Hydrocarbon Generation, Expulsion Processes, and Gas Occurrence Characteristics of Humic Source Rocks: Insights from the Eastern Margin of the Ordos Basin

Abstract

We conducted in-depth research on the hydrocarbon generation and expulsion characteristics of the Permian humic source rocks and the evolution of the reservoir gas content in the eastern margin of the Ordos Basin.Using low-mature shale from the Palougou section, thermal simulation experiments were conducted in a closed system. Rock-Eval pyrolysis, total organic carbon (TOC) analysis, vitrinite reflectance (EasyRo) analysis, and gas chromatography (GC) were performed on sample residues and products. Based on isothermal adsorption experiments combined with the hydrocarbon expulsion conceptual model, we established evolution models for thermogenic gas (G), gas-in-place (GIP), adsorbed gas (Vad), and free gas (Vfree). The results are as follows: (1) The hydrocarbon generation and expulsion stages can be divided into the immature (EasyRo<0.77%), oil generation (EasyRo=0.77–1.54%), oil cracking (EasyRo=1.54–2.51%), secondary heavy hydrocarbon gas cracking (EasyRo=2.51–3.61%), and  organic matter carbonation (EasyRo=3.61–4.05%) stages. Based on aromatic and phenolic features, the humic source rocks have a significant late gas potential, with LGP>0.55 and LGT>1 at EasyRo>1.99%; (2)  The volume of G is 5.89-9.25 cm3/g. The Vad is positively correlated with the pressure and TOC, and negatively with the temperature, ranging from 1.30 to 1.41 cm3/g. The hydrocarbon expulsion efficience ηpg is 65.28–86.62%, and the GIP is 5.02–7.38 cm3/g. Furthermore, the hydrocarbon generation intensity ISt and expulsion intensity IPt are (0.42–8.62)×108/km2 and (1.89–21.75)×108 t/km2, respectively. In the study area of Daning-Jixian, the amount of hydrocarbon generation QSt is 25.94×108 t and expulsion QPt is 9.17×1011t ; (3) A key well Daji3-4 in the study area contains the strata from the Benxi Formation to the P1s22 member, and the main hydrocarbon generation center is located in the lower Shan23 layers, with free gas volume of about 3.85 cm3/g. Rapid subsidence began at around 144 Ma during the Late Cretaceous, which increased the ground temperature gradient, leading to increased maturity and a significant increase in gas content.

Keywords: Humic source rocks, Hydrocarbon generation, Expulsion processes, Gas occurrence, Ordos Basin