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Lower limit of thermal maturity for the carbonization of organic matter in marine shale and its exploration risk
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): 402–411.
RESEARCH PAPER
Lower limit of thermal maturity for the carbonization of organic matter in marine shale and its exploration risk WANG Yuman1, *, LI Xinjing1, CHEN Bo2, WU Wei3, DONG Dazhong1, ZHANG Jian3, HAN Jing2, MA Jie2, DAI Bing2, WANG Hao2, JIANG Shan1 1. PetroChina Research Institute of Petroleum Exploration and Development, Beijing 100083, China; 2. Yangtze University, Wuhan 430100, China; 3. Exploration and Development Research Institute, Southwest Oil & GasField Company, PetroChina, Chengdu 610051, China
Abstract: Based on the drilling data of the Silurian Longmaxi Formation in the Sichuan Basin and periphery, SW China, the Ro lower limits and essential features of the carbonization of organic matter in over-high maturity marine shale were examined using laser Raman, electrical and physical property characterization techniques. Three preliminary conclusions are drawn: (1) The lower limit of Ro for the carbonization of Type I-II1 organic matter in marine shale is 3.5%; when the Ro is less than 3.4%, carbonization of organic matter won’t happen in general; when the Ro ranges from 3.4% to 3.5%, non-carbonization and weak carbonization of organic matter may coexist; when the Ro is higher than 3.5%, the carbonization of organic matter is highly likely to take place. (2) Organic-rich shale entering carbonization phase have three basic characteristics: log resistivity curve showing a general “slender neck” with low-ultralow resistance response, Raman spectra showing a higher graphite peak, and poor physical property (with matrix porosity of only less than 1/2 of the normal level). (3) The quality damage of shale reservoir caused by the carbonization of organic matter is almost fatal, which primarily manifests in depletion of hydrocarbon generation capacity, reduction or disappearance of organic pores and intercrystalline pores of clay minerals, and drop of adsorption capacity to natural gas. Therefore, the lower limit of Ro for the carbonization of Type I-II1 organic matter should be regarded as the theoretically impassable red line of shale gas exploration in the ancient marine shale formations. The organic-rich shale with low-ultralow resistance should be evaluated effectively in area selection to exclude the high risk areas caused by the carbonization of organic matter. The target organic-rich shale layers with low-ultralow resistance drilled during exploration and development should be evaluated on carbonization level of organic matter, and the deployment plan should be adjusted according to the evaluation results in time. Key words: Lower Silurian; Longmaxi Formation; marine shale; thermal maturity; organic matter carbonization; resistivity logging; exploration risk
1.
Background
Carbonization of organic matter is a geological phenomenon that solid organic matter (OM) in overmature shales partially or fully convert to graphite or quasi-graphite matter along with the degradation or composition of such organic matter[14]. The exploration and studies of marine shale gas in south China in recent years have demonstrated that two sets of high-quality shales (Type I–II1 kerogen) in the Yangtze region, i.e., the Lower Cambrian Qiongzhusi Formation and the Lower Silurian Longmaxi Formation (including Upper Ordovician Wufeng Formation) are major shale gas plays. However, their exploration trends are different[1]. The Qiongzhusi Formation, commonly in the context of anthracite to meta anthra-
cite[2,5], reveals extensive carbonization of organic matter (with ultra-low resistivity below 2 Ω·m in organic-rich shale intervals)[1,4,67], and only contains shale gas in local areas such as the Central Sichuan Uplift and northern Guizhou, suggesting unsatisfactory exploration prospect. On the contrary, some high yield prospects, such as Weiyuan, Changning–Zhaotong, Fuling and Fushun–Yongchuan, were discovered in succession in the Longmaxi Formation with relatively low thermal maturity and with proved reserves over 5441×108 m3. Shale gas may accumulate in the whole prospects and local abundance may be high[1, 4, 812]. Recently, a shale interval with low to ultra-low resistivity (Figs. 1 and 2) was penetrated in the Lower Longmaxi sweet spot (in this paper, refers to the or-
Received date: 06 Dec. 2017; Revised date: 17 Apr. 2018. * Corresponding author. E-mail: [email protected] Foundation item: Supported by the CAS Strategic Pilot Project (XDA14010101); National Science and Technology Major Project (2017ZX05035001), and PetroChina Exploration & Production Shale Gas Resource Evaluation and Strategic Selection Project (kt2017-10-02). 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/).
WANG Yuman et al. / Petroleum Exploration and Development, 2018, 45(3): 402–411
Fig. 1. “Sweet spots” and key exploratory wells of the Longmaxi Formation in the Sichuan Basin and the surrounding regions (modified from references [1213]).
ganic-rich shale interval with TOC contents above 3% and brittleness indexes determined by using quartz + dolomite + pyrite over 50%, same below) in Wuxi, western Hubei, Renhuai and Shizhu; this shale interval has electrical properties and gas contents similar to those in the carbonized Qiongzhusi Formation. Such findings reignited interests of researchers for OM carbonization in highly to post mature marine shale[56]. OM carbonization is one of the geologic risks for shale gas exploration in south China; this challenging problem is also important for evaluation of highly to post mature marine shale source rocks and reservoir rocks[1, 4, 68]. There are four issues to be considered: (1) how to calculate Ro (which indicates OM maturity) for highly to post mature marine shale accurately if it is hard to calculate credible reflectance using conventional methods, such as rhabdosome, bituminite and vitrinite; (2) the threshold of Ro at which carbonization of OM of type I-II1 starts; (3) basic features of OM carbonization in marine shale; and (4) how to use the threshold of Ro and basic features of OM carbonization in shale gas exploration. This paper focuses on the Longmaxi Formation in the Sichuan Basin and surrounding regions and deals with laser Raman spectra, resistivity and petrophysics of black shale with different thermal ma-
turities. The objective is to define the threshold of Ro and basic features of OM carbonization in highly to post mature marine shale and make suggestions for marine shale gas exploration.
2. 2.1.
Previous researches Findings
The study of OM carbonization in marine shale is still in its preliminary stages with limited knowledge coming from the data of several exploratory wells targeting the Qiongzhusi Formation in southern Sichuan[1, 4, 7]. OM carbonization in Qiongzhusi shale in southern Sichuan exhibits three features[1, 4, 68]: high-amplitude graphite peak (G' peak) on the Raman spectrum and higher D peak (on irregular band) than G peak (on regular band), log resistivity below 2 Ω·m at organic-rich shale intervals, and resistivity of dry rock samples below 100 Ω·m. These features may indicate high electrical conductivity of OM. The porosity is 1/3 to 1/2 of normal value. There was no gas yield or slight gas yield in production test. The above conclusions, which were drawn in the study of Changning–Zhaotong Qiongzhusi shale with Ro above 4.0%, were used to highlight OM carbonization in highly mature
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Fig. 2. Wufeng–Longmaxi stratigraphic column in Wuxi (GR—Gamma ray; Rt—resistivity; TOC—total organic matter; Ro—organic matter maturity; CNL—neutron porosity).
marine shale. On the other hand, it is yet to know if the conclusions can be used more extensively and if they may apply to the Longmaxi Formation with relatively low Ro and OM carbonization. Consequently, for shale gas exploration in the Lower Silurian formations may involve risks of OM carbonization. In the Wuxi prospect, more than 8 appraisal wells were drilled in succession in the last five years due to the existence of thick Longmaxi “sweet spots”[8, 14]. But in fact, the Wufeng– Longmaxi Formations were deposited in deep water; “sweet spots” which mainly occur in the Katian–Aeronian Stages are composed of siliceous shale (Fig. 2) over 40 m thick[8, 14] with TOC contents of 3.06.5%, siliceous contents above 60% on average, clay content below 30%, resistivity of 37 Ω·m (an order higher than that of the Qiongzhusi shale in Changning[4, 6]) (Fig. 2) and Ro of 3.483.51%. There is a low-magnitude graphite peak at G' on the Raman spectrum (Fig. 3). Total porosity is 2.408.78% (with the average of 3.85%), which is much higher than that of the Qiongzhusi shale in Changning[7] and lower than that in the Changning, Weiyuan and Fuling gas
fields[8, 1115]. Gas contents are 1.383.00 m3/t. A slight amount of gas was produced in post-frac test. The failure was generally attributed to poor structural and preservation conditions in the Dabashan foreland thrust belt[10, 14]. But it was caused by OM carbonization with relatively low Ro in the Longmaxi pay formation (which is different from the scenario in the Qiongzhusi Formation in Changning) in view of low resistivity and low-magnitude graphite peak on the Raman spectrum according to the concerned research. Due to low degrees of exploration and poor knowledge, it is challenging to fully understand the basic features and Ro threshold of OM carbonization in the Longmaxi and Qiongzhusi shales at present stage[1, 4, 7]. 2.2.
