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Natural gas hydrate (NGH) has been widely considered as an alternative to conventional oil and gas resources in the future energy resource supply since Trofimuk’s first resource assessment in 1973. At least 29 global estimates have been published from various studies so far, among which 24 estimates are greater than the total conventional gas resources. If drawn in chronological order, the 29 historical resource estimates show a clear downward trend, reflecting the changes in our percep- tion with respect to its resource potential with increasing our knowledge on the NGH with time. A time series of the 29 estimates was used to establish a statistical model for predict the future trend. The model produces an expected resource 12 3 value of 41.46 × 10 m at the year of 2050. The statistical trend projected future gas hydrate resource is only about 10% of total natural gas resource in conventional reservoir, consistent with estimates of global technically recoverable resources (TRR) in gas hydrate from Monte Carlo technique based on volumetric and material balance approaches. Considering the technical challenges and high cost in commercial production and the lack of competitive advantages compared with rapid growing unconventional and renewable resources, only those on the very top of the gas hydrate resource pyramid will be added to future energy supply. It is unlikely that the NGH will be the major energy source in the future. Keywords Natural gas hydrate · Global gas hydrate resource · Conventional oil and gas resource · Renewable and sustainable energy · Trend analysis method 1 Introduction Afterward, other scholars and government agencies have conducted studies on this issue and obtained at least 29 dif- Trofimuk assessed global natural gas hydrate (NGH) ferent estimates with the maximum and minimum values resources in 1973 with an in-place estimate of 3.02- varying by more than 10,000 times. Many still believed that 18 3 3.09×10 m gas equivalent (Trofimuk et al. 1973). This NGH will be the main energy source in the future for the vast energy resource in NGH was considered as a solution to following reasons. (1) The estimated resource is enormous; future energy shortage (Arthur 2011; Wadham et al. 2012). among the 29 global estimates, 24 (Fig. 1b) are larger than the total conventional gas resource in place of 0.67×10 Edited by Jie Hao and Chun-Yan Tang * Xiong-Qi Pang China National Oil and Gas Corporation, Beijing 100007, pangxq@cup.edu.cn China * Zhuo-Heng Chen School of Earth & Space Sciences, Peking University, Beijing 100871, China * Cheng-Zao Jia Shenzhen Branch of China National Offshore Oil State Key Laboratory of Oil and Gas Resources Corporation Ltd, Guangzhou 510240, China and Prospecting, Beijing 102249, China CSIRO Earth Science and Resource Engineering, College of Geosciences, China University of Petroleum P. O. Box 1130, Bentley, WA 6102, Australia (Beijing), Beijing 102249, China Department of Geosciences and Natural Resource Geological Survey of Canada, Natural Resources Canada, Management, University of Copenhagen, 1350 København, Calgary T2L 2A7, Canada Denmark Vol.:(0123456789) 1 3 324 Petroleum Science (2021) 18:323–338 (a) (b) Data from China National Knowledge Infrastrure (CNKI) Data from Elsevier 15 3 1000 Maximum = 3053 ×10 m 15 3 Average = 1527 ×10 m 15 3 Minimum = 0.2 ×10 m GIP CONtotal GIPCONgas 0.1 123456789 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 Resource size sequence number (c) Mode P50 15 3 Average = 277 ×10 m 15 3 Mode = 5.5 ×10 m 15 3 P50 = 10 ×10 m P10 P90 1961 1971 1981 1991 2001 2011 2021 0.01-0.1 0.1-11-10 10-100 100-1000 1000-10000 15 3 Year Gas resource in place, ×10 m Fig. 1 Basic overview of global NGH researches. a Overview of the growth of papers related to NGH since 1970; b scale sequence distribution of global GIP estimates of NGH since 1973 around the world and statistical analysis parameters; c statistical distribution characteristics of global 15 3 GIP estimates of NGH and relative parameters. GIP —Total conventional oil and gas resource in place, 4.1 × 10 m, GIP —Total CONtotal CONgas 15 3 conventional gas resource in place, 0.67 × 10 m , data from reference (Zou et al. 2015) m (Zou et al. 2015), whose average value, mode value, the major energy source replacing the diminishing petro- middle value and P50 value of 29 estimates are all higher leum resource in the future for three reasons. First, the 29 than the total conventional gas resource (Fig. 1c); (2) The estimates vary up to five orders of magnitude, a reflection USA (Booth et al. 1996), Japan (Konno et al. 2017), Can- of lack of consensus on the character, mode of occurrence ada (Dallimore et al. 2005), India (Sain and Gupta 2012), and a clear definition of the resource. Second, the pattern Korea (Ning et al. 2012) and China (Yang et al. 2015) have that the 29 estimates decrease gradually over time, reflect- launched research programs investigating the NGH as a ing increased knowledge of the resource, improved map- type of potential energy resource since this century, the ping methods and accumulated data for evaluation with number of hydrate papers published every year has been time. Third, although those 29 global resource estimates increasing rapidly (Fig. 1a), indicating increased number of were conducted by different scholars in different periods people involved in and investments to the research in gas using different methods, they have one thing in common: hydrate. Following the Millik-5L-38 well production test The estimates converge with time gradually toward lower of permafrost gas hydrate in northern Canada in 2002, sev- resource abundance with a range of variation equivalent to eral offshore production tests attempted to make technical the uncertainty in petroleum resource assessment. Figure 2 breakthrough in producing the resource, and the Chinese illustrates resource “mapping” methods used and resulting company has successfully conducted a 60-day production estimates in different stages. In the initial stage, the prospec- test in Shenhu Area of the South China Sea in 2017 in verti- tive gas amount of NGH was estimated roughly based on the cal well and completed a second round of 30-day test from a environments and depth in which they may occur, resulting horizontal well in 2020 (Xinhua net 2020). (3) The Chinese in enormous resource volume. Later, studies found that