlation was performed on the shales of the Doushantuo Formation at the Eyangye-1 well,Zidi-1 well, Zidi-2 well and some outcrop profiles in Yichang area for the identification of their sedimentary microfacies characteristics.A comprehensive comparative analysis was made by incorporating the elemental geochemistry, wireline and mud logging data of the well to further identify the sweet-spot interval therein.With the analysis of Eyangye-1 well, the sweet-spot interval of the Doushantuo Formation is 3360-3408 m, of which sedimentary microfacies is deepwater reduced environment with many pyrite laminaes, and the TOC is high to 3.42%, the Ro is 3.3%, the organic and matrix pore are well developed and the aperture more than 50 nm, the porosity is 2.7%, the desorption gas content average is 2.16%, and the main mineral is dolomitic (54%) and siliceous quartz (21%).Additionally, these parameters combined with previous studies will shed light on evaluating and characterizing the layers therein, also provide referential geological data for the following exploration and development activities of this shale system.

1.Introduction

Against the background of the breakup of the supercontinent Rodinia in the late Proterozoic, the middle Yangtze area has been affected by tectonic movements in multiple periods since the Indo-Chinese Epoch and has complex regular structural features (Peng B et al., 2017; Shan CN et al., 2015, 2016; Yong ZQ et al., 2012; Yang P et al.,2012).The Doushantuo Formation is the first period transgressive sedimentary association in this area after the end of Nantuo Ice Age.The bottom deposits were formed approximately 635-551 Ma (Yang AH et al., 2015; Wang L et al., 2011; Liu PJ et al., 2009).Many researchers have studied the mineral identifications of the pyrite, carbonate nodules phosphatic nodules based upon microfossils, carbon isotopes to constraint the palaeogeographic sedimentary environments of the Doushantuo Formation reservoirs (Zhang G et al.,2017; Xu ZX et al., 2015; Dong J et al., 2009; Chen SM et al.,2010; Chen SM et al., 2009; Yan B et al., 2014; Li HH et al,2017).The Doushantuo Formation is generally separated into 4 parts, namely the Dou-1 Member, the Dou-2 Member, the Dou-3 Member and the Dou-4 Member.Macroscopic sedimentary analysis show that the Dou-1 Member of the Doushantuo Formation consists of cap dolomite.The Dou-2 Member consists of organic-rich alternating layers of black shale and greyish black laminated dolomite, containing many siliceous and phosphatic nodules.The Dou-3 Member consists of greyish white massive dolomite containing irregular siliceous nodules or crumbs in the lower part and of grey thin-banded limestone in the upper part and the Dou-4 Member consists of organic-rich black carbonaceous shale containing carbonatite nodules (An ZH et al., 2018; Li XB et al., 2013; Mei MX et al., 2006; Wang CS et al., 2011; Mi WT et al., 2010; Liu PJ et al., 2012; Yin CY et al., 2009; Zhou Y et al., 2004; Zhang HJ et al., 2012).High-quality shales were mainly deposited in deepwater continental shelf environments(Macquake et al., 2007; Abouelresh M.O.et al, 2012;Hammes U, 2012; Jafary H et al, 2013; Slatt R.M., 2015;Liang C et al., 2018; Yang W et al., 2018).Their sequences and interfaces were more sensitive to events such as transgression and regression, oxidation and reduction,biological revolution and sudden events (Jarvie D.M.et al.,2007; Abouelresh M.O.et al, 2012; Slatt R.M.et al, 2012;Wang G et al., 2014; Chen L et al., 2015; Bhattacharya S.et al., 2016; Ma Y et al., 2016).It is hard to identify the sequence interfaces and carry out the sequence stratigraphic classification only using seismic profiles in traditional sequence stratigraphy (Liu JJ et al., 2015; Zhang HJ et al.,2013; Sun ZM et al., 2004; Cai XF et al., 2018; Chen XH et al., 2016; Jing TY et al., 2015; Li ZX et al., 2004; Zhou MJ et al., 1981; Liu YQ et al., 2003;Yuan K et al., 2017; Zhai GY et al., 2017).This study provides a comprehensive comparison analysis based on the area's drilling core observations, well logging parameters, Rocscan while-drilling cutting scanning parameters, wavelet transform analysis.From these, a fifthorder high-frequency sequence stratigraphic classification of the Doushantuo Formation was performed to describe finely the sedimentary microfacies characteristics and accordingly,the high-quality sweet-spot shale interval was evaluated and characterized.

