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Lower limits and grading evaluation criteria of tight oil source rocks of southern Songliao Basin, NE China
PETROLEUM EXPLORATION AND DEVELOPMENT Volume 44, Issue 3, June 2017 Online English edition of the Chinese language journal Cite this article as: PETROL. EXPLOR. DEVELOP., 2017, 44(3): 505–512.
RESEARCH PAPER
Lower limits and grading evaluation criteria of tight oil source rocks of southern Songliao Basin, NE China LU Shuangfang1, HUANG Wenbiao1, *, LI Wenhao1, XUE Haitao1, XIAO Dianshi1, LI Jijun1, WANG Min1, WANG Weiming1, CHEN Fangwen1, ZHANG Jun2, DENG Shouwei3, TANG Zhenxing3 1. Research Institute of Unconventional Petroleum and Renewable Energy, Qingdao, Shandong 266580, China; 2. Sinopec Corp, Beijing 100728, China; 3. Petroleum Exploration & Development Research Institute, PetroChina Jilin Oilfield Company, Songyuan, Jilin 138000, China
Abstract: Taking the southern Songliao Basin as the target area, the hydrocarbon expulsion volume from source rocks has been quantitatively evaluated in this paper, based on the material balance method. Employing the “Overpressure” module of PetroMod software, the overpressure history of the source rocks was evaluated. According to the relationships between hydrocarbon expulsion rate, residual organic carbon content and overpressure within the source rocks, the lower limits of expulsion from tight oil source rocks were determined: a hydrocarbon expulsion amount of 2 mg/g, residual hydrocarbon content of 0.8%, and overpressure of 1 MPa. The source rocks with hydrocarbon expulsion amounts > 8 mg/g, a residual organic carbon content > 2.0%, and overpressure > 7 MPa were defined as excellent source rocks. As a result, tight oil source rocks can be divided into three types, excellent source rocks (typeⅠ), inefficient source rocks (typeⅡ) and non-productive source rocks (type Ⅲ). The actual application in the southern Songliao Basin shows that the excellent source rocks have an obvious control on the planar and vertical distribution of tight oil discoveries, areas with excellent source rocks and nearby formations are favorable for the accumulation of tight oil. Key words: tight oil; source rocks; grading evaluation; hydrocarbon expulsion; overpressure; southern Songliao Basin
Introduction Tight oil and gas has great potential for exploration and development in China[117]. Zou et al. (2011) assessed the tight gas recoverable resources in China to be (1520)×1012 m3 and the tight oil geological resources to be (110135)×108 t [1,34,6]. By analogy, Jia et al. (2012) believed that the geological and recoverable resources of tight gas were of the order of (17.4 25.1)×1012 m3 and (8.812.1)×1012 m3, respectively. The geological and recoverable resources of tight oil were about (7480)×108 t and (1314)×108 t, respectively[7, 8]. Compared with the huge potential of tight oil, their exploration and development are still lagging behind. Therefore, it is necessary to strengthen the exploration and development of tight oil and related evaluation work. According to the relationship between the reservoir development and compaction period and the hydrocarbon migration period, tight oil accumulation can be divided into three types theoretically[2, 3]: (1) Tight oil reservoir formation before reservoir compaction; (2) Tight oil reservoir formation after res-
ervoir compaction; and (3) simultaneous reservoir compaction and hydrocarbon accumulation. In case (1), oil and gas migrate to the structural high driven by buoyancy, where the mechanism and distribution of tight oil reservoirs are the same as those of the conventional oil and gas reservoirs. It is difficult to form a large areal distribution of this kind of tight oil due to the constraints of traps, so it is typically not the main target for tight oil exploration and evaluation. For larger accumulations of tight oil, the essential difference from conventional oil and gas reservoirs is that the migration and accumulation of the oil and gas mainly depends on overpressure, rather than buoyancy[2]. This feature determines that the accumulation, enrichment and distribution of tight oil depends on the source rock quality (determining the size of hydrocarbon accumulation dynamics), reservoir characteristics (determining the difficulty or ease of hydrocarbon accumulation) and their matching relationship in time and space[1822]. Therefore, the lower limits of source rocks and reservoirs for tight oil, as well as their grading evaluation criteria have be-
Received date: 15 Feb. 2016; Revised date: 23 Mar. 2017. * Corresponding author. E-mail: [email protected] Foundation item: Supported by the National Natural Science Foundation of China (41330313, 41402109); China National Science and Technology Major Project (2016ZX05046-001-005). Copyright © 2017, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.
