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Discovery of effective scale source rocks of the Ordovician Majiagou Fm in the Ordos Basin and its geological significance
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ScienceDirect Natural Gas Industry B 3 (2016) 330e338 www.elsevier.com/locate/ngib
Research Article
Discovery of effective scale source rocks of the Ordovician Majiagou Fm in the Ordos Basin and its geological significance* Tu Jianqi a,b,*, Dong Yiguo a,c, Zhang Bin a,b, Nan Hongli a,d, Li Chengjian a,d, Wang Xiaomei a,b, Fei Xuandong a,b, Zhou Wenbao a,b a
Laboratory Research Center of Petroleum Geology, PetroChina Research Institute of Petroleum Exploration and Development, Beijing, 100083, China b CNPC Key Laboratory of Petroleum Geochemistry, Beijing, 100083, China c School of Energy Resources, China University of Geosciences, Beijing, 100083, China d College of Geosciences and Surveying Engineering, China University of Mining & Technology, Beijing, 100083, China Received 17 February 2016; accepted 6 June 2016 Available online 1 March 2017
Abstract There have been different views on the origins of gas reservoirs of Ordovician Majiagou Fm in the Ordos Basin, but none of them supports the opinion that there are scale source rocks (TOC>0.5%) in the Majiagou Fm. In this paper, a series of analysis was made on cores and cutting samples taken from recent newly-drilled wells in the Ordovician reservoirs in the centraleeastern areas of the Ordos Basin. Accordingly, the organic abundance features of different types of source rocks in the Majiagou Fm were presented, and for the first time, it was discovered and verified that there are effective scale source rocks with high organic abundance. Based on these analysis and studies, the following results were achieved. First, the effective source rocks of Majiagou Fm are composed of thinethick layered dark dolomite-bearing mudstones, dolomitic mudstones and argillaceous dolomites, and their enrichment and distribution are obviously controlled by sedimentary facies. During the sedimentation of Majiagou Fm, the effective scale source rocks developed better at the regressive stage than at the transgressive stage. The effective source rocks are primarily distributed in the upper part of Ma 5 Member (especially concentrated in the third and fourth sub-members) and secondly in Ma 3 Member, Ma 1 Member and the middleelower part of Ma 5 Member. Second, the effective source rocks are areally distributed in a dual-girdle form around the Mizhi Salt Depression, and those in the secondary depressions are mediumethick layered, with a large total thickness and high organic abundance. Third, the effective source rocks in the center of Mizhi Depression and the secondary uplifts are thin-layered, with a small total thickness and low organic abundance. And fourth, the organic abundance of the effective source rocks varies from 0.30% to 8.45%. Phytoplankton and Acritarchs are the main parent materials of hydrocarbon, and organic matters are of sapropel or sapropel prone type. To sum up, the Ordovician effective source rocks in the Ordos Basin are generally at an over-mature stage and they contribute greatly to the natural gas in the Majiagou Fm. In these effective source rocks, dry gas is mainly generated at a high rate. © 2017 Sichuan Petroleum Administration. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Keywords: Ordos basin; Ordovician; Effective scale source rocks; Organic abundance; Dolomite-bearing mudstone; Dolomitic mudstone; Argillaceous dolomite; Sedimentary facies
* Project supported by the S&T Project of PetroChina Exploration & Production Company “Evaluation on Ordovician source rocks in the Ordos Basin” (No.: 2012-011) and “Characteristics and comprehensive evaluation of Ordovician gas sources and optimization of favorable zones in the Ordos Basin” (No.: 2014-008). * Corresponding author. Laboratory Research Center of Petroleum Geology, PetroChina Research Institute of Petroleum Exploration and Development, Beijing, 100083, China. E-mail address: [email protected] (Tu JQ). Peer review under responsibility of Sichuan Petroleum Administration.
http://dx.doi.org/10.1016/j.ngib.2016.12.009 2352-8540/© 2017 Sichuan Petroleum Administration. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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1. Background The origins of natural gas in the Ordovician Majiagou Fm have been the controversial topic since the Jinbian Gas Field (i.e., the central gas field) was discovered and explored in the Ordos Basin at the end of 1980s. There are mainly three views. First, it is mainly self-generating and self-preserving oil-type gas, which is generated by highly-mature to over-mature carbonate source rocks with TOC (total organic carbon) lower than 0.2% [1e3]. Second, it is mainly coal-type gas. The mixed gas composed of coal-type gas derived from CarboniferousePermian highly-mature and over-mature coal measure source rocks and a small amount of oil-type gas derived from Carboniferous limestones migrated laterally and accumulated after flowing backward [4e7]. And third, it is mainly outside-generating and inside-preserving oil-type gas. The liquid hydrocarbon generated by the MiddleeUpper Ordovician marls and shales in the western/southwestern Ordos Basin accumulated at the slope controlled by the central paleo-uplift after long-distance migration. After cracking and diffusing, the gaseous hydrocarbon migrated into the Majiagou Fm reservoirs inside the basin, and mixed with the coal-type gas generated by the CarboniferousePermian source rocks [8e10]. These three views are different, but they all agree that the Ordovician Majiagou Fm was deposited in the environment of shallow water carbonate platform, showing the mode of “consumption”e“dilution” with developed non-source rocks, and no scale hydrocarbon source rocks with TOC>0.5% was developed in it [9e17]. According to the researches on the development characteristics and the minimum organic abundance of marine source rocks all around the world in recent years, dark argillaceous rocks and argillaceous carbonates which are distributed extensively with a certain thickness and TOC of 1e2% are the prerequisite for the formation of large marine oil (gas) fields, and the pure carbonate rocks with quite low TOC cannot form large oil and gas fields [18,19]. When the marine source rocks at high maturityeover maturity stage is evaluated, the minimum TOC of effective oil (gas) source rocks should not be lower than 0.5% (0.3%), and the minimum TOC of highquality hydrocarbon (oil) source rocks should not be lower than 1.0% [20e22]. Accordingly, it is generally concerned whether effective scale source rocks with TOC>0.5% and even TOC>1.0% are developed in the Ordovician Majiagou Fm in the Ordos Basin besides the thick marine pure carbonate rocks of low organic abundance (TOC<0.2%). This is related to further understanding on the origins of Ordovician Majiagou Fm natural gas which has been a long-term controversial focus. Moreover, it is directly related to the confirmation of future Ordovician natural gas exploration targets in the Ordos Basin. The Majiagou Fm in the Ordos Basin is located in the central-eastern area of the basin, where the Jinbian Gas Field (i.e., the central gas field) lies. It is divided into 6 apparent lithologic members, i.e., Ma 1eMa 6. Specifically, Ma 2, Ma 4 and Ma 6 Members were deposited in the transgressive stage and they are lithologically composed of limestones and dolomites. Ma 1, Ma 3 and Ma 5 Members were deposited in the
