Abstract
Extremely high porosities and permeabilities are commonly discovered in the sandstones of the Xishanyao Formation in the central Junggar Basin with the burial depth greater than 5500 m, from which hydrocarbons are currently being produced. High content of carbonate cements (up to 20%) is also observed in a similar depth range. Our study aimed to improve our understanding on the origin of carbonate cements in the Xishanyao Formation, in order to provide insights into the existence of high porosity sandstones at greater depths. Integrated analyses including petrographic analysis, isotopic analysis, fluid-inclusion, and core analysis were applied to investigate the distribution and origin of carbonate cements and the influence of high fluid pressure on reservoir quality. Textural evidences demonstrate that there are two generations of carbonate cements, precipitated at the temperature of 90°C and 120°C, respectively. The carbonate cements with low δCPDB13 ranging from −19.07 to -8.95‰ dominantly occurred near the overpressure surface and especially accumulated at approximately 100 m below the surface. Our interpretation is that high content of carbonate cements is significantly influenced by early carbonate cements dissolution and migration under overpressure. Dissolution of plagioclase resulted in the development of internal pores and porosities of as much as 10% at 6500 m depth presumably.
Highlights
Carbonate cements in sandstones have variable mineralogy, texture, and chemical compositions and exhibit significant effects on reservoir properties because it is commonly concentrated rather than being uniformly distributed
It is observed that the favorable sandstone reservoirs are developed at the depth of 4500∼6000 m in the central Junggar Basin
Conventional core samples, epoxy-impregnated thin section analysis, and cathodoluminescence analysis indicate that the growth of carbonate cements has two stages and mostly formed at the late diagenetic stage, generating ferroan calcite and ankerite cements
Summary
Carbonate cements in sandstones have variable mineralogy, texture, and chemical compositions and exhibit significant effects on reservoir properties because it is commonly concentrated rather than being uniformly distributed. If the carbonate cements formed during early diagenetic stage, it could provide a framework that resists burial compaction and retains primary porosity until decarbonatization at greater burial depth [1,2,3]. Microlitic carbonate cements formed at early diagenetic stage can undertake partial overburden load that can slow compaction and can be dissolved into secondary pores under favorable geologic conditions. We attempt to investigate the origin of carbonate cements in deep-buried sandstones in the central Junggar Basin by applying a multidisciplinary approach, including petrographic, microthermometric, fluid-inclusion, and geochemical analysis. The main objectives of the study are as follows: (1) to quantify the chemical composition, size, and spatial distribution of carbonate cements and (2) to provide further insights into the effect of carbonates cements on petrophysical properties of deep-buried reservoirs
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