Evaluation methods
Solid OM carbonization in shale may be characterized using optical, electrical and petrophysical methods. Detailed descriptions are shown in Table 1. Laser Raman spectrum and resistivity logging are two key techniques deployed. To obtain
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Fig. 3. Table 1.
OM laser Raman spectra of the Longmaxi Formation in the Sichuan Basin and its surrounding regions.
Methods to characterize solid OM carbonization in shale.
No. Method 1
2
3
Description Use Raman spectrum and transmission electron microscope Optical to optically image solid OM and identify graphite and quasigraphite in accordance with specific spectral signatures. In view of good electrical conductivity of graphitized solid OM, use logging and dry-sample lab test to obtain shale Elecresistivity and judge the degree of OM carbonization in trical accordance with resistivity curve shape and magnitude. In view of significant decrease in OM-hosted porosity after Petro- graphitization, use porosity lab test and dual-porosity model physical to obtain matrix porosity and OM-hosted porosity of shale and indirectly diagnose OM carbonization in shale.
an accurate result, it is suggested to combine multiple methods for cross validation. OM carbonization is a major geologic risk, which may negatively affect reservoir properties and gas contents in highly to post mature marine shale[1, 4]. How to define the basic features and Ro threshold for OM (type I-II1) carbonization is very important for shale gas exploration in age-old marine formations and shale gas evaluation and deployment in the deep Sichuan Basin and surrounding regions with highly to post mature shale.
3. Basic features and Ro threshold for OM carbonization in marine shale In the concerned study, laser Raman spectrum, resistivity
Merit Demerit References Directly detect graphite and Greatly vary with sample; [13, quasi-graphite and calculate large error for the detec16,17] Ro with high credibility. tion of weak graphitization. Sensitive to a varied degree Hard to estimate of OM carbonization in shale; [4,6, an accurate Ro. the result may be directly 1820] used for evaluation. Apply to the evaluation of the impact of OM carbonization on shale reservoir properties.
Greatly vary with the method of porosity test; hard to estimate Ro.
[7,12, 2125]
logging and petrophysical methods were deployed to define the Ro threshold for OM carbonization in marine shale. The objective is to reveal the features of Raman spectrum, resistivity, and petrophysical property at the Ro threshold for OM carbonization in marine Longmaxi shale. 3.1.
Raman spectrum
Raman spectrum may be used to estimate Ro of shale and thermal maturity of source rocks; its G' (graphite) peak is also a direct indicator for carbonization of solid OM[13, 1617]. The form and shift of Raman peaks (including D, G, and G' peaks) are dependent on the relation between atom and molecule motion in aromatic ring and thermal maturity[13]. In other words, D-G peak separation and peak height ratio increase
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with thermal maturity; G' (graphite) peak occurs in anthracite and increases with the degree of graphitization[13]. Liu et al.[2] presented a method to calculate thermal maturity of shale using Raman spectrum of solid OM. The technique involves D-G peak separation before the appearance of G' peak and uses D-G peak height ratio after the appearance of G' peak to calculate Ro. This method is widely applied to geologic studies[12]. Raman spectra were used to estimate the Ro value of the Silurian and Cambrian shale samples acquired in the Sichuan Basin and its surrounding prospects. During the course, OM carbonization has been detected in the Longmaxi Formation in some prospects (Figs. 2 and 3): In the Block W202 in Weiyuan, Ro values are 2.62.9%; D-G peak separation and peak height ratio are 258.83 cm1 and 0.62, respectively; G' peak does not form yet (Fig. 3a). This indicates low thermal maturity of the Longmaxi Formation with no OM carbonization. In the Block N203 in the Changning–Zhaotong area, Ro value of 3.423.47%, D-G peak separation of 273.29 cm1, peak height ratio of 0.69, and a platform shaped G' peak (Fig. 3b) indicate much higher thermal maturity of the Longmaxi Formation in this prospect compared with those in Weiyuan. There is no evidence of graphitization, but the maturity is close to the bound of OM carbonization. In Block WX2 in Wuxi, Ro values of 3.48 3.51%, D-G peak separation of 272.70 cm1, peak height ratio of 0.63, and the appearance of low-magnitude G' peak (Fig. 3c) show the evidence of graphitization and OM carbonization in anthracite in the Longmaxi Formation with higher maturity than those in the Changning–Zhaotong area. In the Maoba area of Lichuan, Ro values of 3.563.73%, D-G peak height ratio of 0.85 (which is much higher than that in Weiyuan, Changning–Zhaoting, and Wuxi), and medium-magnitude G' peak (Fig. 3d) indicate evident graphitization and OM carbonization in meta anthracite in the Longxima Formation in Lichuan. In view of above Raman spectral and Ro data, Lichuan shows the highest thermal maturity, followed by Wuxi and Changning–Zhaotong. Weiyuan exhibits the lowest thermal maturity. Consequently, the Ro threshold for OM carbonization is expected to be 3.5%. 3.2.
Resistivity
Log-based resistivity responses are important for evaluation of OM carbonization in highly to post mature shale[1,4,6,8,18]. Drilling data and Raman-derived Ro data acquired in Weiyuan, Changning–Zhaotong, Fuling, Wuxi and Lichuan were used to establish log-based resistivity responses of Lower Silurian and Lower Cambrian shale in the Sichuan Basin and surrounding regions (Figs. 4-6); the objective is to determine the Ro threshold for OM carbonization in highly to post mature marine shale. For the Longmaxi shale, forms and magnitudes of resistivity curves varied significantly with Ro values (Fig. 4). In Weiyuan and Fuling, Ro values of 2.6%2.9% and 3.3%, re-
Fig. 4. Resistivity responses of Longmaxi shale in the Sichuan Basin and surrounding regions.
spectively indicated no carbonization and no electrical conductivity; Rt curve exhibits low magnitude at the organic-poor shale interval (Upper Longmaxi) and bell shape with middle to high magnitudes at the organic-rich shale interval (Lower Longmaxi). For Well W202, lower and higher Rt values are observed in the Upper and Lower Longmaxi, respectively[8]. The Rt value is 10 Ω·m at the greyish green grapholith interval in the middle and upper Telychian Stage (with TOC content below 1%) and increases to 2050 Ω·m at the Rhuddanian– Aeronian Stages (with TOC contents of 28%) (Fig. 4a). The Rt values slowly increased with TOC contents (Fig. 4b). Well JY1 drilled in Fuling exhibited similar features (Fig. 4). In the Block N211 in the Changning–Zhaotong area, the Ro value ranges 3.423.47%; flat Rt curve changes slightly at the organic-poor interval in the Upper Longmaxi and decreases at the organic-rich interval in the Lower Longmaxi (Fig. 4). The
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Rt value ranges 830 Ω·m (with the average of 17 Ω·m) at the Aeronian Stage (with TOC content of 0.51.5%) and slightly decreases to 530 Ω·m (with the average of 11 Ω·m) at the Wufeng Formation–Rhuddanian Stage (with TOC content of 1.37.6%). In the Block X202 in Wuxi, the Ro value increases to 3.5%. The Rt curve increases (to 200800 Ω·m) at the interval with middle to low GR value and decreases (to 37 Ω·m) at the interval with high GR value; neck-shaped Rt difference reaches two orders (Figs. 2 and 4a). In addition, the negative correlation between Rt and TOC content also indicates OM carbonization and electrical conductivity at the interval with high GR value. In the Block LY1 Lichuan, Ro increases to 3.6%; the Rt value further increases at the upper interval and decreases at the lower interval (Fig. 4a); Rt decreases faster with TOC contents (Fig. 4b). The Rt value ranges 65100 Ω·m at the upper Aeronian Stage (with TOC contents below 0.5%) and 0.1 20.0 Ω·m from the Wufeng Formation to the lower Aeronian Stage (with TOC contents of 1.06.0%). It generally ranges between 0.1 and 0.9 Ω·m (Fig. 4b). The upper-lower Rt differences of 2-3 orders indicated high electrical conductivity and severe OM carbonization at the high-GR interval. In summary, the Rt values decreased in the organic-rich intervals and increased in the organic-poor intervals with Ro enhanced from 2.6% to over 3.6% in the Longmaxi Formation. The negative correlation between Rt and TOC contents became more distinct (Fig. 4b). The Rt curve changes from bell shape to flat pattern and neck shape. When the Ro is above 3.5%, the Rt curve exhibits a neck shape (Fig. 4a). The Lower Cambrian Series has similar Rt variations (Fig. 5). In the Block W201 in Weiyuan and Qiongzhusi, TOC contents varied at 0.33.6% with Ro value of 3.0%. Due to tight shale property (porosity below 2%), the upper-lower Rt difference is small and Rt values generally varied at 2001 000 Ω·m; this indicates no OM carbonization and no electrical conductivity. In the Block GS17 in Gaoshiti, the Rt curve at the interval of 350m thick (Fig. 5a) in the middle and upper Qiongzhusi Formation (with TOC contents of 0.24.3%, no Raman spectrum to obtain Ro) exhibits small variations, just like the scenarios in the Block W202 and the Block JY1, and generally ranges 1030 Ω·m. There is no evidence of OM carbonization. In the interval of 150 m thick from the lower Qiongzhusi Formation to the upper part of the Lower Cambrian Maidiping Formation (with TOC contents of 1.45.0% and Raman-derived Ro of 3.52%), the Rt curve exhibits a neck shape with large difference in upper-lower magnitudes (Fig. 5) and generally ranges 190 Ω·m (and may reach 12 Ω·m locally). There is a negative correlation between Rt and TOC contents (Fig. 5b) that indicated OM carbonization and electrical conductivity. It is inferred from electrical properties that the depth at 350 m to the top of the Qiongzhusi Formation in Well GS17 may be the limit of OM carbonization in Lower Cambrian source rocks in this prospect. The Qiongzhusi Formation in Changning–Zhaotong has been proved to be highly
Fig. 5. Resistivity responses of Lower Cambrian shale in the Sichuan Basin and surrounding regions.
