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Geological characteristics of shale rock system and shale oil exploration breakthrough in a lacustrine basin: A case study from the Paleogene 1st sub-member of Kong 2 Member in Cangdong sag, Bohai Bay Basin, China
PETROLEUM EXPLORATION AND DEVELOPMENT Volume 45, Issue 3, June 2018 Online English edition of the Chinese language journal Cite this article as: PETROL. EXPLOR. DEVELOP., 2018, 45(3): 377–388.
RESEARCH PAPER
Geological characteristics of shale rock system and shale oil exploration breakthrough in a lacustrine basin: A case study from the Paleogene 1st sub-member of Kong 2 Member in Cangdong sag, Bohai Bay Basin, China ZHAO Xianzheng1,*, ZHOU Lihong1, PU Xiugang1, JIN Fengming1, HAN Wenzhong1, XIAO Dunqing1, CHEN Shiyue2, SHI Zhannan1, ZHANG Wei1, YANG Fei1 1. PetroChina Dagang Oilfield Company, Tianjin 300280, China; 2. China University of Petroleum (East China), Qingdao 266580, China
Abstract: A deep understanding of the basic geologic characteristics of the fine-grained shale layers in the Paleogene 1st sub-member of Kong 2 Member (Ek21) in Cangdong sag, Bohai Bay Basin, is achieved through observation of 140 m continuous cores and systematic analysis of over 1 000 core samples from two wells. Basic geological conditions for shale oil accumulation are proposed based on the unconventional geological theory of oil and gas. The shale rock system mainly developed interbedded formation of felsic shale, calcareous and dolomitic shale and carbonates; high quality hydrocarbon source rock formed in the stable and closed environment is the material base for shale oil enrichment; intergranular pores in analcite, intercrystalline pores in dolomite and interlayer micro-fractures make tight carbonate, calcareous and dolomitic shale and felsic shale effective reservoirs, with brittle mineral content of more than 70%; high abundance laminated shale rock in the lower section of Ek21 is rich in shale oil, with a total thickness of 70 m, burial depth between 2 800 to 4 200 m, an average oil saturation of 50%, a sweet spot area of 260 km2 and predicted resources of over 5108 t. Therefore, this area is a key replacement domain for oil exploration in the Kongdian Formation of the Cangdong sag. At present, the KN9 vertical well has a daily oil production of 29.6 t after fracturing with a 2 mm choke. A breakthrough of continental shale oil exploration in a lacustrine basin is expected to be achieved by volume fracturing in horizontal wells. Key words: shale oil; fine grained deposits; horizontal well; volume fracturing; shale reservoir sweet spot; Paleogene Kongdian Formation; Cangdong sag; Bohai Bay Basin
1.
Research background and geologic overview
With the success of shale gas exploration and commercial development in North America, the world petroleum industry has entered the era in which conventional and unconventional oil and gas are produced simultaneously[13]. The unconventional oil and gas mainly include shale oil and gas, tight oil and gas, oil sand, and coal-bed methane, etc. The shale oil described in this paper is a generalized concept[4], which includes the oil produced not only directly from shale but also from sandstone and carbonate rock interbedded with shale as source rock. Shale oil has abundant resources and large exploration potential. According to the assessment of Energy Information Administration (EIA) of the United States, the technically recoverable reserves of global shale oil may be up to 469108 t, of which Russia, US and China have 105.0108,
67.2108 and 44.8108 t respectively[56]. In 2016, the average daily shale oil production in the United States was 53.3104 t, accounting for 45% of its total daily oil production, and this proportion will reach 60% by 2040. After shale gas, shale oil has become another important unconventional resource[58]. The major series of exploration and development of unconventional oil and gas in North America are Paleozoic marine facies, such as Bakken, Eagle Ford, Barnett, Marcellus, Spraberry, Wolfcamp and Niobrara. Commercial development targets are mainly carbonate rock and sandstone in shale strata, followed by diamictite and shale. Marine shale strata in North America are characterized by stable distribution, high abundance of organic material, moderate thermal maturity, tight reservoirs, high content of brittle minerals and shallow burial
Received date: 21 Nov. 2017; Revised date: 25 Apr. 2018. * Corresponding author. E-mail: [email protected] Foundation item: Supported by the PetroChina Science and Technology Major Project (2017E-11). Copyright © 2018, Research Institute of Petroleum Exploration & Development, PetroChina. Publishing Services provided by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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depth etc.[914] The exploration of shale gas in China mainly focuses on the Upper Ordovician–Lower Silurian marine strata of the Sichuan Basin in South China; while the exploration of tight oil and shale oil in China are mainly located in the Permian and Mesozoic-Cenozoic continental strata in the basins of North China. The exploration targets of shale gas in China include sandstone, carbonate, and diamictite of shale strata. Unlike North American marine facies shale strata (Table 1), China’s continental shale strata are characterized by strong heterogeneity, rapid changes in organic material types and abundance, moderate thermal maturity, tight reservoirs, high content of brittle minerals and deep burial depth[1520]. Due to low oil prices, immature technology and equipment, higher production costs and lack of policy support, China’s shale oil exploration and development are facing challenges. In recent years, exploration of shale oil in the Cangdong sag of the Bohai Bay Basin has achieved great breakthroughs, and commercial oil flows have been successively obtained in several wells such as KN9 and GD6x1, which confirmed that the shale strata in traditional continental source rock areas have bright future for shale oil exploration. In order to break the bottleneck of geological cognition in shale oil exploration, 565 m continuous cores were taken from two wells (G108-8 and GD14) in the lacustrine shale area of low-middle slope region in the Cangdong sag, of which 140 m was from the 1st sub-member of the 2nd member of the Paleogene Kongdian Formation (Ek21). Based on the detailed core description and lab analysis of over 1000 core samples, the lithologies, properties, physical properties, oil content and brittleness of source rock in Ek21 shale formation have been analyzed systematiTable 1.
cally, so as to clarify the basic geologic characteristics of shale oil and sort out the key approaches for beneficial exploration of shale oil. The Cangdong sag is a secondary structure unit in the Huanghua depression of the Bohai Bay Basin, with an area of approximate 1760 km2. Being retained between the Cangxian uplift, the Xuhei bulge, the Kongdian bulge and the Dongguang bulge, it is a Cenozoic intracontinental rift lake basin developing under regional extensional background[21]. Although the current structure of the study area is complex, its structure during the sedimentary period of the Kong 2 Member was relatively stable, being dominated by micro tectonic activities. This laid the foundation for the development of fine-grained sediments. During the deposition period of the Kong 2 Member, the Cangdong sag was an inland enclosed lake basin, located in subtropical zone with semiarid-moist environment. Several lobate deltas were developed around the lake basin. Delta facies were developed on the margin of the lake basin, which is the development zone of conventional middle-fine sandstone; semi-deep lacustrine subfacies was developed in the central part of the lake basin, with an area of 430 km2, which is the development zone of fine-grained sediments (Fig. 1). The top and bottom interfaces of the Kongdian Formation are angle unconformity or parallel unconformity. This formation is a secondary sequence stratigraphic unit, and can be further divided into four third-order sequences: Kong 3 Member, Kong 2 Member, upper Kong 1 sub-member, lower Kong 1 sub-member. The Kong 2 Member (Ek2), 400600 m thick, is different in seismic reflection characteristics from the
Comparison of geologic and reservoir characteristics of typical shale strata in China and abroad (modified from references
[1618]).
Bakken, Williston Basin Eagle Ford, Western Gulf Basin Wolfcape, Permian Basin
5.0 10.0
3 000 5 000
8.0 10.0 6.0 8.0 7.0 11.0 5.0 10.0
1 500 3 500 2 300 4 500 2 000 4 500 1 500 2 400
1.7
2 100 3 300
0.5 2.0 3.0
2 130 3 650 1 680 3 350
Lacustrine muddy shale
Shahejie-1 sub-member, Bohai Bay Basin Yanchang Formation, Ordos Basin Permian, Junggar Basin Jurassic, Sichuan Basin Cretaceous, Songliao Basin
Marine muddy shale
Shale oil zone
Source rock Favorable area/ Buried Litho- Thick- TOC/ Ro/% 104 km2 depth/m logy ness/m % 250 600
2.0 0.6 6.0 1.8
20110 1035 40240 50200
2.0 20.0 3.0 4.0 0.8 3.0 0.7 8.7
5-12
5.0 14.0
3.0 7.0 2.0 60-90 9.0 20-90
Lithology
Fine siltstone
0.7 Fine siltstone 1.2 0.6 Dolomitic siltstone, 1.5 sandy dolomite 0.9 Fine siltstone, 1.4 shell limestone 0.7 Argillaceous 2.0 siltstone, shale Dolomitic/ 0.5 argillaceous 1.3 siltstone 0.7 Muddy 1.3 limestone 0.7 Dolomite 1.0
378
Reservoir bed
Oil and gas reservoir
Permea- BrittleThick- Porobility/ ness ness/m sity/% 3 10 μm2 index/%
Oil Pressure density/ coeffi(g·cm3) cient
80 280
1.0 12.0
0.02 1.00
20 80 80 200 10 60 20 45
2.0 0.01 12.0 1.00 3.0 <1.00 10.0 0.2 1104 7.0 2.10 1.4 0.60 8.7 1.00
5 55
5.0 13.0
30 90 30 150
2.0 12.0 4.0 12.0
>80 >85 >80 >80 >70
0.10 1.00
>90
<0.01
5080
0.01 1.0
>80
0.73 0.94
1.10 1.56
0.80 0.86 0.87 0.92 0.76 0.87 0.78 0.87
0.75 0.85 1.10 1.80 1.23 1.72 1.20 1.58
0.81 0.83
1.35 1.58
0.82 0.87 0.71 0.81
1.35 1.80 1.00 1.20
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Fig. 1.