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come the current research focus. The classification industry standard of conventional oil and gas source rocks has been widely applied. In addition, the characteristics of tight oil source rocks and their relationship with conventional oil and gas reservoirs have also been reported in many examples in the literature[2326]. However, the criteria used in the industry standard of conventional oil and gas source rocks do not apply to tight oil because of the special accumulation mechanism. It is generally recognized that high-quality source rocks are the key controlling factor of hydrocarbon accumulations[27,1416,1822,2731], and the hydrocarbon source conditions of tight oil have been discussed previously[3,6,1420,3236]. However, the research on lower limits and the classification criterion of tight oil source rocks are still reported rarely. Even if some internal standards have been applied, they lack a rigorous and scientific theoretical basis. Pressure difference between source rocks and reservoirs, depends on the overpressure within source rocks, is the main driving force for tight oil migration and accumulation. The formation of overpressure is related to hydrocarbon generation, compaction disequilibrium, thermal maturation, clay mineral dehydration, tectonic stress, etc. Among them, hydrocarbon generation pressurization and compaction disequilibrium are considered to be the most common and important factors. Furthermore, compaction disequilibrium, thermal pressurization, and clay mineral dehydration are coincident with the direction and period of hydrocarbon generation [37-38]. The hydrocarbon expulsion amount is the most direct indicataor to measure the degree of the hydrocarbon generation pressurization. Larger hydrocarbon expulsion amounts indicate the higher driving force of oil and gas for migration and accumulation. There are many methods for the quantitative evaluation of hydrocarbon expulsion, in which the methods of material balance and hydrocarbon generation potential are two widely used and effective approaches[3946]. At present, there is no study to match hydrocarbon expulsion to the lower limits of generation and accumulation and classification evaluation criteria of tight oil source rocks. Based on the actual data of the study area, the geochemical characteristics of source rock were analyzed, and the heterogeneity and distribution of source rocks in the laterally and vertically were characterized by a combination of well logging geochemical techniques[22,47]. Based on thermal simulation experiments on immature source rock samples, a chemical kinetic model was established to evaluate the amount of hydrocarbon generation, residual hydrocarbon and free hydrocarbon expulsion[4346]. Based on the sedimentary burial history, thermal history, and pore evolution history, the overpressure caused by hydrocarbon generation, thermal maturation, clay dehydration, compaction disequilibrium and structural stress was simulated by using the overpressure simulation module of PetroMod software[4852]. On the basis of the above study, the relationship between the amount of hydrocarbon expulsion, total organic carbon content (TOC) and
overpressure within source rocks was established. The lower limits of viable tight oil source rocks were determined according to the inflection points, and provided operable and comparable evaluation criteria for tight oil exploration and evaluation.
1.
Geological setting
The Songliao Basin, a Mesozoic-Cenozoic oil- and gasbearing continental basin, is a faulted-depression composite basin. The depression formations, from old to new strata, are the Lower Cretaceous Denglouku and Quantou Formations and the Upper Cretaceous Qingshankou, Yaojia, Nenjiang, Sifangtai and Mingshui Formations, respectively. Among them, the Qingshankou 1st, 2nd, 3rd Members and the Nenjiang 1st Member are the main source rocks in the depression period (Fig. 1). The Fuyu and Yangdachengzi (F-Y) oil layers of Jilin oilfield, located in the southern Songliao Basin, are developed in the Quantou 4th and 3rd Members, respectively. In addition to the slope and uplift zones, the depth to the main reservoirs vary from 1 750 m to 2 600 m, which are typical tight reservoirs with porosity < 12% and permeability < 1×103 μm2. The sedimentation of the Quantou 4th and 3rd Members was accompanied by the rising of the basin base level and strong fluvial action. The fluvial facies, delta plain and front subfacies are widely developed, and the small area and overlapping sandstones provide the main storage space for tight oil. The overlying source rocks with high TOC, good kerogen type, moderate maturity, huge hydrocarbon generation and expulsion potential, and obvious overpressure in Qingshankou 1st Member not only make important contributions to the accumulation of conventional oil and gas in overlying strata, but also provide sufficient protection for the formation of the underlying F-Y oil layer. A large area of channel sandstones in the F-Y oil layer are adjacent to the overlying source rocks of Qingshankou 1st member, forming a good source-reservoir matching relationship, providing a good foundatation for the formation of tight oil reservoirs[1522]. The research and exploration results have demonstrated that the tight oil in the southern Songliao Basin is mainly distributed in Honggang terrace-Changling sag-Fuxin uplift, and the most prospective area is 1.4×104 km2. As of 2014, the proved tight oil reserves are 2.2×108 t.
2. The lower limits of tight oil source rocks and grading evaluation criteria 2.1. Relationship between hydrocarbon expulsion, overpressure, production and distribution of tight oil A typical positive correlation between the overpressure within the Qingshankou 1st Member at the sedimentary end of the Nenjiang Formation (tight oil accumulation period) and the downward migration depth of tight oil (the maximum distance between the tight reservoirs with oil shows and the bottom of Qingshankou 1st Member) is present (Fig. 2). This indicates that overpressure has an important control on hydro-
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Fig. 1.
Geographical location of the study area and stratigraphy.
the lower limits and ranking evaluation criteria of tight oil source rocks based on the quantitative evaluation of hydrocarbon expulsion and overpressure. 2.2. Lower limits and grading evaluation criteria of tight oil source rocks based on the amount of hydrocarbon expulsion
Fig. 2. Relationship between overpressure within the Qingshankou 1st Member and downward discharge depth of tight oil at the late depositional stage of the Nenjiang Formation.
carbon accumulation. In fact, the enrichment and distribution of tight oil are affected by many factors, such as the heterogeneity of thin mudstone interbedding, vertical fractures, and physical properties of tight reservoirs, etc. The relationship between the tight oil distribution, single well production and hydrocarbon expulsion intensity shows the thicker F-Y oil layer and higher well productivity are often accompanied by strong hydrocarbon expulsion in the Qingshankou 1st Member source rocks (Table 1, Fig. 3). This phenomenon indicates that the hydrocarbon expulsion plays an important role in tight oil accumulation, and provides the basis for the establishment of
Based on the measured pyrolysis data and the material balance method, the scatter plot of the hydrocarbon expulsion amount per unit mass of rock and the original TOC of the Qingshankou 1st Member in the southern Songliao Basin can be obtained (Fig. 4). Based on the original TOC, the hydrocarbon expulsion amount can be classified into three types: (1) when the original TOC is less than 1.0%, the hydrocarbon expulsion amount from the source rocks is very small or alTable 1.
Source rock thickness, hydrocarbon expulsion intensity,
F-Y oil layer thickness and productivity of five wells Well No.
Source rock thickness/m
Hydrocarbon expulsion per unit mass rock/mg·g1
Thickness of F-Y oil layer/m
Productivity/t
Cha 27 Qian 215 Cha 45 Qian 226 Qian 227
75 92 73 69 75
8.6 11.7 20.0 8.3 11.9
56.5 87.5 135.0 67.0 90.0
1.77 12.78 80.80 4.17 60.80
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Fig. 3.
