Abstract: The Yaburai Basin is a Jurassic lacustrine petroliferous basin in western China. In order to assess tight oil potential of the Xinhe Formation in the basin,we employed a geological factor superimposition method to select tight oil play fairway and a small cell volumetric method to calculate the resources based on analyses of source rocks and reservoiring conditions as well as their coupling relationship in the basin. The research shows that high-quality source rocks are mainly located in the lower Xinhe Formation in the Middle Jurassic. Tight oil reservoirs are mainly developed in this source rock interval featuring in large cumulative thickness,rich organic matter of favorable types,and being within oil window. Low-porosity and low-permeability tight reservoirs occur as thin interbeds and interlayers inside the source rocks. Crude found there shows clear signs of self-generation and self-preservation. The large area of contact between matured organic-rich source rocks and reservoirs and the effective sealing of massive mud rock cap are the most important controlling factors of tight oil reservoirs in the basin. The sub-sag Xiaohu was considered to be a play fairway based on analyses of maturity of source rocks. A comprehensive study of mud and wireline logging data as well as oil and gas shows were used to establish a standard (TOC>1%,0.6%<R_o<1.3%,reservoir porosity ≥ 4%,accumulative thickness>10m) to pinpoint the location of play fairway at the central and western part of the subsag. Calculation shows that the target holds an oil-in-place of 92 million tons and resource abundance of 345 thousand tons per square kilometers.

Key words: play fairway, tight oil, Xinhe Formation, Jurassic, Xiaohu Subsag, Yabrai Basin