Geochemical Characteristics of Crude Oil From a Lacustrine Tight-Oil Reservoir in the Lucaogou Formation of the Jimusaer Sag, Junggar Basin

Abstract

The Jimusaer Sag, which lies in the Junggar Basin, is one of the most typical tight oil study areas in China. Commercial oil has been obtained from many wells in the Permian Lucaogou Formation. However, properties and geochemical characteristics of Lucaogou Formation oil have not yet been studied. Lucaogou Formation were formed in a saline lake depositional environment. The lithology is composed of fine grain source rocks (mainly mudstone and dolomite mudstone) interbedded with two sweet spots(mainly alternating thin layers of siltstone, carbonatite and bits of mudstone)having relatively high porosity and permeability. Commercial oils are obtained from both the upper and lower sweet spots. In contrast with representative examples of marine tight oil plays, such as the Bakken Formation, the Lucaogou Formation oil generally has high density and viscosity(average density=0.9 g/cm³, average viscosity=100.18mPa.s in 50°C). Interestingly, the eight samples from the lower sweet spot oil (at greater depth than the upper sweet spot) have higher densities and viscosities than the 15 samples from the upper sweet spot oil. These higher values are not caused by maturity because of the depth of oil. The group component analysis indicates that the lower section oil has higher non-hydrocarbon and asphaltene contents. The mudstone extracted from a similar region exhibits the same features, indicating that the differences in lower and upper section oil is mainly caused by their sources. Oil source correlation based biomakers results show that crude oil from the upper and lower sections originates from thin-bedded mudstones that occur interbedded within sweet spot sand bodies and adjacent to sweet spots(vertical distance<10m). Although the biomarkers of upper and lower section oil(and their adjacent mudstones)bear some similarities(e.g. they both exhibit sub-oxic to sub-anoxic environments, terrestrial organic matter and brackish water environments), obvious differences are also recognized. The lower section oil and mudstone extracts have higher β-carotane and hopane contents and higher Pr/C17 and Ph/C18 values (>1.5), which indicate that the organic matter has more bacterial and algal source contributions. This study reveals that the existence of two oil types (and sources) in one tight oil reservoir is possible. In addition, conventional geochemical analysis methods are applied to investigate this unconventional oil reservoir.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015