Characteristics and Origin of Solid Bitumen in Glutenites: A Case Study from the Baikouquan Formation Reservoirs of the Mahu Sag in the Junggar Basin, China

Abstract

Solid bitumen was identified in the pore spaces of the Triassic Baikouquan Formation glutenite reservoir in the Mahu Sag of the Junggar Basin, northwestern China. Optical microscopy, scanning electron microscopy, and organic geochemical techniques were used to characterize solid bitumen and determine its source and origin. Rock core observations as well as optical microscopy, scanning electron microscopy, and energy spectrum analyses show that the reservoir pores are filled with isotropic solid bitumen. Biomarker geochemistry indicates that the solid bitumen is early mature to mature and formed in a reductive lacustrine environment with saline water. Oil–source correlation suggests that the solid bitumen was sourced from the low-early mature Fengcheng Formation shale. Bulk composition, alkane composition, and terpane analyses indicate that the solid bitumen did not experience oxidation or biodegradation. Geothermal evolution of the Baikouquan Formation confirms that thermal alteration is not effective for bitumen formation. On the basis of the comprehensive analysis, the solid bitumen was formed from early mature crude oil by high pressure and moderate temperatures.