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regressive stage and they are mainly composed of mudstonebearing dolomites, argillaceous dolomites, dolomitic mudstones and gypsum-salt rocks [23]. Ma 5 Member is subdivided into 10 sub-members in its upper, middle and lower parts, namely, the upper part of Ma 5 Member (the first to the fourth sub-member, or Ma 51 to Ma 54), the middle part of Ma 5 Member (the fifth sub-member, or Ma 55), and the lower part of Ma 5 Member (the sixth to the tenth sub-member, or Ma 56 to Ma 510). It is indicated from hydrocarbon researches and exploration practices around the world that petroliferous basins are in close relation to gypsum-salt-bearing basins, and restricted platforms are favorable for the development of hydrocarbon source rocks with high organic abundance. Therefore, this paper focuses on the gypsum-salt lake sedimentation systems of the Ordovician Majiagou Fm in the Ordos Basin. Based on previous research results [24], the cores and cutting samples taken from newly-drilled wells in the Ordovician reservoirs in the centraleeastern area of the Ordos Basin were analyzed in terms of their organic geochemistry. The organic abundances of different types of source rocks in the Ordovician Majiagou Fm were further identified. It was discovered and proved that there are effective scale source rocks with high organic abundance in the Ordovician Majiagou Formation. Furthermore, the development intervals and distribution characteristics of Majiagou Fm effective source rocks were described. The hydrocarbon generation intensity and resources of effective source rocks were calculated, demonstrating the immense hydrocarbon-generating capacity and exploration potential of the formation. 2. Organic geochemical characteristics of source rocks 2.1. Organic abundance Not all the marine sedimentary rocks can act as source rocks. High-energy environments or over-oxidized environments are unfavorable for the development of marine source rocks. Better marine source rocks are usually developed in lowenergy waters and relatively-reducing environments. Therefore, it is discriminated that the possible source rocks of Majiagou Fm in the Ordos Basin are lithologically acted by two types of rocks, i.e., dark (dark grey, grey black and black) pure carbonate rocks (including limestones, dolomites, mudstone-bearing limestones and mudstone-bearing dolomites) and argillaceous carbonate rocks (including dolomitebearing mudstones, dolomitic mudstones, argillaceous dolomites, limy mudstones and marls). The Majiagou Fm in the Ordos Basin is currently at high maturityeover maturity stage. Due to maturity, its geochemical index (e.g. rock pyrolysis index and chloroform bitumen “A” content) can hardly be used for organic abundance evaluation. Therefore, the total organic carbon (TOC ) of rocks is the only available evaluation index. The Majiagou Fm dark argillaceous carbonate rocks are, on the whole, close to the standard of high-quality oil source rocks, for the TOC of their 916 samples varies between 0.17% and 8.45%, averaging 0.83%. Samples with TOC>0.3%, TOC>0.5% and TOC>1.0% account for 86%, 66% and 33%
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of the total samples respectively. Correspondingly, the ratio of effective gas source rocks, effective oil source rocks and highquality oil source rocks is 86%, 66% and 33% respectively. On the contrary, the TOC of 823 samples from the Majiagou Fm dark pure carbonate rocks are almost lower than 0.3%, generally in the range of 0.03e0.32%, averaging 0.18%, and samples with TOC<0.2% account for more than 70%. It is indicated that the Majiagou Fm dark pure carbonate rocks are non-source rocks or poor source rocks, and they can act as neither effective gas source rocks nor effective oil source rocks (Fig. 1). To sum up, there are substantial effective source rocks of dark argillaceous carbonate rocks (including argillaceous rocks) in the Majiagou Fm, Ordos Basin. 2.2. Rock types of effective source rocks The rock types of Majiagou Fm effective source rocks are lithologically distinguished from the overlying source rocks (e.g. limy mudstones and marls) in the Upper Paleozoic CarboniferousePermian coal measures, and they are mainly dark (black, grey black and dark grey) dolomite-bearing mudstones, dolomitic mudstones and argillaceous dolomites. The lamellations or laminae are quite developed and they can be observed directly (Fig. 2aed). It is shown from microscopic analysis that the widely distributed microlaminae is the reason for the well development of lamellations or laminae in effective source rocks. Microlaminae comprise micrite-rich dolomite laminae, organic laminae rich algae laminae and clayrich laminae. All of them are vertically superimposed with each other and form two-layer or three-layer sedimentary patterns, which are quite similar to the principal oil source rocks (the Lower Cretaceous lacustrine dolomitic mudstones and argillaceous dolomites) in the Jiuxi Basin, China, in terms of rock types and microlaminae structures [25]. Algae laminaes are subtranslucent or opaque brown-black without fluorescence due to higher degree of thermal evolution (Fig. 2e). 2.3. Types of organic matters Types of organic matters are the comprehensive reflection of organic biogenesis constitution in source rocks. It is shown from microscopic analysis that planktonic algae and acritarchs
are the main hydrocarbon-generating parent materials of Majiagou Fm effective source rocks in this area. The kerogen maceral of effective source rocks is dominantly sapropelinite. Sapropelic amorphogen is predominant (generally over 85%) with a small amount of better-preserved alginite and acritarchs (Fig. 2-feg). The other macerals are marine vitrinite and marine intertinite, with total content less than 15%, except for the special samples (Fig. 3). The organic matters of effective source rocks are sapropel or sapropel prone type. 2.4. Maturity of organic matters There is no vitrinite in the Early Paleozoic marine source rocks where higher plants are lacking, so vitrinite reflectance cannot be directly used to calibrate the maturity of organic matters. At present, vitrinite-like maceral (marine vitrinite) is selected as the primary object for reflectance measurement. The vitrinite-like maceral reflectance is taken as the organic maturity index of Early Paleozoic source rocks to reflect its thermal evolution degree, especially at high maturityeover maturity stage [26,27]. The Majiagou Fm effective source rocks are at over-maturity stage and dry gas is mainly generated. Their equivalent vitrinite reflectance (Ro) is generally over 2.0%. The equivalent vitrinite reflectance (Ro) of the area to the south of WuqieAnsaieShilou is higher than 2.5% and it is regionally high in the south and low in the north and decreases gradually to the northeast. It is at its early stage of high maturity in the area of ShenmueWell Fu 5 with the equivalent vitrinite reflectance (Ro) of about 1.5% (Fig. 4). 3. Scale development and distribution of source rocks 3.1. Vertical development sections of source rocks Table 1 shows the TOC distribution of possible source rocks in members of the Ordovician Majiagou Fm in the Ordos Basin. 1) There are 449 samples of dark argillaceous carbonate rocks from the upper part of Ma 5 Member, and their TOC varies in the range of 0.18e7.48%, averaging 1.14%. The upper part of Ma 5 Member satisfies the standard of highquality oil source rocks, with effective gas source rocks of
Fig. 1. Distribution frequency of total organic carbon (TOC ) in the possible source rocks of the Ordovician Majiagou Fm.
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Fig. 2. Macro and micro characteristics of core samples from Ordovician effective source rocks.
Fig. 3. Constitutional characteristics of kerogen macerals in Ordovician effective source rocks.
88%, effective oil source rocks of 72% and high-quality oil source rocks of 43%. The effective source rocks are lithologically composed of dolomite-bearing mudstones, dolomitic mudstones and argillaceous dolomites, and they are mostly distributed in a mediumethick layered form. It is demonstrated that there are substantial effective source rocks in the upper part of Ma 5 Member. 2) There are 225 samples of dark argillaceous carbonate rocks from the middleelower part of Ma 5 Member, and their TOC varies in the range of 0.19e8.45%, averaging 0.96% (close to 1.0%). The middleelower part of Ma 5 Member satisfies the standard of high-quality oil source rocks, with
Fig. 4. Contour map of the equivalent vitrinite reflectance (Ro) at the top surface of Ordovician.