Reservoir profile and source rock characteristic of joint wells in the southern Songliao Basin, East China.
respectively. 2.3. Lower limits and grading evaluation criteria of tight oil source rocks based on the overpressure
Fig. 4. Relationship between the hydrocarbon expulsion amount per unit mass source rock and the original TOC of Qingshankou 1st Member.
most nothing. In this case, the material basis of hydrocarbon accumulation is deficient, so an original TOC of 1.0% corresponds to the lower limits of tight oil source rocks. (2) When the original TOC is varying from 1.0% to 2.5%, the hydrocarbon expulsion amount increases slowly, and begins to provide the necessary material for a tight oil reservoir. (3) When the original TOC is larger than 2.5%, the hydrocarbon expulsion amount increases rapidly, and provides sufficient oil and gas for migration and accumulation. In order to facilitate the application, the original TOC needs to be converted to the measured residual TOC. Considering the loss of TOC reduced by hydrocarbon expulsion and the weight loss of rocks due to the compaction, it can be calculated that the original TOC of 1.0% and 2.5% correspond to residual TOC values of 0.8% and 2.0%, respectively. They are the lower and grading limit of residual TOC for tight oil source rocks, respectively. Therefore, results suggest the TOC grading or classification for this basin are: non-productive tight oil source rocks (Type III) are less than 0.8%, inefficient tight oil source rocks (Type II) are between 0.82.0% and excellent tight oil source rocks (Type I) are greater than 2.0%,
Through the recovery of the overpressure history simulated by the overpressure module of Petromod software, the relationship between the overpressure, the hydrocarbon expulsion and the residual TOC can be established (for convenience, the current overpressures are used in this case) (Figs. 5 and 6). Fig. 5 shows that the overpressure corresponding to the inflection point of the curve is about 7 Mpa, and the corresponding hydrocarbon expulsion amount per unit mass rock is 8mg/g. Fig. 6 shows that the residual TOC corresponding to the inflection point of the curve is about 2%, which is consistent with the type I source rock limit determined by the hydrocarbon expulsion method, when the corresponding overpressure is about 7 MPa. When the residual TOC is larger than 2.0%, the overpressure will not continue to increase, indicating the oil and gas is being expelled from the source rocks. At this time, the source rocks can not only discharge a large amount of hydrocarbons, but also provide the effective drive force for oil and gas discharge into the tight reservoirs. The
Fig. 5. Relationship between current overpressure and hydrocarbon expulsion.
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Fig. 6. Relationships between the residual TOC and the current overpressure.
residual TOC of 0.8% is corresponds to an overpressure of 1 Mpa (Fig. 6) and the hydrocarbon expulsion amount of 2 mg/g (Fig. 5), which is consistent with the lower limits of tight oil source rocks determined by the hydrocarbon expulsion method. Based on the above analysis of results and combined with the geochemical parameters, the lower limits and grading evaluation criteria was established. They are classified into three types (Table 2). Type I: excellent source rocks containing mostly type I and some type II1 kerogen and with TOC higher than 2.0%, hydrocarbon expulsion amount per unit mass source rock higher than 8 mg/g rock, organic matter maturity higher than 0.9%, overpressure within source rocks higher than 7 MPa, thickness of source rocks larger than 70 m, and hydrocarbon expulsion intensity higher than 50×104 t/km2. Type II: inefficient source containing mostly type II1 and some type I kerogen and with TOC ranging between 0.8% and 2.0%, hydrocarbon expulsion amount per unit mass source rock varying from 2 mg/g rock to 8mg/g rock, maturity ranging between 0.8% to 0.9%, overpressure within source rocks ranging between 1 MPa to 7 MPa, thickness of source rocks ranging between 30 m to 70 m, and hydrocarbon expulsion intensity of (2550)×104 t/km2. Type III: non-productive source containing mostly type II2 and III kerogen and with TOC below 0.8%, hydrocarbon expulsion amount per unit mass source rock less than 2 mg/g rock, maturity lower than 0.8%, overpressure within source rocks lower than 1MPa, thickness of source rocks less than 30 m, and hydrocarbon expulsion intensity lower than 25×104 t/km2.
3. Application of grading evaluation criteria of tight oil source rocks First of all, logging data and geochemical logging techTable 2.
Grading evaluation criteria of tight oil source rocks in the Songliao Basin
Type of source rocks Ⅰ Ⅱ Ⅲ
nologies were used to carry out the geochemical parameter quantitative characterization of source rocks[22, 47]. Based on the grading evaluation criteria, the residual TOC and the amount of residual hydrocarbon (pyrolysis S1 or chloroform bitumen “A”), the source rocks of the Qingshankou 1st Member were classified. Then, combining with the kerogen type, burial history and thermal history, the amount of hydrocarbon generation was calculated by using the chemical kinetics method. Thirdly, the hydrocarbon expulsion amount of source rocks was evaluated by using the material balance method and the residual hydrocarbon amount[44]. Finally, the overpressure during different geological periods was simulated by the basin simulation software[48]. The relationships between the thicknesses of different types of source rocks and the thickness of the F-Y oil layer (Fig. 7) show a positive correlation between the thickness of type I source rocks and the thickness of the tight oil layer (Fig. 7a). Although a weak positive correlation between the thickness of type II source rocks and the thickness of the tight oil layer can be observed, the small slope indicates the controlling effect of type II source rocks on the thickness of the tight oil layer is not significant (Fig. 7b). There is no correlation between the thickness of type III source rocks and the thickness of the tight oil layer (Fig. 7c). Therefore, it can be concluded that the migration and accumulation of tight oil are controlled by the excellent source rocks. In areas of excellent source rock development, tight oil are likely to accumulate and become enriched. In addition, the accumulation and distribution of tight oil is also controlled by the hydrocarbon expulsion intensity and overpressure within source rocks (Figs. 2 and 3). The higher hydrocarbon expulsion intensity and overpressure easily lead to the larger downward migration distance of tight oil and the thicker tight oil layer. Therefore, prediction of the favorable areas of tight oil should be based on the comprehensive evaluation of the distribution of hydrocarbon expulsion intensity, overpressure and tight reservoirs. On the map (Fig. 8) the superimposed isopach map of F-Y oil layer, the hydrocarbon expulsion isoline of 50×104 t/km2, the overpressure isoline of 10 Mpa at the sedimentary end of Nenjiang Formation (the sedimentary end of Nenjiang Formation is the period of tight oil filling into F-Y oil layer. Based on the characteristics of overpressure evolution, the paleo-overpressure of 10 Mpa in this period is equivalent to the current overpressure of 7 Mpa), and the tight sandstone boundary line is established (Fig. 8), and the distribution range of tight oil favorable areas in the southern Songliao Basin can be obtained.
