Densification and hydrocarbon accumulation of Triassic Yanchang Formation Chang 8 Member, Ordos Basin, NW China: Evidence from geochemistry and fluid inclusions

Abstract

Crushing, acid treatment and step wise separation and oil extraction were employed to obtain the different occurrence state hydrocarbons. All these fractions have been analyzed by Gas Chromatography-Mass Spectrometer (GC-MS). The fractions relationship and related oil charging process can be mirrored through the analysis of fractions weight content and geochemical characteristics, in combination with the research of inclusions homogenization temperature and fluorescence spectrum parameters. Experimental results reveal that there are four state hydrocarbons, i.e. free hydrocarbon, sealed hydrocarbon, hydrocarbon in carbonate cement, and hydrocarbon within inclusions caught by quartz grains and feldspar grains in the oil-rich sandstones of Chang8 Member, Triassic Yanchang Formation in the Ordos Basin. Among them, the overwhelming fraction is the free hydrocarbon, averaged 93.4%. Fluorescence spectrum parameters of γmax, QF535 and Q650/500 show that the crude oil maturity of the inclusions imprisoned in feldspar, quartz, and carbonate cement increase in turn, and the parameter values of the inclusions in feldspar and quartz are similar and much different from those of carbonate cement. Through analysis of C29ββ/(αα+ββ) and C2920S/(20S+20R), together with methylphenanthrene ratio, it is revealed that thermal-evolutionary degree of the hydrocarbon within inclusions, hydrocarbon in carbonate cement, sealed hydrocarbon, free hydrocarbon reflects an upward trend, and the data of the last two type are similar. Integrated study of diagenetic sequence and thermal evolutionary degree suggest that the Chang 8 sandstones had been compacted before reservoir formation and the reservoirs have experienced three phase of charge events in which the third one played the most important role for reservoir formation. Key words: Ordos Basin, Triassic, Yanchang Formation, hydrocarbon occurrence, charge period, major reservoir forming phase, tight reservoir