Structure map of the Cangdong sag and the distribution map of conventional sandstone and fine-grained sediments of EK21.
underlying Kong 3 Member, and in pseudo-conformable contact with the Kong 3 Member; it is in unconformable contact with overlying Kong 1 Member, with apparent overlapping and truncating events on the margin of this basin. From bottom to top, the Kong 2 Member can be further subdivided into four fourth-order sequences (SQEk24, SQEk23, SQEk22 and SQEk21) and ten fifth-order sequences (SQ①-⑩)[22]. SQEk24 is a low-stand systems tract mainly composed of delta front deposits, including gray fine sandstone and gray mudstone. SQEk23–SQEk22 are lacustrine transgressive systems tracts, with the largest lacustrine flooding surface of the Kongdian Formation on the top. Core observation proves that it is dark
grey-black muddy shale with high TOC value, about 30 cm thick. During this period, a set of semi-deep lacustrine fine sediments were deposited, dominated by muddy shale and argillaceous dolomite, and developed fine siltstone of gravity flow on the top. SQEk21 is high-stand systems tract, and the lithology of lower Ek21SQ⑨ is dominated by dolomite interbedded with muddy shale. With the dropping reference plane, a set of shallow lacustrine mudstone was deposited on the upper Ek21SQ⑩, with a thin sand body of delta front at the top[22]. According to lithology assemblage, TOC and mineral composition variation features, Ek21SQ⑨ can be divided into five quasi-sequences.
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2.
Experiment conditions and data
To further understand the geological characteristics of Ek21SQ ⑨ shale system, 140 m cores were taken from SQEk21 of Well G108-8 and Well GD14. Detailed core description was performed then. The lithologies, source rock properties, physical properties, oiliness and brittleness of over 1 000 core samples were analyzed. The lab tests were carried out by two organizations, Basin Analysis and Oil & Gas Reservoir Geology Laboratory and National Key Laboratory of Heavy Oil in China University of Petroleum (East China). X'pert Pro MPD device was used to do X-ray diffraction (XRD) analysis. CuKα radiation ray was used. Other experimental conditions were: electric voltage (40 kV), electric current (40 mA), measurement range of 2θ (mineral diffraction angle) (5° to 60°), and 2θ sampling step width of 2θ (0.016°). QKY-II gas porometer and STY-II gas permeameter were used for physical property analysis, with accuracy of 0.5% and
Fig. 2.
0.01×103 μm2 respectively. The measuring pressures were 0.7 MPa and 1.0 MPa. Core saturation desiccator (type GLY-2) was used for oil saturation analysis, which has operating temperatures between normal and 600°C; dry distillation accuracy: ±3% for dry distillation water, ±5% for dry distillation oil. The analytical data is shown in Figs. 2 and 3.
3. 3.1.
Basic geologic characteristics Lithologic characteristics
The shale strata mainly consist of fine-grained sedimentary rocks with the particle size of less than 0.062 5 mm and particle content of more than 50%[2324]. Due to the extremely fine grain size of shale, it is difficult to accurately identify the mineral contents and determine their names by conventional thin slices. XRD can not only identify mineral types but quantitatively calculate their contents. Therefore, three main minerals, carbonate minerals (calcite and dolomite), felsic minerals
Composite diagram of well logging and experiment data of Well G108-8.
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Fig. 3.
Composite diagram of well logging and experiment data of Well GD14.
(quartz and feldspar) and clay minerals, obtained from XRD analysis were taken as end elements, and the rocks were named according to their specific contents[2526]. In this way, traditional shale, mudstone, carbonate rocks are divided into four basic types: fine-grained felsite sedimentary rock, carbonate rock (mainly dolomite in the study area), clay rock and calcareous and dolomitic shale. Combined with the content of the diagnostic mineral (analcite), the rocks are further subdivided into 24 subtypes[23]. Mud logging data of Well G108-8 shows that Ek21SQ⑨ is an interbedded aggregate structure mainly composed of dark gray mudstone and taupe oil shale, being a mixture of different minerals and structures[27] (Fig. 4). XRD analysis shows that the minerals that make up the shale mainly include quartz, feldspar, calcite, dolomite and clay minerals, and accessory minerals like pyrite, siderite, zeolite and gypsum etc. The average contents of minerals in Ek21SQ⑨ shale layer (Well G108-8) are: clay mineral (12%), quartz+feldspar (28%), calcite+dolomite (40%), analcite (18%) and pyrite+siderite (2%) (Fig. 2). Well GD14 located in Nanpi slope has higher sand content (quartz+feldspar). Gray-white fine-grained sand cuttings in patches or lamina are commonly seen in cores. The
average mineral contents are: clay minerals (7%), quartz+ feldspar (53%), calcite+dolomite (37%), analcite (2%) and pyrite+siderite (1%) (Fig. 3). Similar to the mineral composition of shale layers in different sedimentary environments at home and abroad, Ek21SQ⑨ has no obvious dominant mineral components as a whole. Its clay mineral content is generally less than 20%, which overthrows the traditional understanding that the clay mineral contents of shale layers are greater than 50%[28]. Although there are no dominant minerals in macroscope, the enrichment degree of minerals and the combination of rock types are different in different vertical quasi-sequences. Ek21SQ⑨-1 in Well G108-8 is interbedded dolomite and calcareous and dolomitic shale, in which the dolomite has a total thickness of 14 m and average single layer thickness of 0.6 m, accounting for 56% of the total. Ek21SQ⑨-2 is interbedded dolomite, calcareous and dolomitic shale and felsic shale, accounting for about 1/3 of each; Ek21SQ⑨-3 is interbedded calcareous and dolomitic shale and felsic shale: the former is 9 m thick, accounting for 69%. Ek21SQ⑨-4 is interbedded dolomite (6 m thick, accounting for 43% of the total) and calcareous and dolomitic shale (8 m thick, accounting for 57% of
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Fig. 4. Core and thin slice characteristics of Ek21 shale layer in Well G108-8, Cangdong sag. (a) Well G108-8, 2 973.11 m, felsic dolomite, developed lamination and laminar cracks, cross polarization; (b) Well G108-8, 2 973.41 m, calcareous and dolomitic shale, cross polarization; (c) Well G108-8, 2 973.62 m, felsic shale, cross polarization; (d) Well G108-8, 2 974.01 m, microcrystalline dolomite, yellow-green fluorescence with middle-high luminance, fluorescence thin slice; (e) Well G108-8, 2 974.27 m, calcareous and dolomitic shale, oleaginous bitumen in laminar cracks, yellow-white fluorescence, fluorescence thin slice; (f) Well G108-8, 2 974.81 m, calcareous and dolomitic shale, oleaginous bitumen in fine intergranular pores of zeolite, yellow-white fluorescence, fluorescent thin slice.
the total). Ek21SQ⑨-5 is not cored. According to the logging data, the lithology is dominated by calcareous and dolomitic shale (Fig. 2). From bottom to top, the content of felsic shale of Well GD14 gradually increases. Ek21SQ⑨-3 is mainly felsic shale, which is 9.5 m thick, accounting for 86% of the total (Fig. 3). The higher the contents of brittle minerals (including quartz, calcite, dolomite and analcite), the more likely complex fracture networks will be formed during fracturing, which is favorable for the follow-up development. The contents of brittle minerals in China’s lacustrine shale strata are relatively higher (more than 40%). In Well G108-8 and Well GD14 in the Cangdong sag, the brittle mineral contents of Ek21SQ⑨ shale layer reach 71.8% and 50.3% respectively. According to the analysis of rock mechanics, the dolomite has a Young’s modulus of 26.6 MPa and Poisson’s ratio of 0.20, indicating that the Ek21SQ⑨ shale layer has good crackability.
3.2.
