Methods for Eliminating Oil-Based Drilling Fluid Pollution for Ultradeep Source Rock Cuttings: A Case from the Kuqa...

Obtaining core samples from ultradeep hydrocarbon source rocks poses significant challenges, and the more readily available hydrocarbon source rock cuttings are often contaminated to varying degrees by oil-based drilling fluids (OBDF). This makes it difficult to obtain the real geochemical characteristics of ultradeep hydrocarbon source rocks, which is one of the key problems faced by ultradeep oil and gas exploration at present. To address the issue of OBDF contamination in rock cuttings, this study examines rock cuttings from well T in the Kuqa Depression (∼6980 m) and cores from other wells within the same stratigraphic position as examples. By comparing molecular geochemical parameters from both cores and rock cuttings, this study evaluates the decontamination effects of three different treatment methods: twice-extraction method (TEM), extraction method of hydrocarbons from kerogen adsorption (EMHK), and microablation. Results reveal significant differences in biomarker compounds extracted from source rock cuttings processed by TEM. Biomarkers from the first extraction are largely consistent with those of oil-based drilling fluids, which indicates that the rock cuttings have suffered from different degrees of contamination. Big particle rock cuttings exhibit relatively lower contamination levels, and their biomarker compounds from the second extraction are similar to those from the core. On the other hand, biomarker compounds extracted by EMHK differ notably from those in oil-based drilling fluids and exhibit partial similarity with uncontaminated core samples, but certain parameters remain inconsistent, likely due to incomplete pretreatment (e.g., insufficient solvent washing before kerogen making). In contrast, biomarker parameters and GC-MS spectra of surface-stripping powders obtained via microablation initially show contamination similar to OBDF. However, biomarker parameters and GC-MS spectra from the inner portions of big particles processed via microablation closely align with those of uncontaminated cores, reflecting the authentic geochemical characteristics of source rocks. Comparatively, TEM demonstrates incomplete decontamination, and the effectiveness of EMHK is heavily influenced by the thoroughness of pretreatment. Microablation effectively reveals the true biogenic signature of ultradeep hydrocarbon source rocks. Thus, the broader adoption and application of microablation techniques hold great promise for advancing global ultradeep hydrocarbon exploration.