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Table 1 TOC distribution of possible source rocks in members of the Ordovician Majiagou Fm. Member
Ma 5
Rock type
Upper MiddleeLower
Ma 4 Ma 3 Ma 2 Ma 1
Argillaceous carbonate Pure carbonate rocks Argillaceous carbonate Pure carbonate rocks Argillaceous carbonate Pure carbonate rocks Argillaceous carbonate Pure carbonate rocks Argillaceous carbonate Pure carbonate rocks Argillaceous carbonate Pure carbonate rocks
TOC%
rocks rocks rocks rocks rocks rocks
Effective gas source rocks
Effective oil source rocks
High-quality source rocks
Distribution range
Sample number
Average
TOC>0.3%
TOC>0.5%
TOC>1.0%
0.18e7.48 0.06e0.31 0.19e8.45 0.05e0.29 0.17e1.80 0.06e0.31 0.18e4.15 0.05e0.28 0.17e0.99 0.04e0.26 0.18e1.41 0.06e0.27
449 212 255 189 38 214 98 134 35 54 41 20
1.14 0.18 0.96 0.17 0.53 0.17 0.71 0.17 0.41 0.18 0.42 0.17
88% 5% 93% 0 71% 1% 77% 0 79% 0 58% 0
72% 0 80% 0 39% 0 43% 0 36% 0 19% 0
43% 0 33% 0 10% 0 17% 0 0 0 7% 0
effective gas source rocks of 93%, effective oil source rocks of 80% and high-quality oil source rocks of 33%. The effective source rocks are lithologically composed of dolomite-bearing mudstones, dolomitic mudstones and argillaceous dolomites, and they are mostly distributed in a mediumethick layered form. It is demonstrated that there are substantial effective source rocks in the middleelower part of Ma 5 Member. 3) There are 38 samples of dark argillaceous carbonate rocks from Ma 4 Member, and their TOC varies in the range of 0.17e1.80%, averaging 0.53%. Ma 4 Member satisfies the standard of effective oil source rocks, with effective gas source rocks of 71%, effective oil source rocks of 39% and highquality oil source rocks of 10%. The effective source rocks are lithologically composed of limy mudstones, marls and argillaceous dolomites, and they are mostly distributed in a thin interlayered form. The cumulative thickness of effective source rocks accounts for just 1e2% of total thickness of Ma 4 Member in each well. It is demonstrated that there are only slight effective source rocks in Ma 4 Member. 4) There are 98 samples of dark argillaceous carbonate rocks from Ma 3 Member, and their TOC varies in the range of 0.18e4.15%, averaging 0.71%. Ma 3 Member satisfies the standard of effective oil source rocks, with effective gas source rocks of 77%, effective oil source rocks of 43% and highquality oil source rocks of 17%. The effective source rocks are lithologically composed of dolomite-bearing mudstones, dolomitic mudstones and argillaceous dolomites, and they are mostly distributed in a mediumethick layered form. It is demonstrated that there are substantial effective source rocks in Ma 3 Member. 5) There are 35 samples of dark argillaceous carbonate rocks from Ma 2 Member, and their TOC varies in the range of 0.17e0.99%, averaging 0.41%. Ma 2 Member satisfies the standard of effective gas source rocks, with effective gas source rocks of 79% and effective oil source rocks of 36%. The effective source rocks are lithologically composed of limy mudstones, marls and argillaceous dolomites. Limy mudstones and marls are mostly distributed in a thin interlayered form, and argillaceous dolomites are mostly distributed in a medium-thickness layered form. It is demonstrated that there
are a certain number of effective source rocks in Ma 2 Member. 6) There are 41 samples of dark argillaceous carbonate rocks from Ma 1 Member, and their TOC varies in the range of 0.18e1.41%, averaging 0.42%. Ma 1 Member satisfies the standard of effective gas source rocks, with effective gas source rocks of 58%, effective oil source rocks of 19% and high-quality oil source rocks of 7%. The effective source rocks are lithologically composed of dolomite-bearing mudstones, dolomitic mudstones and argillaceous dolomites, and they are mostly distributed in a mediumethick layered form. It is demonstrated that there are more effective source rocks in Ma 1 Member. 7) The TOC of dark pure carbonate rock samples from the members of Majiagou Fm is almost lower than 0.3%, averaging 0.18%. It is demonstrated that these dark pure carbonate rocks are non-source rocks or poor source rocks, and they can act as neither effective gas source rocks nor effective oil source rocks. It is shown that the development sections of Majiagou Fm effective source rocks are vertically heterogeneous. The effective source rocks are more developed and enriched in Ma 5, Ma 3 and Ma 1 Members which were deposited in the regressive stage than Ma 4 and Ma 2 Members which were deposited in the transgressive stage. It is indicated that regression is more favorable for the development of effective source rocks than transgression. The development sections of effective source rocks are primarily distributed in the upper part of Ma 5 Member (especially concentrated in Ma 53 and Ma 54) and secondarily in the middleelower parts of Ma 5 Member (primarily in Ma 56 and secondarily in Ma 58 and Ma 510), Ma 3 Member, Ma 1 Member and parts of Ma 2 Member. The TOC of effective source rocks varies between 0.30% and 8.45%. It should be noted that Ma 3, Ma 2 and Ma 1 Members are less drilled, so their organic abundance of source rock samples may be currently underestimated. The effective source rocks of the Ordovician Majiagou Fm are tens of centimeters to several meters thick in each layer and they are composed of dark (dark grey, grey black and black) dolomite-bearing mudstones, dolomitic mudstones and
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argillaceous dolomites. Their lithology and thickness are controlled by the sedimentary microfacies during their development. TOC is basically in positive correlation with clay mineral content, but in negative correlation with dolomite content and gypsum content. The GR curve of high-abundance
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source rocks are presented in the shape of saws and fingers. The sedimentary microfacies of dolomite-bearing mud flat and dolomitic mud flat are most favorable for the development of high-abundance effective source rocks, and argillaceous dolomite flat takes the second place (Fig. 5).
Fig. 5. Development section of Ordovician Majiagou Fm effective source rocks in typical exploration wells.
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3.2. Areal distribution and gas-generating intensity of source rocks It is shown from the statistical analysis on drilling data of over 300 wells which encounter the Majiagou Fm that the areal distribution of effective source rocks in the upper and the middleelower parts of Ma 5 Member is strongly heterogeneous. There are numerous effective source rock enrichment zones, and they are distributed areally around the Mizhi Salt Depression in a dual-girdle form (i.e., “North ShenmueWushen BannereSouth SuligeeWest JingbianeWuqie Huangling” girdle and “ShenmueYulineAnsaieYichuan” girdle) (Fig. 6). The effective source rocks are mainly developed in enrichment zones. The effective source rocks in enrichment zones are mostly medium-thick layered with large accumulative thickness and high organic abundance. In the areas of Well Zhao 26 near Wushen Banner, Well Zhao 86 in South Sulige, Well Lian 8 to the east of Wuqi and Well Shaan 303 near Gaoqiao, for example, the effective source rocks in the upper part of Ma 5 Member are totally over 40 m thick, accounting for 40e50% of the thickness of the upper part of Ma 5 Member, and their TOC is generally in the range of 1e4%. In the area of Well Tao 43 in South Sulige, the
effective source rocks in the middleelower part of Ma 5 Member are totally 60 m thick, accounting for about 40% of the thickness of the middleelower part of Ma 5 Member, and their TOC is generally in the range of 1e5%. In the center of Mizhi Salt Depression (e.g. in the area of Well Yu 9) and the secondary uplifts (i.e., the dolomite reservoir development zones, e.g. in the area of Well Shaancan 1), the effective source rocks are thin-layered with a small total thickness and low organic abundance. Obviously, the effective source rocks in these areas are relatively undeveloped. In the areas of Well Yu 9 and Well Shaancan 1, for example, the cumulative thickness of effective source rocks in the upper and the middleelower parts of Ma 5 Member is less than 5 m, and the thickness ratio to the corresponding formations in both wells is lower than 5%. The scale development and distribution of Majiagou Fm effective source rocks are in close relation with the palaeotectonic setting and sedimentary environment at that time. During the sedimentation of the Ordovician Majiagou Fm, it was Yimeng old land in the north of Ordos Basin, “L”-shaped Central Paleo-uplift in the west and southwest, Hancheng old land in the southeast and Lishi uplift (a lower underwater uplift) along the eastern margin. In the central-eastern area
Fig. 6. Isopach map of Ordovician Majiagou Fm effective source rocks.