Features of source rocks
The fine-grained sediments in the shale strata are good to excellent source rocks in general. Statistic results of 356 samples from the Ek21SQ⑨ interval show that they have total organic carbon (TOC) from 0.3% to 12.9% (averaging 3.6%), and hydrocarbon generation potential (S1+S2) from 0.2 to 73.0 mg/g (averaging 24.0 mg/g). Affected by lithology, felsic shale and calcareous and dolomitic shale have better quality, higher TOC values and S1+S2 (Fig. 5). Samples of good source rocks (with TOC>2%, and S1+S2>5 mg/g) account for 70%, while samples of non-source rock (with TOC<0.5%, S1+S2<0.5 mg/g) account for only 3%. The source rocks in different quasi-sequences have different quality. In Well G108-8, the source rocks of Ek21SQ⑨-2 and Ek21SQ⑨-3 are the best in quality, with average TOC of 4.6% and 4.1%
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Fig. 5. Bar charts of hydrocarbon generation potentials for different rock types in Ek21SQ⑨ shale layer in Wells G108-8 and GD14. (a) Well G108-8, 2 9002 990 m, 207 samples, Ro=0.76%; (b) Well GD14, 4 0754 140 m, 112 samples, Ro=1.03%.
respectively; the average TOC of Ek21SQ⑨-1 and Ek21SQ⑨-4 are 3.2% and 1.3% respectively. The dolomite content in Ek21SQ⑨-1 is relatively higher, but the TOC of the source rock in the interbedded layer is not worse (up to 12.9%). The closed environment with poor oxygen and reductive salt water during high-stand period is the key factor for forming high-quality source rock (V/(V+Ni)>0.6, and Sr/Ba>1), being favorable for the preservation and enrichment of organic matter in the lake basin. The Ek21 source rock is good in organic matter type, and the kerogen is mainly type I and type IIA, accounting for 69% and 13%, respectively. This source rock is moderate in maturity, with Ro values between 0.68% and 1.16%, and is right at the stage of massive oil generation. The result of thermal simulation shows that this set of high-quality source rock not only has a long period of hydrocarbon generation, but also a large amount of retained hydrocarbons. The hydrocarbon generation rate of the source rock during low mature stage with Ro less than 0.5% is about 150 mg/g, while the maximum hydrocarbon generation rate in the mature stage is 560 mg/g (averaging about 300 mg/g). The retained hydrocarbons in the source rock are 400 mg/g (averaging 200 mg/g). The ratio of retained hydrocarbons to expelled hydrocarbons is about 2:1 during the thermal evolution stage with Ro values between 0.6% and 1.1%. Calculation shows that the retained hydrocarbons in the Ek21 source rock is about 13.9108 t, suggesting high exploration potential of the shale oil (Fig. 6). 3.3.
Reservoir characteristics
The calcareous and dolomitic shale in the shale is overall tight. Analysis of 90 samples from Ek21 shows that they have porosity values between 1.0%9.0% in general (averaging 3.4%), permeability values between 0.0116.20)×103 μm2 (averaging 0.20×103 μm2). With rich intergranular pores, intercrystalline pores, micro fractures, and laminar cracks etc., the tight shale layers can be effective reservoir beds. The average effective porosities of dolomite, calcareous and dolomitic shale, and felsic shale are 5.8%, 3.3% and 3.1%,
Fig. 6. Hydrocarbon generation and expulsion mode of the source rock in Kong 2 Member.
respectively. The lamellar layer and interlaminar fractures are abundant in the fine-grained sedimentary rock. The mineral components that make up the laminae are mainly dolomite, analcite, felsic debris, and organic matter etc. Hereinto, the granular analcite is mainly distributed in calcareous and dolomitic shale and felsic shale, in layers or in lumps. The observation of thin slices under fluorescent light shows that there are hydrocarbon fillings in both (analcite) lamellae and interlaminar fractures (Fig. 4e and 4f). The average density of macroscopic interlayer fractures is 2 pieces/m, and it is 6 pieces/m in Ek21SQ⑨-1 of Well G108-8. Structural fractures with high-angle, fractures formed under anomalous pressure and differential compaction fractures etc. are also well-developed besides interlayer fractures. Structural fractures are more developed in Well GD14, with an average density of 2/m. Dolomite is an important reservoir rock for shale oil. The dolomite in the Ek21SQ⑨ shale layer is widely developed, being dominated by dolomicrite-micritic dolomite. The degree of crystallization increases gradually with strengthening burial
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diagenesis. Well GD14 is about 1 000 m deeper than Well G108-8, and the dolomite is mainly micritic. Statistics show that the high content of dolomite and high degree of crystallinity can lead to larger reservoir space; the better the fluorescence shows, the higher the oil content will be. 3.4.
Oiliness
The shale layer of the Kong 2 Member in the Cangdong sag is universally oil-bearing and strongly heterogeneous with several sections enriched in oil. In which, the whole Ek21SQ⑨ interval has higher oil content, with core samples showing higher brightness under fluorescence scanning (Fig. 4). Under the microscopic visual field, hydrocarbons are dominated by yellow-white, yellow-green, and blue-white oleaginous asphalt, with medium-strong fluorescent brightness, and oil saturation of 10.0%68.5% (averaging 50.0%). The oiliness of the shale strata is mainly influenced by the degree of thermal evolution. The deeper the burial depth, the higher the thermal maturity, the higher the free hydrocarbon content (S1) will be. As shown in Fig. 3, Well GD14 is 1000 m deeper than Well G108-8, so its Ro value is 0.27% higher than the later, and its average S1 content is 3.5 mg/g, which is 2 to 6 times that of the later. Compared with the S1 contents of other shale strata at home and abroad, the average S1 content of Well GD14 is above the average[28]. According to the relationship between Ro values and S1 values, considering that the development cost of shale gas may increase with bigger depth, we inferred that the interval between 3 3004 000 m (with Ro values between 0.85%1.10%) is the relatively favorable section for shale oil exploration in this area.
4.
Taking Ek21SQ ⑨ -1 and Ek21SQ ⑨ -2 as examples, the source rocks and reservoir beds are laminated like a layered cake, and the area of 30 m thick is 321 km2 ( not including the volcanic rock invaded area). Based on comprehensive analysis of planar distribution of thermal maturity in source rock, the favorable exploration area of shale oil with Ro greater than 0.5% is 260 km2, and the thicker regions are mainly concentrated in the lower parts of the Nanpi slope and the Kongxi slope. Affected by the faulting activities during later period, the buried depth in the lower part of the Nanpi slope is averagely 1000 m deeper than that in the Kongxi slope, and the thermal maturity in the lower part of the Nanpi slope is higher (average Ro=1.1%), so the Nanpi slope is currently the first option for efficient exploration and development (Fig. 7).
5. 5.1.
Shale oil exploration practices Discoveries by straight well drilling
Thirteen exploration wells have been deployed for shale oil exploration in the Kong 2 Member in recent years. Among them, 12 wells have daily oil production of over 5 tons, and 6 wells have daily oil production of over 10 tons. Wells GD6x1, G1608 and KN9 have daily oil production of over 20 t, and the oil production layers are all in Ek21SQ⑨ shale formation. The reservoir buried depth is 2 800 to 4 200 m, the oil density is 0.86 to 0.89 g/cm3 (20 C), and the pressure coefficient of
Evaluation of shale oil sweet spots
With overall high oil content and several high oil production intervals, the shale formation of the Kong 2 Member in the Cangdong sag has shale oil geological resources of over 2108 t according to preliminary estimation, indicating a great potential for profitable shale oil exploration and development in this area. The analysis of geological features of Ek21SQ⑨ shows that this formation has high organic matter abundance, good organic matter type, and moderate maturity, thus it is at the stage of massive oil generation. It is a set of high-quality source rock with large amount of retained hydrocarbons. It has more laminar dolomite, high dolomite crystallization degree, large reservoir space, and high oil content; moreover, it has high contents of brittle minerals (such as quartz, calcite and analcite), which is conducive to later fracturing. It has the geological conditions for forming good reservoir beds. During the depositional period of this formation, the tectonic activities were weak, so few faults were developed and the formation was stable in lateral distribution. The formation is thick in local parts because of the effects of provenance and paleogeomorphology etc. It is concluded by comprehensive evaluation that four quasi-sequences in Ek21SQ⑨ are all oil-rich, with thicknesses of 3793 m (averaging 70 m).
Fig. 7. Distribution of sweet spot thickness of Ek21SQ⑨-1 and Ek21SQ⑨-2 in Cangdong sag.