the government has listed it as a new mineral species and is NGH can only be formed in some favorable environments, ready to organize larger-scale exploration and development such as in marine and deep permafrost regions. In the sec- (Ministry of Natural Resources of China 2017). ond stage, resource estimate was constrained by the genetic Until now, there is no definite answer as whether or not mechanisms (Falenty et al. 2014) and formation conditions NGH can be an alternative to replace conventional oil and (Sloan 2003) of NGH, based on the sediment volume of gas in the future and the question is still being explored. the gas hydrate stable zone (GHSZ). It was realized that However, the time series of 29 global estimates reveals a GHSZ is only a necessary condition, and the formation of general trend with an indication that NGH may not become NGH still requires gas source and other conditions (Dai et al. 1 3 Publications Gas resource in place, 15 3 ×10 m Frequency, % Petroleum Science (2021) 18:323–338 325 Marine area: Possible-HG GHSZ: Potential-HG Source rock area-HGIP HG-RIP HG-TRR Marine Marine deposits Deep resource rock Fault Buoyance-driven Basin bottom Technically recoverable resource Hydrate gas resource in place Hydrate gas in natural occurrence Fig. 2 NGH occurrence in different forms and their relationships to each other with major factors. Marine area is favorable for NGH (possible HG), but can only formed and distributed in gas hydrate stable zone (GHSZ) with high pressure and low temperature (potential HG). All hydrate gas in place (HGIP) exists in strata with gas coming from degradation of organic matter in shallow or deep source rocks; hydrate gas resource in place (RIP) and hydrate gas technically recoverable resource (TRR) are distributed in sandstone with high porosity or mudstone with fractures 2017). In the third stage, the in-place gas hydrate content As discussed above, the current NGH estimates are more (GIP) was estimated based on geophysical indication of the reliable than previous results. It is possible to obtain more presence of natural gas—bottom simulating reflector (BSR) reliable results of the NGH by statistical analysis of the pre- in the seismic profile. However, even small amount of free vious estimates. In this study, the trend analysis method was gas in GHSZ may cause significant geophysical anomalies, utilized to predict the future resources of the global GIP and resulting in inclusion of areas of low concentration hydrate TRR, and the corresponding trend values are determined occurrence without economic significance in resource cal- as their objectively existing actual values when the varia- culation (Boswell 2009; Boswell and Collett 2011). In the tion of the 29 estimates is almost the constant in the future. latest stage, the hydrate gas resource is evaluated with con- This method can overcome problems in assessment, such as sidering reservoir characteristics. Only those stored in high insufficient data, incorrect parameter values and immature porosity in sandstone and the mudstones with fractures are understanding of the NGH in previous studies, and the esti- considered technically recoverable resources. The deepen- mated results are more representative and reliable than the ing of such understanding and the continuously refining results obtained by a single approach or several methods. evaluation parameters led to the gradual reduction of the estimated results of NGH resources. For example, the global area extent of NGH in GHSZ decreases from an initial value 4 2 4 2 of 220×10 km to current about 20×10 km , the average 2 Method and results thickness dropped from about 450 m to less than 50 m, the average hydrate saturation in reservoir pores declined from 2.1 Trend analysis method principle nearly 100% to less than 40%, etc., resulting in significant decreases in global NGH resources. All these changes reflect Taking the previous 29 estimates of the global NGH since the continuous progress of human understanding of natural 1973 as a time series (Table 1), a statistical trend analysis is gas hydrate resources (Chong et al. 2016), some scholars performed to project present and future resources simultane- introduced the concept of TRR for NGH resource assess- ously. Previous estimates were obtained by different schol- ment (Boswell 2009; Boswell and Collett 2011) and con- ars with different methods and techniques differing in levels cluded that only a few concentrated NGH in high-porosity and credibility. Before 1980, only a rough prospective and and permeable sandstone reservoirs or fractured muds are potential hydrate gas content of NGH was estimated by a recoverable resources, and their estimated global GIP and few data. During 1980–2000, the GIP resource assessment 14 3 14 3 TRR is about (3-6)×10 m and 3×10 m , respectively. was obtained by including BSR constraint from seismic data. During 2000–2010, the resource in place (RIP) was obtained 1 3 326 Petroleum Science (2021) 18:323–338 Table 1 Estimated results and variation characteristics of global NGH resources by previous studies. V—volumetric method; C—comprehensive analysis method; D—particle organic carbon deposition rate method. 6 2 No. Methods Data resourcesArea, × 10 km Thickness, m GIP of References NGH, 15 3 × 10 m 1 V Doklady Akademii Nauk SSSR 335.71 300 3053 Trofimuk et al. (1973) 2 V Doklady Akademii Nauk SSSR 225 360.2 300 1135 Trofimuk et al. (1975) 3 V Geologiyai Geofizika 360.2 85 1573 Tsarev and Cherskiy et al. (1977) 4 V Priroda 1 280.5 300 120 Trofimuk et al. (1979) 5 V Long-Term Energy Resources – – 1550 Nesterov and Salmanov, (1981) 6 V Long-Term Energy Resources 51.25 400 3.1 McIver (1981) 7 V Proceedings of the Fifth II ASA Conference – 15 Trofimuk et al. (1983) on Energy Resources 8 V Chemical Geology 10 500 40 Kvenvolden (1988) 9 V Annual Review of Energy 62.4 500 20 MacDonald (1990) 10 V Global Biogeochemical Cycles 36.3–54.7 379 ~ 453.4 26.4 Gornitz and Fung (1994) 11 V Journal of Geophysical Research 59.6 277 0.48 Harvey and Huang (1995) 12 V Science 10.5 400 6.8 Holbrook et al. (1996) 13 V Nature 10.5 400 15 Dickens et al. (1997) 14 V Penn Well – – 15 Makogon (1997) 15 C Proceedings of the National Academy of – – 21 Kvenvolden (1999) Sciences 16 V Organic Geochemistry 17.5 400 4 Dickens (2001) 17 V Russian Geology and Geophysics 35.7 2.8 0.21 Soloviev (2002) 18 V Geology 7 300 4 Milkov et al. (2003) 19 V Earth-Science Reviews 4.5 