Authigenic Illite in the Sandstone Reservoirs of Taiyuan Formation, Northeast Ordos Basin

The Upper Palaeozoic sandstones are important hydrocarbon reservoirs in the Northeast Ordos basin. More extensive illitization is observed in the Upper Carboniferous Taiyuan Formation than those in other reservoir units. In this research, the nature and porosity effects of illite in the Taiyuan Formation of Northeast Ordos basin have been investigated by means of thin-section observations, scanning electron microscope (SEM), and X-ray diffraction techniques. We have shown different types of illite in the sandstones based on their textural features. Detrital illite, the least abundant component, frequently occurs as tangentially arranged, grain-lining, ragged plates. A detrital origin can be distinguished by its poorly-crystalline morphology with wrinkled surface and blurred boundary, suggesting it was suffering from sedimentary transport. Diagenetic illite, recognized by their delicate, fibrous or lathy morphology, occurs commonly as pore-filling phases formed by feldspar alteration and radially-oriented, grain-rimming cements. It is also observed replacing kaolinite, as laths are extending from pseudohexagonal plates.

It is only in the sandstone of Taiyuan Formation that kaolinite concentration dose not increase at the expense of feldspar. This intense illitization, and relative scarcity of kaolinite, may be due to the prolonged exposure to saline depositional pore-waters during Miaogou transgression happened in early Taiyuan period. The growth of authigenic K-feldspar, occurring as intergrowth with grain-lining illite, implies that a high concentration of potassium ions was present in the original ground waters. K-feldspar dissolution, inhibited or slowed down by early penetration of K-rich fluids, may provide other potential sources of potassium for illite formation at the expense of kaolinite during burial diagenesis. This process has a greater temperature dependency and burial histories indicate that Taiyuan Formation entered illitization window (130°C and 140°C) at 108-135 Ma.

There is a positive correlation (R²= 0.55) between the abundance of authigenic illite and variations in thin-section porosity for Taiyuan sandstone reservoirs. This is interpreted to indicate that majority of the total sandstone volume is secondary porosity, produced primarily by illitization at the expense of kaolinite and K-feldspar.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California