Thermal cracking history by laboratory kinetic simulation of Paleozoic oil in eastern Tarim Basin, NW China, implications for the occurrence of residual oil reservoirs

The thermal stability of Paleozoic oil in eastern Tarim Basin, NW China was investigated through laboratory kinetic simulation experiments. Laboratory cracking of a selected marine oil sample from Ordovician strata in well LG-1 of Tarim Basin was performed by confined, dry pyrolysis system at T = 300-650 degrees C, P = 50 MPa. Results indicated the oil required higher temperature for cracking. At laboratory heating rates, oil cracking started at 390-400 degrees C and the laboratory cracking was completed at around 650 degrees C. At geological heating rates, the onset temperature is about 148-162 degrees C for cracking start and was completed at 245-276 degrees C. The oil-cracking history was recovered using the acquired kinetic parameters and the aeothermal history of TD-2, and the threshold temperature for oil cracking under geological conditions was calculated. The oil cracking started at 165 degrees C (R-o = 1.45%) and stopped in early Devonian (390 Ma), and the oil-cracking rates in the strata of C-O-1 reached 60-70% at the end of Silurian. The calculated oil generation and oil cracking windows overlapped to some extent and were completed rapidly. The possible geological controls for the occurrence of residual oil reservoirs in Eastern Tarim basin have been discussed, including the high stability of the Paleozoic oil in Tarim Basin, the fast heating rate and longer duration time for oil cracking, the slight biodegradation in later uplift, the good preservation of the paleoreservoirs and the moderate structural adjustment, which were critical for the exploration of residual oil and gases in this area. (c) 2006 Elsevier Ltd. All rights reserved.