carbonized[1,4] with Ro up to 4.09% and large differences in upper-lower Rt values (Fig. 5). In view of the above electrical properties, the upper-lower Rt difference is small before OM carbonization and becomes large after OM carbonization. Thermal maturity at the turning point accords with the Ro value of the Longmaxi Formation in Wuxi and Qiongzhusi Formation in the Block GS17 in Gaoshiti; thus, this may be the threshold of Ro for OM (of type I-II1) carbonization. To highlight the Ro threshold accurately, average Rt and Ro at the organic-rich shale intervals in 14 major exploratory wells have been correlated (10 targeting the Longmaxi Formation and 4 targeting the Qiongzhusi Formation) (Fig. 6). Both trend lines show three sections: (1) section with medium to high Rt at Ro below 3.4%, where there may be no OM carbonization; (2) section with significant Rt decreases and variations of 650 Ω·m at Ro of 3.43.5%,
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Fig. 6. Average Rt-Ro correlation of organic-rich shale in the Lower Paleozoic Erathem in the Sichuan Basin and surrounding regions.
where there may be no OM carbonization or weak carbonization; and (3) section with extremely low Rt below 2 Ω·m at Ro above 3.5%, where there may be serious OM carbonization. This means OM carbonization in marine shale is liable to occur at Ro exceeding 3.5%; thus, Ro of 3.5% may be considered as the threshold of thermal maturity for carbonization of OM (of type I-II1). These conclusions coincided well with the results of Raman-based evaluation. 3.3.
Petrophysics
In the process of OM carbonization, the matrix porosity of shale may decrease to the 1/3-1/2 of the normal level and there may be no gas yield or slight gas yield in production test[1, 4, 7]. Under such circumstances, properties of marine shale can be deployed to confirm the Ro threshold for OM (of type I-II1) carbonization. Average porosity and Ro have been correlated (Fig. 7) by using the data from the wells shown in Fig. 6 and wells in the Woodford gas field[2627]. Most data are core data and only the porosity (of 3.8%) at Ro of 2.9% is derived through log data interpretation. It skewed from the trend line but still maintained in the normal range of 3.86.0%[12, 15, 21]. Fig. 7 also shows three sections on the trend line: (1) section with porosity of 3.86.0% at Ro below 3.4%, where there is no OM carbonization; (2) section with great porosity decrease and variations
Fig. 7. Average porosity-Ro correlation of marine organic-rich shale.
of 2.55.0% at Ro of 3.43.5%, where there may be weak OM carbonization; and (3) section with porosity below 2.5% at Ro higher than 3.5%, where the porosity is only 1/4-1/2 of the normal level and black shale becomes tight. OM carbonization may deteriorate reservoir properties and thus cause large upper-lower Rt difference at the Longmaxi and Qiongzhusi shale intervals (Figs. 2, 4a, and 5a) showing middle to high resistivity at the tight organic-poor shale intervals and low to extremely low resistivity at the carbonized organic-rich intervals. The Rt difference may exceed 2 orders. In conclusion, large upper-lower Rt difference is a direct evidence denoting OM carbonization and consequent petrophysical deterioration. To get clear on why OM carbonization may negatively affect shale reservoir properties, the dual-porosity model[12, 15] was used to estimate the pore volume per unit mass and porosity of three components (i.e. brittle minerals, clay minerals and OM) in the Longmaxi samples acquired from Well X202 in Wuxi and Well LY1 in Lichuan. Data acquired from additional prospects or formations are shown in Table 2. The dual-porosity model developed in the last few years may be used to quantitatively estimate shale matrix porosity and fracture porosity[12, 15]. The first step is to calculate and calibrate the pore volume per unit mass of brittle minerals, clay minerals, and OM (denoted as VBri, VClay and VTOC) using credible data points. The second step is to calculate matrix porosity (including internal porosity of brittle minerals, OM porosity, and intercrystalline porosity of clay minerals) in accordance with calibrated VBri, VClay and VTOC as well as mineral composition and TOC data at the zone of interest. Total porosity from core test (helium penetration or mercury penetration) should be deployed to calculate fracture porosity. Refer to references [12, 15, 21] for more details related to the algorithm and parameters. The results (Table 2) show that for the carbonized intervals in the Longmaxi Formation in Wuxi and Lichuan, OM and clay minerals (which provide most pore spaces) make a small contribution to the total porosity; the matrix porosity of organic-rich shale is small; some key indicators are much smaller than those in Woodford, Changning and Fuling, but equivalent to those of Qiongzhusi shale in the Changning– Zhaotong area (Table 2). For example, the VTOC value of the Longmaxi Formation in Wuxi is 0.088 m3/t, only 63% of that in the Changning field, 50% of that in the Fuling field, and slightly larger than that of the Qiongzhusi Formation in the Changning–Zhaotong area. The VClay value is 0.020 m3/t, only 51% of that in the Changning field. The VBri value is 0.005 m3/t, which is equivalent to that in the Fuling field and generally distributes within the normal range of 0.000 40.007 9 m3/t. Matrix porosity is 2.30%3.20% (with the average of 2.90%, far below the normal level of 3.8%6.0%), in which OM porosity is 0.12%1.43% (with the average of 0.98%, less than the normal level of 1.28%1.68%). At the same time, intercrystalline porosity of clay mineral is 0.41%1.96% (with the average of 0.96%, much lower than the normal
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Table 2.
Reservoir properties of highly to post mature marine organic-rich shale (modified from references [12, 15, 21]).