Hydrocarbon exHydrocarbon expulsion Maturity/ Overpressure within Thickness of Residual pulsion intensity/ amount per unit mass % source rocks/MPa source rocks/m TOC/% (104 t·km2) source rock/(mg·g1) Mostly Ⅰ and someⅡ1 >2.0 >8 >0.9 >7 70130 >50 Mostly Ⅱ1 and someⅠ 0.82.0 28 0.80.9 17 3070 2550 Mostly Ⅱ2, Ⅲ and someⅡ1 <0.8 <2 <0.8 <1 1030 <25 Kerogen type
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Fig. 8.
Fig. 7. Relationship between the thicknesses of different type source rocks and the thickness of F-Y oil layer.
4.
Conclusion and discussion
Based on the quantitative evaluation of hydrocarbon expulsion and overpressure of the Qingshankou 1st Member, the lower limits and grading evaluation criteria of tight oil source rocks in the southern Songliao Basin were established. The lower limits of tight oil source rocks are hydrocarbon expulsion amount of 2 mg/g rock, residual TOC of 0.8%, and overpressure of 1 MPa. Excellent source rocks (type I) contain TOC >2.0%, overpressure >7 Mpa, mainly type I and some type II1 kerogen, maturity >0.9%, hydrocarbon expulsion >8mg/g rock, thickness of source rocks >70 m, and hydrocarbon expulsion intensity >50×104 t/km2. Inefficient source rocks (type II) contain TOC ranging between 0.8% and 2.0%, overpressure varies from 1 Mpa to 7 Mpa, mainly type II1 and some type I kerogen, maturity ranging between 0.8% and 0.9%, hydrocarbon expulsion of 2-8mg/g rock, thickness of source rocks vary from 30 m to 70 m, and hydrocarbon expulsion intensity of
Tight oil favorable areas in the southern Songliao Basin.
(2550)×104 t/km2. Non-productive source rocks (type III) contain TOC <0.8%, overpressure <1Mpa, mainly type II2, III and some type II1 kerogen, maturity <0.8%, hydrocarbon expulsion <2 mg/g rock, thickness of source rocks >70 m, and hydrocarbon expulsion intensity <25×104 t/km2. On the basis of the source rock classification and the development and distribution of tight reservoirs, the tight oil favorable exploration areas can be predicted. The lower limits (e.g., the residual TOC of 0.8%) and the grading limits (e.g., the residual TOC of 2.0%) of tight oil source rocks are higher than the lower limits of conventional oil and gas source rocks (e.g., the residual TOC of 0.4%) and the boundary of good source rocks (e.g., the residual TOC of 1.0%), respectively[2325], and are comparable to that of highquality source rocks (the residual TOC of 2.0%)[25]. Conventional oil and gas, which can be expelled from the source rocks, is possible to accumulate and enrich gradually driven by buoyancy. But for tight oil, hydrocarbon fluids discharged from the source rock must overcome the capillary resistance beyond the buoyancy to fill into a tight reservoir, so the accumulation dynamics can only come from overpressure which drives the oil and gas from the source rocks. Therefore, it is not enough for tight oil source rocks to have adequate hydrocarbon expulsion, which should have high overpressure. This suggests that the lower limits and grading limits of tight oil source rocks must be higher than that of conventional oil and gas. It should be point out that due to the differences in molecular size, interfacial tension among oil, gas, water and rock, and
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expansion coefficients of oil and gas during the generation process, may result in some differences in the driving force and resistance of oil and gas migration and accumulation in tight reservoirs. Therefore, the lower limits and grading criteria of source rocks for tight oil and/or tight gas will be different in principle. The object of this paper is to report on the source rocks with mainly oil generation and some gas generation. However, for the source rocks with mainly gas generation (with type III kerogen or higher maturity), it is necessary to reevaluate these rocks according to the research method of this paper and combine it with the actual geological conditions (e.g., geochemical characteristics of source rocks, burial history, thermal history, hydrocarbon generation and expulsion, overpressure development, etc.). Moreover, this is the next step we need to study. In addition, since the amount of hydrocarbon generation and residual hydrocarbon is related to the abundance, type and maturity of organic matter, mineral composition, hydrocarbon expulsion conditions (such as presence of sand-mud interbeds as favorable rock types for hydrocarbon expulsion) and so on, the relationship between hydrocarbon expulsion and residual TOC will be different in varying geological conditions. Therefore, the lower limits and grading criteria of tight oil source rocks in varying geological conditions should be reevaluated according to the method presented in this paper. But in areas with limited data or without relevant previous evaluation, the lower limits and grading evaluation criteria provided in this paper can be used in similar geological conditions (such as oil and gas accumulation in tight reservoirs beyond large set of mudstones).
