Origin and reservoir properties of deep-water gravity flow sediments in the Upper Triassic Ch6–Ch7 members of the Yanchang Formation in the Jinghe Oilfield, the Southern Ordos Basin, China

Abstract

In recent years, the Ch6–Ch7 deep-water gravity flow deposits of the Yanchang Formation in the Southern Ordos Basin have been characterised as sandy debris flow sediments, a modification from previous sedimentary facies characterisations of braided river delta fronts, deep lacustrine turbidite fans and seismite deposits. This leads to the necessity of a detailed interpretation of the origin and reservoir properties of the deep-water deposits of the Jinghe Oil Field in the Ordos Basin. A large number of core images were analysed, identifying 15 lithofacies and 3 main sedimentary facies, including the sandy debris flow microfacies, turbidite microfacies and seismite-slump microfacies. Sedimentary facies determination was proved by particle size analysis and vertical and horizontal microfacies distributions. The sedimentary process can be described by earthquake and gravity deformation inducing a slide of the large deposit of delta front sediments on the slope break down the slope. Simultaneously, ambient lake water penetrated the sediments, forming seismite-slump microfacies with load structures, liquefied structures and slump deformation structures. With continuous sediment liquefaction, sandy debris flow microfacies, which were massive bedding sandstones, were formed during transportation. Leading the sediments, turbidite flows resulted from flow transformation which were possibly remolded by weak bottom currents in intermittent periods. Studies of reservoir properties and oil shows indicate that sandy debris flow sandstones have the best reservoir properties and oil shows, followed by turbidite sandstones, with seismite-slump sandstones being the poorest. The sandy debris flow and part of the turbidite sandstones have good oil production potential.