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the reservoir is 0.9 to 1.2. Well GD6x1 had an initial daily oil production of 32.6 t after fracturing with 3 mm choke. It was tested with 44 mm choke for 20 days, at the pump stroke of 5.2 m, the pumping rate of 2 times/min, the cumulative oil production was 115 t, and the average daily oil production was 5.8 t. Well G1608 had an initial daily oil production of 53.13 t after fracturing with 3 mm choke. It was tested for 105 days at the pump drainage pressure of 10 MPa, and produced 1540.7 t oil in total, with an average daily oil production of 14.7 t. Well KN9 had a daily oil production of 29.6 t after fracturing with 2 mm choke (Fig. 8). The data from the producing test shows that the shale formation of Ek21SQ⑨ has stable industrial oil productivity. Breakthroughs have been made in the Ek21, Ek22 and Ek23 shale formations. Well G1508 produced 14.0 t of oil a day from Ek22 with 2 mm choke, and Well Z1605 produced 14.9 t of oil a day from Ek23 after fracturing at the pumping drainage pressure of 25 MPa. 5.2. Breakthroughs in preliminary horizontal exploration wells 5.2.1. Characteristics of oil layers encountered by horizontal wells Horizontal well GD1701H was drilled in the G1608-G1508 well field in the lower part of the Nanpi slope targeting the Ek21SQ⑨-2. This well has a finished depth of 5 465.49 m, vertical depth of 3851.5 m, and deviation angle of 21.6° 91.6°. This well encountered 228 m of Ek21SQ⑨-1, 716 m of Ek21SQ⑨-2, 530 m of E k21SQ⑨-3, with the cumulative length of horizontal section of 1474 m. The gas logging in the horizontal section was abnormally active, with all gas hydrocarbons ranging from 1.74% to 18.98%; there are 1237.4 m/76 intervals of Type I oil layers and 175 m/2 intervals of Type II oil layers from comprehensive interpretation, and high-quality reservoir encountering rate is 96.25%. The well logging interpretation shows that the lithology is mainly composed of 714 m/56 layers of argillaceous dolomite, 150 m/32 layers of calcareous and dolomitic shale, and 512 m/47 layers of felsic shale. This formation has an average TOC of 3.0%, average S1 of 5 mg/g, average effective porosity of 7%, and average generalized brittleness index of 70%. 5.2.2. Perforation cluster optimization and volume fracturing in horizontal wells In order to improve the producing efficiency of shale oil in horizontal wells, locations of perforation clusters were selected under the guidance of “intensive cutting and volume stimulation”, based on source-reservoir combination model, oiliness and brittleness, the points with good source-reservoir combination, high resistivity, high gas logging value, high S1 content, and higher brittleness were selected as positions for perforation clusters. The total selected perforation interval length is 1 240 m, including 69 clusters in 21 intervals, at the average cluster spacing of 18 m (Fig. 9). Fracturing of this well has started with a total liquid volume of 43 300 m3, av-
erage meter volume of 35 m3, total sanding volume of 2 044.4 m3, and average sanding volume of 1.6 m3/m. 5.3.
Enlightenment from shale oil exploration
5.3.1. Comprehensive geological research is the important foundation for shale oil exploration breakthrough During the period of structural and lithologic reservoir exploration, 76 wells were drilled in the shale strata in the central lake basin. The mud logging data showed that they were all mudstone or oil shale. Although there were hydrocarbon shows in several wells, this shale formation had not received enough attention because of the traditional understanding that shale strata were source rock but not reservoirs, and less ideal oil testing results of the shale strata in some individual wells. A completely new understanding on the geological characteristics of shale strata has been obtained through systematic analysis of cores and experimental data of Well G108-8 and Well GD14: the shale strata not only have high-quality source rocks, but also effective reservoirs, which are combined or adjacent to each other, with active oil and gas shows. Several exploration wells based on this understanding have obtained industrial oil flows and even high production, which revealed the history of shale oil exploration in the Kong 2 Member of the Cangdong sag. 5.3.2. Volumetric fracturing is an effective means to increase single well production Mathematical model based on rock mechanics under XMAC well logging is established to optimize the fracturing operation parameters, according to the rock mechanics experimental data of Well G108-8. Multiple sand addition-hybrid hydraulic fracturing technology has been worked out: firstly, to pump low-viscosity linear gel fracturing fluid and fine ceramic beads for fracturing at high pumping rate to create fracture network, to stop pumping for 1-2 hours, and then add sand and pump again, to reduce the horizontal stress difference coefficient through the previous fracture interference on the ground stress and improve the formation of fracture network. The composite fracturing process has been applied 24 layer-times in the Kong 2 Member of the Cangdong sag, and industrial oil flow has been obtained in 17 layer-times, which accounts for 70.8% of the total layers. The average daily liquid production before fracturing was 1.45 m3, and the average daily production liquid after fracturing was 20.1 m3, that is 12.9 times of the daily liquid production before fracturing. The average daily oil production before fracturing was 0.4 t, and the average daily oil production after fracturing was 8.74 t, that is 20.9 times of the former oil production. The composite fracturing technology can increase oil production significantly. 5.3.3. Geology-engineering integration is an effective way for efficient producing
385
Geology-engineering integration is centered on the same
Fig. 8. EW-striking correlation profile of shale oil enrichment intervals in Ek21SQ⑨ shale formation (see Fig. 1 for the location. Interpretation of logging lithology can be seen in reference [22]).
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Fig. 9. Table 2.
Comparison of conventional fracturing and the secondary sanding + hybrid fluid fracturing.
Fracturing technology KN9
Fracturing fluid volume/m3
Proppant volume/m3
Cost/10000 Yuan
Slick Low Total liquid 0.110.21 mm 0.110.21 mm 0.210.38 0.270.55 mm Total sand FracturProppant Total water damage volume quartz sand fine pottery clay mm ceramsite ceramsite volume ing fluid 1 903
Conventional 922 Difference
Wellbore trajectory of horizontal well GD1701H.
233
2 136
1 214
2 136
+981 981
67.1 +67.1
24.5
14.5
106.1 106.1
7
65.4
33.7
7
40.9
19.2
exploration and development objective, with the idea of geology serving engineering, engineering relying on geology, geology guiding engineering, and engineering feeding back geology, to reach the goal of developing engineering technology and deepening geologic understanding[6]. Only by combining the two aspects closely, can the efficient exploration and development of shale oil be achieved. As shale oil exploration in China is still at an exploratory stage, current research priority is to innovate geological cognition and optimize operation parameters. Based on geological understanding, optimization of the process and fracturing system will both reduce the cost and increase efficiency. Well KN 9 is a typical exploratory well, treated with optimized process based on innovative understanding of shale strata successfully. It was perforated for oil testing in the interval from 3402 to 3424 m in 2012 (not fractured), at pump drainage pressure of 25 MPa, producing 5.42 t of oil a day (no water). It was fractured in September 2017, with “slickwater+low-damage fracturing fluid system”, with a total fluid volume of 2136 m3. In the treatment, the proportion of slickwater was increased from 43.2% for conventional fracturing to 89.1%. The proppant was quartz sand+ceramsite, with a total sand volume of 106.1 m3, and the proportion of proppant changed from 0% of powder ceramsite to 63.2% of quartz sand. The total saved cost of the fracturing was 505700 Yuan (56.5% down) compared with the conventional fracturing material costs of the same scale (Table 2). Fracturing simulation shows that the cracks are 179 m long and 40 m high,
23.30
15.65
38.95
58.29
31.23
89.52
34.99
15.58 50.57
forming a more complex fracture network. The initial daily oil production of the well after fracturing by 2 mm choke was 29.6 tons, which was nearly 5 times of that before fracturing.
6.
Conclusions
The shale layer of the Kong 2 Member in the Cangdong sag of the Bohai Bay Basin, China, is a complex lamellar aggregate of felsic shales, calcareous and dolomitic shale, and carbonate rocks. With rich laminae, clay mineral content of less than 20%, and the content of brittle minerals (such as quartz, calcite, dolomite, and analcite) more than 70%, it is likely to form complex fracture network after fracturing. The shale layer is overall good-very good source rock, with mainly Type I and Type IIA kerogen, an average TOC of 3.6%, average S1+S2 of 24 mg/g, Ro value of 0.68% to 1.16%, and average hydrocarbon generation intensity of 300 mg/g. The reservoir matrix has a porosity of less than 10% and permeability of less than 1×103 μm2 in general. But with abundant intergranular pores, intercrystalline pores, micro fractures, and laminar cracks, the dense shale strata can be effective reservoir beds for shale oil. The reservoir beds are commonly filled with oil, with high content of free hydrocarbon S1 and an average oil saturation of 50%. Exploration practices show that under the pertinent fracturing measures, this shale layer can yield stable industrial productivity and considerable economic value, breaking the preliminary understanding that this shale layer was mainly source rock, not reservoir, without movable oil.
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Nomenclature GAS—gas logging total hydrocarbon, %; GR—gamma-ray, API; Ro—vitrinite reflectance, %; Rt—resistivity, ·m; S1—free hydrocarbon content, mg/g; S2—pyrolytic hydrocarbon content, mg/g; SP—self-potential, mV; TOC—total organic content, %; Δt—interval travel time, s/m.
