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

Abstract

Unconventional oil and gas resources require petrophysical logs to answer the question “How to optimize the geological and engineering sweet spots?”. Therefore the establishment of key well with comprehensive descriptions of lithology, reservoir property, hydrocarbon bearing property, electronic well log responses, source rock property, brittleness, and in situ stress magnitude and direction is important for exploration and production of unconventional hydrocarbon resources. Cores, thin section, scanning electron microscopy (SEM) and comprehensive well log suits are used to build a key well of the Permian Lucaogou Formation, Jimusar Sag of Junggar Basin. The results show that there are mainly three 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 the T2 components >0.3 ms is used to calculate oil saturation. TOC is calculated using ΔlogR method. Brittleness index is calculated using Poisson‐ Young's method, and brittleness index ranges 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. Then geological and engineering (geomechanical) sweet spots are optimized from the key well by fully analyzing lithology, reservoir property, oil bearing potential, in situ stress magnitude and brittleness. The results are hoped to support engineers' and geologists' decisions for exploitation of unconventional hydrocarbon resources.

Control of Physical Properties by the Matching between Rock Components and Pore Structure in Shale Reservoirs of the Permian Lucaogou Formation, Jimusar Sag

“…The primary carbonate minerals in the P 2 l are dolomite, which exhibits mud-crystal and microcrystalline structures. The dolomite’s strong compressive resistance effectively preserves regular-shaped pores between crystals, mostly classified as P mes (Figure h,i). Similarly, organic acids can dissolve pores between dolomite particles, forming P mac (Figure j), although these are less common.…”

“…The development of these pores is closely related to the content of OM and thermal maturity. Additionally, high dolomite content can react with iron elements in the fluid to form ankerite, undergoing transitional cementation, which significantly reduces the number of intergranular pores (Figure k). Calcite content is lower, mostly acting as a cement to fill pores and developing only a few pores between other minerals (Figure c).…”