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around it, there is a NS trending large sag (MizhieYan'an Sag) which is gentle on the western flank and steep on the eastern flank, dipping slowly from the west to the east. The center of the sag is located in the area of MizhieSuide. Numerous secondary uplifts and secondary depressions with different axial directions are developed and distributed in the sag, and paleo-uplifts and secondary uplifts act as barriers or semi-barriers to sea water [28]. At the sedimentary regressive stage (i.e., low sea level stage, the sedimentation period of Ma 5, Ma 3 and Ma 1 Members), the sedimentary environments of restricted evaporate platform (e.g. semi-restrictederestricted evaporate tidal flatelagoon) which are favorable for the development of source rocks with high organic abundance were formed in large sags. From the central paleo-uplift in the west and the underwater uplift in the east to the sag center in the central part, the sedimentary facies belts are areally gypsum-bearing dolomite flat at the basin margin / gypseous dolomite flat at the basin margin / gypsum rock and salt rock low in sequence. Correspondingly, they are lithologically presented as dolomite, argillaceous dolomite, dolomitic mudstone and dolomitebearing mudstone / argillaceous dolomite, gypseous dolomite, dolomitic gypsum / gypsum rock and salt rock. The closer it is to the sag center, the thinner the deposited dolomites are, indicating the increasing tendency of seawater salinity. The secondary depressions at the basin-margin gypsum-bearingegypseous dolomite flat with “medium salinity” (brine salinity 4e12%) around the salt depression along the eastern and western flanks in large gentle sags are favorable for the scale development of highquality source rocks and effective source rocks with high organic abundance. Moreover, oscillating transgressioneregression varied frequently in short periods during the sedimentation of Majiagou Fm. As a result, the effective source rocks are vertically developed in the pattern of multilayer superimposition and laterally distributed around the Mizhi Salt Depression in the form of wide-coverage girdle. It is shown from the basin simulation calculation results that the Majiagou Fm effective source rocks reached the hydrocarbon generation and expulsion peak in the Late JurassiceCretaceous with gas-generating intensity of 2 108e 26 108 m3/km2, total gas generation quantity of 56.6 1012 m3 and total resources of 2.8 1012 m3. There are three main hydrocarbon generation centers, including Suligee Wushen Banner, WuqieAnsai and HuanglingeLuochuan zones from the north to the south (Fig. 7). The hydrocarbon generated and expelled by the Majiagou effective source rocks tends to migrate vertically or laterally a short distance into the dolomite reservoirs inside the Majiagou Fm for accumulation. Thus, natural gas reservoirs of self-generating & self-preserving and natural gas enrichment zones are formed.
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Fig. 7. Hydrocarbon-generating intensity of Ordovician Majiagou Fm effective source rocks.
4. Conclusions 1) It is, for the first time, discovered and verified that scale effective source rocks with TOC>0.5% (even TOC>1.0% in many zones) are developed in the Ordovician Majiagou Fm in the Ordos Basin. The effective source rocks of Majiagou Fm are composed of thinethick layered dark dolomite-bearing mudstones, dolomitic mudstones and argillaceous dolomites, and their enrichment and distribution are obviously controlled by sedimentary facies, presenting apparent heterogeneity vertically and laterally. The effective scale source rocks are developed better in the regressive stage than in the transgressive stage. The effective source rocks are primarily distributed in the upper part of Ma 5 Member (especially concentrated Ma 53 and Ma 54) and secondarily in Ma 3 Member, Ma 1 Member and the middleelower part of Ma 5 Member (primarily Ma 56 and secondarily Ma 58 and Ma 510). The effective source rocks are areally distributed in a dual-
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girdle form around the Mizhi Salt Depression, and those in the secondary depressions are mediumethick layered, with large total thickness and high organic abundance. The effective source rocks in the center of Mizhi Salt Depression and the secondary uplifts are thin-layered, with small total thickness and low organic abundance. The TOC of the effective source rocks varies from 0.30% to 8.45%. 2) Planktonic algae and acritarchs are the main hydrocarbongenerating parent materials of Ordovician effective source rocks. The kerogen maceral of effective source rocks is dominantly sapropel. The organic matters are sapropel or sapropel prone type. The effective source rocks are mostly at overmature stage (high in the south and low in the north), and dry gas is mainly generated. Total gas generation quantity is 56.6 1012 m3 and total resources is 2.8 1012 m3. There are three main hydrocarbon generation centers, including SuligeeWushen Banner, WuqieAnsai and HuanglingeLuochuan zones from the north to the south. 3) It is demonstrated by the discovery and confirmation of Majiagou Fm scale effective source rocks in the Ordos Basin that Majiagou Fm itself has important hydrocarbon source rocks, so it really contributes a lot to the Majiagou Fm gas reservoirs. Acknowledgements We appreciate the leaders and experts of PetroChina Exploration & Production Company, PetroChina Research Institute of Petroleum Exploration & Development and PetroChina Changqing Oilfield Company for their great support and help during ten years' research. We also thank the Geochemistry Analysis Department of Laboratory Research Center of Petroleum Geology, PetroChina Research Institute of Petroleum Exploration & Development for the sample testing and analysis. References [1] Yang Junjie. Discovery of the natural gas in lower Palaeozoic in Shaanganning basin. Nat Gas Ind 1991;11(2):1e6. [2] Huang Difan, Xiong Chuanwu, Yang Junjie, Xu Zhengqiu, Wang Keren. Gas source discrimination and natural gas genetic types of central gas field in Ordos Basin. Nat Gas Ind 1996;16(6):1e5. [3] Chen Anding, Dai Jinyou, Wang Wenyue. Characteristics and origin of gas reservoirs and the favorable geological conditions in Jingbian Gasfield, Ordos Basin. Mar Orig Pet Geol 2010;15(2):45e55. [4] Guan Deshi, Zhang Wenzheng, Pei Ge. Oil-gas sources of Ordovician reservoir in gas field of central Ordos Basin. Oil Gas Geol 1993;14(3):191e9. [5] Zhang Shiya. Natural gas source and explorative direction in Ordos Basin. Nat Gas Ind 1994;14(3):1e4. [6] Dai Jinxing, Li Jian, Luo Xia, Zhang Wenzheng, Hu Guoyi, Ma Chenghua, et al. Stable carbon isotope compositions and source rock geochemistry of the giant gas accumulations in the Ordos Basin, China. Org Geochem 2005;36(12):1617e35. [7] Yang Hua, Zhang Wenzheng, Zan Chuanli, Ma Jun. Geochemical characteristics of Ordovician subsalt gas reservoir and their significance for re-understanding the gas source of Jingbian Gasfield, East Ordos Basin. Nat Gas Geosci 2009;20(1):8e14. [8] Ning Ning, Chen Mengjin, Sun Fenjin, Xu Huazheng. Determination and its significance of ancient oil pools in Ordovician weathering crust, Ordos Basin. Oil Gas Geol 2007;28(2):280e6.
