SCIENCE CHINA Earth Sciences • RESEARCH PAPER •
January 2011 Vol.54 No.1: 73–83 doi: 10.1007/s11430-010-4080-2
Evolution of kaolinite subgroup minerals and mixed-layer illite/smectite in the Paleogene Damintun Depression in Liaohe Basin of China and its implication for paleotemperature ZHAO Ming1*, JI JunFeng1, CHEN ZhenYan2, CHEN XiaoMing1, CUI XiangDong2 & WANG YanShan2 1
School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, China; 2 Exploration and Development Institute, Liaohe Oilfield, Panjin 124000, China
Received August 17, 2009; accepted March 1, 2010; published online November 24, 2010
The oil-rich Damintun Depression is located in the Liaohe Basin, Northeast China, and was formed during the Paleogene. The major oil-producing strata in the depression are mudstone and shale. To explore the burial diagenetic history of the basin and the formation thresholds of hydrocarbons, the characters of the kaolinite subgroup minerals and mixed-layer illite/smectite in the mudstone and the shale are studied by using X-ray diffraction, electron probe, scanning electron microscope, and Fourier infrared spectrum. The kaolinite subgroup consists of kaolinite and halloysite. The kaolinite is flake-like or vermiform-like. The halloysite is in long tubular shape and its length is related to its iron content. A longer tube has lower iron content. The crystallinity of kaolinite is 0.40 °2θ, and its degree of order increases from 0.03 to 1.17 with the burial depth. Kaolinite is in disorder when the buried depth is less than or equal to 2479 m , and it is partially ordered when the buried depth is greater than 2479 m. Kaolinite is supposed to turn into dickite when the depth is greater than 2550 m, but low penetrability and low porosity of the shale and mudstone prevent such a change. The mixed-layer illite/smectite changes from disorder to order continually as the buried depth increases. Its disorder (R0I/S), as defined by illite layer content (I%), is smaller than 50% at depths less than 2550.25 m. Based on Hoffman & Hower’s model, the paleo-geothermal gradients of 3.37–3.76°C/100 m (3.57°C /100 m on average) can be derived in the Paleocene Damintun Depression, which is significantly higher than the present geothermal gradient (2.9°C/100 m). The threshold depth of the oil formation in the depression is about 2550 m. Damintun Depression, Paleogene, kaolinite, halloysite, mixed-layer illite/smectite, mixed-layer illite/smectite geothermometer, paleo-geothermal gradient Citation:
Zhao M, Ji J F, Chen Z Y, et al. Evolution of kaolinite subgroup minerals and mixed-layer illite/smectite in the Paleogene Damintun Depression in Liaohe Basin of China and its implication for paleotemperature. Sci China Earth Sci, 2011, 54: 73–83, doi: 10.1007/s11430-010-4080-2
Previous studies suggest that the burial diagenetic history in a sedimentary basin can be revealed through the characteristics of clay minerals in mudstone and shale [1–7]. Reactions frequently happened in clay minerals’ generation with diagenetic enhancement, resulting in more and more newly generated crystal layers, higher degree of order in mixed*Corresponding author (email:
[email protected])
© Science China Press and Springer-Verlag Berlin Heidelberg 2010
layer illite/smectite (I/S) and the better crystallinity of kaolinite. These changes are irreversible processes and comprise a gradual reaction series. Different reaction progresses in clay minerals appear in different sedimentary basins because of different thermodynamic conditions. Kaolinite and mixed-layer illite/smectite (I/S) are two common minerals in the series of diagenetic reactions. Kaolinite subgroup minerals are dioctahedral minerals of 1:1type structural layer, including kaolinite, dickite, nakrite, earth.scichina.com
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and halloysite [8, 9]. Dickite and nakrite are different polytypes of kaolinite. Kaolinite and dickite widely exist in the crust, often formed during diagenesis [10–12]. Kaolinite is far more common than dakrite. Many studies have proved that kaolinite is the only stable mineral in kaolinite subgroup, whereas dickite, nakrite and halloysite are all in metastability [13–15]. Studies on shales have verified that structural and textural changes of kaolinite subgroup minerals are related to temperature [16]. Mixed-layer illite/smectite was created from the clay mineral reaction during the diagenesis process through the sedimentary burying history. Because of strong diagenesis, semctite turned into mixed-layer llite/smectite, and then became illite. This reaction is called the diagenesis or illiteization of smectite [1, 5, 7]. With the increase of illiteization, the structure