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RESEARCH PAPER
Lower limits and grading evaluation criteria of tight oil source rocks of southern Songliao Basin, NE China LU Shuangfang1, HUANG Wenbiao1, *, LI Wenhao1, XUE Haitao1, XIAO Dianshi1, LI Jijun1, WANG Min1, WANG Weiming1, CHEN Fangwen1, ZHANG Jun2, DENG Shouwei3, TANG Zhenxing3 1. Research Institute of Unconventional Petroleum and Renewable Energy, Qingdao, Shandong 266580, China; 2. Sinopec Corp, Beijing 100728, China; 3. Petroleum Exploration & Development Research Institute, PetroChina Jilin Oilfield Company, Songyuan, Jilin 138000, China
Abstract: Taking the southern Songliao Basin as the target area, the hydrocarbon expulsion volume from source rocks has been quantitatively evaluated in this paper, based on the material balance method. Employing the “Overpressure” module of PetroMod software, the overpressure history of the source rocks was evaluated. According to the relationships between hydrocarbon expulsion rate, residual organic carbon content and overpressure within the source rocks, the lower limits of expulsion from tight oil source rocks were determined: a hydrocarbon expulsion amount of 2 mg/g, residual hydrocarbon content of 0.8%, and overpressure of 1 MPa. The source rocks with hydrocarbon expulsion amounts > 8 mg/g, a residual organic carbon content > 2.0%, and overpressure > 7 MPa were defined as excellent source rocks. As a result, tight oil source rocks can be divided into three types, excellent source rocks (typeⅠ), inefficient source rocks (typeⅡ) and non-productive source rocks (type Ⅲ). The actual application in the southern Songliao Basin shows that the excellent source rocks have an obvious control on the planar and vertical distribution of tight oil discoveries, areas with excellent source rocks and nearby formations are favorable for the accumulation of tight oil. Key words: tight oil; source rocks; grading evaluation; hydrocarbon expulsion; overpressure; southern Songliao Basin
Introduction Tight oil and gas has great potential for exploration and development in China[117]. Zou et al. (2011) assessed the tight gas recoverable resources in China to be (1520)×1012 m3 and the tight oil geological resources to be (110135)×108 t [1,34,6]. By analogy, Jia et al. (2012) believed that the geological and recoverable resources of tight gas were of the order of (17.4 25.1)×1012 m3 and (8.812.1)×1012 m3, respectively. The geological and recoverable resources of tight oil were about (7480)×108 t and (1314)×108 t, respectively[7, 8]. Compared with the huge potential of tight oil, their exploration and development are still lagging behind. Therefore, it is necessary to strengthen the exploration and development of tight oil and related evaluation work. According to the relationship between the reservoir development and compaction period and the hydrocarbon migration period, tight oil accumulation can be divided into three types theoretically[2, 3]: (1) Tight oil reservoir formation before reservoir compaction; (2) Tight oil reservoir formation after res-
ervoir compaction; and (3) simultaneous reservoir compaction and hydrocarbon accumulation. In case (1), oil and gas migrate to the structural high driven by buoyancy, where the mechanism and distribution of tight oil reservoirs are the same as those of the conventional oil and gas reservoirs. It is difficult to form a large areal distribution of this kind of tight oil due to the constraints of traps, so it is typically not the main target for tight oil exploration and evaluation. For larger accumulations of tight oil, the essential difference from conventional oil and gas reservoirs is that the migration and accumulation of the oil and gas mainly depends on overpressure, rather than buoyancy[2]. This feature determines that the accumulation, enrichment and distribution of tight oil depends on the source rock quality (determining the size of hydrocarbon accumulation dynamics), reservoir characteristics (determining the difficulty or ease of hydrocarbon accumulation) and their matching relationship in time and space[1822]. Therefore, the lower limits of source rocks and reservoirs for tight oil, as well as their grading evaluation criteria have be-
Received date: 15 Feb. 2016; Revised date: 23 Mar. 2017. * Corresponding author. E-mail: [email protected] Foundation item: Supported by the National Natural Science Foundation of China (41330313, 41402109); China National Science and Technology Major Project (2016ZX05046-001-005). Copyright © 2017, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.
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come the current research focus. The classification industry standard of conventional oil and gas source rocks has been widely applied. In addition, the characteristics of tight oil source rocks and their relationship with conventional oil and gas reservoirs have also been reported in many examples in the literature[2326]. However, the criteria used in the industry standard of conventional oil and gas source rocks do not apply to tight oil because of the special accumulation mechanism. It is generally recognized that high-quality source rocks are the key controlling factor of hydrocarbon accumulations[27,1416,1822,2731], and the hydrocarbon source conditions of tight oil have been discussed previously[3,6,1420,3236]. However, the research on lower limits and the classification criterion of tight oil source rocks are still reported rarely. Even if some internal standards have been applied, they lack a rigorous and scientific theoretical basis. Pressure difference between source rocks and reservoirs, depends on the overpressure within source rocks, is the main driving force for tight oil migration and accumulation. The formation of overpressure is related to hydrocarbon generation, compaction disequilibrium, thermal maturation, clay mineral dehydration, tectonic stress, etc. Among them, hydrocarbon generation pressurization and compaction disequilibrium are considered to be the most common and important factors. Furthermore, compaction disequilibrium, thermal pressurization, and clay mineral dehydration are coincident with the direction and period of hydrocarbon generation [37-38]. The hydrocarbon expulsion amount is the most direct indicataor to measure the degree of the hydrocarbon generation pressurization. Larger hydrocarbon expulsion amounts indicate the higher driving force of oil and gas for migration and accumulation. There are many methods for the quantitative evaluation of hydrocarbon expulsion, in which the methods of material balance and hydrocarbon generation potential are two widely used and effective approaches[3946]. At present, there is no study to match hydrocarbon expulsion to the lower limits of generation and accumulation and classification evaluation criteria of tight oil source rocks. Based on the actual data of the study area, the geochemical characteristics of source rock were analyzed, and the heterogeneity and distribution of source rocks in the laterally and vertically were characterized by a combination of well logging geochemical techniques[22,47]. Based on thermal simulation experiments on immature source rock samples, a chemical kinetic model was established to evaluate the amount of hydrocarbon generation, residual hydrocarbon and free hydrocarbon expulsion[4346]. Based on the sedimentary burial history, thermal history, and pore evolution history, the overpressure caused by hydrocarbon generation, thermal maturation, clay dehydration, compaction disequilibrium and structural stress was simulated by using the overpressure simulation module of PetroMod software[4852]. On the basis of the above study, the relationship between the amount of hydrocarbon expulsion, total organic carbon content (TOC) and
overpressure within source rocks was established. The lower limits of viable tight oil source rocks were determined according to the inflection points, and provided operable and comparable evaluation criteria for tight oil exploration and evaluation.