Total porosity/% Ro/ Prospect/Block Lithofacies % Internal porosity Intercrystalline poroOM of brittle mineral sity of clay mineral porosity Wuxi, Well X202, Siliceous 3.48 0.661.23 0.411.96 0.121.43 Longmaxi Fm. shale 3.51 (0.96) (0.96) (0.98) Lichuan, Well LY1, Siliceous 3.56 0.260.42 0.571.80 0.351.28 Longmaxi Fm. shale 3.73 (0.32) (1.37) (0.66) Siliceous 2.00 0.070.08 1.992.88 0.902.36 Woodford shale 2.20 (0.07) (2.52) (1.68) Fuling, Well JY4, Siliceous 0.631.23 1.233.63 0.581.95 3.30 Longmaxi Fm. shale (0.93) (2.36) (1.29) Changning–Zhaotong Hybrid cal3.42 0.741.78 0.785.83 0.711.90 careous-siliWell CX1, 3.47 (1.30) (2.81) (1.28) ceous shale Longmaxi Fm. Changning–Zhaotong, Siliceous 0.030.05 0.811.56 0.410.66 4.09 Qiongzhusi Fm. shale (0.04) (1.07) (0.55)
Pore volume per unit mass/(m3·t1) Fracture porosity 05.60 (0.95) 02.50 (0.41) 04.46 (1.55) 03.28 (1.26)
Sum 2.408.78 (3.85) 1.904.77 (2.76) 4.207.50 (5.83) 4.577.80 (5.83)
VBri
VClay VTOC
References
0.005 0 0.020 0.088 0.002 0 0.012 0.082 0.000 4 0.035 0.120 [2627] 0.006 1 0.025 0.170 [8,15]
01.16 3.428.35 0.007 9 0.039 0.140 [8,15] (0.12) (5.51) 1.432.01 0.000 2 0.022 0.069 [1,7] (1.66)
Note: The mean values are in brackets.
level of 2.36%2.81%), internal porosity of brittle mineral is 0.66%1.23% (with the average of 0.96%, within the normal range of 0.07%1.30%). This means the destructive effect of OM carbonization resulting in that OM porosity and intercrystalline porosity of clay mineral may decline significantly or even vanish. Accordingly, matrix porosity may decrease to below half the normal level. The Fuling and Woodford shales have the largest fracture porosity and Longmaxi shale in Changning has the smallest fracture porosity. The Longmaxi shales in Wuxi and Lichuan fall into between (Table 2). The existence of fractures is mainly dependent on structure. In addition, matrix porosity accounts for the most of total porosity in many prospects in Yangtze. Thus, fracture porosity is often neglected. In view of optical, electrical, and petrophysical results, it can be concluded that the Ro threshold for OM (of type I-II1) carbonization in highly to post mature marine shale may be 3.5%. Organic-rich shale with OM carbonization may exhibit three basic features: (1) Low to ultra-low resistivity, which is usually below 8 Ω·m and may at least be 2 orders lower than that at the organic-poor interval. There are negative correlations among Rt and TOC contents. Rt values may fall below 2 Ω·m in cases with serious OM carbonization. (2) Abnormal Raman spectrum, in which a graphite peak may occur at G' peak and D-G peak height ratio is generally greater than 0.63. D peak may be higher than G peak in cases with serious OM carbonization. (3) Poor petrophysical properties and gas contents. Matrix porosity may be half the normal value or even lower. There is no gas yield or slight gas yield.
4.
Exploration risks and suggestions
Exploration activities targeting carbonized Longmaxi shale (with low to ultra-low resistivity, as shown in Fig. 1 and Table 3) in Wuxi, western Hubei and Renhuai in the last few years show that there was no gas yield or slight gas yield no matter how favorable the structural and preservation conditions are.
Recent exploration and studies show that OM carbonization may have the following negative effects on marine shale gas accumulation. (1) Exhausted capacity to generate hydrocarbon. Natural gas in highly to post mature marine shale mainly is originated from kerogen degradation and secondary pyrolysis of dispersed liquid hydrocarbon with Ro of 1.353.2%. The Ro threshold for gas generation is 3.5%[1, 5], which coincided well with the Ro threshold of solid OM (of type I-II1) carbonization. This means carbonized solid OM may not generate natural gas any longer and dispersed liquid hydrocarbon has been entirely cracked and disappeared; thus, residual hydrocarbon in organic-rich shale may not be supplemented and preserved. (2) Dramatic decrease in OM porosity and intercrystalline porosity of clay mineral. During OM carbonization, OM pores may shrink and be filled with interstitial materials; consequently, macropores reduce greatly or even vanish[1, 4, 7]. Due to increased crystallinity of clay minerals in shale, intercrystalline pore spaces also reduced greatly[4, 7]. For example, in the Longmaxi Formation discussed above, OM porosity and intercrystalline porosity of clay minerals decreased to 0.98% and 0.96%, respectively on average (Table 2), whereas average matrix porosity is less than 3%; these values are far less than those in the Changning and Fuling gas fields. (3) Decrease in gas adsorptive capacity. This is related to the decrease in OM pores and intercrystalline pores of clay minerals and subsequent specific surface area of black shale. For example, the specific surface area of Qiongzhusi shale in Changning is generally 1.68.3 cm2/g, far below the 9.522.1 cm2/g in the Longmaxi shale in Changning; thus, nitrogen and methane adsorptive capacities of Qiongzhusi shale are 1/3-1/2 and 80% of those of Longmaxi shale, respectively[4]. As OM carbonization may damage the source and reservoir properties of organic-rich shale, the Ro threshold of 3.5% for solid OM (of type I-II1) carbonization may be theoretically taken as an alert for age-old marine shale gas exploration.
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Table 3.
Geologic parameters of Longmaxi organic-rich shale drilled in some typical dry wells in the Sichuan Basin and surrounding
regions. Well Buried Prospect TOC/% number depth/m 2 790 LY1 Lichuan 1.16.0 2 830 2 142 HY1 Enshi 1.55.3 2 166 X202
Wuxi
1 965 1 989
0.56.4
TY1
Fengdu >3 900 2.05.0
RY1
Renhuai
4 030 4 055
1.96.5
Ro/% 3.56 3.73
3.48 3.51
Porosity/ GR/API Rt/(Ω·m) % 1.90 150270 0.10.9 4.77(2.76) 0.010.30 150270 (0.2) 2.40 145300 8.78(3.85)
37
150350
26(4)
0.50 180250 1.88.0 2.30(0.74)
Gas content/ (m3·t1)
OM carbonization
0.13-0.48
Severe
Slight gas yield 1.383.00 m3/t, slight gas yield in production test Slight gas yield, pressure coefficient below 1 0.51
Severe Weak
Preservation Well preserved in offbasin synclinal zone Well preserved in offbasin synclinal zone Moderately preserved in off-basin fold belt with the self-sealed Longmaxi Fm.
References
[10]
Weak
Well preserved inside basin
[28]
Weak
Well preserved inside basin
[910, 28]
Note: The mean values are in brackets.
Here the following suggestions are made for marine shale gas exploration in South China. (1) Geologic evaluation of prospect areas may focus on Lower Cambrian and Lower Silurian organic-rich shale. Source properties, especially in the prospect with low resistivity responses, may be evaluated by measuring laser Raman spectrum, petrophysical properties and resistivity of dry samples, which may then be combined with burial history and geothermal history to diagnose the origin, degree, and extent of OM carbonization. Areas with high carbonization risks may be excluded from shale gas evaluation and exploration. (2) If the intervals with low resistivity are drilled, it is suggested conducting OM carbonization assessment for a timely adjustment of deployment. The Lower Cambrian and Lower Silurian shale in confirmed prospect areas is mostly within the gas window (Ro of 2.53.5%)[1, 5, 8, 10], but locally the Ro value of 3.33.52% (for example, in the Longmaxi Formation in Changning, Fuling, and Wuxi) is close to the threshold (Fig. 1). If the intervals drilled with low resistivity universally exhibit above three basic features of OM carbonization, it is necessary to suspend exploratory drilling and adjust deployment to minimize risks related to OM carbonization and investment regardless of Ro value and gas yield. If above three basic features of OM carbonization are not detected, which usually corresponds to flat Rt curve of 520 Ω·m and average porosity above 4%, exploratory drilling may continue; meanwhile, single-well geologic evaluation and economic evaluation should be conducted to gain an insight into the cause of low resistivity and exploration potential.
5.
three basic features: (1) neck-shaped resistivity log curve, which is negatively correlated with TOC content, with the magnitude below 8 Ω·m and at least 2 orders lower than that at the organic-poor interval; (2) graphite peak at G' on Raman spectrum; and (3) matrix porosity smaller than half the normal value and no gas yield or slight gas yield. OM carbonization may damage the source and reservoir properties of shale by inducing significant decline of hydrocarbon generation capacity, OM porosity, intercrystalline porosity of clay minerals and gas adsorptive capacity. The Ro threshold for solid OM (of type I-II1) carbonization may be theoretically taken as an alert for age-old marine shale gas exploration. In the process of geologic evaluation of prospect areas, efforts should be devoted to source properties of low-resistivity shale to exclude the zones with OM carbonization risks. In shale gas exploration and production, efforts should focus on the evaluation of OM carbonization for low-resistivity intervals drilled to adjust deployment quickly.
