Geochemical Characterization of Organic Matter and Depositional Environment of Tuffaceous Source Rocks in the Jimsar Sag of Junggar Basin, NW China

Abstract

The Jimsar Sag has been the focus of tight oil exploration and development in Junggar Basin, Northwestern China. Lucaogou Formation, with a thickness of 200-300 m, is made up of thin lamellate tuffite and tuffaceous dolomite interbedded with mudstone and siltstone and is the primary exploratory stratum of tight oil reservoirs in the Jimsar Sag. A combined investigation of bulk geochemical study and molecular geochemical observation were carried out on the tuffaceous sediments of the Lower Permian Lucaogou Formation in the Jimsar Sag, to define tuffaceous source rocks geochemical characteristics, depositional environment and hydrocarbon potential. The biomarkers of mudstone extracts are characterized by short to long chain n-alkanes, high CPI values (0.65-1.65), a wide range of Pr/Ph (0.80-1.42), high concentrations of C29 sterane and the presence of relatively low tricyclic terpanes and middle gammacerane index values (0.07-0.36), which are consistent with a relatively low oxidizing to low reducing depositional environment of fresh to brackish water conditions, with a major contribution of terrigenous higher plant and a low contribution of phytoplankton, aquatic algal-bacterial organisms. Cross-plots of biomarker parameters and TOC show that the water salinity and primary productivity are controlling factors on organic matter accumulation in the tuffaceous source rocks, rather than the redox condition of deposition environment. With high TOC, high hydrocarbon potential index (S₁+S₂) and low-maturity to maturity, the tuffaceous sediments in the Lucaogou Formation are defined as good source rock with high hydrocarbon generation potential.

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