Laminae combination and shale oil enrichment patterns of Chang 73 sub-member organic-rich shales in the Triassic Yanchang Formation, Ordos Basin, NW China
Kelai Xi, Ke Li, Yingchang Cao, Miruo Lin, Xiaobing Niu, Rukai Zhu, Xinzhuo Wei, Yuan You, Xiaowei Liang, Shengbin Feng
Abstract
The Chang 73 sub-member of Triassic Yanchang Formation in the Ordos Basin was taken as an example and the lamina types and combinations, reservoir space features and shale oil enrichment patterns in organic-rich shale strata were investigated using core observation, thin section analysis, XRF element measurement, XRD analysis, SEM, high solution laser Raman spectroscopy analysis, and micro-FTIR spectroscopy analysis, etc. According to the mineral composition and thickness of the laminae, the Chang 73 organic-rich shales have four major types of laminae, tuff-rich lamina, organic-rich lamina, silt-grade feldspar-quartz lamina and clay lamina. They have two kinds of shale oil-bearing layers, “organic-rich lamina + silt-grade feldspar-quartz lamina” and “organic-rich lamina + tuff-rich lamina” layers. In the “organic-rich + silt-grade feldspar-quartz” laminae combination shale strata, oil was characterized by relative high maturation, and always filled in K-feldspar dissolution pores in the silt-grade feldspar-quartz laminae, forming oil generation, migration and accumulation process between laminae inside the organic shales. In the “organic-rich + tuff-rich lamina” binary laminae combination shale strata, however, the reservoir properties were poor in organic-rich shales, the oil maturation was relatively lower, and mainly accumulated in the intergranular pores of interbedded thin-layered sandstones. The oil generation, migration and accumulation mainly occurred between organic-rich shales and interbedded thin-layered sandstones.