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RESEARCH PAPER
Geological characteristics of shale rock system and shale oil exploration breakthrough in a lacustrine basin: A case study from the Paleogene 1st sub-member of Kong 2 Member in Cangdong sag, Bohai Bay Basin, China ZHAO Xianzheng1,*, ZHOU Lihong1, PU Xiugang1, JIN Fengming1, HAN Wenzhong1, XIAO Dunqing1, CHEN Shiyue2, SHI Zhannan1, ZHANG Wei1, YANG Fei1 1. PetroChina Dagang Oilfield Company, Tianjin 300280, China; 2. China University of Petroleum (East China), Qingdao 266580, China
Abstract: A deep understanding of the basic geologic characteristics of the fine-grained shale layers in the Paleogene 1st sub-member of Kong 2 Member (Ek21) in Cangdong sag, Bohai Bay Basin, is achieved through observation of 140 m continuous cores and systematic analysis of over 1 000 core samples from two wells. Basic geological conditions for shale oil accumulation are proposed based on the unconventional geological theory of oil and gas. The shale rock system mainly developed interbedded formation of felsic shale, calcareous and dolomitic shale and carbonates; high quality hydrocarbon source rock formed in the stable and closed environment is the material base for shale oil enrichment; intergranular pores in analcite, intercrystalline pores in dolomite and interlayer micro-fractures make tight carbonate, calcareous and dolomitic shale and felsic shale effective reservoirs, with brittle mineral content of more than 70%; high abundance laminated shale rock in the lower section of Ek21 is rich in shale oil, with a total thickness of 70 m, burial depth between 2 800 to 4 200 m, an average oil saturation of 50%, a sweet spot area of 260 km2 and predicted resources of over 5108 t. Therefore, this area is a key replacement domain for oil exploration in the Kongdian Formation of the Cangdong sag. At present, the KN9 vertical well has a daily oil production of 29.6 t after fracturing with a 2 mm choke. A breakthrough of continental shale oil exploration in a lacustrine basin is expected to be achieved by volume fracturing in horizontal wells. Key words: shale oil; fine grained deposits; horizontal well; volume fracturing; shale reservoir sweet spot; Paleogene Kongdian Formation; Cangdong sag; Bohai Bay Basin
1.
Research background and geologic overview
With the success of shale gas exploration and commercial development in North America, the world petroleum industry has entered the era in which conventional and unconventional oil and gas are produced simultaneously[13]. The unconventional oil and gas mainly include shale oil and gas, tight oil and gas, oil sand, and coal-bed methane, etc. The shale oil described in this paper is a generalized concept[4], which includes the oil produced not only directly from shale but also from sandstone and carbonate rock interbedded with shale as source rock. Shale oil has abundant resources and large exploration potential. According to the assessment of Energy Information Administration (EIA) of the United States, the technically recoverable reserves of global shale oil may be up to 469108 t, of which Russia, US and China have 105.0108,
67.2108 and 44.8108 t respectively[56]. In 2016, the average daily shale oil production in the United States was 53.3104 t, accounting for 45% of its total daily oil production, and this proportion will reach 60% by 2040. After shale gas, shale oil has become another important unconventional resource[58]. The major series of exploration and development of unconventional oil and gas in North America are Paleozoic marine facies, such as Bakken, Eagle Ford, Barnett, Marcellus, Spraberry, Wolfcamp and Niobrara. Commercial development targets are mainly carbonate rock and sandstone in shale strata, followed by diamictite and shale. Marine shale strata in North America are characterized by stable distribution, high abundance of organic material, moderate thermal maturity, tight reservoirs, high content of brittle minerals and shallow burial
Received date: 21 Nov. 2017; Revised date: 25 Apr. 2018. * Corresponding author. E-mail: [email protected] Foundation item: Supported by the PetroChina Science and Technology Major Project (2017E-11). Copyright © 2018, Research Institute of Petroleum Exploration & Development, PetroChina. Publishing Services provided by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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depth etc.[914] The exploration of shale gas in China mainly focuses on the Upper Ordovician–Lower Silurian marine strata of the Sichuan Basin in South China; while the exploration of tight oil and shale oil in China are mainly located in the Permian and Mesozoic-Cenozoic continental strata in the basins of North China. The exploration targets of shale gas in China include sandstone, carbonate, and diamictite of shale strata. Unlike North American marine facies shale strata (Table 1), China’s continental shale strata are characterized by strong heterogeneity, rapid changes in organic material types and abundance, moderate thermal maturity, tight reservoirs, high content of brittle minerals and deep burial depth[1520]. Due to low oil prices, immature technology and equipment, higher production costs and lack of policy support, China’s shale oil exploration and development are facing challenges. In recent years, exploration of shale oil in the Cangdong sag of the Bohai Bay Basin has achieved great breakthroughs, and commercial oil flows have been successively obtained in several wells such as KN9 and GD6x1, which confirmed that the shale strata in traditional continental source rock areas have bright future for shale oil exploration. In order to break the bottleneck of geological cognition in shale oil exploration, 565 m continuous cores were taken from two wells (G108-8 and GD14) in the lacustrine shale area of low-middle slope region in the Cangdong sag, of which 140 m was from the 1st sub-member of the 2nd member of the Paleogene Kongdian Formation (Ek21). Based on the detailed core description and lab analysis of over 1000 core samples, the lithologies, properties, physical properties, oil content and brittleness of source rock in Ek21 shale formation have been analyzed systematiTable 1.
cally, so as to clarify the basic geologic characteristics of shale oil and sort out the key approaches for beneficial exploration of shale oil. The Cangdong sag is a secondary structure unit in the Huanghua depression of the Bohai Bay Basin, with an area of approximate 1760 km2. Being retained between the Cangxian uplift, the Xuhei bulge, the Kongdian bulge and the Dongguang bulge, it is a Cenozoic intracontinental rift lake basin developing under regional extensional background[21]. Although the current structure of the study area is complex, its structure during the sedimentary period of the Kong 2 Member was relatively stable, being dominated by micro tectonic activities. This laid the foundation for the development of fine-grained sediments. During the deposition period of the Kong 2 Member, the Cangdong sag was an inland enclosed lake basin, located in subtropical zone with semiarid-moist environment. Several lobate deltas were developed around the lake basin. Delta facies were developed on the margin of the lake basin, which is the development zone of conventional middle-fine sandstone; semi-deep lacustrine subfacies was developed in the central part of the lake basin, with an area of 430 km2, which is the development zone of fine-grained sediments (Fig. 1). The top and bottom interfaces of the Kongdian Formation are angle unconformity or parallel unconformity. This formation is a secondary sequence stratigraphic unit, and can be further divided into four third-order sequences: Kong 3 Member, Kong 2 Member, upper Kong 1 sub-member, lower Kong 1 sub-member. The Kong 2 Member (Ek2), 400600 m thick, is different in seismic reflection characteristics from the
Comparison of geologic and reservoir characteristics of typical shale strata in China and abroad (modified from references
[1618]).
Bakken, Williston Basin Eagle Ford, Western Gulf Basin Wolfcape, Permian Basin
5.0 10.0
3 000 5 000
8.0 10.0 6.0 8.0 7.0 11.0 5.0 10.0
1 500 3 500 2 300 4 500 2 000 4 500 1 500 2 400
1.7
2 100 3 300
0.5 2.0 3.0
2 130 3 650 1 680 3 350
Lacustrine muddy shale
Shahejie-1 sub-member, Bohai Bay Basin Yanchang Formation, Ordos Basin Permian, Junggar Basin Jurassic, Sichuan Basin Cretaceous, Songliao Basin
Marine muddy shale
Shale oil zone
Source rock Favorable area/ Buried Litho- Thick- TOC/ Ro/% 104 km2 depth/m logy ness/m % 250 600
2.0 0.6 6.0 1.8
20110 1035 40240 50200
2.0 20.0 3.0 4.0 0.8 3.0 0.7 8.7
5-12
5.0 14.0
3.0 7.0 2.0 60-90 9.0 20-90
Lithology
Fine siltstone
0.7 Fine siltstone 1.2 0.6 Dolomitic siltstone, 1.5 sandy dolomite 0.9 Fine siltstone, 1.4 shell limestone 0.7 Argillaceous 2.0 siltstone, shale Dolomitic/ 0.5 argillaceous 1.3 siltstone 0.7 Muddy 1.3 limestone 0.7 Dolomite 1.0
378
Reservoir bed
Oil and gas reservoir
Permea- BrittleThick- Porobility/ ness ness/m sity/% 3 10 μm2 index/%
Oil Pressure density/ coeffi(g·cm3) cient
80 280
1.0 12.0
0.02 1.00
20 80 80 200 10 60 20 45
2.0 0.01 12.0 1.00 3.0 <1.00 10.0 0.2 1104 7.0 2.10 1.4 0.60 8.7 1.00
5 55
5.0 13.0
30 90 30 150
2.0 12.0 4.0 12.0
>80 >85 >80 >80 >70
0.10 1.00
>90
<0.01
5080
0.01 1.0
>80
0.73 0.94
1.10 1.56
0.80 0.86 0.87 0.92 0.76 0.87 0.78 0.87
0.75 0.85 1.10 1.80 1.23 1.72 1.20 1.58
0.81 0.83
1.35 1.58
0.82 0.87 0.71 0.81
1.35 1.80 1.00 1.20
ZHAO Xianzheng et al. / Petroleum Exploration and Development, 2018, 45(3): 377–388
Fig. 1.
