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AAPG Bulletin


Volume: 80 (1996)

Issue: 2. (February)

First Page: 157

Last Page: 172

Title: Illite/Smectite Diagenesis in the NanXiang, Yitong, and North China Permian-Carboniferous Basins: Application to Petroleum Exploration in China

Author(s): Xinhua Deng (2), Youngchuan Sun (3), Xinrong Lei (3), Qi Lu (3)


The NanXiang basin in Henan Province and the Yitong basin in Jilin Province, located in the eastern part of China, are composed mainly of Tertiary nonmarine clastic sequences that thicken to more than 4000-5000 m. The North China Permian-Carboniferous basin consists of Permian-Carboniferous systems composed of marine and nonmarine coal-bearing sequences. Clay diagenesis in these basins was studied by systematic x-ray diffraction analysis. The clay mineralogy consists of illite, smectite, mixed-layered illite/smectite (I/S), kaolinite, and chlorite. The major clay reaction is the illitization of smectite with depth and temperature. The reactions generally involve discrete smectite stage to smectite, plus the randomly interstratified I/S (R = 0 in the Reichweite nomenclatur ), followed by the first rapid change (I) to the ordered I/S of short-range stacking (R = 1) and the second rapid change (II) to the metastable stage (with the ordered I/S of long-range stacking--R >= 3). The first rapid change leads to the appearance of I/S-ordered I/S and disappearance of discrete smectite. The proportion of smectite layers (Ps) in the I/S in the metastable stage remained unchanged (15-20%) over a thick burial interval. The discrete illite stage finally occurs in highly mature areas.

Potassium-feldspar (K-feldspar) in rocks and K ions in present formation waters are available for the I/S reactions; I/S reactions in these areas responded mainly to temperature. The I/S transition from the randomly interstratified I/S (R = 0) to the ordered I/S occurred at 80-95°C and coincided with the onset of the oil window in source rocks and with secondary porosity enhancement of sandstones. The transition from the R = 1 to R >= 3 happened around 130-165°C, corresponding to the wet gas stage. The illite stage corresponds to the dry gas stage of organic materials. The duration of burial did not contribute significantly to the reaction. Integrating the I/S reactions with thermal histories, hydrological regimes, structure, sedimentation, and sandstone diagenesis of eac basin allows formulation of clay diagenetic models for each specific basin.

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