Unraveling Oil Migration and Mixing History From Different Structural Belts in Mahu Depression of Junggar Basin, NW China: Constraints From Molecular Geochemistry and Fluid Inclusions

Abstract

Mahu foreland depression has always been the exploration focus of Junggar basin in recent years for its rich hydrocarbon resources. With the discovery of crude oils in Triassic and Permian formations, it is increasingly important to know the fluid migration history and mixing process, which is crucial to find more hydrocarbons. Previous research of our team mainly based on biomarkers and carbon isotopes of oils from Triassic Baikouquan formation (T₁b) and Permian source rocks, and the conclusion was the oil in fault belt mainly came from the Permian Fengcheng source rock (P₁f) and the oil in deep sag was from Permian Wuerhe source rock(P₂w), whereas the oil in slope area was mixing of P₁f and P₂w oils. This time we analyzed the pyrrolic nitrogen compounds(NC) in oil samples and fluid inclusions of typical reservoirs to study oil migration direction and mixing stages.

Preliminary results show that the oils from different source facies and maturity have disparate amounts of NC. By doing the scatter plots of NC migration-indicators with source facies and maturity, three ratios such as 1-MC/4-MC, 1,8-DMC/2,7-DMC and [a]/([a]+[c])-BC were selected to study the migration directions. The oils in fault belts mainly vertically migrated through large faults and the oils in deep sag only migrated to neighboring formations near source rock, whereas the oils in slope area show two of them. Secondly, the petrographic observation were conducted on fluid inclusions from typical T₁b and Permian reservoirs, and the diagenetic periods were classified and homogenization temperatures (HT) of aqueous inclusions were measured to identify the accumulation stages of fluids. In view of common mixing phenomena and multi-stages accumulation in this area, FTIR was conducted on hydrocarbon inclusions to attempt to set up relationships of absorption spectrophotometric parameters and different stages of inclusions, thus to study the fluid history microscopically. The relevant work is in progress.

In summary, there are at least three models of oil migration in Mahu foreland depression and the fault belts, slope area and deep sag show distinct migration directions and muiti-stages accumulation. Understanding the history of migration and accumulation will shed light on the future exploration targets.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018