Dolomitization history and porosity evolution of the deeply buried Cambrian Xixiangchi Formation, Sichuan Basin, SW China

Abstract

The deeply buried (>4500 m) Cambrian Xixiangchi Formation in the Sichuan Basin, southwestern China, hosts significant reserves of natural gas. A comprehensive analysis combining petrographic, paleo-thermometric, geochemical, and petrophysical materials of the dolostone from the Xixiangchi Formation was conducted to provide insight into deeply buried carbonate reservoirs and decipher the complex diagenetic history. Dolomite-mudstone, fine-crystalline dolostone, and fine-crystalline dolomite cement of the Xixiangchi Formation underwent sabkha and reflux dolomitization. Medium- to coarse-crystalline dolostone, dolo-grainstone, and medium- to coarse-crystalline dolomite cements were then formed by burial dolomitization. These dolomites display δ¹³C ratios and REE patterns comparable to seawater, with progressively depleted δ¹⁸O ratios at greater burial depths. Dolo-grainstone originating from the platform shoal facies exhibits higher primary porosities and well-developed inter-particle pores compared to dolo-mudstone and crystalline dolostone lithologies, which are typically associated with the low-energy tidal flat and/or restricted platform environments. The initial spatial heterogeneity of primary porosity was subsequently modified by meteoric alteration and repeated episodes of dolomitization, which contributed to the development of secondary porosity. These processes increased the resistivity to compaction, and open fractures increased reservoir permeability. During the deep burial regime, saddle dolomite and calcite cements were precipitated at high fluid temperatures (up to 220 °C). Thermochemical sulfate reduction is characterized by the occurrence of anhydrite, hydrocarbon, and high homogenization temperatures and significantly low δ¹³C ratios (av.=−23.7‰) of calcite cements. Deep burial dissolution is significantly constrained by: corrosion of late diagenetic minerals, and the occurrence of bitumen in the center of pores. Mechanisms for the deep-burial dissolution include hydrothermal alteration and thermochemical sulfate reduction. This study indicates the complex diagenetic evolution of Cambrian Xixiangchi Formation, providing significant insights into global deep-burial carbonate reservoir potential.