2.Regional geology

The Yichang Region in western Hubei is on the south edge of the Huangling Anticline which is located in a relatively stable area between two large orogenic belts, the Jiangnan intracontinental orogenic belt in the south, and the Qinling-Dabie orogenic belt in the north.The Huangling Anticline is a double-layer platform structure consisting of a basement and cap (Xu DL et al., 2013; Ge XH et al., 2010).The basement crops out of the core of the anticline and consists of the Kongling Group metamorphic rocks from the late Archeozoic and early Proterozoic, and Neoproterozoic Huangling granite (Xiong CY et al., 2004; Liu ZH et al.,2015).Meng QF et al.(2012) thought that the Huangling Anticline has been in an uplifted state for a long period during its evolution according to the lithologic character of the strata in the adjacent regions and to the thickness distribution features of the residual strata therein.Huangling Anticline and the folds in adjacent areas experienced a complex tectonic evolution, including early Proterozoic uplift due to the Zhongtiao Orogeny, the Palaeozoic inclining south-westward as palaeoslope, late Triassic uplift, Late Jurassic to early Cretaceous steadily subsidence and late Cretaceous significantly uplift until its current tectonic framework.In the Yichang Region, there are four main faults (Fig.1):Tongcheng River Fault (F1), Wudu River Fault (F2),Tianyangping Fault (F3) and Xiannüfeng Fault (F4).The Eyangye-1 well is located in the Huaguoping Composite Syncline, west of the Tianyangping Fault and east of the Xiannüfeng Fault (Fig.1).

Fig.1.Structure outline map of the Yichang region in western Hubei.1-Quaternary; 2-Palaeocene-Neocene; 3-Cretaceous; 4-Jurassic;5-Triassic; 6-Devonian-Permian; 7-Silurian; 8-Cambian-Ordovician; 9-Nanhua-Sinian; 10-Neoproterozoic granite; 11-syncline axis;12-normal fault; 13-unidentified fault; 14-boundary of the structural unit, 15-well name; 16-area name.F1-Tongcheng river fault; F2-Wudu river fault; F3-Tianyangping fault; F4-Xiannüfeng fault

Due to the influence of the large-scale faulting and depression events in the Nanhua Period, the Upper Yangtze Block consists of distinct platform, trough and basin divisions as well as two nearly north-south fault depression troughs,one of which is located in middle Sichuan and the other of which is in western Hunan and western Hubei (Pi DH et al.,2013).The development of the Sinian and Cambrian shales in western Hubei was controlled by the fault depression trough located in western Hubei and western Hunan.The shale gas surveys conducted in western Hubei in recent years prove that the Sinian Doutuoshan Formation and Cambrian Niutitang Formation are mostly in a banded fault depression trough,which is about 400 km long and about 160 km wide (Zhai GY et al., 2018).All the high-quality sweet-spot shale members deposited in the Sinian-Cambrian fault depression trough are located in the transgressive system tracts (TST) consisting mainly of black shales.The high-quality shales (total thickness up to 40-60 m) contain many pyrite laminae and phosphatic nodules, which indicates that prevailed reducing sedimentary environments (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, 2015, 2018).

3.Sequence classification and correlation of the Doushantuo Formation

Sequence interface identification is the key to sequence stratigraphy research.In this study, a high-frequency sequence stratigraphic classification of the Doushantuo Formation was performed according to the area's drilling core observations and sensitive well logging parameters.Accordingly, a regional stratigraphic correlation and analysis based on the drilling core data from the wells Eyangye-1, Zidi-1 and Zidi-2, as well as the gamma, uranium, thorium, potassium and energy spectrum logs in the well-logging-characteristic curves, were performed in this paper.