300 2.5 Milkov (2004) 20 D Earth and Planetary Science Letters 13.1 300 5.7 Buffett and Archer (2004) 21 V Energy & Fuels - 115.4 Klauda and Sandler (2005) 22 V Marine Geology and Quaternary Geology 0.397 Ge et al. (2005) 23 D Proceedings of the National Academy of 13.1 300 3.4 Archer et al. (2009) Sciences 24 D Geochimica Et Cosmochimica Acta – 0.5 Burwicz (2011) 25 C Energy & Environmental Science – 0.45 Boswell and Collett (2011) 26 D Energies 147 300 1.0 Wallmann et al., (2012) 27 D Biogeosciences 26 300 1.05 Piñero et al., (2013) 28 C Advances in New and Renewable Energy – 5 Cong et al. (2014) 29 D Global Biogeochemical Cycles - 3.5 Kretschmer et al. (2015) by seismic profile data and drilling wells. After 2010, the volumetric parameters validated by drilling results, such TRR resource was evaluated by using the recovery factors as occurrence area, net reservoir thickness, saturation and obtained through laboratory physical and numerical simu- others. Since 2010, the decline rate of estimated results lations (Konno et al. 2014). This implies that the level and has slowed down significantly and is nearly constant in reliability of the later estimated results is much higher than 2050. In this study, the corresponding trend value at 2050 the previous estimates. is roughly regarded as the theoretical maximum value of The 29 estimates of the global NGH were fitted to an estimated global RIP of NGH. empirical model as a function of time (Year) as shown in Fig. 3a. Before 1980, the global estimates of NGH were 2.2 Estimate of global RIP and TRR 18 3 in the order of 8×10 m , declining gradually to around 14 3 7.0×10 m in 2010s, about four orders of magnitude Based on the statistical trend analysis model, the final simula- less. The drastic drop is due to improved understand- tion of the global GIP and TRR of NGH at 2020 and 2050 ing of the nature of NGH occurrence and much reliable 1 3 Upper bound Lower bound Petroleum Science (2021) 18:323–338 327 Global data Shenhu estimates estimated in trend analysis accounted for only 3.2% of all these Global trend model Matter balance method Volumetric approach Data range conventional resources, as shown in Fig. 3b. Trend values 19 16 10 10 1 NGH 3.2% 5 Bitumen Y=8.1e+18/(X-1971)2.4 21.0% 18 R=0.8078 15 10 10 Oil 3 Reliability validation—compared 42.3% 17 14 10 Gas 10 with volumetric method 10 33.5% (b) 16 13 10 10 The latest drilling results, volumetric methods and Monte 15 6 12 Carlo technique were combined to evaluate the global 10 10 A1 NGH resources, and the result was then compared with that 14 11 10 10 obtained by trend analysis, which were both tested and vali- (a) 13 10 10 10 dated mutually. 1970 1980 1990 2000 2010 2020 2030 2040 2050 Year 3.1 Volumetric method principle Fig. 3 Estimated results of the global resource of NGH by trend anal- For a proven NGH reservoir, the GIP and RIP are related to ysis method and compared to other results. a Historical global GIP hydrate distribution area (A), average thickness (H), poros- estimates of NGH showing a general tendency of decreasing resource with time (A), superimposed with fitted statistical trends to project ity (Ø), gas hydrate saturation (S ), gas volume conversion gh future estimates, projected resource estimates of mode values for RIP coefficient (B ) and conversion coefficient (K ). K is the gh R R of NGH at 2020 and 2050 and their uncertainty range from the sta- ratio of enriched NGH resource to total NGH resource, and tistical trend analysis are added to the plot (A, A ). The estimate of 1 2 the enriched NGH resource is identified by criteria of ∑H > mode value for RIP of NGH from volumetric approach is shown in red star (B), and the average value for the RIP of NGH from matter 2 m, S > 10%, and Ø > 15%; here, ∑H refers to accumula- balance approach is shown in pink dot (C). The historical resource tive net thickness of the target layers (i) with enriched NGH, estimates for GIP of NGH from 1999 to 2017 in Shenhu area of and the GIP and RIP calculations are expressed in Eqs. 1 and South China Sea are plotted to show the learning curve in orange 2. After obtaining the relative data, the TRR of the NGH was dotted line (D). b The proportion of global TRR of NGH in the total amount of four types of conventional oil and gas resources is shown calculated according to Eq. 3. After converting the RIP and in the pie chart at the upper right TRR in the hydrate reservoirs into the ratios of resources for per unit volume in GHSZ, the global RIP and TRR can be evaluated by multiplying the total volume of GHSZ in are conducted. The recovery parameters obtained by physical different regions with these two ratios. simulation laboratory are used in the calculation of the TRR, GIP = A × H × × S × B varying from 15% to 70% with a mode of 30%. The projected (1) 12 3 mode of global RIP of NGH resource is 711.20×10 m at 2020, and the projected RIP at 2050 ranges from 45 (F90) to RIP = K × GIP (2) 12 12 3 367 (F10) ×10 with a mode of 148.22×10 m (Fig. 4a and b). The projected TRR at 2050 ranges from 28 (F90) to 145 TRR = K × RIP (3) 12 3 (F10) with a mode of 41.46×10 m (Fig. 4c and d), indicat- ing the NGH resources will continue to decline in the next The above calculation is too crude because it does not 30 years, from 2020 to 2050 (A and A in Fig. 3a). As NGH consider the formation conditions of NGH in different 1 2 exploration and research results are increasingly disclosed to GHSZ. In this study, this method is further improved by the public, it will guarantee better distinguishing potential eco- considering both GHSZ volume and formation conditions nomic resources from the entire NGH occurrence, allowing using probability distributions. The relevant geological the estimates being more consistent, realistic and less uncer- model and calculation equations are shown in Fig. 2 and tain. However, as suggested by the resource estimate pat- Eqs. 4–10. Equation 4 is used to calculate the global pos- terns during the last 20-year NGH exploration in the Shenhu sible hydrate gas content, and mainly considering the rock Area (Orange dotted line D in Fig. 3a), it may take decades volume in GHSZ area, B is hydrate gas volume coefficient gh to achieve this effect. The projected trend of the global NGH in the earth’s surface; Eq. 5 is used to calculate the GIP of resources from 2020 to 2050 also supports a long time period NGH, further considering the gas source conditions, and 12 3 of