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411
RESEARCH PAPER
Lower limit of thermal maturity for the carbonization of organic matter in marine shale and its exploration risk WANG Yuman1, *, LI Xinjing1, CHEN Bo2, WU Wei3, DONG Dazhong1, ZHANG Jian3, HAN Jing2, MA Jie2, DAI Bing2, WANG Hao2, JIANG Shan1 1. PetroChina Research Institute of Petroleum Exploration and Development, Beijing 100083, China; 2. Yangtze University, Wuhan 430100, China; 3. Exploration and Development Research Institute, Southwest Oil & GasField Company, PetroChina, Chengdu 610051, China
Abstract: Based on the drilling data of the Silurian Longmaxi Formation in the Sichuan Basin and periphery, SW China, the Ro lower limits and essential features of the carbonization of organic matter in over-high maturity marine shale were examined using laser Raman, electrical and physical property characterization techniques. Three preliminary conclusions are drawn: (1) The lower limit of Ro for the carbonization of Type I-II1 organic matter in marine shale is 3.5%; when the Ro is less than 3.4%, carbonization of organic matter won’t happen in general; when the Ro ranges from 3.4% to 3.5%, non-carbonization and weak carbonization of organic matter may coexist; when the Ro is higher than 3.5%, the carbonization of organic matter is highly likely to take place. (2) Organic-rich shale entering carbonization phase have three basic characteristics: log resistivity curve showing a general “slender neck” with low-ultralow resistance response, Raman spectra showing a higher graphite peak, and poor physical property (with matrix porosity of only less than 1/2 of the normal level). (3) The quality damage of shale reservoir caused by the carbonization of organic matter is almost fatal, which primarily manifests in depletion of hydrocarbon generation capacity, reduction or disappearance of organic pores and intercrystalline pores of clay minerals, and drop of adsorption capacity to natural gas. Therefore, the lower limit of Ro for the carbonization of Type I-II1 organic matter should be regarded as the theoretically impassable red line of shale gas exploration in the ancient marine shale formations. The organic-rich shale with low-ultralow resistance should be evaluated effectively in area selection to exclude the high risk areas caused by the carbonization of organic matter. The target organic-rich shale layers with low-ultralow resistance drilled during exploration and development should be evaluated on carbonization level of organic matter, and the deployment plan should be adjusted according to the evaluation results in time. Key words: Lower Silurian; Longmaxi Formation; marine shale; thermal maturity; organic matter carbonization; resistivity logging; exploration risk
1.
Background
Carbonization of organic matter is a geological phenomenon that solid organic matter (OM) in overmature shales partially or fully convert to graphite or quasi-graphite matter along with the degradation or composition of such organic matter[14]. The exploration and studies of marine shale gas in south China in recent years have demonstrated that two sets of high-quality shales (Type I–II1 kerogen) in the Yangtze region, i.e., the Lower Cambrian Qiongzhusi Formation and the Lower Silurian Longmaxi Formation (including Upper Ordovician Wufeng Formation) are major shale gas plays. However, their exploration trends are different[1]. The Qiongzhusi Formation, commonly in the context of anthracite to meta anthra-
cite[2,5], reveals extensive carbonization of organic matter (with ultra-low resistivity below 2 Ω·m in organic-rich shale intervals)[1,4,67], and only contains shale gas in local areas such as the Central Sichuan Uplift and northern Guizhou, suggesting unsatisfactory exploration prospect. On the contrary, some high yield prospects, such as Weiyuan, Changning–Zhaotong, Fuling and Fushun–Yongchuan, were discovered in succession in the Longmaxi Formation with relatively low thermal maturity and with proved reserves over 5441×108 m3. Shale gas may accumulate in the whole prospects and local abundance may be high[1, 4, 812]. Recently, a shale interval with low to ultra-low resistivity (Figs. 1 and 2) was penetrated in the Lower Longmaxi sweet spot (in this paper, refers to the or-
Received date: 06 Dec. 2017; Revised date: 17 Apr. 2018. * Corresponding author. E-mail: [email protected] Foundation item: Supported by the CAS Strategic Pilot Project (XDA14010101); National Science and Technology Major Project (2017ZX05035001), and PetroChina Exploration & Production Shale Gas Resource Evaluation and Strategic Selection Project (kt2017-10-02). 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/).
WANG Yuman et al. / Petroleum Exploration and Development, 2018, 45(3): 402–411
Fig. 1. “Sweet spots” and key exploratory wells of the Longmaxi Formation in the Sichuan Basin and the surrounding regions (modified from references [1213]).
ganic-rich shale interval with TOC contents above 3% and brittleness indexes determined by using quartz + dolomite + pyrite over 50%, same below) in Wuxi, western Hubei, Renhuai and Shizhu; this shale interval has electrical properties and gas contents similar to those in the carbonized Qiongzhusi Formation. Such findings reignited interests of researchers for OM carbonization in highly to post mature marine shale[56]. OM carbonization is one of the geologic risks for shale gas exploration in south China; this challenging problem is also important for evaluation of highly to post mature marine shale source rocks and reservoir rocks[1, 4, 68]. There are four issues to be considered: (1) how to calculate Ro (which indicates OM maturity) for highly to post mature marine shale accurately if it is hard to calculate credible reflectance using conventional methods, such as rhabdosome, bituminite and vitrinite; (2) the threshold of Ro at which carbonization of OM of type I-II1 starts; (3) basic features of OM carbonization in marine shale; and (4) how to use the threshold of Ro and basic features of OM carbonization in shale gas exploration. This paper focuses on the Longmaxi Formation in the Sichuan Basin and surrounding regions and deals with laser Raman spectra, resistivity and petrophysics of black shale with different thermal ma-
turities. The objective is to define the threshold of Ro and basic features of OM carbonization in highly to post mature marine shale and make suggestions for marine shale gas exploration.
2. 2.1.
Previous researches Findings
The study of OM carbonization in marine shale is still in its preliminary stages with limited knowledge coming from the data of several exploratory wells targeting the Qiongzhusi Formation in southern Sichuan[1, 4, 7]. OM carbonization in Qiongzhusi shale in southern Sichuan exhibits three features[1, 4, 68]: high-amplitude graphite peak (G' peak) on the Raman spectrum and higher D peak (on irregular band) than G peak (on regular band), log resistivity below 2 Ω·m at organic-rich shale intervals, and resistivity of dry rock samples below 100 Ω·m. These features may indicate high electrical conductivity of OM. The porosity is 1/3 to 1/2 of normal value. There was no gas yield or slight gas yield in production test. The above conclusions, which were drawn in the study of Changning–Zhaotong Qiongzhusi shale with Ro above 4.0%, were used to highlight OM carbonization in highly mature
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Fig. 2. Wufeng–Longmaxi stratigraphic column in Wuxi (GR—Gamma ray; Rt—resistivity; TOC—total organic matter; Ro—organic matter maturity; CNL—neutron porosity).
marine shale. On the other hand, it is yet to know if the conclusions can be used more extensively and if they may apply to the Longmaxi Formation with relatively low Ro and OM carbonization. Consequently, for shale gas exploration in the Lower Silurian formations may involve risks of OM carbonization. In the Wuxi prospect, more than 8 appraisal wells were drilled in succession in the last five years due to the existence of thick Longmaxi “sweet spots”[8, 14]. But in fact, the Wufeng– Longmaxi Formations were deposited in deep water; “sweet spots” which mainly occur in the Katian–Aeronian Stages are composed of siliceous shale (Fig. 2) over 40 m thick[8, 14] with TOC contents of 3.06.5%, siliceous contents above 60% on average, clay content below 30%, resistivity of 37 Ω·m (an order higher than that of the Qiongzhusi shale in Changning[4, 6]) (Fig. 2) and Ro of 3.483.51%. There is a low-magnitude graphite peak at G' on the Raman spectrum (Fig. 3). Total porosity is 2.408.78% (with the average of 3.85%), which is much higher than that of the Qiongzhusi shale in Changning[7] and lower than that in the Changning, Weiyuan and Fuling gas
fields[8, 1115]. Gas contents are 1.383.00 m3/t. A slight amount of gas was produced in post-frac test. The failure was generally attributed to poor structural and preservation conditions in the Dabashan foreland thrust belt[10, 14]. But it was caused by OM carbonization with relatively low Ro in the Longmaxi pay formation (which is different from the scenario in the Qiongzhusi Formation in Changning) in view of low resistivity and low-magnitude graphite peak on the Raman spectrum according to the concerned research. Due to low degrees of exploration and poor knowledge, it is challenging to fully understand the basic features and Ro threshold of OM carbonization in the Longmaxi and Qiongzhusi shales at present stage[1, 4, 7]. 2.2.