3.1.Development characteristics of different sedimentary sequences

The Doushantuo Formation sequence classification of the Eyangye-1, Zidi-1 and Zidi-2 wells show that the Doushantuo Formation consists of three third-order sequences (i.e.Dsq1,Dsq2 and Dsq3 in Fig.2, Fig.3 and Fig.4, respectively) and each of the third-order sequences consists of a TST and a highstand system tract (HST).The sequence of Dsq1 corresponds to the Dou-1 Member and lower Dou-1 Member of the Doushantuo Formation in the lower Sinian and the thickness is 50-90 m.The sequence, ranging from 30 m to 50 m in thickness, consists of a TST and HST.The TST, which consists mainly of dolomite and mudstone in the lower Doushantuo Formation, is characterized by mostly basin facies deposit and gradually increasing GR values on the well logs (Fig.2, Fig.3 and Fig.4).The grey shales prevail and multi pyrite laminae are visible in the TST (Fig.5), generally showing that the sedimentary environment is deep and still waters.In addition, many phosphatic nodules can be seen in the system tract.This indicates that the water bodies were rich in nutrients and were favourable for organic matter concentration.The HST corresponds to the middle and lower strata in the Dou-2 Member of the Doushantuo Formation.The strata are shown not only in the drilling profiles, but also in the field outcrop profiles, with the thickness range of 30-40 m.The deposits therein are mainly light grey mudstones, which were generally deposited in relatively shallow water clinothem environments.The HST can be classified into two parasequences, and is characterized by quickly decreasing GR values in the well logs (Fig.2, Fig.3 and Fig.4).

Fig.3.Generallized stratigraphic column of the Doushantuo Formation in Zidi-1.

Fig.4.Generallized stratigraphic column of the Doushantuo Formation in Zidi-2.

Fig.5.Pyrite laminae in the Doushantuo Formation in Eyangye-1.

The sequence of Dsq2 is the high-quality shale member in the Doushantuo Formation.It corresponds to the middle and upper parts of the Dou-2 Member of the Doushantuo Formation, with an approximate thickness range of 90-110 m and consisting typically of black shales.It is also composed of a TST and a HST.Corresponding to the middle and upper parts of the Dou-2 Member of the Doushantuo Formation, its TST is a basin facies of stable thickness (about 50 m) in which black shales prevail and pyrite laminae (Fig.6) and phosphatic nodules (Fig.7) can be seen in large quantities.The black shales suggest that the environment was of marine deepwater origin, while the laminae indicate that the deepwaters were still waters.Large quantities of phosphatic nodules mean that the sea waters were typically rich in nutrients during the deposition of these strata.The nutrientrich sea water was favourable for the flourishing of surface life and thus caused the surfaces of the water bodies to be more productive.Thus, we can infer that the strata developed during the period feature basin facies deposition in a reducing environment.On the well logs, the system tract is generally characterized by steadily increasing GR, U and Th values,indicating that the changes in water body during that period were relatively gradual.According to the well logs, the TST can be further classified into three parasequences, as shown in Fig.2-Fig.4.

Fig.6.Black shale laminae in the Doushantuo Formation in Eyangye-1.

Fig.7.Phosphatic nodules in the Doushantuo Formation in Eyangye-1.

The lower part of the HST of Dsq2 is mainly composed of slope-facies shales and silty shales.The existence of the silty shales means an enhanced terrestrial input and indirectly reflects that the cause for this may be a fall of the sea surface.On the well logs, the GR curves of the system tract are characterized by the inverted Christmas trees shown in Fig.2,Fig.3 and Fig.4, indicating that this is a deposition process featuring a water body of decreasing depth and increasing sand content.The system tract can also be further classified into three parasequences.