learning. So far, 1362.0×10 m gas equivalent of global K refers to the ratio of source rock area to GHSZ area; source TRR of conventional oil and gas has been found worldwide, Eq. 6 is used to calculate the global migration gas content 12 3 12 including 595.0×10 m gas equivalent of oil and 470.5×10 3 12 3 m gas in normal traps, and 296.5×10 m gas equivalent of bitumen and heavy oil in reformed traps. The TRR of NGH 1 3 Global NGH resource, m Shenhu NGH resource estimate, m 328 Petroleum Science (2021) 18:323–338 10000 1.0 (a) (b) Mode P90 8000 0.8 90% 50% 10% Mean Mode 6000 0.6 95 225 536 283 148.22 P50 4000 0.4 2000 0.2 P10 0 0 1 2 3 100 200 300 400 500 600 700 800 900 1000 1100 1200 10 10 10 12 3 12 3 NGH in-place, ×10 m NGH in-place, ×10 m 10000 1.0 (c) (d) Mode P90 8000 0.8 90% 50% 10% Mean Mode 6000 0.6 28 70 145 91 41.46 P50 4000 0.4 2000 0.2 P10 0 0 0 1 2 3 0 50 100 150 200 250 300 350 400 10 10 10 12 3 12 3 NGH in-place, ×10 m NGH in-place, ×10 m Fig. 4 Estimated results of the global RIP of NGH by trend analysis method, indicating statistical distributions of RIP and TRR for global NGH resources from statistical trend analysis of 29 previous estimates at the year of 2050. a RIP histogram; b RIP cumulative probability; c TRR his- togram; d TRR cumulative probability in place (MIP), the area with no gas expulsion from source AIP = K × MIP play (7) rocks (Pang et al. 2005) is left out from favorable area, and K refers to the ratio of area with gas expulsion from migra RIP = K × AIP (8) enrich effective source rocks to all source rock area. Eq. 7 is used to calculate the global accumulated gas content in place (AIP) RIP = K × K × K × K × A × H × × S × B enrich play migra source GHSZ of NGH, and further considering the hydrate accumulation (9) conditions, K refers to the ratio of reservoir volume to all play rock volume in area with effective source rocks or the prod- TRR = K × K × K × K × K recov enrich play migra source uct of plane reservoir area ratio K and vertical thickness play-A × A × H × × S × B GHSZ (10) ratio K ; Eqs. 8 and 9 are used to calculate the global play-H RIP of NGH, only considering the accumulated NGH with The reservoir volumetric parameters are determined from enrichment degree greater than specified requirement, and statistics of well-studied NGH reservoirs and production test K refers to the ratio of enriched resource to all accumu- enrich data, mainly from Shenhu Area in South China Sea (Fig. 5) lated resource; and Eq. 10 is used to calculate global TRR of and permafrost region of the Beaufort-Mackenzie Basin NGH, and considering the resource favorable for exploita- in Canada (Katsube et al. 2005). The history of the NGH tion currently, K refers to the ratio of recoverable NGH recov resource evaluation in the Shenhu Area is an epitome of the resource to all enriched NGH resources. global NGH assessment. The Shenhu site in the Pearl River Mouth Basin, South China Sea, covers about 3000 km . In Q = A × H × × S × B possible GHSZ GHSZ (4) 1999, the first NGH resource assessment was conducted based on BSR anomalies from seismic survey and obtained GIP = K × Q 12 3 source possible (5) an overly optimistic prospective gas content of 29×10 m . During 1999–2007, 5 exploration wells were drilled and a MIP = K × GIP migra (6) 1 3 # of occurrence # of occurrence Cum. prob. greater than Cum. prob. greater than Petroleum Science (2021) 18:323–338 329 resource reevaluation based on newly drilled data lead to a 4 Reliability validation–compared 9 3 reduced gas content of 199×10 m . In the following decade, with matter balance approach 14 more wells were drilled and 60-day production test was conducted in 2017, the updated GIP resource was further 4.1 Mass balance method principle reduced to one-third of the previous estimate, with a mode 9 3 of 66×10 m . The estimate shrunk almost three orders of At least 13 NGH accumulations have been extensively stud- magnitude compared with the first assessment. The Mallik is ied around the world (Table 3). Methane carbon isotope data one of the best-studied NGH accumulations in a permafrost from samples in the 13 sites show that all δ C values are environment worldwide. The NGH studies in Mallik site < −30 ‰, indicative of organic origin. Among them, 55% were carried out in the 1980 and continued to 2010’s. The is of biogenic origin with δ C ≤−55‰, and 20% is ther- Mallik NGH accumulation occurs below thick permafrost mogenic with δ C varying between −55‰ and −30‰; and overlays directly on a free gas field sourced from deep and the other 25% show signatures of mixed biogenic and thermal genetic gas of the basin. A basin wide study in the thermogenic origins having δ C ranging from −80‰ to Beaufort-Mackenzie Basin (BMB) based on petrophysical −30‰. Biodegradation and thermal degradation provide logs from 251 oil and gas exploration wells showed that, about 60% and 40% of natural gas hydrate, respectively, indi- although the NGH occurs in 122 wells (50%), only 7 contain cating homologous with conventional and unconventional oil net thickness greater than 5 meters, accounting for < 3% of and gas in petroliferous basins (Dai et al. 2017). the total wells studied (Osadetz and Chen 2010). The geochemical characteristics of NGH indicate that Based on the analysis of the above examples, NGH resource the total potential resource of NGH cannot exceed the total coefficient was determined as 18%, and the RIP and TRR per amount of oil and gas generated by the organic matter in 8 3 3 cubic kilometer in GHSZ are estimated as 0.12×10 m /km and the sedimentary basin before entering the active source rock 8 3 3 0.036×10 m /km , respectively. When the areal extent of global depth limit (Pang et al. 2020a), expressed quantitatively as 6 2 GHSZ is determined to be about 50×10 km and 350 m in in Eq. 11 and Fig. 7a. The NGH enrichment occurs in the thickness, the corresponding global total RIP and TRR of NGH sandstones with high porosity and fractured mudstones, 12 3 12 3 are 210×10 m and 63×10 m by analogy, and the TRR showing that the gas within NGH is generated and expelled accounts for 4.4% of total conventional oil and gas resources. from both biogenic and thermogenic source rocks, migrated Formation conditions of global NGH in different regions dif- through fault, unconformity or reservoir layers, and accumu- fer greatly from the drilling results, and the mode values and lated in reservoirs with high porosity and permeability. This