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

Abstract

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 (R 0 I/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: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][2][3][4][5][6][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 mixedlayer 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 1:1-type structural layer, including kaolinite, dickite, nakrite, 74 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][11][12]. Kaolinite is far more common than dakrite. Many studies have proved that kaolinite is the only stab...

Oscillation of mineral compositions in Core SG-1b, western Qaidam Basin, NE Tibetan Plateau

“…On the other hand, temperature is an important control of the crystallinity and chemistry of illite and chlorite, the formation of I/S mixed minerals, and the switch from disorder I/S to order I/S 5 6 7 8 9 32 33 34 . For example, low IC is preserved in the cold climate sections.…”

“…The S-I reaction is an abiotic dissolution re-crystallization process that typically requires temperature conditions of 300–350 °C 9 . The transformation temperature from disorder R 0 I/S to order R 1 I/S is ~100–110 °C 32 33 , and R 1 I/S is stable between ~150 °C and ~225 °C 34 . The Fe-Mg contents of illite and chlorite will increase with increasing temperature 6 .…”