A complex and tight lacustrine carbonate reservoir is the main target reservoir in Qaidam Basin. The diagenesis of shallow-shore lake deposition has created a highly heterogeneous reservoir in both vertical and horizontal directions in terms of lithologies, pore structures, and permeability. The core analysis shows a wide variety of lithologies that include terrigenous clastics (quartz, orthoclase, and plagioclase), carbonate (calcite, dolomite, and ankerite), and evaporite minerals such as anhydrite, etc. Electron microscope analysis shows mainly intercrystallite pores with dolomite, dissolved pores, and microfractures. Associated with the complex lithology and pore structure, reservoir permeability is varied and is mostly less than 0.02mD. Evaluating and characterizing this complex reservoir to find a better pay zone was the objective of this study.
To evaluate the lithologies, pore structures, porosities, and permeability in quantities accurately, the advanced gamma-ray spectroscopy logs, nuclear magnetic resonance (NMR) logs, and high-resolution micro-resistivity borehole image logs were acquired in some key wells. Integrated with core data such as X-ray diffraction (XRD), mercury injection capillary (MICP), the lithofacies, texture facies, and pore facies were classified based on the minerals from gamma-ray spectroscopy, the textures from borehole image, and the pore sizes from NMR. The combination of these three facies represents the reservoir quality.
In the study area, the lithofacies were classified into four types, which are high carbonate content facies, middle carbonate content facies, high clay content facies, and high clastic content facies. The texture facies were classified into four types, which are massive texture, layered texture, dissolved texture, and algal texture. The pore structure facies were classified into four types, which are macro facies, meso facies, meso-micro facies, and microfacies. The better pay zone is the combination of high carbonate content facies, algal texture, and macro facies. The pay zone of the reservoir was well defined and mapped with the combination facies vertically and horizontally in the whole study area.
The integrated solution described in this study leverages the advantages of advanced gamma-ray spectroscopy, NMR, borehole images, and core measurements. It develops a comprehensive understanding of the complex carbonate reservoir and provides the solution to pinpoint the sweet spots and place the horizontal well.