Kinetics of oil-cracking of ultra-deep Ordovician oil in the North Shuntuoguole area of Tarim Basin and its geological implications

Abstract

With the expansion to the deep strata for the exploration of marine oil and gas in the Tarim Basin, the upper limit of temperature for the occurrence of ultra-deep reservoirs has become a scientific concerning of organic geochemistry and petroleum geology. Thermal simulations of the ultra-deep Ordovician oil from well SB 7 in the North Shuntuoguole area of Tarim Basin were carried out using a gold-tube confined system under two different pressures of 50 and 90 MPa and two different heating rates of 2 and 20℃/h, respectively. According to the results of simulation experiments, Kinetics software was used to calculate the chemical kinetics, and the mass yield of gas generation during oil-cracking under different temperature and pressure conditions was compared, and its geological significance was discussed. Under different temperature and pressure stages, the same oil sample experienced a similar process of oil-cracking as well as of total volume and mass yield of gas generation. Heavier gas compounts were generated in the early stage of oil-cracking and further transformed to form methane in the later stage. Heating rates impacted greatly on oil-cracking processes. Under a higher heating rate, the oil-cracking process moved to higher temperature and the separate oil phase can be kept with a higher temperature limit. High pressures only played minor roles on oil-cracking. With the same heating rate, higher pressure negligibly suppressed oil-cracking in the early stage of oil-cracking, whereas in the later stage, higher pressure promoted oil-cracking a bit. The difference of oil-cracking process under different temperatures and pressures can be explained by the distribution of activation energy. The distribution of activation energy of C1-C5 gas mass yield of oil-cracking of well SB 7 is relatively more concentrated, suggesting that the "temperature window" of oil-cracking is relatively narrower. According to the thermal simulation and kinetics calculation results, the maximum temperature of oil phase in the block 1 in the North Shuntuoguole area is greater than 180℃, and oil phase can be maintained at a depth greater than 9 000 m.

Keywords

• oil cracking
• kinetics simulation
• reservoir fluid phase
• ordovician
• ultra-deep
• north shuntuoguole area
• tarim basin