of mixed-layer llite/semctiteis converted from disordered form (R0I/S) to ordered form (R1I/S). Based on Hoffman & Howerr’s model [1], the temperature of the transformation during the conversion can be obtained [1, 7]. Therefore, it is an effective method to utilize the mixed-layer illite/smectite to retrieve the thermal history of basin. This method has been widely used abroad but very limited in China. Moreover, the related depth of the transformation temperature during the conversion is in accordance with the depth of the oil formation, which can be used to explore the degree of maturity on organic substances [5, 7]. Therefore, the study of the characteristics of the mixedlayer illite/smectite has significant impact on diagenesis, oil and gas formation and exploration. Damingtun Paleogene Depression is a famous petroleum basin in Northeast China [17–19]. In order to understand the characteristics and diagenetic evolvement of kaolinite subgroup minerals in the shale and mudstone, their morphology, chemical composition, crystallization and particles’ ordering are studied. The characteristics of kaolinite minerals and mixed-layer illite/smectite are used in discussion of the related paleo-temperature and paleo-geothermal gradient in the basin as well as the etched thickness.
1 Geological background The Damintum Depression underwent the rifting stage in Paleogene and the post-rifting stage in Neogene [18, 20]. The sedimentary strata in the depression were deposited in Paleogene and Neogene. The Paleogene strata consist of Fangshengpao Formation (Ef), the member four of Shahejie Formation (Es4), the member three of Shahejie Formation (Es3), the member one of Shahejie Formation (Es1), and Dongying Formation (Ed). Their lithology was mainly sandstone and dark mudstone. The Neogene strata consist of Guantao Formation (Ng) and Minghuazhen Formation (Nm). Their lithology is mostly glutenite, gravel bed, sandy mudstone, and subclay. There are two unconformable contacts,
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one between the member three of Shahejie Formation (Es3) and the member one of Shahejie Formation (Es1) and the other between the Paleogene and the Neogene. The unconformable contacts were caused by the crust uplift and erosion from the textural movement. The dark mudstone in the member four of Shahejie Formation (Es4) and the member three of Shahejie Formation (Es3) was the main reservoir [19], which was an independent oil and gas formation unit [20, 21].
2
Samples and analyses
Samples were collected from eight wells (Shen Well 203#, 80#, 97#, 13#, 166#, 223#, 150#, and Sheng Well 3#) at different depths from 1443.8 to 3074 m. They belong to the member four of Shahejie Formation (Es4) and the member three of Shahejie Formation (Es3), and the lithology is shale and mudstone. (1) Sample preparation. The preparation method is based on the recommendation of Warr and Rice [22]. The clay fractions smaller than 2 μm were extracted and then were used to make air-dried orientated flakes, glycolated flakes, and some non-orientated powder samples for X-ray diffraction (XRD). The rock samples and probe flakes were prepared for scanning electron microscope and electron probe, respectively. (2) Analytical method. The phase of the clay mineral was identified by the orientated flakes on a D/Max-Ra X-Ray Diffraction Machine at the Analytical Center, Nanjing University with Cu target, voltage of 50 kV, currency of 150 mA, step width of 0.02°2θ, and scan range of 3–40 °2θ. The contents of the clay minerals were estimated in semi-quantitative way by the intensity of diffraction peak [23]. The chemical composition of the kaolinite was analyzed on JEOLJXA-8800M Electronic Probe Instrument at the State Key Laboratory for Mineral Deposits Research, Nanjing University, using the mineral standards of the National Bureau of Standards (NBS) in USA. The acceleration voltage was 15kV, the probe currency 10 mA, and the beam spot diameter smaller than 1 μm. Using the non-orientated powder samples through removing chlorite, the order-disorder of kaolinites was measured on a D/Max-Ra X-Ray Diffraction Machine at the Analytical Center, Nanjing University. The condition is the same as the phase analysis of the clay mineral. The infrared spectrum analysis was accomplished on NICOLET6700 Fourier Infrared Spectrum Machine (FTIR) at the State Key Laboratory for Mineral Deposits Research, Nanjing University. The configurations of kaolinite and halloysite were observed on JSM-6490 Scanning Electron Microscope Instrument (SEM) with voltage of 20 kV at the State Key
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Laboratory for Mineral Deposits Research, Nanjing University.
