Upper Paleozoic Transitional Shale Gas Enrichment Factors: A Case Study of Typical Areas in China

Abstract

Based on the shale gas research experience in North America, large-scale geological evaluations have been conducted in China to determine the enrichment characteristics of deep marine shale gas, leading to the discovery of the Fuling, Changning and Weiyuan shale gas fields. However, research on Upper Paleozoic transitional shale gas remains limited, restricting the subsequent exploration and development. Therefore, taking the Lower Permian Shanxi and Pennsylvanian Taiyuan Formations in the northeastern Ordos Basin and the Upper Permian Longtan Formation in southwestern Guizhou as examples, gas logging, gas desorption, thermal simulation, maximum vitrinite reflectance (Rmax), and X-ray diffraction (XRD) were used to study the influence of lithological associations, sedimentary facies, gas generation thresholds, and pore evolution on transitional shale gas, and then Upper Paleozoic transitional shale gas enrichment factors of the northeastern Ordos Basin and southwestern Guizhou were analysed. The results show that carbonaceous mudstone adjacent to coal seams presents a high gas content level, and is primarily developed in swamps in the delta plain environment, and swamps and lagoons in the barrier coastal environment. The gas generation threshold maturity (Rmax) of transitional shale is 1.6% and the corresponding threshold depths of the northeastern Ordos Basin and southwestern Guizhou are estimated to be 2265 m and 1050 m. Transitional shale pore evolution is jointly controlled by hydrocarbon generation, clay minerals transformation, and compaction, and may have the tendency to decrease when Rmax < 1.6% or Rmax > 3.0%, but increase when Rmax ranges between 1.6% and 3.0%, while the main influential factors of pore evolution differ in each period. Continuous distribution of transitional shale gas enrichment areas can be formed along the slope adjacent to coal seams with a moderate maturity range (1.6%–3.0%) in the northeastern Ordos Basin, and transitional shale gas can be enriched in the areas adjacent to coal seams with a moderate maturity range (1.6%–3.0%), abundant fractures, and favorable sealing faults in southwestern Guizhou.

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

“…The particles are in linear–concave–convex contact, and the primary pores are greatly reduced. Chemical compaction proceeds, the space of clay mineral layers is further reduced, and the interlayer water is removed; only the last layer of structural water remained. − Face porosity of clay minerals is approximately 10%. In period B of the middle diagenetic stage, organic matter in shale evolves to the high maturity stage, and a large number of gaseous hydrocarbons forms.…”

Numerical Investigation of Fluid Flow Instabilities in Pore-Scale With Heterogeneities in Permeability and Wettability

“…This field is developed in swamps/lagoons in the delta plain and coastal environments. Lithology and sedimentary facies, pore spaces, structural geometries, and long-term tectonic movements are the main factors of shale gas enrichment, hydrocarbon migration, and entrapment (Wang and Guo, 2020). Paradox Basin is another example of silica-clastic rocks contiguous to chemical rocks (e.g.…”