Evaluation methods
Solid OM carbonization in shale may be characterized using optical, electrical and petrophysical methods. Detailed descriptions are shown in Table 1. Laser Raman spectrum and resistivity logging are two key techniques deployed. To obtain
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Fig. 3. Table 1.
OM laser Raman spectra of the Longmaxi Formation in the Sichuan Basin and its surrounding regions.
Methods to characterize solid OM carbonization in shale.
No. Method 1
2
3
Description Use Raman spectrum and transmission electron microscope Optical to optically image solid OM and identify graphite and quasigraphite in accordance with specific spectral signatures. In view of good electrical conductivity of graphitized solid OM, use logging and dry-sample lab test to obtain shale Elecresistivity and judge the degree of OM carbonization in trical accordance with resistivity curve shape and magnitude. In view of significant decrease in OM-hosted porosity after Petro- graphitization, use porosity lab test and dual-porosity model physical to obtain matrix porosity and OM-hosted porosity of shale and indirectly diagnose OM carbonization in shale.
an accurate result, it is suggested to combine multiple methods for cross validation. OM carbonization is a major geologic risk, which may negatively affect reservoir properties and gas contents in highly to post mature marine shale[1, 4]. How to define the basic features and Ro threshold for OM (type I-II1) carbonization is very important for shale gas exploration in age-old marine formations and shale gas evaluation and deployment in the deep Sichuan Basin and surrounding regions with highly to post mature shale.
3. Basic features and Ro threshold for OM carbonization in marine shale In the concerned study, laser Raman spectrum, resistivity
Merit Demerit References Directly detect graphite and Greatly vary with sample; [13, quasi-graphite and calculate large error for the detec16,17] Ro with high credibility. tion of weak graphitization. Sensitive to a varied degree Hard to estimate of OM carbonization in shale; [4,6, an accurate Ro. the result may be directly 1820] used for evaluation. Apply to the evaluation of the impact of OM carbonization on shale reservoir properties.
Greatly vary with the method of porosity test; hard to estimate Ro.
[7,12, 2125]
logging and petrophysical methods were deployed to define the Ro threshold for OM carbonization in marine shale. The objective is to reveal the features of Raman spectrum, resistivity, and petrophysical property at the Ro threshold for OM carbonization in marine Longmaxi shale. 3.1.
Raman spectrum
Raman spectrum may be used to estimate Ro of shale and thermal maturity of source rocks; its G' (graphite) peak is also a direct indicator for carbonization of solid OM[13, 1617]. The form and shift of Raman peaks (including D, G, and G' peaks) are dependent on the relation between atom and molecule motion in aromatic ring and thermal maturity[13]. In other words, D-G peak separation and peak height ratio increase
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with thermal maturity; G' (graphite) peak occurs in anthracite and increases with the degree of graphitization[13]. Liu et al.[2] presented a method to calculate thermal maturity of shale using Raman spectrum of solid OM. The technique involves D-G peak separation before the appearance of G' peak and uses D-G peak height ratio after the appearance of G' peak to calculate Ro. This method is widely applied to geologic studies[12]. Raman spectra were used to estimate the Ro value of the Silurian and Cambrian shale samples acquired in the Sichuan Basin and its surrounding prospects. During the course, OM carbonization has been detected in the Longmaxi Formation in some prospects (Figs. 2 and 3): In the Block W202 in Weiyuan, Ro values are 2.62.9%; D-G peak separation and peak height ratio are 258.83 cm1 and 0.62, respectively; G' peak does not form yet (Fig. 3a). This indicates low thermal maturity of the Longmaxi Formation with no OM carbonization. In the Block N203 in the Changning–Zhaotong area, Ro value of 3.423.47%, D-G peak separation of 273.29 cm1, peak height ratio of 0.69, and a platform shaped G' peak (Fig. 3b) indicate much higher thermal maturity of the Longmaxi Formation in this prospect compared with those in Weiyuan. There is no evidence of graphitization, but the maturity is close to the bound of OM carbonization. In Block WX2 in Wuxi, Ro values of 3.48 3.51%, D-G peak separation of 272.70 cm1, peak height ratio of 0.63, and the appearance of low-magnitude G' peak (Fig. 3c) show the evidence of graphitization and OM carbonization in anthracite in the Longmaxi Formation with higher maturity than those in the Changning–Zhaotong area. In the Maoba area of Lichuan, Ro values of 3.563.73%, D-G peak height ratio of 0.85 (which is much higher than that in Weiyuan, Changning–Zhaoting, and Wuxi), and medium-magnitude G' peak (Fig. 3d) indicate evident graphitization and OM carbonization in meta anthracite in the Longxima Formation in Lichuan. In view of above Raman spectral and Ro data, Lichuan shows the highest thermal maturity, followed by Wuxi and Changning–Zhaotong. Weiyuan exhibits the lowest thermal maturity. Consequently, the Ro threshold for OM carbonization is expected to be 3.5%. 3.2.
Resistivity
Log-based resistivity responses are important for evaluation of OM carbonization in highly to post mature shale[1,4,6,8,18]. Drilling data and Raman-derived Ro data acquired in Weiyuan, Changning–Zhaotong, Fuling, Wuxi and Lichuan were used to establish log-based resistivity responses of Lower Silurian and Lower Cambrian shale in the Sichuan Basin and surrounding regions (Figs. 4-6); the objective is to determine the Ro threshold for OM carbonization in highly to post mature marine shale. For the Longmaxi shale, forms and magnitudes of resistivity curves varied significantly with Ro values (Fig. 4). In Weiyuan and Fuling, Ro values of 2.6%2.9% and 3.3%, re-
Fig. 4. Resistivity responses of Longmaxi shale in the Sichuan Basin and surrounding regions.
spectively indicated no carbonization and no electrical conductivity; Rt curve exhibits low magnitude at the organic-poor shale interval (Upper Longmaxi) and bell shape with middle to high magnitudes at the organic-rich shale interval (Lower Longmaxi). For Well W202, lower and higher Rt values are observed in the Upper and Lower Longmaxi, respectively[8]. The Rt value is 10 Ω·m at the greyish green grapholith interval in the middle and upper Telychian Stage (with TOC content below 1%) and increases to 2050 Ω·m at the Rhuddanian– Aeronian Stages (with TOC contents of 28%) (Fig. 4a). The Rt values slowly increased with TOC contents (Fig. 4b). Well JY1 drilled in Fuling exhibited similar features (Fig. 4). In the Block N211 in the Changning–Zhaotong area, the Ro value ranges 3.423.47%; flat Rt curve changes slightly at the organic-poor interval in the Upper Longmaxi and decreases at the organic-rich interval in the Lower Longmaxi (Fig. 4). The
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Rt value ranges 830 Ω·m (with the average of 17 Ω·m) at the Aeronian Stage (with TOC content of 0.51.5%) and slightly decreases to 530 Ω·m (with the average of 11 Ω·m) at the Wufeng Formation–Rhuddanian Stage (with TOC content of 1.37.6%). In the Block X202 in Wuxi, the Ro value increases to 3.5%. The Rt curve increases (to 200800 Ω·m) at the interval with middle to low GR value and decreases (to 37 Ω·m) at the interval with high GR value; neck-shaped Rt difference reaches two orders (Figs. 2 and 4a). In addition, the negative correlation between Rt and TOC content also indicates OM carbonization and electrical conductivity at the interval with high GR value. In the Block LY1 Lichuan, Ro increases to 3.6%; the Rt value further increases at the upper interval and decreases at the lower interval (Fig. 4a); Rt decreases faster with TOC contents (Fig. 4b). The Rt value ranges 65100 Ω·m at the upper Aeronian Stage (with TOC contents below 0.5%) and 0.1 20.0 Ω·m from the Wufeng Formation to the lower Aeronian Stage (with TOC contents of 1.06.0%). It generally ranges between 0.1 and 0.9 Ω·m (Fig. 4b). The upper-lower Rt differences of 2-3 orders indicated high electrical conductivity and severe OM carbonization at the high-GR interval. In summary, the Rt values decreased in the organic-rich intervals and increased in the organic-poor intervals with Ro enhanced from 2.6% to over 3.6% in the Longmaxi Formation. The negative correlation between Rt and TOC contents became more distinct (Fig. 4b). The Rt curve changes from bell shape to flat pattern and neck shape. When the Ro is above 3.5%, the Rt curve exhibits a neck shape (Fig. 4a). The Lower Cambrian Series has similar Rt variations (Fig. 5). In the Block W201 in Weiyuan and Qiongzhusi, TOC contents varied at 0.33.6% with Ro value of 3.0%. Due to tight shale property (porosity below 2%), the upper-lower Rt difference is small and Rt values generally varied at 2001 000 Ω·m; this indicates no OM carbonization and no electrical conductivity. In the Block GS17 in Gaoshiti, the Rt curve at the interval of 350m thick (Fig. 5a) in the middle and upper Qiongzhusi Formation (with TOC contents of 0.24.3%, no Raman spectrum to obtain Ro) exhibits small variations, just like the scenarios in the Block W202 and the Block JY1, and generally ranges 1030 Ω·m. There is no evidence of OM carbonization. In the interval of 150 m thick from the lower Qiongzhusi Formation to the upper part of the Lower Cambrian Maidiping Formation (with TOC contents of 1.45.0% and Raman-derived Ro of 3.52%), the Rt curve exhibits a neck shape with large difference in upper-lower magnitudes (Fig. 5) and generally ranges 190 Ω·m (and may reach 12 Ω·m locally). There is a negative correlation between Rt and TOC contents (Fig. 5b) that indicated OM carbonization and electrical conductivity. It is inferred from electrical properties that the depth at 350 m to the top of the Qiongzhusi Formation in Well GS17 may be the limit of OM carbonization in Lower Cambrian source rocks in this prospect. The Qiongzhusi Formation in Changning–Zhaotong has been proved to be highly
Fig. 5. Resistivity responses of Lower Cambrian shale in the Sichuan Basin and surrounding regions.