With a thickness range of 80-90 m, the sequence of Dsq3 corresponds to the Dou-3 Member of the Doushantuo Formation and consists mainly of limestone as well as silty and crystalline dolomite.The existence of carbonatite in large quantities indicates that the sea level in this period was lower than those of the former two sequences.Similarly, the sequence can be classified into two system tracts: a TST and HST.With a thickness range of 30-50 m and the carbonate platform as the sedimentary environment, its TST consists mainly of limestone.Its changes on the well logs are relatively smooth, indicating that its lithologic changes upward were relatively uniform and that the water body changes were not considerable.According to its shapes on the well logs, the sequence can be shown to have the three parasequences shown in Fig.2, Fig.3 and Fig.4.With a thickness of roughly 30 m, its HST still has the then carbonate platform as the sedimentary environment.The lithologic changes of the HST consisting mainly of silty and crystalline dolomite were relatively smooth and the water body changes were uniform.According to its combination pattern on the well logs, the system tract has one parasequence (Fig.2, Fig.3 and Fig.4).

3.2.Regional sequence stratigraphic correlation

The correlation profiles of the wells Eyangye-1, Zidi-1 and Zidi-2 (Fig.8) show that the sequence of Dsq1 has stable presence across the study area, with a thickness range of 50-80 m.Its TST gradually becomes thinner from south to north.The lithologic correlation of the main strata in its parasequences shows that the black shales are increasingly thinner from Eyangye-1 to Zidi-2; and thus, that there is a possibility that the two parasequences in Eyangye-1 might be reduced to one parasequence.The changes in thickness of its HST are not considerable and the number of parasequences therein does not change significantly.

Fig.8.Multi-well columnar section of the Doushantuo Formation (Eyangye-1, Zidi-1 and Zidi-2).

Being the same as Dsq1, Dsq2 has stable thickness(approximately 80 m) across the study area.However, it has the following differences from Dsq1: the parasequences within its TST are relatively stable across the area, their changes in total thickness from north to south are not significant (generally about 50 m) and they are mainly composed of basin-facies carbonaceous mudstone.Its HST is relatively stable and relatively thin, with a thickness of nearly 30 m.

The thickness changes of Dsq3 are obvious.It has the tendency to be increasingly thinner from south to north.Its thickness at Eyangye-1 in the south is approximately 80 m while at Zidi-2 it is approximately 30 m.Its TST has the tendency to be increasingly thinner from south to north.The thickness of its TST at Eyangye-1 is approximately 50 m while that at Zidi-2 is about 20 m.There are three parasequences at Eyangye-1 while there are two at Zidi-2.The general characteristics of its HST are similar to those of its TST: its HST also has the feature of being thick in the south and being thin in the north.The number of parasequences therein is reduced from two at Eyangye-1 to one at Zidi-2.

3.3.Palaeogeographic characteristics of the rock lithofacies

The lithofacies paleogeography and deposition mode maps of the Doushantuo Formation and its high-quality target sequence (Dsq2) in Yichang and adjacent areas were drawn on the basis of the study and analysis of more than 20 wells including Eyangye-1 and Zidi-1 and 16 profiles including the Shangyang Profile.

The lithofacies distribution of the Doushantuo Formation in the Yichang Region successively includes platform facies,platform margin facies, upper continental slope facies and lower continental slope-deepwater trough basin facies from east to west.In the East part, the southeast area of the Eyangye-1 and Zidi-2 wells belong to the platform facies which occupied the largest area.The wells of Yiqian-1 and Yidi-3 are in the platform margin facies and the west of the platform facies and the platform margin facies run southward and northward in a banded pattern.The wells of Ehongdi-1,Xingdi-1, Zidi-1, Zidi-2 and Yican-1 are in the upper continental slope facies which is located in west of the platform margin facies.The upper continental slope facies also run southward and northward in the banded pattern.Its area is larger than that of the platform margin facies.The other drill holes, including Eyangye-1, are in the lower continental slope-deepwater trough basin facies, which are located in the southwest part of the study area (Fig.9).