1.
Geological setting
The Songliao Basin, a Mesozoic-Cenozoic oil- and gasbearing continental basin, is a faulted-depression composite basin. The depression formations, from old to new strata, are the Lower Cretaceous Denglouku and Quantou Formations and the Upper Cretaceous Qingshankou, Yaojia, Nenjiang, Sifangtai and Mingshui Formations, respectively. Among them, the Qingshankou 1st, 2nd, 3rd Members and the Nenjiang 1st Member are the main source rocks in the depression period (Fig. 1). The Fuyu and Yangdachengzi (F-Y) oil layers of Jilin oilfield, located in the southern Songliao Basin, are developed in the Quantou 4th and 3rd Members, respectively. In addition to the slope and uplift zones, the depth to the main reservoirs vary from 1 750 m to 2 600 m, which are typical tight reservoirs with porosity < 12% and permeability < 1×103 μm2. The sedimentation of the Quantou 4th and 3rd Members was accompanied by the rising of the basin base level and strong fluvial action. The fluvial facies, delta plain and front subfacies are widely developed, and the small area and overlapping sandstones provide the main storage space for tight oil. The overlying source rocks with high TOC, good kerogen type, moderate maturity, huge hydrocarbon generation and expulsion potential, and obvious overpressure in Qingshankou 1st Member not only make important contributions to the accumulation of conventional oil and gas in overlying strata, but also provide sufficient protection for the formation of the underlying F-Y oil layer. A large area of channel sandstones in the F-Y oil layer are adjacent to the overlying source rocks of Qingshankou 1st member, forming a good source-reservoir matching relationship, providing a good foundatation for the formation of tight oil reservoirs[1522]. The research and exploration results have demonstrated that the tight oil in the southern Songliao Basin is mainly distributed in Honggang terrace-Changling sag-Fuxin uplift, and the most prospective area is 1.4×104 km2. As of 2014, the proved tight oil reserves are 2.2×108 t.
2. The lower limits of tight oil source rocks and grading evaluation criteria 2.1. Relationship between hydrocarbon expulsion, overpressure, production and distribution of tight oil A typical positive correlation between the overpressure within the Qingshankou 1st Member at the sedimentary end of the Nenjiang Formation (tight oil accumulation period) and the downward migration depth of tight oil (the maximum distance between the tight reservoirs with oil shows and the bottom of Qingshankou 1st Member) is present (Fig. 2). This indicates that overpressure has an important control on hydro-
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Fig. 1.
Geographical location of the study area and stratigraphy.
the lower limits and ranking evaluation criteria of tight oil source rocks based on the quantitative evaluation of hydrocarbon expulsion and overpressure. 2.2. Lower limits and grading evaluation criteria of tight oil source rocks based on the amount of hydrocarbon expulsion
Fig. 2. Relationship between overpressure within the Qingshankou 1st Member and downward discharge depth of tight oil at the late depositional stage of the Nenjiang Formation.
carbon accumulation. In fact, the enrichment and distribution of tight oil are affected by many factors, such as the heterogeneity of thin mudstone interbedding, vertical fractures, and physical properties of tight reservoirs, etc. The relationship between the tight oil distribution, single well production and hydrocarbon expulsion intensity shows the thicker F-Y oil layer and higher well productivity are often accompanied by strong hydrocarbon expulsion in the Qingshankou 1st Member source rocks (Table 1, Fig. 3). This phenomenon indicates that the hydrocarbon expulsion plays an important role in tight oil accumulation, and provides the basis for the establishment of
Based on the measured pyrolysis data and the material balance method, the scatter plot of the hydrocarbon expulsion amount per unit mass of rock and the original TOC of the Qingshankou 1st Member in the southern Songliao Basin can be obtained (Fig. 4). Based on the original TOC, the hydrocarbon expulsion amount can be classified into three types: (1) when the original TOC is less than 1.0%, the hydrocarbon expulsion amount from the source rocks is very small or alTable 1.
Source rock thickness, hydrocarbon expulsion intensity,
F-Y oil layer thickness and productivity of five wells Well No.
Source rock thickness/m
Hydrocarbon expulsion per unit mass rock/mg·g1
Thickness of F-Y oil layer/m
Productivity/t
Cha 27 Qian 215 Cha 45 Qian 226 Qian 227
75 92 73 69 75
8.6 11.7 20.0 8.3 11.9
56.5 87.5 135.0 67.0 90.0
1.77 12.78 80.80 4.17 60.80
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Fig. 3.
