Abstract
Feldspar dissolution is a common feature in clastic rock reservoirs of petroliferous basins and has an important influence on reservoir quality. However, the effect of feldspar dissolution on reservoir quality varies under different depositional environments and diagenetic systems. The study area in this paper is located in the Baikouquan Formation in the northwestern margin of the Junggar Basin, which is significantly influenced by feldspar dissolution. Based on the analyses of core and thin section observations, QEMSEM, XRD, SEM, CL, fluorescence, and image analysis software combined with logging and physical property data, this study shows that feldspar dissolution in the subaqueous distributary channel of a fan delta plain, which has good original physical properties and low mud contents, significantly improves the properties of the reservoir. The main reasons for this are as follows: (1) the sedimentary facies with good original properties and low mud content is a relatively open system in the burial stage. The acidic fluids needed for feldspar dissolution are mostly derived from organic acids associated with the source rocks and migrate to the good-permeability area of the reservoir; (2) the by-products of feldspar dissolution, such as authigenic clay minerals and authigenic quartz, are transported by pore water in a relatively open diagenetic system and then precipitated in a relatively closed diagenetic system; and (3) the clay minerals produced by feldspar dissolution in different diagenetic environments and diagenetic stages have different effects on the reservoir. When the kaolinite content is less than 3%, the illite content is less than 4%, and the chlorite content is less than 12%, the clay minerals have a positive effect on the porosity. These clay minerals can reduce porosity and block pore throats when their contents are larger than these values.
Highlights
One-third of clastic rock oil and gas storage space is secondary porosity formed by mineral dissolution [1]
Feldspar dissolution in clastic reservoirs is a common geological phenomenon that results in dissolution pores, which improve the physical properties of the reservoir [13,14,15], and a popular research topic in oil and gas reservoir geology [2, 6, 16]
The conglomerate, sandstones, and clays analysed in this study are from the Upper Triassic Baikouquan Formation of the Xinjiang oil field, which is located in the northwestern margin of the Junggar Basin
Summary
One-third of clastic rock oil and gas storage space is secondary porosity formed by mineral dissolution [1]. Some scholars [25, 26] believe that grain particle dissolution, especially feldspar dissolution, can effectively improve the physical properties of the reservoir. Precipitation occurs when the pore water reaches saturation and forms authigenic minerals, such as kaolinite, chlorite, illite, and authigenic quartz, which can occupy pore space, block throats, and reduce the quality of the reservoir properties. This saturation of the solution is related to temperature, pressure, flow velocity, and ionic species. This paper carries out a large number of core observations, intensive and representative sample collection, advanced research methods, and experimental means to analyse the influence of feldspar dissolution, its by-products on the reservoir, and provide a reliable basis for the future exploration of favourable reservoirs
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