Phase fractionation controlling regional distribution of diamondoids: A case study from the Halahatang oil field, Tarim Basin, China

Abstract

Large volumes of black and volatile oils have been recently discovered from 7000 m to 8400 m below surface in the Tarim Basin. Diamondoids, compounds recalcitrant to thermal stress, have been applied to estimate the extent of oil-cracking, but uncertainties remain concerning additional influences, such as evaporative fractionation and biodegradation. To investigate controls on diamondoid abundance in oil reservoirs, we performed detailed geochemical characterizations of biomarkers and diamondoids of twenty-eight oils from the Halahatang sub-Depression in the Tabei area of the Tarim Basin. Based on the results, the oils belong to one population. Three oils in the northeastern Halahatang area show the coexistence of n-alkanes and 25-norhopanes, indicating that early-expelled oils were severely biodegraded and then mixed with later, fresh charges of oil. Multiple molecular maturity indices suggest that oil maturities increase with depth. Interestingly, 3- + 4-methyldiamantanes, which should be enriched in highly mature oils, are found to be more abundant in less mature oils from shallower reservoirs, with exceptions of only four samples in the northeast. Published data for the eastern Lunnan area display a similar pattern that diamondoids are progressively enriched in shallower oils. The special pattern is interpreted to be the result of phase fractionation induced by a gas-dominated late charge. The late charge of gas firstly extracted diamondoids from deep-buried, highly-cracked oils through evaporative fractionation. As the vapor migrated towards the up-dip direction, decrease of pressure and temperature led to condensation of the diamondoids in the vapor. Briefly, the diamondoids were transferred to relatively shallow reservoirs through a combination of evaporative fractionation and subsequent phase separation. Alternatively, the abnormal enrichment of diamondoids in the northern part may be because of locally vertical charges of highly mature, diamondoid-rich gases through large faults.