Structure map of the Cangdong sag and the distribution map of conventional sandstone and fine-grained sediments of EK21.
underlying Kong 3 Member, and in pseudo-conformable contact with the Kong 3 Member; it is in unconformable contact with overlying Kong 1 Member, with apparent overlapping and truncating events on the margin of this basin. From bottom to top, the Kong 2 Member can be further subdivided into four fourth-order sequences (SQEk24, SQEk23, SQEk22 and SQEk21) and ten fifth-order sequences (SQ①-⑩)[22]. SQEk24 is a low-stand systems tract mainly composed of delta front deposits, including gray fine sandstone and gray mudstone. SQEk23–SQEk22 are lacustrine transgressive systems tracts, with the largest lacustrine flooding surface of the Kongdian Formation on the top. Core observation proves that it is dark
grey-black muddy shale with high TOC value, about 30 cm thick. During this period, a set of semi-deep lacustrine fine sediments were deposited, dominated by muddy shale and argillaceous dolomite, and developed fine siltstone of gravity flow on the top. SQEk21 is high-stand systems tract, and the lithology of lower Ek21SQ⑨ is dominated by dolomite interbedded with muddy shale. With the dropping reference plane, a set of shallow lacustrine mudstone was deposited on the upper Ek21SQ⑩, with a thin sand body of delta front at the top[22]. According to lithology assemblage, TOC and mineral composition variation features, Ek21SQ⑨ can be divided into five quasi-sequences.
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2.
Experiment conditions and data
To further understand the geological characteristics of Ek21SQ ⑨ shale system, 140 m cores were taken from SQEk21 of Well G108-8 and Well GD14. Detailed core description was performed then. The lithologies, source rock properties, physical properties, oiliness and brittleness of over 1 000 core samples were analyzed. The lab tests were carried out by two organizations, Basin Analysis and Oil & Gas Reservoir Geology Laboratory and National Key Laboratory of Heavy Oil in China University of Petroleum (East China). X'pert Pro MPD device was used to do X-ray diffraction (XRD) analysis. CuKα radiation ray was used. Other experimental conditions were: electric voltage (40 kV), electric current (40 mA), measurement range of 2θ (mineral diffraction angle) (5° to 60°), and 2θ sampling step width of 2θ (0.016°). QKY-II gas porometer and STY-II gas permeameter were used for physical property analysis, with accuracy of 0.5% and
Fig. 2.
0.01×103 μm2 respectively. The measuring pressures were 0.7 MPa and 1.0 MPa. Core saturation desiccator (type GLY-2) was used for oil saturation analysis, which has operating temperatures between normal and 600°C; dry distillation accuracy: ±3% for dry distillation water, ±5% for dry distillation oil. The analytical data is shown in Figs. 2 and 3.
3. 3.1.
Basic geologic characteristics Lithologic characteristics
The shale strata mainly consist of fine-grained sedimentary rocks with the particle size of less than 0.062 5 mm and particle content of more than 50%[2324]. Due to the extremely fine grain size of shale, it is difficult to accurately identify the mineral contents and determine their names by conventional thin slices. XRD can not only identify mineral types but quantitatively calculate their contents. Therefore, three main minerals, carbonate minerals (calcite and dolomite), felsic minerals
Composite diagram of well logging and experiment data of Well G108-8.
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Fig. 3.
Composite diagram of well logging and experiment data of Well GD14.
(quartz and feldspar) and clay minerals, obtained from XRD analysis were taken as end elements, and the rocks were named according to their specific contents[2526]. In this way, traditional shale, mudstone, carbonate rocks are divided into four basic types: fine-grained felsite sedimentary rock, carbonate rock (mainly dolomite in the study area), clay rock and calcareous and dolomitic shale. Combined with the content of the diagnostic mineral (analcite), the rocks are further subdivided into 24 subtypes[23]. Mud logging data of Well G108-8 shows that Ek21SQ⑨ is an interbedded aggregate structure mainly composed of dark gray mudstone and taupe oil shale, being a mixture of different minerals and structures[27] (Fig. 4). XRD analysis shows that the minerals that make up the shale mainly include quartz, feldspar, calcite, dolomite and clay minerals, and accessory minerals like pyrite, siderite, zeolite and gypsum etc. The average contents of minerals in Ek21SQ⑨ shale layer (Well G108-8) are: clay mineral (12%), quartz+feldspar (28%), calcite+dolomite (40%), analcite (18%) and pyrite+siderite (2%) (Fig. 2). Well GD14 located in Nanpi slope has higher sand content (quartz+feldspar). Gray-white fine-grained sand cuttings in patches or lamina are commonly seen in cores. The
average mineral contents are: clay minerals (7%), quartz+ feldspar (53%), calcite+dolomite (37%), analcite (2%) and pyrite+siderite (1%) (Fig. 3). Similar to the mineral composition of shale layers in different sedimentary environments at home and abroad, Ek21SQ⑨ has no obvious dominant mineral components as a whole. Its clay mineral content is generally less than 20%, which overthrows the traditional understanding that the clay mineral contents of shale layers are greater than 50%[28]. Although there are no dominant minerals in macroscope, the enrichment degree of minerals and the combination of rock types are different in different vertical quasi-sequences. Ek21SQ⑨-1 in Well G108-8 is interbedded dolomite and calcareous and dolomitic shale, in which the dolomite has a total thickness of 14 m and average single layer thickness of 0.6 m, accounting for 56% of the total. Ek21SQ⑨-2 is interbedded dolomite, calcareous and dolomitic shale and felsic shale, accounting for about 1/3 of each; Ek21SQ⑨-3 is interbedded calcareous and dolomitic shale and felsic shale: the former is 9 m thick, accounting for 69%. Ek21SQ⑨-4 is interbedded dolomite (6 m thick, accounting for 43% of the total) and calcareous and dolomitic shale (8 m thick, accounting for 57% of
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Fig. 4. Core and thin slice characteristics of Ek21 shale layer in Well G108-8, Cangdong sag. (a) Well G108-8, 2 973.11 m, felsic dolomite, developed lamination and laminar cracks, cross polarization; (b) Well G108-8, 2 973.41 m, calcareous and dolomitic shale, cross polarization; (c) Well G108-8, 2 973.62 m, felsic shale, cross polarization; (d) Well G108-8, 2 974.01 m, microcrystalline dolomite, yellow-green fluorescence with middle-high luminance, fluorescence thin slice; (e) Well G108-8, 2 974.27 m, calcareous and dolomitic shale, oleaginous bitumen in laminar cracks, yellow-white fluorescence, fluorescence thin slice; (f) Well G108-8, 2 974.81 m, calcareous and dolomitic shale, oleaginous bitumen in fine intergranular pores of zeolite, yellow-white fluorescence, fluorescent thin slice.
the total). Ek21SQ⑨-5 is not cored. According to the logging data, the lithology is dominated by calcareous and dolomitic shale (Fig. 2). From bottom to top, the content of felsic shale of Well GD14 gradually increases. Ek21SQ⑨-3 is mainly felsic shale, which is 9.5 m thick, accounting for 86% of the total (Fig. 3). The higher the contents of brittle minerals (including quartz, calcite, dolomite and analcite), the more likely complex fracture networks will be formed during fracturing, which is favorable for the follow-up development. The contents of brittle minerals in China’s lacustrine shale strata are relatively higher (more than 40%). In Well G108-8 and Well GD14 in the Cangdong sag, the brittle mineral contents of Ek21SQ⑨ shale layer reach 71.8% and 50.3% respectively. According to the analysis of rock mechanics, the dolomite has a Young’s modulus of 26.6 MPa and Poisson’s ratio of 0.20, indicating that the Ek21SQ⑨ shale layer has good crackability.
3.2.
