Genetic origin of sour gas condensates in the Paleozoic dolomite reservoirs of the Tazhong Uplift, Tarim Basin

Abstract

Twenty gas condensates, six oils and their associated formation water from the Tazhong Uplift in the Tarim Basin, NW China were characterized for their molecular and isotopic compositions and water chemistry to elucidate the origins of the sour condensates. Thermochemical sulfate reduction (TSR) is the main secondary alteration process relevant to the generation of sour gas condensates. Those condensates are slightly heavier than regular condensates with a wide range of physical property and chemical composition variations. The hydrogen sulfide (H2S) concentrations in gas phase show a strong positive correlation to gas/oil ratio, organosulfur compound (OSC) concentrations in the liquid phase. While methane isotopic values are mainly controlled by thermal maturity, wet gas components show unusual 13C enrichment at a low degree of thermal cracking. Geochemical anomalies occur mainly in the Lower Ordovician Yingshan Fm. where more H2S has been discovered, while the Upper Ordovician Lianglitage Fm. shows less irregularity and H2S concentration is much lower. Generation of sour gas condensate is most likely caused by TSR alteration, which could oxidize liquid hydrocarbons and form a large quantity of OSCs. Isotopic enrichment of 34S in formation water sulfate and elevated OSC concentrations provide supplemental evidence of TSR alteration. Both burial history reconstructions based on fluid inclusion analysis and PVT simulation indicate that in situ TSR reactions mainly occurred around last 10 Ma during the late Himalayan orogeny and overall TSR extent is relatively low in the Tazhong Uplift.