variation ranges of these parameters are finally determined after process is mainly driven by the buoyancy, and the enriched analyzing the formation conditions of NGH and the variation of NGH resource is controlled by the gas amount expelled from essential parameters about NGH reservoirs (Table 2). Finally, all source rocks above buoyance-driven hydrocarbon accu- Eqs. 9 and 10 as well as Monte Carlo technology were utilized mulation depth (Pang et al. 2021). These findings suggest to calculate the distributions of global RIP and TRR of NGH that the total potential resources of all kinds of oil and gas with different probability. This method can also be used to evalu- resource, including NGH, cannot exceed the total amount ate the potential resources of NGH in any specific sedimentary of oil and gas expelled from the source rocks, quantitatively basin in the world. expressed as in Eq. 12 and Fig. 7a2/a3. Previous studies show that NGH is a special type of conventional oil and gas 3.2 Estimates of global RIP and TRR reservoir, whose migration and enrichment are dominated by buoyancy (Zhang et al. 2017), so it can only be formed The statistic results of parameters from the Shenhu Area, BMB in stratigraphic area above the BHAD. Therefore, it can and other well-studied NGH sites, coupled with governing prin- be inferred that their maximum potential resource cannot ciples of deposition of source and reservoir rocks in sedimentary exceed the hydrocarbon amount expelled from source rocks basins, as well as those parameters including area, thickness above the BHAD in the petroliferous basin, which can be of the GHSZ in the world are used to evaluate the global RIP quantitatively represented by Eq. 13 and Fig. 7a2. In six and TRR of NGH resource. The volumetric method combined representative petroliferous basins of China, including the with Monte Carlo simulation yielded distributions of RIP and Tarim, Junggar, Ordos, Sichuan, Bohai Bay and Songliao TRR of NGH resource, respectively (Fig. 6); the global RIP Basins, the amount of oil and gas expelled from their source 12 3 12 3 estimates range from 45×10 m (F90) to 367×10 m (F10) rocks before buried under the BHAD accounts for about 12 3 with a mode of 64.51×10 m (Figs. 6a and 5b), and the TRR 10.8% of the total amount of generated oil and gas, which 12 3 12 3 estimates range from 14×10 m (F90) to 119×10 m (F10) theoretically represents the maximum potential limit of NGH 12 3 with a mode of 22.09×10 m (Fig. 6c and d). The TRR of resources. The formation and distribution of various oil and NGH obtained by the volumetric method accounts for 1.6% to 4.4% of the total conventional oil and gas resources, respectively. 1 3 330 Petroleum Science (2021) 18:323–338 1 3 Xisha Trough (a) (b) Chinese Mainland Dongsha Islands Well name about 3000 km 727.7 km (c) Hainan Island 45 Basin boundaryS Coastline henhu area Xisha Islands 50 km Obvious BSRs Distributions of BSRs Well name Natural G- Wave velocity, Resistivity, Density, Caliper log, Tempera- Depth Porosity Saturability (d) amma(AP) cm/s ·m g/cm mm ture, °C GR Vp Rs DEN Den Sh-Qarchie CALI TEMP Porosity, % 30 60 1500 2500 1 4 1.5 2.2 200 4008 18 0 1 0 30 40 50 60 353 km Rt Sh-Indonesian Rd 5 km 2 km 1 0 0 4 11 0 1 x1 Obvious BSRs Sites with hydrates 22 km x2 Drillingsites Submarine canyons Sites without hydrates Fig. 5 Latest drilling results of NGH in Shenhu Area of Pearl River Mouth Basin, South China Sea (Wang et al. 2010; Gong et al. 2013; Zhang et al. 2018). a Location map of Shenhu Explo- 2 2 ration, modified from the literature, red frame covers an area of about 3000 km ; BSR in Shenhu exploration area shows that the hydrate area (green area) is 727.7 km ; b statistical charts of effective hydrate thickness from each well in Shenhu Exploration Area, data from the literature; c statistical results of hydrate saturation in pores of each well in Shenhu Exploration area, data from literature; d hydrate target zone porosity logging map of Well W2 in Shenhu Area SH7 SH6 SH3 SH2 SH9 SH1 SH4 SH5 Pearl River Mouth Basin Qiongdongnan Basin SH-W01-2015 SH-W02-2015 SH-W04-2015 SH-W05-2015 SH-W06-2015 SH-W07-2015 SH-W08A-2015 SH-W10-2015 SH-W11-2015 SH-W12-2015 SH-W14-2015 SH-W16-2015 SH-W17-2015 SH-W18-2015 SH-W19-2015 SH-W22-2015 SH-W23-2015 SH-W24A-2015 SH-W25-2015 SH-W01-2015 SH-W02-2015 SH-W04-2015 SH-W05-2015 SH-W06-2015 SH-W07-2015 SH-W08A-2015 SH-W10-2015 SH-W11-2015 SH-W12-2015 SH-W14-2015 SH-W16-2015 SH-W17-2015 SH-W18-2015 SH-W19-2015 SH-W22-2015 SH-W23-2015 SH-W24A-2015 SH-W25-2015 Southwest Taiwan Basin Saturability, % Effective thickness, m Petroleum Science (2021) 18:323–338 331 gas resources in petroliferous basins and their relationship (R ) of global NGH resources to total conventional oil NGH with the dynamic boundaries and hydrocarbon dynamic and gas resources varies between 2.2% and 6.7%. As only fields can be characterized by a joint model in Fig. 7b (Pang gas hydrates can be formed in GHSZ, this ratio varies from et al. 2020b). The stratigraphic area above the BHAD is a about 1.1% to 3.4% with an average of 2.3%, the global 12 3 free dynamic field (F-HDF) for oil and gas migration and TRR of NGH resource is derived to be 15–46 ×10 m accumulation dominated by buoyancy, forming three kinds based on Eq. 14, and the global RIP of NGH resources is 12 3 of conventional oil and gas resources with different char - (44.0–134.8)×10 m . acteristics, including normal trapped oil and gas resources, In petroliferous basins, more than 90% of NGH are dis- reformed heavy oil and bitumen resources, and solid NGH persed in mud or mudstone with hydrate saturation less resources. This imply that the potential NGH resource can- than 5%. Even for those accumulated resources in reservoir not exceed the total amount of all kinds of conventional oil layers, only small part of them can become recoverable and gas resources in the F-HDF, and the total potential NGH resource with porosity > 12%, saturation > 20% and accu- resource can be roughly estimated according to the percent- mulative thickness > 2m. The drilling results reveal that age of GHSZ volume in the volume of free dynamic field, these highly enriched NGH resources account for about 20% expressed quantitatively as in Eqs. 14 and 15. of total accumulated NGH in reservoir layers. In Shenhu Area of China, it is about 18%, and in the world, it is about Q < Q NGH p (11) 22% (Boswell 2009; Boswell and Collett 2011), implying