3
Analysis results
3.1
Reaction series of the clay minerals
3.1.1 Assemblages of the clay minerals The clay minerals in the Paleogene Damintun Depression (Es3 and Es4) are composed mostly of mixed-layer illite/ smectite (I/S), kaolinite (Ka), halloysite (H), illite (I) and chlorite (Chl) (Table 1). All samples contain kaolinite and mixed-layer illite/smectite. Halloysite only appeared in the depth of 1443.8 m. Chlorite content is extremely low in the area, only 0–7.5%. The assemblages of the clay minerals in the depth range of 1443.8 to 2479 m are R0I/S+Ka±I± H±Chl, 2550.25 to 3074 m are R1I/S+Ka±Chl±I except for two samples that are R0I/S +Ka+I +Chl. The low evolvement of clay minerals represents the relatively low thermal evolvement in the basin. 3.1.2 Kaolinite (1) Morphologies of kaolinite and halloysite. The energy spectrum data from SEM show that the components of kaolinite are mainly Si and Al, with a few other elements. The morphologies of kaolinite are generally irregular flakes (Figure 1(a), (c) and (d)) and vermiform-like (Figure 1(b)). Table 1 Well No.
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The halloysite was found at the depth of 1443.8 m. Its energy spectrum data are the same as the kaolinite’s, but its morphology is a long tubular shape (Figure 1(e)). (2) Chemical compositions of kaolinite. Chemical compositions of kaolinites obtained by using the electron probe method are listed in Table 2. The mean values from different depths show that the main components, SiO2 and Al2O3, account for 45.450%－47.025% and 37.190%－38.602% respectively, and other oxides are all lower than 1%. It also shows that MgO is greater than FeO, CaO greater than FeO, and K2O greater than Na2O in content. The correlations have not been observed between the burial depths and the contents of various ions (Table 2). (3) Kaolinite crystallinity. The kaolinite crystallinity was determined by measuring the half width of the X-ray 001 diffraction peaks of random clay powder samples. Bauluz et al. [24] requires that the samples must be chlorite and halloysite free for kaolinite crystallinity measurement. For the purpose of comparison, the half widths of the 002 diffraction peaks were measured at the same time. The results of four samples from different depths are shown in Table 3 and Figures 2 and 3. It can be seen that the kaolinite crystallinity from 001 is all the same (0.40 °2θ) while the kaolinite crystallinity from 002 decreases from 0.42 °2θ to 0.20 °2θ as the sample depth increases. The difference between 001 and 002 is caused by non-uniformity in the structures.
Clay mineral assemblages of the Paleogene strata from a part of wells in Damintun Depression Horizon
Es4
Depth (m)
Lithology
Assemblages
1443.8 1545 1636 1670 1553.7 1611.41 1756.86 1859.36 1959.7 1999 2025 2170 2218
black mudstone black mudstone black mudstone green mudstone brown shale brown shale celadon shale dark grey shale celadon mudstone black shale black shale black shale dark grey mudstone
R0I/S+Ka+I+H R0I/S+Ka+Chl R0I/S+ Ka R0I/S+Ka R0I/S+I+Ka R0I/S+I+Ka R0I/S+Ka R0I/S+Ka R0I/S+I+Ka R0I/S+Ka R0I/S+Ka R0I/S+Ka R0I/S+Ka+I+Chl
Ka 10 53 61 49 28 23 19 46 48 48 43 33 28
Content (%) I/S 32 46 39 51 59 61 81 54 31 52 57 67 52
I 58
I% (in I/S)
15
5
55 50