[9] Wang Chuangang, Wang Yi, Xu Huazheng, Sun Yipu, Yang Weili, Wu Tianhong. Discussion on evolution of source rocks in lower Paleozoic of Ordos Basin. Acta Pet Sin 2009;30(1):38e45. [10] Ma Chunsheng, Xu Huazheng, Gong Changhong, Sun Lianzhong. Paleooil reservoir and Jingbian natural gasfield of Ordovician weathering crust in Ordos Basin. Nat Gas Geosci 2011;22(2):280e6. [11] Chen Anding. Carbonate source-rocks of the Ordovician in Shaanganning basin and discussion on the hydrocarbon generation of carbonates. Acta Sedimentol Sin 1996;14(S1):90e9. [12] Xia Xinyu, Hong Feng, Zhao Lin, Zhang Wenzheng. Organic facies type and hydrocarbon potential of carbonates in Majiagou Formation, lower Ordovician in Ordos Basin. Acta Sedimentol Sin 1999;17(4):638e50. [13] Li Xianqing, Hou Dujie, Hu Guoyi, Tang Youjun, Xie Zengye, Zhang Aiyun. The discussion on hydrocarbon-generated potential of lower Paleozoic carbonates in the central part of Ordos Basin. Bull Mineralogy Petrology Geochem 2002;21(3):152e7. [14] Xie Zengye, Hu Guoyi, Li Jian, Jiang Zhusheng, Wu Damao, Yan Qituan. A study on validity identification of source rock by new experimental techniques for Ordovician in Ordos Basin. Petroleum Explor Dev 2002;29(2):29e32. [15] Liu Dehan, Fu Jinhua, Zheng Congbin, Xiao Xianming, Mi Jingkui. Research on hydrocarbon generation of Ordovician marine carbonatite of the Ordos Basin and genesis of Changqing gas fields. Acta Geol Sin 2004;78(4):542e50. [16] Wang Chuangang. Availability analysis of oil pool forming for marine source rock in Ordos Basin. Earth Sci Front 2012;19(1):253e63. [17] Xia Xinyu, Zeng Fangang, Hong Feng. Hydrocarbon-generating potential of organic matter in epicontinental sea carbonate rocks in China. Oil Gas Geol 2001;22(4):287e92. [18] Qiu Zhongjian, Zhang Yiwei, Li Guoyu, Liang Digang, Wu Qizhi, Wang Zhaoming, et al. Enlightenment from petroleum geology investigation of Tengiz and Yurubchenskoye carbonate oil-gas fields on exploring giant oil-gas fields in Tarim Basin. Mar Orig Pet Geol 1998;3(1):49e56. [19] Tenger. Progress and challenges in the research of marine hydrocarbon source rocks. Nat Gas Ind 2011;31(1):20e5. [20] Zhang Shuichang, Liang Digang, Zhang Dajiang. Evaluation criteria for Paleozoic effective hydrocarbon source rocks. Petroleum Explor Dev 2002;29(2):8e12. [21] Wang Zhaoyun, Zhao Wenzhi, Wang Yunpeng. Evaluation indicators for marine carbonate as gas source rock in China. Prog Nat Sci 2004;14(11):1236e43. [22] Peng Ping’an, Liu Dayong, Qin Yan, Yu Chiling, Zhang Shanwen, Sui Fenggui, et al. Low limits of organic carbon content in carbonate as oil and gas source rocks. Geochimica 2008;37(4):415e22. [23] Hou Fanghao, Fang Shaoxian, Dong Zhaoxiong, Zhao Jingsong, Lu Shuxiu, Wu Yi, et al. The developmental characters of sedimentary environments and lithofacies of middle Ordovician Majiagou Formation in Ordos Basin. Acta Sedimentol Sin 2003;21(1):106e12. [24] Tu Jianqi, Huang Difan, Chen Jianping, Zhang Bin, He Zhonghua, Tang Liping, et al. The formation of high quality lower Paleozoic source rocks and favorable distribution area evaluation in the Ordos Basin. Beijing: PetroChina Research Institute of Petroleum Exploration and Development; 2008. [25] Tu Jianqi, Chen Jianping, Zhang Dajiang, Cheng Keming, Chen Jianjun, Yang Zhiming. A petrographic classification of macerals in lacustrine carbonate source rocks and their organic petrological characteristics: a case study on Jiuxi Basin, NW China. Acta Petrol Sin 2012;28(3):917e26. [26] Buchardt BJ, Lewan MD. Reflectance of vitrinite-like macerals as a thermal maturity index for CambrianeOrdovician Alum shale, southern Scandinavia. AAPG Bull 1990;74(4):394e406. [27] Tu Jianqi, Jin Kuili. Study and comparison on some important indicators with the reference to the degree of organic maturation of marine hydrocarbon source rocks. Adv Earth Sci 1999;14(1):18e23. [28] Zhang Jun, Xu Liming, Bao Guomin. The formation of central uplift and its relationship with natural gas in Ordos Basin. Nat Gas Ind 1994;14(Suppl.):19e23.
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Research Article
Discovery of effective scale source rocks of the Ordovician Majiagou Fm in the Ordos Basin and its geological significance* Tu Jianqi a,b,*, Dong Yiguo a,c, Zhang Bin a,b, Nan Hongli a,d, Li Chengjian a,d, Wang Xiaomei a,b, Fei Xuandong a,b, Zhou Wenbao a,b a
Laboratory Research Center of Petroleum Geology, PetroChina Research Institute of Petroleum Exploration and Development, Beijing, 100083, China b CNPC Key Laboratory of Petroleum Geochemistry, Beijing, 100083, China c School of Energy Resources, China University of Geosciences, Beijing, 100083, China d College of Geosciences and Surveying Engineering, China University of Mining & Technology, Beijing, 100083, China Received 17 February 2016; accepted 6 June 2016 Available online 1 March 2017
Abstract There have been different views on the origins of gas reservoirs of Ordovician Majiagou Fm in the Ordos Basin, but none of them supports the opinion that there are scale source rocks (TOC>0.5%) in the Majiagou Fm. In this paper, a series of analysis was made on cores and cutting samples taken from recent newly-drilled wells in the Ordovician reservoirs in the centraleeastern areas of the Ordos Basin. Accordingly, the organic abundance features of different types of source rocks in the Majiagou Fm were presented, and for the first time, it was discovered and verified that there are effective scale source rocks with high organic abundance. Based on these analysis and studies, the following results were achieved. First, the effective source rocks of Majiagou Fm are composed of thinethick layered dark dolomite-bearing mudstones, dolomitic mudstones and argillaceous dolomites, and their enrichment and distribution are obviously controlled by sedimentary facies. During the sedimentation of Majiagou Fm, the effective scale source rocks developed better at the regressive stage than at the transgressive stage. The effective source rocks are primarily distributed in the upper part of Ma 5 Member (especially concentrated in the third and fourth sub-members) and secondly in Ma 3 Member, Ma 1 Member and the middleelower part of Ma 5 Member. Second, the effective source rocks are areally distributed in a dual-girdle form around the Mizhi Salt Depression, and those in the secondary depressions are mediumethick layered, with a large total thickness and high organic abundance. Third, the effective source rocks in the center of Mizhi Depression and the secondary uplifts are thin-layered, with a small total thickness and low organic abundance. And fourth, the organic abundance of the effective source rocks varies from 0.30% to 8.45%. Phytoplankton and Acritarchs are the main parent materials of hydrocarbon, and organic matters are of sapropel or sapropel prone type. To sum up, the Ordovician effective source rocks in the Ordos Basin are generally at an over-mature stage and they contribute greatly to the natural gas in the Majiagou Fm. In these effective source rocks, dry gas is mainly generated at a high rate. © 2017 Sichuan Petroleum Administration. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Keywords: Ordos basin; Ordovician; Effective scale source rocks; Organic abundance; Dolomite-bearing mudstone; Dolomitic mudstone; Argillaceous dolomite; Sedimentary facies
* Project supported by the S&T Project of PetroChina Exploration & Production Company “Evaluation on Ordovician source rocks in the Ordos Basin” (No.: 2012-011) and “Characteristics and comprehensive evaluation of Ordovician gas sources and optimization of favorable zones in the Ordos Basin” (No.: 2014-008). * Corresponding author. Laboratory Research Center of Petroleum Geology, PetroChina Research Institute of Petroleum Exploration and Development, Beijing, 100083, China. E-mail address: [email protected] (Tu JQ). Peer review under responsibility of Sichuan Petroleum Administration.
