Tectono-thermal impacts on the formation of a heavy oil in the eastern Tarim Basin (China): Implications for oil and gas potential

Abstract

Unraveling the accumulation and alteration process of reservoired petroleum is significant for the evaluation of petroleum potential. In this study, a paleo heavy oil from the TD2 well in the eastern Tarim Basin was investigated through integrated geochemical methods including two-dimensional gas chromatography coupled with time-of-flight mass spectrometry (GC × GC-TOFMS), gas chromatography-mass spectrometry (GC-MS) and compound specific carbon isotope analysis (CSCIA). Oil-source correlation analysis indicates that the TD2 oil accumulation is “self-generation and self-accumulation” within the Cambrian strata. Combined with tectonic evolution analysis, it is suggested that the oil experienced significant impacts due to changes in tectono-thermal process. Abnormally enriched polycyclic aromatic hydrocarbons (62.5 mg/g in total, 4- to 6- ring account for ~41 %) with few substituents suggest oil cracking due to a short-time high-temperature heating during rapid tectonic subsidence under higher geothermal gradient. Phase fractionation, as indicated by the physiochemical features and heavy isotopic values, was induced by severe tectonic uplift and denudation after accumulation, and resulted in the thickening, re-distribution, and destruction of the primary oil pool. After that, the TD2 oil pool was preserved from further alterations under steady tectonic setting with a low geothermal gradient. Four accumulation assemblages in the eastern Tarim Basin were identified and compared, indicating that great oil and gas potential remains including oil/heavy oil in the Cambrian assemblage and light oil/condensate in the Jurassic and Silurian assemblages. Future exploration should focus on locally developed favorable reservoirs with good preservation conditions.