carbonized[1,4] with Ro up to 4.09% and large differences in upper-lower Rt values (Fig. 5). In view of the above electrical properties, the upper-lower Rt difference is small before OM carbonization and becomes large after OM carbonization. Thermal maturity at the turning point accords with the Ro value of the Longmaxi Formation in Wuxi and Qiongzhusi Formation in the Block GS17 in Gaoshiti; thus, this may be the threshold of Ro for OM (of type I-II1) carbonization. To highlight the Ro threshold accurately, average Rt and Ro at the organic-rich shale intervals in 14 major exploratory wells have been correlated (10 targeting the Longmaxi Formation and 4 targeting the Qiongzhusi Formation) (Fig. 6). Both trend lines show three sections: (1) section with medium to high Rt at Ro below 3.4%, where there may be no OM carbonization; (2) section with significant Rt decreases and variations of 650 Ω·m at Ro of 3.43.5%,
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Fig. 6. Average Rt-Ro correlation of organic-rich shale in the Lower Paleozoic Erathem in the Sichuan Basin and surrounding regions.
where there may be no OM carbonization or weak carbonization; and (3) section with extremely low Rt below 2 Ω·m at Ro above 3.5%, where there may be serious OM carbonization. This means OM carbonization in marine shale is liable to occur at Ro exceeding 3.5%; thus, Ro of 3.5% may be considered as the threshold of thermal maturity for carbonization of OM (of type I-II1). These conclusions coincided well with the results of Raman-based evaluation. 3.3.
Petrophysics
In the process of OM carbonization, the matrix porosity of shale may decrease to the 1/3-1/2 of the normal level and there may be no gas yield or slight gas yield in production test[1, 4, 7]. Under such circumstances, properties of marine shale can be deployed to confirm the Ro threshold for OM (of type I-II1) carbonization. Average porosity and Ro have been correlated (Fig. 7) by using the data from the wells shown in Fig. 6 and wells in the Woodford gas field[2627]. Most data are core data and only the porosity (of 3.8%) at Ro of 2.9% is derived through log data interpretation. It skewed from the trend line but still maintained in the normal range of 3.86.0%[12, 15, 21]. Fig. 7 also shows three sections on the trend line: (1) section with porosity of 3.86.0% at Ro below 3.4%, where there is no OM carbonization; (2) section with great porosity decrease and variations
Fig. 7. Average porosity-Ro correlation of marine organic-rich shale.
of 2.55.0% at Ro of 3.43.5%, where there may be weak OM carbonization; and (3) section with porosity below 2.5% at Ro higher than 3.5%, where the porosity is only 1/4-1/2 of the normal level and black shale becomes tight. OM carbonization may deteriorate reservoir properties and thus cause large upper-lower Rt difference at the Longmaxi and Qiongzhusi shale intervals (Figs. 2, 4a, and 5a) showing middle to high resistivity at the tight organic-poor shale intervals and low to extremely low resistivity at the carbonized organic-rich intervals. The Rt difference may exceed 2 orders. In conclusion, large upper-lower Rt difference is a direct evidence denoting OM carbonization and consequent petrophysical deterioration. To get clear on why OM carbonization may negatively affect shale reservoir properties, the dual-porosity model[12, 15] was used to estimate the pore volume per unit mass and porosity of three components (i.e. brittle minerals, clay minerals and OM) in the Longmaxi samples acquired from Well X202 in Wuxi and Well LY1 in Lichuan. Data acquired from additional prospects or formations are shown in Table 2. The dual-porosity model developed in the last few years may be used to quantitatively estimate shale matrix porosity and fracture porosity[12, 15]. The first step is to calculate and calibrate the pore volume per unit mass of brittle minerals, clay minerals, and OM (denoted as VBri, VClay and VTOC) using credible data points. The second step is to calculate matrix porosity (including internal porosity of brittle minerals, OM porosity, and intercrystalline porosity of clay minerals) in accordance with calibrated VBri, VClay and VTOC as well as mineral composition and TOC data at the zone of interest. Total porosity from core test (helium penetration or mercury penetration) should be deployed to calculate fracture porosity. Refer to references [12, 15, 21] for more details related to the algorithm and parameters. The results (Table 2) show that for the carbonized intervals in the Longmaxi Formation in Wuxi and Lichuan, OM and clay minerals (which provide most pore spaces) make a small contribution to the total porosity; the matrix porosity of organic-rich shale is small; some key indicators are much smaller than those in Woodford, Changning and Fuling, but equivalent to those of Qiongzhusi shale in the Changning– Zhaotong area (Table 2). For example, the VTOC value of the Longmaxi Formation in Wuxi is 0.088 m3/t, only 63% of that in the Changning field, 50% of that in the Fuling field, and slightly larger than that of the Qiongzhusi Formation in the Changning–Zhaotong area. The VClay value is 0.020 m3/t, only 51% of that in the Changning field. The VBri value is 0.005 m3/t, which is equivalent to that in the Fuling field and generally distributes within the normal range of 0.000 40.007 9 m3/t. Matrix porosity is 2.30%3.20% (with the average of 2.90%, far below the normal level of 3.8%6.0%), in which OM porosity is 0.12%1.43% (with the average of 0.98%, less than the normal level of 1.28%1.68%). At the same time, intercrystalline porosity of clay mineral is 0.41%1.96% (with the average of 0.96%, much lower than the normal
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Table 2.
Reservoir properties of highly to post mature marine organic-rich shale (modified from references [12, 15, 21]).
