Pore type and pore size distribution of tight reservoirs in the Permian Lucaogou Formation of the Jimsar Sag, Junggar Basin, NW China

The tight reservoir of the Lucaogou Formation in the Jimsar sag, located in the southeastern Junggar Basin, northwest China, has been investigated for its great potential tight oil resources. In this study, petrography and mineralogy, pore microstructures, and pore size distributions of the samples collected from nine wells, were analyzed using X-ray diffraction (XRD), total organic carbon (TOC), thin sections and scanning electron microscopy (SEM). The major rock types were identified as dolomite and siltstone, which had different pore type assemblages and various pore size distribution patterns. Based on the pore origin, pores were identified as: primary mineral pores, dissolved mineral pores, primary organic matter pores and secondary organic matter pores. Their sizes followed a power law distribution, regardless of lithological variability or SEM magnification. However, the power law exponent D was associated with lithology and diagenesis as well as hydrocarbon generation. In dolomite, the decrease in dolomite content and the increase in dolomite particle size were commonly in accordance with the decrease in the power law exponent D. Compaction and cementation were the principal diagenetic processes that had a crucial effect on the pore size distribution with a large D equal to 2.77. In siltstone, the influence of various pore types on the pore size distribution was controlled by diagenesis, in which case dissolution and cementation were compensating for their influence with a fluctuating power law exponent D ranging from 1.5 to 2.7.