the resource ratio coefficient of NGH is 20 ± 2% (Fig. 8a). Q < Q (12) The proportion of NGH in total conventional oil and gas NGH e resources varied from 1.1% to 4.4%, with an average of less than 3%, and their proportion in total oil and gas resources Q < Q (13) NGH was even lower (Fig. 8b). The total potential resources of unconventional oil and gas (shale oil and gas, tight oil and Q =(V ∕V )× Q (14) NGH GHSZ F−HDF conv gas) are estimated to be about 3–5 times that of conventional oil and gas resources (McGlade et al. 2013; Zou et al. 2015); R = V ∕V ≈(A × H )∕(A × H ) NGH GHSZ F−HDF GHSZ GHSZ F−HDF F−HDF thus, it is reasonable that the NGH resources account for less (15) than 1% of all kinds of oil and gas resources in petroliferous where Q —total potential NGH resource; Q —total basins. NGH p generated hydrocarbon amount, including the hydrocarbon amounts remained in source rocks and expelled from source rocks; Q —hydrocarbon amount expelled from source 5 Discussions and implications rocks; Q —hydrocarbon amount expelled from source rocks 12 3 in the F-HDF above the BHAD; V —stratigraphic vol- The mode value of 41×10 m for TRR of global NGH GHSZ ume of gas hydrate stable zone; V —stratigraphic vol- resource obtained by trend analysis method accounts for F-HDF ume of free hydrocarbon dynamic field above the BHAD; 3.2% of the TRR of global conventional oil and gas resource 12 3 R —ratio of total NGH resources to the total oil and gas (1403×10 m gas equivalent). It is in close agreement with NGH resources in F-HDF; A and H —average area and the results obtained by the volumetric method and mass bal- GHSZ GHSZ thickness of GHSZ, respectively; and A and H — ance method, much less than previous estimates. The three F-HDF F-HDF average and thickness of F-HDF, respectively. kinds of results from three different methods and their aver - age comprehensive results are listed in Table 4. The compre- 12 3 4.2 Estimates of global RIP and TRR hensive TRR for global NGH varies from (15-63)×10 m , whose ratios to the total conventional oil and gas resource According to the original data of 29 groups of previous changes from 1.1% to 4.4%, based on the world oil and gas 12 3 estimates, it is found that the average values of GHSZ area consumption level of 7.6×10 m gas equivalent in 2017 6 2 (A ) used by different scholars range from 50×10 km (BP 2018), can only meet demands of our society for 2 to GHSZ 6 2 to 68×10 km , and the average values of GHSZ thickness 8 years. (H ) range from 324m to 400m. Based on the statistic Evaluation results of three different methods reflect the GHSZ results of 52,926 conventional oil and gas reservoirs (IHS, limitation of NGH resources from three different aspects. 2016) in the world, the average areas of free hydrocarbon The matter balance method reveals the limitation of the dynamic field (F-HDF) in petroliferous basins (A ) potential NGH resource in terms of the genetic mechanism, F-HDF 6 2 range from 162 to 245×10 km , and the average buried and the theoretical calculation results show that the potential depths (H ) range from 2500 m to 3000 m. In accord- NGH resource cannot exceed 3.4% of the total conventional F-HDF ance with these data and Eq. 15, it is estimated that the ratio oil and gas resources in F-HDF or 1% of the total oil and gas 1 3 332 Petroleum Science (2021) 18:323–338 1 3 Table 2 Parameters and risk factors for global NGH resource assessment by volumetric method Key parameters Symbol Min Mode Max Physical significance and data source References 6 2 GHSZ area extent, 10 km A 5 50 150 Statistical analysis of previous available and effective data Table 1 GHSZ Net reservoir thickness, m H 2 15 80 Statistical analysis of drilling results from Shenhu of China and Katsube et al. (2005), Osadetz and Chen (2010), Wang et al. BMB of Canada (2010), Gong et al. (2013),Zhang et al. (2018) Porosity, % Ø 15 35 50 Statistical analysis of drilling results from Shenhu of China and Katsube et al. (2005), Osadetz and Chen (2010), Wang et al. BMB of Canada (2010), Gong et al. (2013), Zhang et al. (2018) Hydrate saturation, % S 10 35 90 Statistical analysis of drilling results from Shenhu of China and Katsube et al. (2005), Osadetz and Chen (2010), Wang et al. gh BMB of Canada (2010), Gong et al. (2013), Zhang et al. (2018) Hydrate volume factor B 160 164 168 Based on laboratory results in the relevant studies Sloan (2003), Chong et al. (2016) gh Probability of Source rock P 0.15 0.35 0.65 Statistical analysis of drilling results from Shenhu area of China, Katsube et al. (2005), Osadetz and Chen (2010), Wang et al. source BMB area of Canada and other areas with proven NGH reser- (2010), Gong et al. (2013), Zhang et al. (2018), Table 3 voirs in studies Probability of Reservoir P 0.05 0.2 0.35 Statistical analysis of drilling results from Shenhu area of China, Katsube et al. (2005), Osadetz and Chen (2010), Wang et al. play-A BMB area of Canada and other areas with proven NGH reser- (2010), Gong et al. (2013), Zhang et al. (2018), Table 3 voirs in studies Probability of Migration P 0.05 0.5 0.75 Statistical analysis of drilling results from Shenhu area of China, Katsube et al. (2005), Osadetz and Chen (2010), Wang et al. migra BMB area of Canada and other areas with proven NGH reser- (2010), Gong et al. (2013), Zhang et al. (2018), Table 3 voirs in studies Probability of Enrichment P 0.1 0.25 0.35 Statistical analysis of drilling results from Shenhu area of China, Katsube et al. (2005), Osadetz and Chen (2010), Wang et al. enrich BMB area of Canada and other areas with proven NGH reser- (2010), Gong et al. (2013), Zhang et al. (2018), Table 3 voirs in studies Recovery factor, % K 10 30 70 Based on laboratory results in the relevant studies Konno et al. (2015) recovery Petroleum Science (2021) 18:323–338 333 Global NGH resource (Shenhu analogue) 10000 1.0 (a) (b) Mode P90 8000 0.8 90% 50% 10% Mean Mode 6000 0.6 45 135 367 180 64.51 P50 4000 0.4 2000 0.2 P10 0 0 1 2 3 0 100 200 300 400 500 600 700 10 10 10 12 3 12 3 In-place NGH, ×10 m In-place NGH, ×10 m 10000 1.0 (c) (d) Mode P90 8000 0.8 90% 50% 10% Mean Mode 6000 0.6 14 42 11957 22.09 P50 4000 0.4 2000 0.2 P10 0 0 -1 0 1 2 3 020406080 100 120 140 160 180 200 10 10 10 10 10 12 3 12 3 NGH TRR, ×10 m NGH TRR, ×10 m Fig. 6 Estimated results of RIP of global NGH by volumetric method, indicating the statistical distributions of results for GIP and TRR by reser- voir volumetric approach using