Types, Characteristics and Origin of Kaolinite in Sandstone Reservoir of Shahejie Formation, Dongying Sag

Key words: 
• kaolinite /
• types /
• origin /
• sandstone reservoir /
• Shahejie Formation /
• Dongying sag

Abstract: Kaolinite is a common clay mineral of the Shahejie sandstone reservoir in Dongying sag, which has an important influence on the reservoir properties. However, the characteristics and origin of kaolinite are poorly understood. Based on data of structural evolution, fluid history and diagenetic sequence of Dongying sag, types, characteristics and origin of kaolinite were studied by using a series of techniques-rock slice, scanning electron microscope(SEM), energy dispersive X-ray(EDX) and cathodoluminescence(CL). Research results show that there are mainly two kinds of kaolinite. The first kind kaolinite has perfect crystal form and pure elements, it appears in scaly-vermiform shape under single polar and the tightly packed aggregate of kaolinite occurs as vermiform or booklet under SEM. The crystal form of the second kind kaolinite is not so perfect as the first, it appears in scaly shape under single polar light. Under the SEM some laminae is curved and the aggregate of kaolinite occurs as accordion which is packed loosely. Except for Al, Si, O, the second kind of kaolinite contains little Fe and Ca. These two kinds kaolinite in Dongying sag are mainly authigenic kaolinite, whick have been precipitated from the dissolution of feldspar in the strata acidic media relevant to the evolution of organic matter. These two kinds of kaolinite have different formation period, origin and formation circumstance, the first and the second kinds kaolinite are relative to the twice oil and gas charging of Dongying sag, respectively, which occurs in late Dongying period, late Guantao and Minghuazhen period. When the second kind kaolinite forms, sandstone enters the late diagenesis A substage period and large amounts of smectite transfer to illite, causing more Fe and Ca cation dissolved in the acid fluid.