Reservoir profile and source rock characteristic of joint wells in the southern Songliao Basin, East China.
respectively. 2.3. Lower limits and grading evaluation criteria of tight oil source rocks based on the overpressure
Fig. 4. Relationship between the hydrocarbon expulsion amount per unit mass source rock and the original TOC of Qingshankou 1st Member.
most nothing. In this case, the material basis of hydrocarbon accumulation is deficient, so an original TOC of 1.0% corresponds to the lower limits of tight oil source rocks. (2) When the original TOC is varying from 1.0% to 2.5%, the hydrocarbon expulsion amount increases slowly, and begins to provide the necessary material for a tight oil reservoir. (3) When the original TOC is larger than 2.5%, the hydrocarbon expulsion amount increases rapidly, and provides sufficient oil and gas for migration and accumulation. In order to facilitate the application, the original TOC needs to be converted to the measured residual TOC. Considering the loss of TOC reduced by hydrocarbon expulsion and the weight loss of rocks due to the compaction, it can be calculated that the original TOC of 1.0% and 2.5% correspond to residual TOC values of 0.8% and 2.0%, respectively. They are the lower and grading limit of residual TOC for tight oil source rocks, respectively. Therefore, results suggest the TOC grading or classification for this basin are: non-productive tight oil source rocks (Type III) are less than 0.8%, inefficient tight oil source rocks (Type II) are between 0.82.0% and excellent tight oil source rocks (Type I) are greater than 2.0%,
Through the recovery of the overpressure history simulated by the overpressure module of Petromod software, the relationship between the overpressure, the hydrocarbon expulsion and the residual TOC can be established (for convenience, the current overpressures are used in this case) (Figs. 5 and 6). Fig. 5 shows that the overpressure corresponding to the inflection point of the curve is about 7 Mpa, and the corresponding hydrocarbon expulsion amount per unit mass rock is 8mg/g. Fig. 6 shows that the residual TOC corresponding to the inflection point of the curve is about 2%, which is consistent with the type I source rock limit determined by the hydrocarbon expulsion method, when the corresponding overpressure is about 7 MPa. When the residual TOC is larger than 2.0%, the overpressure will not continue to increase, indicating the oil and gas is being expelled from the source rocks. At this time, the source rocks can not only discharge a large amount of hydrocarbons, but also provide the effective drive force for oil and gas discharge into the tight reservoirs. The
Fig. 5. Relationship between current overpressure and hydrocarbon expulsion.
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Fig. 6. Relationships between the residual TOC and the current overpressure.
residual TOC of 0.8% is corresponds to an overpressure of 1 Mpa (Fig. 6) and the hydrocarbon expulsion amount of 2 mg/g (Fig. 5), which is consistent with the lower limits of tight oil source rocks determined by the hydrocarbon expulsion method. Based on the above analysis of results and combined with the geochemical parameters, the lower limits and grading evaluation criteria was established. They are classified into three types (Table 2). Type I: excellent source rocks containing mostly type I and some type II1 kerogen and with TOC higher than 2.0%, hydrocarbon expulsion amount per unit mass source rock higher than 8 mg/g rock, organic matter maturity higher than 0.9%, overpressure within source rocks higher than 7 MPa, thickness of source rocks larger than 70 m, and hydrocarbon expulsion intensity higher than 50×104 t/km2. Type II: inefficient source containing mostly type II1 and some type I kerogen and with TOC ranging between 0.8% and 2.0%, hydrocarbon expulsion amount per unit mass source rock varying from 2 mg/g rock to 8mg/g rock, maturity ranging between 0.8% to 0.9%, overpressure within source rocks ranging between 1 MPa to 7 MPa, thickness of source rocks ranging between 30 m to 70 m, and hydrocarbon expulsion intensity of (2550)×104 t/km2. Type III: non-productive source containing mostly type II2 and III kerogen and with TOC below 0.8%, hydrocarbon expulsion amount per unit mass source rock less than 2 mg/g rock, maturity lower than 0.8%, overpressure within source rocks lower than 1MPa, thickness of source rocks less than 30 m, and hydrocarbon expulsion intensity lower than 25×104 t/km2.
3. Application of grading evaluation criteria of tight oil source rocks First of all, logging data and geochemical logging techTable 2.
Grading evaluation criteria of tight oil source rocks in the Songliao Basin
Type of source rocks Ⅰ Ⅱ Ⅲ
nologies were used to carry out the geochemical parameter quantitative characterization of source rocks[22, 47]. Based on the grading evaluation criteria, the residual TOC and the amount of residual hydrocarbon (pyrolysis S1 or chloroform bitumen “A”), the source rocks of the Qingshankou 1st Member were classified. Then, combining with the kerogen type, burial history and thermal history, the amount of hydrocarbon generation was calculated by using the chemical kinetics method. Thirdly, the hydrocarbon expulsion amount of source rocks was evaluated by using the material balance method and the residual hydrocarbon amount[44]. Finally, the overpressure during different geological periods was simulated by the basin simulation software[48]. The relationships between the thicknesses of different types of source rocks and the thickness of the F-Y oil layer (Fig. 7) show a positive correlation between the thickness of type I source rocks and the thickness of the tight oil layer (Fig. 7a). Although a weak positive correlation between the thickness of type II source rocks and the thickness of the tight oil layer can be observed, the small slope indicates the controlling effect of type II source rocks on the thickness of the tight oil layer is not significant (Fig. 7b). There is no correlation between the thickness of type III source rocks and the thickness of the tight oil layer (Fig. 7c). Therefore, it can be concluded that the migration and accumulation of tight oil are controlled by the excellent source rocks. In areas of excellent source rock development, tight oil are likely to accumulate and become enriched. In addition, the accumulation and distribution of tight oil is also controlled by the hydrocarbon expulsion intensity and overpressure within source rocks (Figs. 2 and 3). The higher hydrocarbon expulsion intensity and overpressure easily lead to the larger downward migration distance of tight oil and the thicker tight oil layer. Therefore, prediction of the favorable areas of tight oil should be based on the comprehensive evaluation of the distribution of hydrocarbon expulsion intensity, overpressure and tight reservoirs. On the map (Fig. 8) the superimposed isopach map of F-Y oil layer, the hydrocarbon expulsion isoline of 50×104 t/km2, the overpressure isoline of 10 Mpa at the sedimentary end of Nenjiang Formation (the sedimentary end of Nenjiang Formation is the period of tight oil filling into F-Y oil layer. Based on the characteristics of overpressure evolution, the paleo-overpressure of 10 Mpa in this period is equivalent to the current overpressure of 7 Mpa), and the tight sandstone boundary line is established (Fig. 8), and the distribution range of tight oil favorable areas in the southern Songliao Basin can be obtained.
