Petroleum generation potentials and kinetics of coaly source rocks in the Kuqa Depression of Tarim Basin, northwest China

Abstract

Abstract Confined pyrolysis experiments (gold capsules) were performed to determine the yields and kinetic features for petroleum formation for seven coal samples with hydrogen index (HI) ranging from 57 to 278 mg HC/g TOC and maturities of 0.58–0.74 %Ro from coal pits within Triassic–Jurassic strata in the Kuqa Depression, China. Gases and liquid yields were measured at regular intervals as the sealed tubes were heated at 2 and 20 °C/min and total thermal stress calculated as a vitrinte reflectance equivalent (%Re) using Easy%Ro. The total confined pyrolysate yields of oil and gaseous hydrocarbons at 1.19–1.50 %Re only account for a portion (38–53%) of the releasable moieties in measured by Rock-Eval (open) pyrolysis, suggesting that a substantial portion of (47–62%) of these moieties was rearranged and incorporated into polyaromatic residual solids. At maturities >1.87 %Re, the solid residues of the seven coals have very similar gas generative potentials (ΣC 1-5 ), which are substantially higher than their quality index (QI = (S1+S2)/TOC) with differences ranging from 20 to 40 mg/g TOC. This result can be mainly ascribed to the differences both in methane formation mechanisms and final thermal stress levels between open (2.25 %Re) and confined pyrolysis (4.44 %Re). Only a minor portion of gaseous hydrocarbons (∼32% and 44% for the Jurassic and Triassic coals, respectively) was generated up to 2.19 %Re while the major portion was generated at higher maturities. Under a heating rate of 5 °C/My, the Jurassic and Triassic coals are modeled to become effective gas source rocks with gas yield (ΣC 1-5 ) >20 mg/g TOC at maturities of >1.76 %Re and 1.59 %Re, respectively. The abundant gaseous hydrocarbons found in the Kuqa Depression can be mainly ascribed to the high maturities of coal source rocks (>2.0 %Ro), in combination with excellent seal of thick salt and gypsum for the gas reservoirs.