analogues from well-studied gas hydrate accumulations in the world: Shenhu Area of China in marine and Mal- lik area of Canada in permafrost. a GIP histogram; b GIP cumulative probability; c TRR histogram; d TRR cumulative probability resources in the basin. Volumetric method reveals the limi- is difficult for the NGH to compete with other alternative tation of potential NGH resource from exploration practice energy by 2050. and drilling results, the Shenhu exploration area in China Although Japan and China have made progresses in and the MBM area in Canada are the most favorable areas marine NGH production tests, and the USA and Canada have for gas hydrate accumulation in the ocean and the permafrost made achievements in permafrost sites, there is still a long region, respectively, and the results obtained by comparing way to achieve commercial production status. The current them with other regions in the world represent the upper costs of the NGH production are much higher than that of limit of the potential NGH resource to some extent. As the conventional oil and gas reservoirs, and significantly higher results obtained by trend analysis method eliminate the esti- than those from unconventional oil and gas reservoirs as mate differences caused by varied methods, data, technology additional energy inputs are required to disassociate NGH and purposes in previous studies, it is more representative, prior to production. Harsh environments of deep water or more predictive and closer to the objective reality than any remote permafrost region and associated geohazards with other results. dissociation of NGH bring further challenges. In addi- In comparison, unconventional oil and gas resources have tion, the NGH exploitation may have environmental con- much great potential, which are replacing conventional oil sequences. Methane is one of the most potent greenhouse and gas resources in North America, and will account for gasses with capacity of absorbing heat about 21 times of car- over 65%–75% of the total annual energy consumption bon dioxide. The NGH dissociation in production, hydrate before 2050 (Schelly 2016). Besides, the world annual structural transition throughout differentiation and critical renewable energy production, such as nuclear, solar, wind, pressure change of free-gas reservoirs below NGH prov- geothermal and hydropower, accounted for 15.1% in total inces could lead to methane leakage, causing greenhouse energy consumption in 2017, and it will reach 30% by 2050 effect (Hope 2006). Increased concentration of methane in (Energy Information Administration 2010). Obviously, it water due to NGH production could reduce water density, causing marine acidification, and the NGH dissociation in 1 3 # of occurrence # of occurrence Cum. prob. greater than Cum. prob. greater than 334 Petroleum Science (2021) 18:323–338 1 3 Table 3 Hydrogen and carbon isotope characteristics of hydrocarbon in proven 13 NGH reservoirs in the world No. Area Strata Depth below sea Natural gas component (%) Isotope Genetic Reference floor, m type 13 13 Methane Heavy hydrocarbon gas Non-hydrocarbon gas δ D (VSMOW)/‰ δ C (VPDB)/‰ 1 1 1 Oregon – 16–101/M = 72 99.14 ~ 100.0/M = 99.65 0.002 ~ 0.161/M = 0.024 0.012 ~ 0.699/M = 0.256 −208 to −188/M = −200 −70.0 to −63.2/M−67.9 CO Wallmann coastal reduc- et al. (2012) hydration tion roof biogenic gas 2 Vancouver Miocene– – 68.1 ~ 85.1/M = 79.85 13.8 ~ 29.7/M = 18.9 – − 143 to − 138/M = − 140 − 43.4 to − 42.6/M = − 42.9 Mixed gas Piñero et al. Island Oligo- (2013) nearshore cene Barkley canyon 3 Coastal Sino- – 120 ~ − 259 99.87 ~ 99.98 0.0213 ~ 0.135 – − 206 ~ −189 − 67.7 ~ − 63.7 CO Cong et al. US trench reduc- (2014) M = 194 M = 99.92 M = 0.066 Me = − 198 M = − 65.24 in Costa tion Rica biogenic gas 4 South Korea Pliocene– – 79.91 0.034 – − 189 − 63.6 CO Kretschmer East, Quater- reduc- et al. (2015) nary tion Yuling Basin biogenic gas 5 Kitami in – – 97.8–100 0.0022–0.112 0.3–2.2 − 203 to − 197 − 65.7 to − 63.6 CO Konno et al. Sakhalin reduc- (2014) M = 99.1 M = 0.015 M = 1.0 M = − 200 M = − 64.6 Island tion nearshore biogenic gas 6 Norwegian Pliocene– – 99.67–99.71 0.329–0.290 – − 202 to − 198 − 70.8 to − 69.9 CO Pang et al. sea Nyegga Pleisto- reduc- (2005), Kat- M = 99.69 M = 309 M = 200 M = − 70.35 cene tion sube et al. biogenic (2005) gas 7 The Gulf of Upper – 71.66–99.00 1.0–28.15 0–0.4 − 190 to − 148 − 50.1 to − 42.2 Pyrolysis Osadetz et al. Mexico Pleisto- gas (2010), cene Wang et al. (2010), Holocene M = 73.5 M = 16.0 M = 0.20 M = − 173 M = − 42.7 and Mixed Gong, et al. gas (2013), Boswell (2009) 8 Black Ridge Pleistocene 259 99.62–99.98/M = 99.73 0.018–0.0772/M = 0.0729 0.2–0.3/M = 0.25 − 205 to − 175/M = − 193 − 70.7 to − 66.6/M = − 67.4 biogenic Zhang et al. in South gas (2018) Carolina Miocene– 330–331/M = 331 98.38–99.98 0.019–0.024 0–1.6 − 206 to − 196 − 66.5 to − 65.8 biogenic Pang et al. coast Holo- gas (2020a, b) cene 9 Baikal Basin – 85.22–99.98/M = 95.64 0.02–14.78/M = 4.34 0–0.007 − 305 to − 302/M-303.5 − 66.6 to − 57.0/M = 60.5 biogenetic Pang et al. gas (2020a, b) Petroleum Science (2021) 18:323–338 335 marine sediments would weaken slope stability, leading to submarine landslides, triggering earthquakes and tsunamis (Knittel and Boetius 2009). The increasing temperature of the earth caused by the massive oil and gas consumption is much higher than we once estimated, and it is necessary to drastically reduce it to be compatible with a global warm- ing limit of 2℃ (Mcglade and Ekins 2015). Meanwhile, the exploitation cost for the NGH is extremely high and the tech- nology is not yet mature. All of these are unfavorable for the NGH exploitation. It is emphasized here that as a kind of fossil energy, NGH is unlikely to replace conventional and unconventional oil and gas under current conditions, but it does not mean that this kind of resource will not be used on a large scale in the future. As human beings continue to search for renewable energy and make a transition to low carbon economy, the coal, oil, natural gas and NGH may be continuously exploited and utilized on a large scale as the main chemical materials or other valuable raw materials in the future. 