http://dx.doi.org/10.1016/j.ngib.2016.12.009 2352-8540/© 2017 Sichuan Petroleum Administration. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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1. Background The origins of natural gas in the Ordovician Majiagou Fm have been the controversial topic since the Jinbian Gas Field (i.e., the central gas field) was discovered and explored in the Ordos Basin at the end of 1980s. There are mainly three views. First, it is mainly self-generating and self-preserving oil-type gas, which is generated by highly-mature to over-mature carbonate source rocks with TOC (total organic carbon) lower than 0.2% [1e3]. Second, it is mainly coal-type gas. The mixed gas composed of coal-type gas derived from CarboniferousePermian highly-mature and over-mature coal measure source rocks and a small amount of oil-type gas derived from Carboniferous limestones migrated laterally and accumulated after flowing backward [4e7]. And third, it is mainly outside-generating and inside-preserving oil-type gas. The liquid hydrocarbon generated by the MiddleeUpper Ordovician marls and shales in the western/southwestern Ordos Basin accumulated at the slope controlled by the central paleo-uplift after long-distance migration. After cracking and diffusing, the gaseous hydrocarbon migrated into the Majiagou Fm reservoirs inside the basin, and mixed with the coal-type gas generated by the CarboniferousePermian source rocks [8e10]. These three views are different, but they all agree that the Ordovician Majiagou Fm was deposited in the environment of shallow water carbonate platform, showing the mode of “consumption”e“dilution” with developed non-source rocks, and no scale hydrocarbon source rocks with TOC>0.5% was developed in it [9e17]. According to the researches on the development characteristics and the minimum organic abundance of marine source rocks all around the world in recent years, dark argillaceous rocks and argillaceous carbonates which are distributed extensively with a certain thickness and TOC of 1e2% are the prerequisite for the formation of large marine oil (gas) fields, and the pure carbonate rocks with quite low TOC cannot form large oil and gas fields [18,19]. When the marine source rocks at high maturityeover maturity stage is evaluated, the minimum TOC of effective oil (gas) source rocks should not be lower than 0.5% (0.3%), and the minimum TOC of highquality hydrocarbon (oil) source rocks should not be lower than 1.0% [20e22]. Accordingly, it is generally concerned whether effective scale source rocks with TOC>0.5% and even TOC>1.0% are developed in the Ordovician Majiagou Fm in the Ordos Basin besides the thick marine pure carbonate rocks of low organic abundance (TOC<0.2%). This is related to further understanding on the origins of Ordovician Majiagou Fm natural gas which has been a long-term controversial focus. Moreover, it is directly related to the confirmation of future Ordovician natural gas exploration targets in the Ordos Basin. The Majiagou Fm in the Ordos Basin is located in the central-eastern area of the basin, where the Jinbian Gas Field (i.e., the central gas field) lies. It is divided into 6 apparent lithologic members, i.e., Ma 1eMa 6. Specifically, Ma 2, Ma 4 and Ma 6 Members were deposited in the transgressive stage and they are lithologically composed of limestones and dolomites. Ma 1, Ma 3 and Ma 5 Members were deposited in the
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regressive stage and they are mainly composed of mudstonebearing dolomites, argillaceous dolomites, dolomitic mudstones and gypsum-salt rocks [23]. Ma 5 Member is subdivided into 10 sub-members in its upper, middle and lower parts, namely, the upper part of Ma 5 Member (the first to the fourth sub-member, or Ma 51 to Ma 54), the middle part of Ma 5 Member (the fifth sub-member, or Ma 55), and the lower part of Ma 5 Member (the sixth to the tenth sub-member, or Ma 56 to Ma 510). It is indicated from hydrocarbon researches and exploration practices around the world that petroliferous basins are in close relation to gypsum-salt-bearing basins, and restricted platforms are favorable for the development of hydrocarbon source rocks with high organic abundance. Therefore, this paper focuses on the gypsum-salt lake sedimentation systems of the Ordovician Majiagou Fm in the Ordos Basin. Based on previous research results [24], the cores and cutting samples taken from newly-drilled wells in the Ordovician reservoirs in the centraleeastern area of the Ordos Basin were analyzed in terms of their organic geochemistry. The organic abundances of different types of source rocks in the Ordovician Majiagou Fm were further identified. It was discovered and proved that there are effective scale source rocks with high organic abundance in the Ordovician Majiagou Formation. Furthermore, the development intervals and distribution characteristics of Majiagou Fm effective source rocks were described. The hydrocarbon generation intensity and resources of effective source rocks were calculated, demonstrating the immense hydrocarbon-generating capacity and exploration potential of the formation. 2. Organic geochemical characteristics of source rocks 2.1. Organic abundance Not all the marine sedimentary rocks can act as source rocks. High-energy environments or over-oxidized environments are unfavorable for the development of marine source rocks. Better marine source rocks are usually developed in lowenergy waters and relatively-reducing environments. Therefore, it is discriminated that the possible source rocks of Majiagou Fm in the Ordos Basin are lithologically acted by two types of rocks, i.e., dark (dark grey, grey black and black) pure carbonate rocks (including limestones, dolomites, mudstone-bearing limestones and mudstone-bearing dolomites) and argillaceous carbonate rocks (including dolomitebearing mudstones, dolomitic mudstones, argillaceous dolomites, limy mudstones and marls). The Majiagou Fm in the Ordos Basin is currently at high maturityeover maturity stage. Due to maturity, its geochemical index (e.g. rock pyrolysis index and chloroform bitumen “A” content) can hardly be used for organic abundance evaluation. Therefore, the total organic carbon (TOC ) of rocks is the only available evaluation index. The Majiagou Fm dark argillaceous carbonate rocks are, on the whole, close to the standard of high-quality oil source rocks, for the TOC of their 916 samples varies between 0.17% and 8.45%, averaging 0.83%. Samples with TOC>0.3%, TOC>0.5% and TOC>1.0% account for 86%, 66% and 33%
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of the total samples respectively. Correspondingly, the ratio of effective gas source rocks, effective oil source rocks and highquality oil source rocks is 86%, 66% and 33% respectively. On the contrary, the TOC of 823 samples from the Majiagou Fm dark pure carbonate rocks are almost lower than 0.3%, generally in the range of 0.03e0.32%, averaging 0.18%, and samples with TOC<0.2% account for more than 70%. It is indicated that the Majiagou Fm dark pure carbonate rocks are non-source rocks or poor source rocks, and they can act as neither effective gas source rocks nor effective oil source rocks (Fig. 1). To sum up, there are substantial effective source rocks of dark argillaceous carbonate rocks (including argillaceous rocks) in the Majiagou Fm, Ordos Basin. 2.2. Rock types of effective source rocks The rock types of Majiagou Fm effective source rocks are lithologically distinguished from the overlying source rocks (e.g. limy mudstones and marls) in the Upper Paleozoic CarboniferousePermian coal measures, and they are mainly dark (black, grey black and dark grey) dolomite-bearing mudstones, dolomitic mudstones and argillaceous dolomites. The lamellations or laminae are quite developed and they can be observed directly (Fig. 2aed). It is shown from microscopic analysis that the widely distributed microlaminae is the reason for the well development of lamellations or laminae in effective source rocks. Microlaminae comprise micrite-rich dolomite laminae, organic laminae rich algae laminae and clayrich laminae. All of them are vertically superimposed with each other and form two-layer or three-layer sedimentary patterns, which are quite similar to the principal oil source rocks (the Lower Cretaceous lacustrine dolomitic mudstones and argillaceous dolomites) in the Jiuxi Basin, China, in terms of rock types and microlaminae structures [25]. Algae laminaes are subtranslucent or opaque brown-black without fluorescence due to higher degree of thermal evolution (Fig. 2e). 2.3. Types of organic matters Types of organic matters are the comprehensive reflection of organic biogenesis constitution in source rocks. It is shown from microscopic analysis that planktonic algae and acritarchs
are the main hydrocarbon-generating parent materials of Majiagou Fm effective source rocks in this area. The kerogen maceral of effective source rocks is dominantly sapropelinite. Sapropelic amorphogen is predominant (generally over 85%) with a small amount of better-preserved alginite and acritarchs (Fig. 2-feg). The other macerals are marine vitrinite and marine intertinite, with total content less than 15%, except for the special samples (Fig. 3). The organic matters of effective source rocks are sapropel or sapropel prone type. 2.4. Maturity of organic matters There is no vitrinite in the Early Paleozoic marine source rocks where higher plants are lacking, so vitrinite reflectance cannot be directly used to calibrate the maturity of organic matters. At present, vitrinite-like maceral (marine vitrinite) is selected as the primary object for reflectance measurement. The vitrinite-like maceral reflectance is taken as the organic maturity index of Early Paleozoic source rocks to reflect its thermal evolution degree, especially at high maturityeover maturity stage [26,27]. The Majiagou Fm effective source rocks are at over-maturity stage and dry gas is mainly generated. Their equivalent vitrinite reflectance (Ro) is generally over 2.0%. The equivalent vitrinite reflectance (Ro) of the area to the south of WuqieAnsaieShilou is higher than 2.5% and it is regionally high in the south and low in the north and decreases gradually to the northeast. It is at its early stage of high maturity in the area of ShenmueWell Fu 5 with the equivalent vitrinite reflectance (Ro) of about 1.5% (Fig. 4). 3. Scale development and distribution of source rocks 3.1. Vertical development sections of source rocks Table 1 shows the TOC distribution of possible source rocks in members of the Ordovician Majiagou Fm in the Ordos Basin. 1) There are 449 samples of dark argillaceous carbonate rocks from the upper part of Ma 5 Member, and their TOC varies in the range of 0.18e7.48%, averaging 1.14%. The upper part of Ma 5 Member satisfies the standard of highquality oil source rocks, with effective gas source rocks of
Fig. 1. Distribution frequency of total organic carbon (TOC ) in the possible source rocks of the Ordovician Majiagou Fm.
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Fig. 2. Macro and micro characteristics of core samples from Ordovician effective source rocks.