Fig.9.Paleogeography of lithofacies in the Sinian Doushantuo Formation in Yichang and adjacent areas.

The deposition mode map of the high-quality shale member Dsq2, which was drawn according to multiple constraints including its lithofacies distribution, drilling columns, multi-well profile, seismic data and fault structure distribution, can intuitively reflect the distribution characteristics of its lithofacies.The platform facies, platform margin facies, upper continental slope facies and lower continental slope-deepwater trough basin facies that exist successively from east to west were always controlled by factors such as paleotopography and sea level.The platform facies were developed below the wave base and above the storm wave base.The platform margin facies were developed at the structural high below the sea level and above the wave base.The upper continental slope facies were developed below the wave base and above the storm wave base due to fault control and the lower continental slope-deepwater trough basin facies were developed below the storm wave base (Fig.10).

Fig.10.Deposition model of the targets sequence (Dsq2) of the Sinian Doushantuo Formation in Yichang and adjacent areas.

4.Identification and characterization of the high-quality sweet-spot shale interval

Chinese researchers such as Li YX and Li XB evaluated Doushantuo Formation's favourable shale gas members and their forming mechanisms and then predicted their exploration prospects (Li YX et al., 2010; Cai XF et al., 2018; Li XB et al., 2014).The high-quality sweet-spot shale interval was further identified through sequence stratigraphic correlation,chemical element stratigraphic correlation and comprehensive well/mud logging interpretation correlation of the Dou-2 Member of the Doushantuo Formation at Eyangye-1, on the basis of the results of the high-frequency sequence analysis of the Doushantuo Formation in the area examined during this study.

4.1.Sequence stratigraphic correlation and sweet-spot interval identification

The sedimentary sequence correlation and analysis results show that the Doushantuo Formation's sweet-spot member with exploration prospects is only its Dou-2 Member (Dsq2,Fig.11).In this member, consisting mainly of deepwater and semi-deepwater deposits in its TST, black shales prevail and pyrite laminae and phosphatic nodules can be seen in large quantities.This indicates that the water body in the deposition period was deep and a reducing sedimentary environment prevailed therein.The TST of the member Dsq1 is thin and unfavourable for follow-up large-scale horizontal drilling and staged fracturing stimulation activities.The member Dsq2 is stably distributed across the study area.Its sweet-spot interval thickness at Eyangye-1 is 48 m (3360-3408 m), that at Zidi-1 is 46 m (722-768 m) and that at Zidi-2 is 44 m (1338-1382 m).Generally, the high-quality shales in this member feature wide distribution, considerable thickness and great exploration and development potential.

Fig.11.Sequence stratigraphic correlation column of the Doushantuo Formation in Eyangye-1.

4.2.Chemical stratigraphic correlation and sweet-spot interval identification

According to the results of core element scanning and Rocscan cuttings scanning for the Doushantuo Formation at Eyangye-1, a chemical element stratigraphic correlation study of the Formation was carried out as the first attempt.The stratigraphic correlation shows that further fine classification of sedimentary microfacies for sweet-spot interval identification is possible if content change patterns of different elements, which can represent different sedimentary microfacies, are used for the analysis.The vertical content change patterns of elements (including U, Th, Fe, Mn, K, Si,Al, V and Cr) in the Doushantuo Formation at Eyangye-1(Fig.12) can reflect the water body's depth change patterns as follows: the contents of these elements have a marked tendency to increase in funnel form in the TST while they have the tendency to decrease gradually in the regressive system tract (shallower water body) (Pi DH et al., 2013; Liu ZH et al., 2015).In the Dou-2 Member (3360-3410 m), three distinctive sequence characteristics representing transgression can be identified.This happens to coincide well with the results of the well logging sequence stratigraphic correlation.The elemental analysis shows that the member is in the intraplatform basin facies featuring deep water body deposition and prevailing reducing sediment environment.The fact that the contents of the elements are stable reflects that the water body depth was stable.In addition, it is possible to clearly identify the obvious interbed characteristics caused by changes in the water body.Therefore, chemical-element facies analysis is also a means of identifying shale microfacies characteristics and sweet-spot intervals in the future.