Features of source rocks
The fine-grained sediments in the shale strata are good to excellent source rocks in general. Statistic results of 356 samples from the Ek21SQ⑨ interval show that they have total organic carbon (TOC) from 0.3% to 12.9% (averaging 3.6%), and hydrocarbon generation potential (S1+S2) from 0.2 to 73.0 mg/g (averaging 24.0 mg/g). Affected by lithology, felsic shale and calcareous and dolomitic shale have better quality, higher TOC values and S1+S2 (Fig. 5). Samples of good source rocks (with TOC>2%, and S1+S2>5 mg/g) account for 70%, while samples of non-source rock (with TOC<0.5%, S1+S2<0.5 mg/g) account for only 3%. The source rocks in different quasi-sequences have different quality. In Well G108-8, the source rocks of Ek21SQ⑨-2 and Ek21SQ⑨-3 are the best in quality, with average TOC of 4.6% and 4.1%
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Fig. 5. Bar charts of hydrocarbon generation potentials for different rock types in Ek21SQ⑨ shale layer in Wells G108-8 and GD14. (a) Well G108-8, 2 9002 990 m, 207 samples, Ro=0.76%; (b) Well GD14, 4 0754 140 m, 112 samples, Ro=1.03%.
respectively; the average TOC of Ek21SQ⑨-1 and Ek21SQ⑨-4 are 3.2% and 1.3% respectively. The dolomite content in Ek21SQ⑨-1 is relatively higher, but the TOC of the source rock in the interbedded layer is not worse (up to 12.9%). The closed environment with poor oxygen and reductive salt water during high-stand period is the key factor for forming high-quality source rock (V/(V+Ni)>0.6, and Sr/Ba>1), being favorable for the preservation and enrichment of organic matter in the lake basin. The Ek21 source rock is good in organic matter type, and the kerogen is mainly type I and type IIA, accounting for 69% and 13%, respectively. This source rock is moderate in maturity, with Ro values between 0.68% and 1.16%, and is right at the stage of massive oil generation. The result of thermal simulation shows that this set of high-quality source rock not only has a long period of hydrocarbon generation, but also a large amount of retained hydrocarbons. The hydrocarbon generation rate of the source rock during low mature stage with Ro less than 0.5% is about 150 mg/g, while the maximum hydrocarbon generation rate in the mature stage is 560 mg/g (averaging about 300 mg/g). The retained hydrocarbons in the source rock are 400 mg/g (averaging 200 mg/g). The ratio of retained hydrocarbons to expelled hydrocarbons is about 2:1 during the thermal evolution stage with Ro values between 0.6% and 1.1%. Calculation shows that the retained hydrocarbons in the Ek21 source rock is about 13.9108 t, suggesting high exploration potential of the shale oil (Fig. 6). 3.3.
Reservoir characteristics
The calcareous and dolomitic shale in the shale is overall tight. Analysis of 90 samples from Ek21 shows that they have porosity values between 1.0%9.0% in general (averaging 3.4%), permeability values between 0.0116.20)×103 μm2 (averaging 0.20×103 μm2). With rich intergranular pores, intercrystalline pores, micro fractures, and laminar cracks etc., the tight shale layers can be effective reservoir beds. The average effective porosities of dolomite, calcareous and dolomitic shale, and felsic shale are 5.8%, 3.3% and 3.1%,
Fig. 6. Hydrocarbon generation and expulsion mode of the source rock in Kong 2 Member.
respectively. The lamellar layer and interlaminar fractures are abundant in the fine-grained sedimentary rock. The mineral components that make up the laminae are mainly dolomite, analcite, felsic debris, and organic matter etc. Hereinto, the granular analcite is mainly distributed in calcareous and dolomitic shale and felsic shale, in layers or in lumps. The observation of thin slices under fluorescent light shows that there are hydrocarbon fillings in both (analcite) lamellae and interlaminar fractures (Fig. 4e and 4f). The average density of macroscopic interlayer fractures is 2 pieces/m, and it is 6 pieces/m in Ek21SQ⑨-1 of Well G108-8. Structural fractures with high-angle, fractures formed under anomalous pressure and differential compaction fractures etc. are also well-developed besides interlayer fractures. Structural fractures are more developed in Well GD14, with an average density of 2/m. Dolomite is an important reservoir rock for shale oil. The dolomite in the Ek21SQ⑨ shale layer is widely developed, being dominated by dolomicrite-micritic dolomite. The degree of crystallization increases gradually with strengthening burial
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diagenesis. Well GD14 is about 1 000 m deeper than Well G108-8, and the dolomite is mainly micritic. Statistics show that the high content of dolomite and high degree of crystallinity can lead to larger reservoir space; the better the fluorescence shows, the higher the oil content will be. 3.4.
Oiliness
The shale layer of the Kong 2 Member in the Cangdong sag is universally oil-bearing and strongly heterogeneous with several sections enriched in oil. In which, the whole Ek21SQ⑨ interval has higher oil content, with core samples showing higher brightness under fluorescence scanning (Fig. 4). Under the microscopic visual field, hydrocarbons are dominated by yellow-white, yellow-green, and blue-white oleaginous asphalt, with medium-strong fluorescent brightness, and oil saturation of 10.0%68.5% (averaging 50.0%). The oiliness of the shale strata is mainly influenced by the degree of thermal evolution. The deeper the burial depth, the higher the thermal maturity, the higher the free hydrocarbon content (S1) will be. As shown in Fig. 3, Well GD14 is 1000 m deeper than Well G108-8, so its Ro value is 0.27% higher than the later, and its average S1 content is 3.5 mg/g, which is 2 to 6 times that of the later. Compared with the S1 contents of other shale strata at home and abroad, the average S1 content of Well GD14 is above the average[28]. According to the relationship between Ro values and S1 values, considering that the development cost of shale gas may increase with bigger depth, we inferred that the interval between 3 3004 000 m (with Ro values between 0.85%1.10%) is the relatively favorable section for shale oil exploration in this area.
4.
Taking Ek21SQ ⑨ -1 and Ek21SQ ⑨ -2 as examples, the source rocks and reservoir beds are laminated like a layered cake, and the area of 30 m thick is 321 km2 ( not including the volcanic rock invaded area). Based on comprehensive analysis of planar distribution of thermal maturity in source rock, the favorable exploration area of shale oil with Ro greater than 0.5% is 260 km2, and the thicker regions are mainly concentrated in the lower parts of the Nanpi slope and the Kongxi slope. Affected by the faulting activities during later period, the buried depth in the lower part of the Nanpi slope is averagely 1000 m deeper than that in the Kongxi slope, and the thermal maturity in the lower part of the Nanpi slope is higher (average Ro=1.1%), so the Nanpi slope is currently the first option for efficient exploration and development (Fig. 7).
5. 5.1.
Shale oil exploration practices Discoveries by straight well drilling
Thirteen exploration wells have been deployed for shale oil exploration in the Kong 2 Member in recent years. Among them, 12 wells have daily oil production of over 5 tons, and 6 wells have daily oil production of over 10 tons. Wells GD6x1, G1608 and KN9 have daily oil production of over 20 t, and the oil production layers are all in Ek21SQ⑨ shale formation. The reservoir buried depth is 2 800 to 4 200 m, the oil density is 0.86 to 0.89 g/cm3 (20 C), and the pressure coefficient of
Evaluation of shale oil sweet spots
With overall high oil content and several high oil production intervals, the shale formation of the Kong 2 Member in the Cangdong sag has shale oil geological resources of over 2108 t according to preliminary estimation, indicating a great potential for profitable shale oil exploration and development in this area. The analysis of geological features of Ek21SQ⑨ shows that this formation has high organic matter abundance, good organic matter type, and moderate maturity, thus it is at the stage of massive oil generation. It is a set of high-quality source rock with large amount of retained hydrocarbons. It has more laminar dolomite, high dolomite crystallization degree, large reservoir space, and high oil content; moreover, it has high contents of brittle minerals (such as quartz, calcite and analcite), which is conducive to later fracturing. It has the geological conditions for forming good reservoir beds. During the depositional period of this formation, the tectonic activities were weak, so few faults were developed and the formation was stable in lateral distribution. The formation is thick in local parts because of the effects of provenance and paleogeomorphology etc. It is concluded by comprehensive evaluation that four quasi-sequences in Ek21SQ⑨ are all oil-rich, with thicknesses of 3793 m (averaging 70 m).
Fig. 7. Distribution of sweet spot thickness of Ek21SQ⑨-1 and Ek21SQ⑨-2 in Cangdong sag.