6 Conclusion There are at least 29 estimates of global NGH resource in the literature, and their variation over time records how our perceptions with respect to its potential resource change with increasing knowledge. Based on the statistical model, pro- jecting the trend into future time could provide insights for repositioning the NGH in the future global energy resource system, and the global NGH resource cannot be predicted by the current or previous estimated values. From the trend model, the global potential TRR of NGH will decrease gradually in next 30 years. It is projected to 12 3 be 41×10 m at 2050, accounting for 3.2% of the global conventional oil and gas resource, which is consistent with the 1.6%–4.4% from volumetric approach and 1.1% – 3.4% from matter balance approach, and can only support human society for less than 8 years based on the current energy consumption level. In addition to much reduced potential resource esti- mates, technical challenges and high cost in commercial production, environmental risks and lack of competitive advantage against fast growing unconventional and renew- able resources suggest that NGH resource will be difficult to replace conventional oil and gas resources and become major energy source in the future. Acknowledgements This research was financially supported by the CAS consultation project (Grant number-2019-ZW11-Z-035) and the National Basic Research Program of China (973) (Projects: 2006CB202300, 2011CB201100) and China High-Tech R&D (863) Program Project (2013AA092600). We would like to thank Gao Deli, Academician of Chinese Academy of Sciences, for his comments and recommendation in publishing this paper in Petroleum Science. 1 3 Table 3 (continued) No. Area Strata Depth below sea Natural gas component (%) Isotope Genetic Reference floor, m type 13 13 Methane Heavy hydrocarbon gas Non-hydrocarbon gas δ D (VSMOW)/‰ δ C (VPDB)/‰ 1 1 10 Marmara Sea – – 66.1 29.72 4 − 219 − 44.1 Pyrolysis Pang et al. gas (2020a, b), 11 Mackenzie Upper Oli- 898–915(908) * 99.62–99.89/M = 99.78 0.02–0.13/M = 0.047 0.09–0.36/M = 0.17 − 242 to − 214/M = − 227 − 48.7 Zhang et al. Basin gocene to − 39.6/M = − 42.11 (2017), Miocene Zhang et al. (2017) 12 Tundra of Jurassic 267–395/M347 * 34.85–86.95/M60.0 3.64–41.39/M = 27.61 0.11–27.6/M = 13.5 −285 to − 227/M = 250.2 − 52.6 Mixed and BP (2018), Qilian forma- to − 39.5/M = − 45.56 Pyroly- Schelly Mountains tion sis gas (2016), Energy 13 Pearl River Upper 97.0–99.97/M = 99.82 0.02–1.31/M = 0.051 – −226 to − 180/M = − 203 − 71.2 to − 31.1/M-54.7 Biogenic, Information Mouth Mio- pyroly- Administra- Basin cene–Pli- sis and tion, (2010), ocene mixed Hope gas (2006), Knittel and Boetius (2009), Mcglade and Ekins (2015) 336 Petroleum Science (2021) 18:323–338 Low heat flow Medium heat flow High heat flow Original hydrocarbons Original hydrocarbons basin/ Weak basin/ Moderate basin/ Strong (a) remaining in source expelled from source (b) compaction degree compaction degree compaction degree 9 9 rocks (10 t) rocks (10 t) 2 2 2 2 (<40 mW/m ) (40 mW/m -60 mW/m ) (>60 mW/m ) 450 350 250 150 500 0 100 200 300 400 500 600 GHSZ 0.2 (a3) 0.4 ab cd ef 0.6 Conventional oil and 0.8 gas reservoirs 1.0 BHAD BHAD 1.2 1.4 1.6 1.8 2.0 (a1) 9000 (a2) 2.2 ab cd ef a bcd e f 2.4 Shale oil and gas Tight oil and gas 2.6 reservoirs reservoirs 2.8 BHAD F-HDF HADL ASDL Bottom HADL HADL of GHSZ 3.0 ASDL ASDL Source rock C-HDF B1-HDF B2-HDF Shallow Enriched R , % source rock NGH a. Tarim Basin b. Junggar Basin c. Ordos Basin d. Sichuan Basin e. Bohai Bay Basin f. Songliao Basin 1234 5 BHAD: Buoyance-driven Hydrocarbon Accumulation Depth HADL: Hydrocarbon Accumulation Depth Limit ASDL: Active Source Rock Depth Limit Geothermal gradient, °C/100 m Fig. 7 Formation and distribution model of oil and gas resources controlled jointly by hydrocarbon thresholds and dynamic fields in petrolifer - ous basins. a hydrocarbon thresholds controlling the distribution hydrocarbon generation and expulsion from source rocks in six representative petroliferous basins in China: hydrocarbons retained in source rocks (a1); hydrocarbons expelled above BHAD (a2). Hydrocarbons expelled under the BHAD (a3). b Dynamic boundaries and dynamic fields jointly controlling the formation and distribution of different oil and gas resources: free hydrocarbon dynamic field (F-HDF, blue) controlling conventional oil and gas; confined hydrocarbon dynamic fields (C-HDF, yellow) controlling tight oil and gas; bound hydrocarbon dynamic field (B2-HDF, gray) controlling shale oil and gas. Gas hydrate stable zone (GHSZ, top of F-HDF) controlling NGH resources (a) Potential NGH Enriched NGH (b) 14 3 TRR=3× 10 m Resource Gas hydrate<5% Gas hydrate<1% 18%-22% Normal trap Conventional Pore filling in sands bitumen and oil and gas 13.6 Grain-displacing in heavy oil formed by buoyance 14 3 ×10 m “fractured” muds >90% <29% Pore-filling in muds Porosity<12% NGH saturation<20% Accumulative thickness<2 m Unconventional 14 3 3-6×10 m oil and gas formed by (Boswell and Timothy, 2011) Shale oil and gas non-buoyance >60% Tight oil and gas 16 3 2×10 m 3~5 times of (MacDonald, 1990) Conventional resources 18 3 3.1×10 m Dispersed (Trofimuk, 1973) NGH Fig. 8 Global NGH resource and its relative amount. a Estimated TRR of NGH resource and its relative amount in the total global estimated NGH amount in place (modified from Boswell 2009; Boswell and Collett 2011); b estimated global NGH resource, its relative amounts in both the total global conventional resource and the total global conventional and unconventional resources 1 3 ASDL BHAD HADL Conventional resource All kinds of oil and gas resources Depth, m Petroleum Science (2021) 18:323–338 337 Table 4 Global resource in place and technically recoverable resource of NGH estimated by three methods and their comparison Estimate methods Trend analysis Volumetric (aver- Matter balance (average) Comprehensive (mode value) age, mode) (average, mode) 12 3 RIP, × 10 m 148.2 64.5–184.7* 44.0*–134.8* 115.2 12 3 TRR, × 10 m 41.46 22–63 15–46 15–63 Ratios of NGH TRR to total conventional, % 3.2 1.6–4.4 1.1–3.4 1.1–4.4 Sustainable time at current consumption, years 5.5 2.9–8.3 2.0–6.0 2.0–8.3 12 3 Remarks about calculations The TRR of global conventional oil and gas resource (× 10 m gas equivalent) is 1362.0, including normal oil 595.0, normal gas 470.5 and bitumen 296.5, data from the literature (Zou et al. 2015); the global oil and gas consumption level of 7.6 × 10 m gas equivalent in 2017 (BP 2018) Number with * extrapolated from other data Open Access This article is licensed under a Creative Commons Attri- Cong XR, Wu NY, Su M, et al. 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