Fig.12.Elemental correlation column of the Doushantuo Formationin Eyangye-1.

4.3.Well/mud logging stratigraphic correlation and sweetspot interval identification

The well/mud logging data were used for further fine classification of the Dou-2 Member of the Doushantuo Formation into 10 layers (Fig.13).Layers 1 to 4, which are in the sequence Dsq1, had unusually high gas survey values while drilling and were interpreted to have high TOC content and good reservoir physical properties.However, their total high-quality gas-bearing thickness was only about 20 m.Layers 5 to 10 are in the sequence of Dsq2.Layers 5-9 had unusually high gas survey values while drilling and layers 5, 9 and 10 had low resistivity values.The drill cores from layers 9 and 10 were badly broken.Layer 5 had hole enlargement.The interval consisting of the relatively stable layers 6-8, the total thickness of which is 41.5 m, features high fragile mineral content (up to ＞ 70%), high TOC content, good physical reservoir properties (high porosity and high permeability), a while-drilling gas survey total hydrocarbon anomaly up to 5%-9% and high gas content.According to the comparative analysis, it was put forward that the interval of layers 6-8 was the high-quality sweet-spot shale interval and that they would be the main development targets of future horizontal wells.Therefore, it was determined that the trajectory of the horizontal well of Eyangye-2HF should run through Layer 7.The horizontal drilling and staged fracturing practices of the well Eyangye-2HF show that fracturing stimulation of layers 6-8 was effective.The reservoir has high gas content.After fracturing of the 1410 m-long horizontal section, the daily gas productivity of the well was up to 55300 m3/d,thereby making a major breakthrough in shale gas productivity.This proves that the shale gas in the Doushantuo Formation has great exploration and development potential.

Fig.13.Well/mud logging column of the Dou-2 member of the Doushantuo Formation in Eyangye-1.

5.Conclusions

(i) Analysis of the data from the Sinian Doushantuo Formation in the Yichang Region of Hubei Province, revealed that the organic-rich black shales were deposited in a deepwater trough basin-slope setting against a background of a fault depression trough and that their distribution belts and sedimentation centres exist mainly along the fault depression trough.

(ii) Through high-frequency sequence stratigraphic correlation and comparative study, three Third-order sequences were identified in the Doushantuo Formation and each of them is composed of a TST and a HST.The layers of the organic-rich high-quality sweet-spot shale interval therein,are mainly in the TST and early HST.

(iii) The chemo-stratigraphic correlation results coincide well with the well logging sequence stratigraphic correlation results.The elemental profiles can reflect change in a water body and the sedimentation characteristics of the sedimentary microfacies.In the future, chemoelement-stratigraphic correlation may be used as a means for finely characterizing sedimentary microfacies of shales and identifying sweet-spot intervals in the study area.

(iv) The drilling and well/mud logging results prove that the high-quality sweet-spot shale interval in the Doushantuo Formation features high fragile-mineral content (up to ＞70%), high TOC content (up to 3.42%), good physical reservoir properties (the aperture more than 50 nm and the porosity is 2.7%), the abnormal values of total hydrocarbon gas measurement during horizontal well drilling is up to 5%-9% and high gas content.This makes it highly favourable for follow-up staged fracturing stimulation activities.Well testing at Eyangye-2HF shows that the interval is commercially viable of being explored and developed.

Acknowledgement

This study was supported by the Evaluation Procedure and Exploration Technology Development of Shale Gas Resources (2016ZX05034) under Major National Science and Technology Projects, Basic Geological Survey for Shale Gas in South China (121201229000160021), Strategic Survey in Favourable Shale Gas Areas in Zigui and Changyang, Hubei(DD20179623) and Sino-US Cooperative Research on Technologies of Evaluating Shale Reservoirs in Carboniferous and Permian Systems (2017YFE0106300).