Sedimentology and reservoir characteristics of a Middle Jurassic fluvial system, Datong Basin, northern China

Abstract

ABSTRACT An outcrop of the Middle Jurassic Yungang Formation in Datong Basin of northern China was studied in order to provide a better understanding of the architecture and heterogeneity of hydrocarbon reservoirs associated with braided-fluvial deposits. The main section of outcrop, which is continuously exposed over 350 m, is interpreted to have been deposited in the middle-stream reaches of a sandy braided river in a proximal area at the eastern edge of the basin. Seventeen lithofacies were identified, based on petrology and sedimentary structures. These lithofacies were grouped into seven architectural elements, including channel fill, transverse bar, longitudinal bar, diagonal bar, abandoned channel fill, levee, and overbank fines. These sediments were deposited in three stages (active, abandonment, and reactivation), which are related to sediment channel switching and waxing and waning of the fluvial system. The sandstones are mainly arkoses or lithic arkoses, with high matrix content, low compositional and textural maturity, and a variety of cements. A total of 282 specimens were collected from outcrops and analyzed for porosity and permeability. The results show that sandstones from longitudinal bar and channel fills have the highest porosity and permeability while abandoned channels and levee sediments have the lowest. In cross-section, porosity and permeability are generally highest in the middle of the sand bodies, reflecting the spatial distribution of architectural elements and petrophysical properties. End_Page 105------------------------