Fig. 3. Constitutional characteristics of kerogen macerals in Ordovician effective source rocks.
88%, effective oil source rocks of 72% and high-quality oil source rocks of 43%. The effective source rocks are lithologically composed of dolomite-bearing mudstones, dolomitic mudstones and argillaceous dolomites, and they are mostly distributed in a mediumethick layered form. It is demonstrated that there are substantial effective source rocks in the upper part of Ma 5 Member. 2) There are 225 samples of dark argillaceous carbonate rocks from the middleelower part of Ma 5 Member, and their TOC varies in the range of 0.19e8.45%, averaging 0.96% (close to 1.0%). The middleelower part of Ma 5 Member satisfies the standard of high-quality oil source rocks, with
Fig. 4. Contour map of the equivalent vitrinite reflectance (Ro) at the top surface of Ordovician.
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Table 1 TOC distribution of possible source rocks in members of the Ordovician Majiagou Fm. Member
Ma 5
Rock type
Upper MiddleeLower
Ma 4 Ma 3 Ma 2 Ma 1
Argillaceous carbonate Pure carbonate rocks Argillaceous carbonate Pure carbonate rocks Argillaceous carbonate Pure carbonate rocks Argillaceous carbonate Pure carbonate rocks Argillaceous carbonate Pure carbonate rocks Argillaceous carbonate Pure carbonate rocks
TOC%
rocks rocks rocks rocks rocks rocks
Effective gas source rocks
Effective oil source rocks
High-quality source rocks
Distribution range
Sample number
Average
TOC>0.3%
TOC>0.5%
TOC>1.0%
0.18e7.48 0.06e0.31 0.19e8.45 0.05e0.29 0.17e1.80 0.06e0.31 0.18e4.15 0.05e0.28 0.17e0.99 0.04e0.26 0.18e1.41 0.06e0.27
449 212 255 189 38 214 98 134 35 54 41 20
1.14 0.18 0.96 0.17 0.53 0.17 0.71 0.17 0.41 0.18 0.42 0.17
88% 5% 93% 0 71% 1% 77% 0 79% 0 58% 0
72% 0 80% 0 39% 0 43% 0 36% 0 19% 0
43% 0 33% 0 10% 0 17% 0 0 0 7% 0
effective gas source rocks of 93%, effective oil source rocks of 80% and high-quality oil source rocks of 33%. The effective source rocks are lithologically composed of dolomite-bearing mudstones, dolomitic mudstones and argillaceous dolomites, and they are mostly distributed in a mediumethick layered form. It is demonstrated that there are substantial effective source rocks in the middleelower part of Ma 5 Member. 3) There are 38 samples of dark argillaceous carbonate rocks from Ma 4 Member, and their TOC varies in the range of 0.17e1.80%, averaging 0.53%. Ma 4 Member satisfies the standard of effective oil source rocks, with effective gas source rocks of 71%, effective oil source rocks of 39% and highquality oil source rocks of 10%. The effective source rocks are lithologically composed of limy mudstones, marls and argillaceous dolomites, and they are mostly distributed in a thin interlayered form. The cumulative thickness of effective source rocks accounts for just 1e2% of total thickness of Ma 4 Member in each well. It is demonstrated that there are only slight effective source rocks in Ma 4 Member. 4) There are 98 samples of dark argillaceous carbonate rocks from Ma 3 Member, and their TOC varies in the range of 0.18e4.15%, averaging 0.71%. Ma 3 Member satisfies the standard of effective oil source rocks, with effective gas source rocks of 77%, effective oil source rocks of 43% and highquality oil source rocks of 17%. The effective source rocks are lithologically composed of dolomite-bearing mudstones, dolomitic mudstones and argillaceous dolomites, and they are mostly distributed in a mediumethick layered form. It is demonstrated that there are substantial effective source rocks in Ma 3 Member. 5) There are 35 samples of dark argillaceous carbonate rocks from Ma 2 Member, and their TOC varies in the range of 0.17e0.99%, averaging 0.41%. Ma 2 Member satisfies the standard of effective gas source rocks, with effective gas source rocks of 79% and effective oil source rocks of 36%. The effective source rocks are lithologically composed of limy mudstones, marls and argillaceous dolomites. Limy mudstones and marls are mostly distributed in a thin interlayered form, and argillaceous dolomites are mostly distributed in a medium-thickness layered form. It is demonstrated that there
are a certain number of effective source rocks in Ma 2 Member. 6) There are 41 samples of dark argillaceous carbonate rocks from Ma 1 Member, and their TOC varies in the range of 0.18e1.41%, averaging 0.42%. Ma 1 Member satisfies the standard of effective gas source rocks, with effective gas source rocks of 58%, effective oil source rocks of 19% and high-quality oil source rocks of 7%. The effective source rocks are lithologically composed of dolomite-bearing mudstones, dolomitic mudstones and argillaceous dolomites, and they are mostly distributed in a mediumethick layered form. It is demonstrated that there are more effective source rocks in Ma 1 Member. 7) The TOC of dark pure carbonate rock samples from the members of Majiagou Fm is almost lower than 0.3%, averaging 0.18%. It is demonstrated that these dark pure carbonate rocks are non-source rocks or poor source rocks, and they can act as neither effective gas source rocks nor effective oil source rocks. It is shown that the development sections of Majiagou Fm effective source rocks are vertically heterogeneous. The effective source rocks are more developed and enriched in Ma 5, Ma 3 and Ma 1 Members which were deposited in the regressive stage than Ma 4 and Ma 2 Members which were deposited in the transgressive stage. It is indicated that regression is more favorable for the development of effective source rocks than transgression. The development sections of effective source rocks are primarily distributed in the upper part of Ma 5 Member (especially concentrated in Ma 53 and Ma 54) and secondarily in the middleelower parts of Ma 5 Member (primarily in Ma 56 and secondarily in Ma 58 and Ma 510), Ma 3 Member, Ma 1 Member and parts of Ma 2 Member. The TOC of effective source rocks varies between 0.30% and 8.45%. It should be noted that Ma 3, Ma 2 and Ma 1 Members are less drilled, so their organic abundance of source rock samples may be currently underestimated. The effective source rocks of the Ordovician Majiagou Fm are tens of centimeters to several meters thick in each layer and they are composed of dark (dark grey, grey black and black) dolomite-bearing mudstones, dolomitic mudstones and
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argillaceous dolomites. Their lithology and thickness are controlled by the sedimentary microfacies during their development. TOC is basically in positive correlation with clay mineral content, but in negative correlation with dolomite content and gypsum content. The GR curve of high-abundance
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source rocks are presented in the shape of saws and fingers. The sedimentary microfacies of dolomite-bearing mud flat and dolomitic mud flat are most favorable for the development of high-abundance effective source rocks, and argillaceous dolomite flat takes the second place (Fig. 5).
Fig. 5. Development section of Ordovician Majiagou Fm effective source rocks in typical exploration wells.
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3.2. Areal distribution and gas-generating intensity of source rocks It is shown from the statistical analysis on drilling data of over 300 wells which encounter the Majiagou Fm that the areal distribution of effective source rocks in the upper and the middleelower parts of Ma 5 Member is strongly heterogeneous. There are numerous effective source rock enrichment zones, and they are distributed areally around the Mizhi Salt Depression in a dual-girdle form (i.e., “North ShenmueWushen BannereSouth SuligeeWest JingbianeWuqie Huangling” girdle and “ShenmueYulineAnsaieYichuan” girdle) (Fig. 6). The effective source rocks are mainly developed in enrichment zones. The effective source rocks in enrichment zones are mostly medium-thick layered with large accumulative thickness and high organic abundance. In the areas of Well Zhao 26 near Wushen Banner, Well Zhao 86 in South Sulige, Well Lian 8 to the east of Wuqi and Well Shaan 303 near Gaoqiao, for example, the effective source rocks in the upper part of Ma 5 Member are totally over 40 m thick, accounting for 40e50% of the thickness of the upper part of Ma 5 Member, and their TOC is generally in the range of 1e4%. In the area of Well Tao 43 in South Sulige, the
effective source rocks in the middleelower part of Ma 5 Member are totally 60 m thick, accounting for about 40% of the thickness of the middleelower part of Ma 5 Member, and their TOC is generally in the range of 1e5%. In the center of Mizhi Salt Depression (e.g. in the area of Well Yu 9) and the secondary uplifts (i.e., the dolomite reservoir development zones, e.g. in the area of Well Shaancan 1), the effective source rocks are thin-layered with a small total thickness and low organic abundance. Obviously, the effective source rocks in these areas are relatively undeveloped. In the areas of Well Yu 9 and Well Shaancan 1, for example, the cumulative thickness of effective source rocks in the upper and the middleelower parts of Ma 5 Member is less than 5 m, and the thickness ratio to the corresponding formations in both wells is lower than 5%. The scale development and distribution of Majiagou Fm effective source rocks are in close relation with the palaeotectonic setting and sedimentary environment at that time. During the sedimentation of the Ordovician Majiagou Fm, it was Yimeng old land in the north of Ordos Basin, “L”-shaped Central Paleo-uplift in the west and southwest, Hancheng old land in the southeast and Lishi uplift (a lower underwater uplift) along the eastern margin. In the central-eastern area
Fig. 6. Isopach map of Ordovician Majiagou Fm effective source rocks.
