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Carbonate Cementation in the Middle Jurassic Reservoir Sandstone, Changji Depression, Junggar Basin, NW China

Abstract

The top seal of overpressured compartment in Changji Depression lies in Xishanyao Group and Toutunhe Group, buried between 5000∼6000km. frequent geo-fluid activities had been taken place in pressure transformation zoon and its vicinity area, as a result fluid-rock interaction are significant and complex. Carbonate cements are commonly used as indicator of paleo-fluid to rebuilt the diagenesis process and reveal the mechanism of cement and secondary porosity generation. In this study transmitted light microscopy, cathodoluminescence, electron probe, stable isotopes and other tests are carried out. Carbonate cements can be divided into two types. Type A carbonate constitutes 15∼30% of the average sandstone volume, sufficiently filling primary and feldspar dissolution pores and having bright orange cathodoluminescence color. The cements have ferrum (Fe) content about 0.45% in average, forming temperature is 15 ∼ 30 °C(calculated by oxygen isotope) and δ13C values cluster closely around – 5‰(PDB). Type B carbonate cements constitute less than 10% of the average sandstone volume and the cathodoluminescence color is dark red. Fe content of Type B is about 1.2% in average, forming temperature is 70∼100°C and δ13C values more depleted than – 10‰(PDB) genrally. Considering oil charging history, paleo-climate during unconformity formation and coal maturation evolution, we deduce that there had been two phases of carbonate cementation. Organic acids were produced by oil biodegradation during tectonic uplift and erosion in Late Jurassic. The acids dissolute feldspar, generating plenty of secondary porosity, and then Type A carbonate cement formed and blocked late mechanical compaction. During the period of coral maturating, related organic acids and CO2 charging into the sandstone, feldspar as a pH buffer was consumed, resulting in clay minerals generation and Type B carbonate cements precipitation. Consequently, porosity and permeability conditions of high feldspar content sandstone change worse in this process, while reservoir quality of low feldspar content sandstone are improved by acidic fluid dissolution of early generated carbonate cements.