Hydrocarbon exHydrocarbon expulsion Maturity/ Overpressure within Thickness of Residual pulsion intensity/ amount per unit mass % source rocks/MPa source rocks/m TOC/% (104 t·km2) source rock/(mg·g1) Mostly Ⅰ and someⅡ1 >2.0 >8 >0.9 >7 70130 >50 Mostly Ⅱ1 and someⅠ 0.82.0 28 0.80.9 17 3070 2550 Mostly Ⅱ2, Ⅲ and someⅡ1 <0.8 <2 <0.8 <1 1030 <25 Kerogen type
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Fig. 8.
Fig. 7. Relationship between the thicknesses of different type source rocks and the thickness of F-Y oil layer.
4.
Conclusion and discussion
Based on the quantitative evaluation of hydrocarbon expulsion and overpressure of the Qingshankou 1st Member, the lower limits and grading evaluation criteria of tight oil source rocks in the southern Songliao Basin were established. The lower limits of tight oil source rocks are hydrocarbon expulsion amount of 2 mg/g rock, residual TOC of 0.8%, and overpressure of 1 MPa. Excellent source rocks (type I) contain TOC >2.0%, overpressure >7 Mpa, mainly type I and some type II1 kerogen, maturity >0.9%, hydrocarbon expulsion >8mg/g rock, thickness of source rocks >70 m, and hydrocarbon expulsion intensity >50×104 t/km2. Inefficient source rocks (type II) contain TOC ranging between 0.8% and 2.0%, overpressure varies from 1 Mpa to 7 Mpa, mainly type II1 and some type I kerogen, maturity ranging between 0.8% and 0.9%, hydrocarbon expulsion of 2-8mg/g rock, thickness of source rocks vary from 30 m to 70 m, and hydrocarbon expulsion intensity of
Tight oil favorable areas in the southern Songliao Basin.
(2550)×104 t/km2. Non-productive source rocks (type III) contain TOC <0.8%, overpressure <1Mpa, mainly type II2, III and some type II1 kerogen, maturity <0.8%, hydrocarbon expulsion <2 mg/g rock, thickness of source rocks >70 m, and hydrocarbon expulsion intensity <25×104 t/km2. On the basis of the source rock classification and the development and distribution of tight reservoirs, the tight oil favorable exploration areas can be predicted. The lower limits (e.g., the residual TOC of 0.8%) and the grading limits (e.g., the residual TOC of 2.0%) of tight oil source rocks are higher than the lower limits of conventional oil and gas source rocks (e.g., the residual TOC of 0.4%) and the boundary of good source rocks (e.g., the residual TOC of 1.0%), respectively[2325], and are comparable to that of highquality source rocks (the residual TOC of 2.0%)[25]. Conventional oil and gas, which can be expelled from the source rocks, is possible to accumulate and enrich gradually driven by buoyancy. But for tight oil, hydrocarbon fluids discharged from the source rock must overcome the capillary resistance beyond the buoyancy to fill into a tight reservoir, so the accumulation dynamics can only come from overpressure which drives the oil and gas from the source rocks. Therefore, it is not enough for tight oil source rocks to have adequate hydrocarbon expulsion, which should have high overpressure. This suggests that the lower limits and grading limits of tight oil source rocks must be higher than that of conventional oil and gas. It should be point out that due to the differences in molecular size, interfacial tension among oil, gas, water and rock, and
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expansion coefficients of oil and gas during the generation process, may result in some differences in the driving force and resistance of oil and gas migration and accumulation in tight reservoirs. Therefore, the lower limits and grading criteria of source rocks for tight oil and/or tight gas will be different in principle. The object of this paper is to report on the source rocks with mainly oil generation and some gas generation. However, for the source rocks with mainly gas generation (with type III kerogen or higher maturity), it is necessary to reevaluate these rocks according to the research method of this paper and combine it with the actual geological conditions (e.g., geochemical characteristics of source rocks, burial history, thermal history, hydrocarbon generation and expulsion, overpressure development, etc.). Moreover, this is the next step we need to study. In addition, since the amount of hydrocarbon generation and residual hydrocarbon is related to the abundance, type and maturity of organic matter, mineral composition, hydrocarbon expulsion conditions (such as presence of sand-mud interbeds as favorable rock types for hydrocarbon expulsion) and so on, the relationship between hydrocarbon expulsion and residual TOC will be different in varying geological conditions. Therefore, the lower limits and grading criteria of tight oil source rocks in varying geological conditions should be reevaluated according to the method presented in this paper. But in areas with limited data or without relevant previous evaluation, the lower limits and grading evaluation criteria provided in this paper can be used in similar geological conditions (such as oil and gas accumulation in tight reservoirs beyond large set of mudstones).
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