Total porosity/% Ro/ Prospect/Block Lithofacies % Internal porosity Intercrystalline poroOM of brittle mineral sity of clay mineral porosity Wuxi, Well X202, Siliceous 3.48 0.661.23 0.411.96 0.121.43 Longmaxi Fm. shale 3.51 (0.96) (0.96) (0.98) Lichuan, Well LY1, Siliceous 3.56 0.260.42 0.571.80 0.351.28 Longmaxi Fm. shale 3.73 (0.32) (1.37) (0.66) Siliceous 2.00 0.070.08 1.992.88 0.902.36 Woodford shale 2.20 (0.07) (2.52) (1.68) Fuling, Well JY4, Siliceous 0.631.23 1.233.63 0.581.95 3.30 Longmaxi Fm. shale (0.93) (2.36) (1.29) Changning–Zhaotong Hybrid cal3.42 0.741.78 0.785.83 0.711.90 careous-siliWell CX1, 3.47 (1.30) (2.81) (1.28) ceous shale Longmaxi Fm. Changning–Zhaotong, Siliceous 0.030.05 0.811.56 0.410.66 4.09 Qiongzhusi Fm. shale (0.04) (1.07) (0.55)
Pore volume per unit mass/(m3·t1) Fracture porosity 05.60 (0.95) 02.50 (0.41) 04.46 (1.55) 03.28 (1.26)
Sum 2.408.78 (3.85) 1.904.77 (2.76) 4.207.50 (5.83) 4.577.80 (5.83)
VBri
VClay VTOC
References
0.005 0 0.020 0.088 0.002 0 0.012 0.082 0.000 4 0.035 0.120 [2627] 0.006 1 0.025 0.170 [8,15]
01.16 3.428.35 0.007 9 0.039 0.140 [8,15] (0.12) (5.51) 1.432.01 0.000 2 0.022 0.069 [1,7] (1.66)
Note: The mean values are in brackets.
level of 2.36%2.81%), internal porosity of brittle mineral is 0.66%1.23% (with the average of 0.96%, within the normal range of 0.07%1.30%). This means the destructive effect of OM carbonization resulting in that OM porosity and intercrystalline porosity of clay mineral may decline significantly or even vanish. Accordingly, matrix porosity may decrease to below half the normal level. The Fuling and Woodford shales have the largest fracture porosity and Longmaxi shale in Changning has the smallest fracture porosity. The Longmaxi shales in Wuxi and Lichuan fall into between (Table 2). The existence of fractures is mainly dependent on structure. In addition, matrix porosity accounts for the most of total porosity in many prospects in Yangtze. Thus, fracture porosity is often neglected. In view of optical, electrical, and petrophysical results, it can be concluded that the Ro threshold for OM (of type I-II1) carbonization in highly to post mature marine shale may be 3.5%. Organic-rich shale with OM carbonization may exhibit three basic features: (1) Low to ultra-low resistivity, which is usually below 8 Ω·m and may at least be 2 orders lower than that at the organic-poor interval. There are negative correlations among Rt and TOC contents. Rt values may fall below 2 Ω·m in cases with serious OM carbonization. (2) Abnormal Raman spectrum, in which a graphite peak may occur at G' peak and D-G peak height ratio is generally greater than 0.63. D peak may be higher than G peak in cases with serious OM carbonization. (3) Poor petrophysical properties and gas contents. Matrix porosity may be half the normal value or even lower. There is no gas yield or slight gas yield.
4.
Exploration risks and suggestions
Exploration activities targeting carbonized Longmaxi shale (with low to ultra-low resistivity, as shown in Fig. 1 and Table 3) in Wuxi, western Hubei and Renhuai in the last few years show that there was no gas yield or slight gas yield no matter how favorable the structural and preservation conditions are.
Recent exploration and studies show that OM carbonization may have the following negative effects on marine shale gas accumulation. (1) Exhausted capacity to generate hydrocarbon. Natural gas in highly to post mature marine shale mainly is originated from kerogen degradation and secondary pyrolysis of dispersed liquid hydrocarbon with Ro of 1.353.2%. The Ro threshold for gas generation is 3.5%[1, 5], which coincided well with the Ro threshold of solid OM (of type I-II1) carbonization. This means carbonized solid OM may not generate natural gas any longer and dispersed liquid hydrocarbon has been entirely cracked and disappeared; thus, residual hydrocarbon in organic-rich shale may not be supplemented and preserved. (2) Dramatic decrease in OM porosity and intercrystalline porosity of clay mineral. During OM carbonization, OM pores may shrink and be filled with interstitial materials; consequently, macropores reduce greatly or even vanish[1, 4, 7]. Due to increased crystallinity of clay minerals in shale, intercrystalline pore spaces also reduced greatly[4, 7]. For example, in the Longmaxi Formation discussed above, OM porosity and intercrystalline porosity of clay minerals decreased to 0.98% and 0.96%, respectively on average (Table 2), whereas average matrix porosity is less than 3%; these values are far less than those in the Changning and Fuling gas fields. (3) Decrease in gas adsorptive capacity. This is related to the decrease in OM pores and intercrystalline pores of clay minerals and subsequent specific surface area of black shale. For example, the specific surface area of Qiongzhusi shale in Changning is generally 1.68.3 cm2/g, far below the 9.522.1 cm2/g in the Longmaxi shale in Changning; thus, nitrogen and methane adsorptive capacities of Qiongzhusi shale are 1/3-1/2 and 80% of those of Longmaxi shale, respectively[4]. As OM carbonization may damage the source and reservoir properties of organic-rich shale, the Ro threshold of 3.5% for solid OM (of type I-II1) carbonization may be theoretically taken as an alert for age-old marine shale gas exploration.
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Table 3.
Geologic parameters of Longmaxi organic-rich shale drilled in some typical dry wells in the Sichuan Basin and surrounding
regions. Well Buried Prospect TOC/% number depth/m 2 790 LY1 Lichuan 1.16.0 2 830 2 142 HY1 Enshi 1.55.3 2 166 X202
Wuxi
1 965 1 989
0.56.4
TY1
Fengdu >3 900 2.05.0
RY1
Renhuai
4 030 4 055
1.96.5
Ro/% 3.56 3.73
3.48 3.51
Porosity/ GR/API Rt/(Ω·m) % 1.90 150270 0.10.9 4.77(2.76) 0.010.30 150270 (0.2) 2.40 145300 8.78(3.85)
37
150350
26(4)
0.50 180250 1.88.0 2.30(0.74)
Gas content/ (m3·t1)
OM carbonization
0.13-0.48
Severe
Slight gas yield 1.383.00 m3/t, slight gas yield in production test Slight gas yield, pressure coefficient below 1 0.51
Severe Weak
Preservation Well preserved in offbasin synclinal zone Well preserved in offbasin synclinal zone Moderately preserved in off-basin fold belt with the self-sealed Longmaxi Fm.
References
[10]
Weak
Well preserved inside basin
[28]
Weak
Well preserved inside basin
[910, 28]
Note: The mean values are in brackets.
Here the following suggestions are made for marine shale gas exploration in South China. (1) Geologic evaluation of prospect areas may focus on Lower Cambrian and Lower Silurian organic-rich shale. Source properties, especially in the prospect with low resistivity responses, may be evaluated by measuring laser Raman spectrum, petrophysical properties and resistivity of dry samples, which may then be combined with burial history and geothermal history to diagnose the origin, degree, and extent of OM carbonization. Areas with high carbonization risks may be excluded from shale gas evaluation and exploration. (2) If the intervals with low resistivity are drilled, it is suggested conducting OM carbonization assessment for a timely adjustment of deployment. The Lower Cambrian and Lower Silurian shale in confirmed prospect areas is mostly within the gas window (Ro of 2.53.5%)[1, 5, 8, 10], but locally the Ro value of 3.33.52% (for example, in the Longmaxi Formation in Changning, Fuling, and Wuxi) is close to the threshold (Fig. 1). If the intervals drilled with low resistivity universally exhibit above three basic features of OM carbonization, it is necessary to suspend exploratory drilling and adjust deployment to minimize risks related to OM carbonization and investment regardless of Ro value and gas yield. If above three basic features of OM carbonization are not detected, which usually corresponds to flat Rt curve of 520 Ω·m and average porosity above 4%, exploratory drilling may continue; meanwhile, single-well geologic evaluation and economic evaluation should be conducted to gain an insight into the cause of low resistivity and exploration potential.
5.
three basic features: (1) neck-shaped resistivity log curve, which is negatively correlated with TOC content, with the magnitude below 8 Ω·m and at least 2 orders lower than that at the organic-poor interval; (2) graphite peak at G' on Raman spectrum; and (3) matrix porosity smaller than half the normal value and no gas yield or slight gas yield. OM carbonization may damage the source and reservoir properties of shale by inducing significant decline of hydrocarbon generation capacity, OM porosity, intercrystalline porosity of clay minerals and gas adsorptive capacity. The Ro threshold for solid OM (of type I-II1) carbonization may be theoretically taken as an alert for age-old marine shale gas exploration. In the process of geologic evaluation of prospect areas, efforts should be devoted to source properties of low-resistivity shale to exclude the zones with OM carbonization risks. In shale gas exploration and production, efforts should focus on the evaluation of OM carbonization for low-resistivity intervals drilled to adjust deployment quickly.
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