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around it, there is a NS trending large sag (MizhieYan'an Sag) which is gentle on the western flank and steep on the eastern flank, dipping slowly from the west to the east. The center of the sag is located in the area of MizhieSuide. Numerous secondary uplifts and secondary depressions with different axial directions are developed and distributed in the sag, and paleo-uplifts and secondary uplifts act as barriers or semi-barriers to sea water [28]. At the sedimentary regressive stage (i.e., low sea level stage, the sedimentation period of Ma 5, Ma 3 and Ma 1 Members), the sedimentary environments of restricted evaporate platform (e.g. semi-restrictederestricted evaporate tidal flatelagoon) which are favorable for the development of source rocks with high organic abundance were formed in large sags. From the central paleo-uplift in the west and the underwater uplift in the east to the sag center in the central part, the sedimentary facies belts are areally gypsum-bearing dolomite flat at the basin margin / gypseous dolomite flat at the basin margin / gypsum rock and salt rock low in sequence. Correspondingly, they are lithologically presented as dolomite, argillaceous dolomite, dolomitic mudstone and dolomitebearing mudstone / argillaceous dolomite, gypseous dolomite, dolomitic gypsum / gypsum rock and salt rock. The closer it is to the sag center, the thinner the deposited dolomites are, indicating the increasing tendency of seawater salinity. The secondary depressions at the basin-margin gypsum-bearingegypseous dolomite flat with “medium salinity” (brine salinity 4e12%) around the salt depression along the eastern and western flanks in large gentle sags are favorable for the scale development of highquality source rocks and effective source rocks with high organic abundance. Moreover, oscillating transgressioneregression varied frequently in short periods during the sedimentation of Majiagou Fm. As a result, the effective source rocks are vertically developed in the pattern of multilayer superimposition and laterally distributed around the Mizhi Salt Depression in the form of wide-coverage girdle. It is shown from the basin simulation calculation results that the Majiagou Fm effective source rocks reached the hydrocarbon generation and expulsion peak in the Late JurassiceCretaceous with gas-generating intensity of 2 108e 26 108 m3/km2, total gas generation quantity of 56.6 1012 m3 and total resources of 2.8 1012 m3. There are three main hydrocarbon generation centers, including Suligee Wushen Banner, WuqieAnsai and HuanglingeLuochuan zones from the north to the south (Fig. 7). The hydrocarbon generated and expelled by the Majiagou effective source rocks tends to migrate vertically or laterally a short distance into the dolomite reservoirs inside the Majiagou Fm for accumulation. Thus, natural gas reservoirs of self-generating & self-preserving and natural gas enrichment zones are formed.
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Fig. 7. Hydrocarbon-generating intensity of Ordovician Majiagou Fm effective source rocks.
4. Conclusions 1) It is, for the first time, discovered and verified that scale effective source rocks with TOC>0.5% (even TOC>1.0% in many zones) are developed in the Ordovician Majiagou Fm in the Ordos Basin. The effective source rocks of Majiagou Fm are composed of thinethick layered dark dolomite-bearing mudstones, dolomitic mudstones and argillaceous dolomites, and their enrichment and distribution are obviously controlled by sedimentary facies, presenting apparent heterogeneity vertically and laterally. The effective scale source rocks are developed better in the regressive stage than in the transgressive stage. The effective source rocks are primarily distributed in the upper part of Ma 5 Member (especially concentrated Ma 53 and Ma 54) and secondarily in Ma 3 Member, Ma 1 Member and the middleelower part of Ma 5 Member (primarily Ma 56 and secondarily Ma 58 and Ma 510). The effective source rocks are areally distributed in a dual-
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girdle form around the Mizhi Salt Depression, and those in the secondary depressions are mediumethick layered, with large total thickness and high organic abundance. The effective source rocks in the center of Mizhi Salt Depression and the secondary uplifts are thin-layered, with small total thickness and low organic abundance. The TOC of the effective source rocks varies from 0.30% to 8.45%. 2) Planktonic algae and acritarchs are the main hydrocarbongenerating parent materials of Ordovician effective source rocks. The kerogen maceral of effective source rocks is dominantly sapropel. The organic matters are sapropel or sapropel prone type. The effective source rocks are mostly at overmature stage (high in the south and low in the north), and dry gas is mainly generated. Total gas generation quantity is 56.6 1012 m3 and total resources is 2.8 1012 m3. There are three main hydrocarbon generation centers, including SuligeeWushen Banner, WuqieAnsai and HuanglingeLuochuan zones from the north to the south. 3) It is demonstrated by the discovery and confirmation of Majiagou Fm scale effective source rocks in the Ordos Basin that Majiagou Fm itself has important hydrocarbon source rocks, so it really contributes a lot to the Majiagou Fm gas reservoirs. Acknowledgements We appreciate the leaders and experts of PetroChina Exploration & Production Company, PetroChina Research Institute of Petroleum Exploration & Development and PetroChina Changqing Oilfield Company for their great support and help during ten years' research. We also thank the Geochemistry Analysis Department of Laboratory Research Center of Petroleum Geology, PetroChina Research Institute of Petroleum Exploration & Development for the sample testing and analysis. References [1] Yang Junjie. Discovery of the natural gas in lower Palaeozoic in Shaanganning basin. Nat Gas Ind 1991;11(2):1e6. [2] Huang Difan, Xiong Chuanwu, Yang Junjie, Xu Zhengqiu, Wang Keren. Gas source discrimination and natural gas genetic types of central gas field in Ordos Basin. Nat Gas Ind 1996;16(6):1e5. [3] Chen Anding, Dai Jinyou, Wang Wenyue. Characteristics and origin of gas reservoirs and the favorable geological conditions in Jingbian Gasfield, Ordos Basin. Mar Orig Pet Geol 2010;15(2):45e55. [4] Guan Deshi, Zhang Wenzheng, Pei Ge. Oil-gas sources of Ordovician reservoir in gas field of central Ordos Basin. Oil Gas Geol 1993;14(3):191e9. [5] Zhang Shiya. Natural gas source and explorative direction in Ordos Basin. Nat Gas Ind 1994;14(3):1e4. [6] Dai Jinxing, Li Jian, Luo Xia, Zhang Wenzheng, Hu Guoyi, Ma Chenghua, et al. Stable carbon isotope compositions and source rock geochemistry of the giant gas accumulations in the Ordos Basin, China. Org Geochem 2005;36(12):1617e35. [7] Yang Hua, Zhang Wenzheng, Zan Chuanli, Ma Jun. Geochemical characteristics of Ordovician subsalt gas reservoir and their significance for re-understanding the gas source of Jingbian Gasfield, East Ordos Basin. Nat Gas Geosci 2009;20(1):8e14. [8] Ning Ning, Chen Mengjin, Sun Fenjin, Xu Huazheng. Determination and its significance of ancient oil pools in Ordovician weathering crust, Ordos Basin. Oil Gas Geol 2007;28(2):280e6.
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