Geological and Engineering ‘Sweet Spots’ in the Permian Lucaogou Formation, Jimusar Sag, Junggar Basin

Abstract

AbstractUnconventional oil and gas resources require petrophysical logs to answer the question of how best to optimize geological and engineering ‘sweet spots’. Therefore, the establishment of a key well with comprehensive descriptions of lithology, reservoir properties, hydrocarbon‐bearing properties, electronic well log responses, source rock properties, brittleness, and in situ stress magnitude and direction is important for the effective exploration and production of unconventional hydrocarbon resources. Cores, thin sections, scanning electron microscopy (SEM) and comprehensive well log suites are used to build a key well for the Permian Lucaogou Formation, Jimusar Sag of the Junggar Basin. The results show that there are three main types of lithologies, including siltstone, mudstone and dolostone. Lithologies can be predicted using the combination of conventional well and image logs. The pore spaces consist of interparticle pores, intragranular dissolution pores and micropores. Nuclear Magnetic Resonance (NMR) T2 components longer than 1.7 ms are superposed as effective porosity. Permeability is calculated using the Coates model from NMR T2 spectra. The ratio of T2 components >7.0 ms to T2 components >0.3 ms is used to calculate oil saturation. TOC is calculated using the ΔlogR method. Brittleness index is calculated using Poisson‐Young's method, ranging from 13.42%–70.53%. In situ stress direction is determined, and in situ stress magnitudes (maximum horizontal stress SHmax, minimum horizontal stress Shmin, vertical stress Sv) are calculated using density and sonic logs. The strike‐slip stress type (SHmax > Sv > Shmin) is encountered. The key well which comprehensively includes the above seven properties is established. Geological and engineering (geomechanical) ‘sweet spots’ are then optimized from the key well by fully analyzing lithology, reservoir property, oil‐bearing potential, in situ stress magnitude and brittleness. It is hoped that the results support engineers' and geologists' decisions for the future exploitation of unconventional hydrocarbon resources.