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the reservoir is 0.9 to 1.2. Well GD6x1 had an initial daily oil production of 32.6 t after fracturing with 3 mm choke. It was tested with 44 mm choke for 20 days, at the pump stroke of 5.2 m, the pumping rate of 2 times/min, the cumulative oil production was 115 t, and the average daily oil production was 5.8 t. Well G1608 had an initial daily oil production of 53.13 t after fracturing with 3 mm choke. It was tested for 105 days at the pump drainage pressure of 10 MPa, and produced 1540.7 t oil in total, with an average daily oil production of 14.7 t. Well KN9 had a daily oil production of 29.6 t after fracturing with 2 mm choke (Fig. 8). The data from the producing test shows that the shale formation of Ek21SQ⑨ has stable industrial oil productivity. Breakthroughs have been made in the Ek21, Ek22 and Ek23 shale formations. Well G1508 produced 14.0 t of oil a day from Ek22 with 2 mm choke, and Well Z1605 produced 14.9 t of oil a day from Ek23 after fracturing at the pumping drainage pressure of 25 MPa. 5.2. Breakthroughs in preliminary horizontal exploration wells 5.2.1. Characteristics of oil layers encountered by horizontal wells Horizontal well GD1701H was drilled in the G1608-G1508 well field in the lower part of the Nanpi slope targeting the Ek21SQ⑨-2. This well has a finished depth of 5 465.49 m, vertical depth of 3851.5 m, and deviation angle of 21.6° 91.6°. This well encountered 228 m of Ek21SQ⑨-1, 716 m of Ek21SQ⑨-2, 530 m of E k21SQ⑨-3, with the cumulative length of horizontal section of 1474 m. The gas logging in the horizontal section was abnormally active, with all gas hydrocarbons ranging from 1.74% to 18.98%; there are 1237.4 m/76 intervals of Type I oil layers and 175 m/2 intervals of Type II oil layers from comprehensive interpretation, and high-quality reservoir encountering rate is 96.25%. The well logging interpretation shows that the lithology is mainly composed of 714 m/56 layers of argillaceous dolomite, 150 m/32 layers of calcareous and dolomitic shale, and 512 m/47 layers of felsic shale. This formation has an average TOC of 3.0%, average S1 of 5 mg/g, average effective porosity of 7%, and average generalized brittleness index of 70%. 5.2.2. Perforation cluster optimization and volume fracturing in horizontal wells In order to improve the producing efficiency of shale oil in horizontal wells, locations of perforation clusters were selected under the guidance of “intensive cutting and volume stimulation”, based on source-reservoir combination model, oiliness and brittleness, the points with good source-reservoir combination, high resistivity, high gas logging value, high S1 content, and higher brittleness were selected as positions for perforation clusters. The total selected perforation interval length is 1 240 m, including 69 clusters in 21 intervals, at the average cluster spacing of 18 m (Fig. 9). Fracturing of this well has started with a total liquid volume of 43 300 m3, av-
erage meter volume of 35 m3, total sanding volume of 2 044.4 m3, and average sanding volume of 1.6 m3/m. 5.3.
Enlightenment from shale oil exploration
5.3.1. Comprehensive geological research is the important foundation for shale oil exploration breakthrough During the period of structural and lithologic reservoir exploration, 76 wells were drilled in the shale strata in the central lake basin. The mud logging data showed that they were all mudstone or oil shale. Although there were hydrocarbon shows in several wells, this shale formation had not received enough attention because of the traditional understanding that shale strata were source rock but not reservoirs, and less ideal oil testing results of the shale strata in some individual wells. A completely new understanding on the geological characteristics of shale strata has been obtained through systematic analysis of cores and experimental data of Well G108-8 and Well GD14: the shale strata not only have high-quality source rocks, but also effective reservoirs, which are combined or adjacent to each other, with active oil and gas shows. Several exploration wells based on this understanding have obtained industrial oil flows and even high production, which revealed the history of shale oil exploration in the Kong 2 Member of the Cangdong sag. 5.3.2. Volumetric fracturing is an effective means to increase single well production Mathematical model based on rock mechanics under XMAC well logging is established to optimize the fracturing operation parameters, according to the rock mechanics experimental data of Well G108-8. Multiple sand addition-hybrid hydraulic fracturing technology has been worked out: firstly, to pump low-viscosity linear gel fracturing fluid and fine ceramic beads for fracturing at high pumping rate to create fracture network, to stop pumping for 1-2 hours, and then add sand and pump again, to reduce the horizontal stress difference coefficient through the previous fracture interference on the ground stress and improve the formation of fracture network. The composite fracturing process has been applied 24 layer-times in the Kong 2 Member of the Cangdong sag, and industrial oil flow has been obtained in 17 layer-times, which accounts for 70.8% of the total layers. The average daily liquid production before fracturing was 1.45 m3, and the average daily production liquid after fracturing was 20.1 m3, that is 12.9 times of the daily liquid production before fracturing. The average daily oil production before fracturing was 0.4 t, and the average daily oil production after fracturing was 8.74 t, that is 20.9 times of the former oil production. The composite fracturing technology can increase oil production significantly. 5.3.3. Geology-engineering integration is an effective way for efficient producing
385
Geology-engineering integration is centered on the same
Fig. 8. EW-striking correlation profile of shale oil enrichment intervals in Ek21SQ⑨ shale formation (see Fig. 1 for the location. Interpretation of logging lithology can be seen in reference [22]).
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Fig. 9. Table 2.
Comparison of conventional fracturing and the secondary sanding + hybrid fluid fracturing.
Fracturing technology KN9
Fracturing fluid volume/m3
Proppant volume/m3
Cost/10000 Yuan
Slick Low Total liquid 0.110.21 mm 0.110.21 mm 0.210.38 0.270.55 mm Total sand FracturProppant Total water damage volume quartz sand fine pottery clay mm ceramsite ceramsite volume ing fluid 1 903
Conventional 922 Difference
Wellbore trajectory of horizontal well GD1701H.
233
2 136
1 214
2 136
+981 981
67.1 +67.1
24.5
14.5
106.1 106.1
7
65.4
33.7
7
40.9
19.2
exploration and development objective, with the idea of geology serving engineering, engineering relying on geology, geology guiding engineering, and engineering feeding back geology, to reach the goal of developing engineering technology and deepening geologic understanding[6]. Only by combining the two aspects closely, can the efficient exploration and development of shale oil be achieved. As shale oil exploration in China is still at an exploratory stage, current research priority is to innovate geological cognition and optimize operation parameters. Based on geological understanding, optimization of the process and fracturing system will both reduce the cost and increase efficiency. Well KN 9 is a typical exploratory well, treated with optimized process based on innovative understanding of shale strata successfully. It was perforated for oil testing in the interval from 3402 to 3424 m in 2012 (not fractured), at pump drainage pressure of 25 MPa, producing 5.42 t of oil a day (no water). It was fractured in September 2017, with “slickwater+low-damage fracturing fluid system”, with a total fluid volume of 2136 m3. In the treatment, the proportion of slickwater was increased from 43.2% for conventional fracturing to 89.1%. The proppant was quartz sand+ceramsite, with a total sand volume of 106.1 m3, and the proportion of proppant changed from 0% of powder ceramsite to 63.2% of quartz sand. The total saved cost of the fracturing was 505700 Yuan (56.5% down) compared with the conventional fracturing material costs of the same scale (Table 2). Fracturing simulation shows that the cracks are 179 m long and 40 m high,
23.30
15.65
38.95
58.29
31.23
89.52
34.99
15.58 50.57
forming a more complex fracture network. The initial daily oil production of the well after fracturing by 2 mm choke was 29.6 tons, which was nearly 5 times of that before fracturing.
6.
Conclusions
The shale layer of the Kong 2 Member in the Cangdong sag of the Bohai Bay Basin, China, is a complex lamellar aggregate of felsic shales, calcareous and dolomitic shale, and carbonate rocks. With rich laminae, clay mineral content of less than 20%, and the content of brittle minerals (such as quartz, calcite, dolomite, and analcite) more than 70%, it is likely to form complex fracture network after fracturing. The shale layer is overall good-very good source rock, with mainly Type I and Type IIA kerogen, an average TOC of 3.6%, average S1+S2 of 24 mg/g, Ro value of 0.68% to 1.16%, and average hydrocarbon generation intensity of 300 mg/g. The reservoir matrix has a porosity of less than 10% and permeability of less than 1×103 μm2 in general. But with abundant intergranular pores, intercrystalline pores, micro fractures, and laminar cracks, the dense shale strata can be effective reservoir beds for shale oil. The reservoir beds are commonly filled with oil, with high content of free hydrocarbon S1 and an average oil saturation of 50%. Exploration practices show that under the pertinent fracturing measures, this shale layer can yield stable industrial productivity and considerable economic value, breaking the preliminary understanding that this shale layer was mainly source rock, not reservoir, without movable oil.
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Nomenclature GAS—gas logging total hydrocarbon, %; GR—gamma-ray, API; Ro—vitrinite reflectance, %; Rt—resistivity, ·m; S1—free hydrocarbon content, mg/g; S2—pyrolytic hydrocarbon content, mg/g; SP—self-potential, mV; TOC—total organic content, %; Δt—interval travel time, s/m.
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