Porosity model and pore evolution of transitional shales: an example from the Southern North China Basin

Abstract

The evolution of shale reservoirs is mainly related to two functions: mechanical compaction controlled by ground stress and chemical compaction controlled by thermal effect. Thermal simulation experiments were conducted to simulate the chemical compaction of marine-continental transitional shale, and X-ray diffraction (XRD), CO 2 adsorption, N 2 adsorption and high-pressure mercury injection (MIP) were then used to characterize shale diagenesis and porosity. Moreover, simulations of mechanical compaction adhering to mathematical models were performed, and a shale compaction model was proposed considering clay content and kaolinite proportions. The advantage of this model is that the change in shale compressibility, which is caused by the transformation of clay minerals during thermal evolution, may be considered. The combination of the thermal simulation and compaction model may depict the interactions between chemical and mechanical compaction. Such interactions may then express the pore evolution of shale in actual conditions of formation. Accordingly, the obtained results demonstrated that shales having low kaolinite possess higher porosity at the same burial depth and clay mineral content, proving that other clay minerals such as illite-smectite mixed layers (I/S) and illite are conducive to the development of pores. Shales possessing a high clay mineral content have a higher porosity in shallow layers (< 3500 m) and a lower porosity in deep layers (> 3500 m). Both the amount and location of the increase in porosity differ at different geothermal gradients. High geothermal gradients favor the preservation of high porosity in shale at an appropriate R o. The pore evolution of the marine-continental transitional shale is divided into five stages. Stage 2 possesses an R o of 1.0%-1.6% and has high porosity along with a high specific surface area. Stage 3 has an R o of 1.6%-2.0% and contains a higher porosity with a low specific surface area. Finally, Stage 4 has an R o of 2.0%-2.9% with a low porosity and high specific surface area.

Pore Connectivity of Deep Lacustrine Shale and its Effect on Gas‐bearing Characteristics in the Songliao Basin: Implications from Continental Scientific Drilling

“…The development and evolution of shale pores in deep lacustrine Shahezi Formation of Well SK-2 in the Songliao basin are primarily affected by compaction and high-temperature corrosion. With the increase of burial depth of shale, the compaction and high temperature corrosion are stronger, and smectite in I/S is gradually transformed into illite (Yang and Guo, 2020;Deng et al, 2021;Dong et al, 2021). Illite gradually participates in the formation of pores as the main clay mineral.…”

Review of the Effect of Diagenetic Evolution of Shale Reservoir on the Pore Structure and Adsorption Capacity of Clay Minerals

“…In the late diagenetic stage, illitization of smectite proceeds with removal of the last structural water, and the pore size and porosity of clay minerals is further reduced. − Additionally, chemical compaction is a deionization process of calcium, iron, magnesium, and silicon. Silicon ions form silicate cement, calcium, magnesium, iron, and ions form carbonate cement. − Generally, cementation is the filling and plugging of pores and fractures by diagenetic authigenic minerals, resulting in the reduction of porosity and permeability, and the deterioration of reservoir performance.…”

Pore Structure Characteristics, Genesis, and Its Controlling Effect on Gas Migration of Quaternary Mudstone Reservoir in Qaidam Basin

“…With the continuous advancement of exploration and development technology, primarily the breakthrough of horizontal wells and hydraulic fracturing technology, the abundant hydrocarbon resources in mudstone, which is the source rock or cap layer of conventional oil and gas reservoirs, have received widespread attention. Micro-nanopores are developed in mudstone reservoirs, and the pore structure controls the occurrence state, gas content, seepage capacity, and microhydrocarbon migration and accumulation mechanism of mudstone gas [6][7][8]. Therefore, the study of micro-nanopore structure characterization of mudstone is helpful to improve the prediction and characterization of the gas storage, hydrocarbon migration, and accumulation properties of mudstone reservoirs.…”