Resource potential and core area prediction of lacustrine tight oil: The Triassic Yanchang Formation in Ordos Basin, China

Abstract

Unlike liquid-rich hydrocarbons in marine shale systems in North America, the Triassic Yanchang oil is preserved in tight sandstone reservoirs deposited in a lacustrine setting. The tight reservoir covers an area of 30,000 km2 (11,600 mi2) and is characterized by ultralow permeability (∼0.001–1.00 md [∼0.001 × 10−3–1.00 × 10−3 μm2]), a low subsurface pressure coefficient (0.6–0.8), and low single-well yield (several tons [10–40 bbl] to >10 tons [>70 bbl] per day). The reservoir also features a high content of quartz, feldspar, and carbonate (averaging 70%–80%) and low crude oil density of 0.84 g/cm3 (36.5° API) with an oil saturation of approximately 60%–80%. For the first time, this study employs the assessment of petroleum in-place resources based on the genesis and statistics predicting method (GSPM) and estimated ultimate recovery method for the tight oil in the Ordos Basin. The advantage of GSPM over previously reported methodologies is the suitable combination of genetic and statistical methods to estimate proximally sourced tight oil. The results show that the total in-place resource potential of the investigated tight oil is approximately 42.26 × 108 t (∼295.82 × 108 bbl), with an average abundance of 15 × 104 t/km2 (99 × 104 bbl/mi2). The recoverable resources for the 50% probability of the Chang 7 tight oil accumulation is 3.11 × 108 t (21.77 × 108 bbl) in the Ordos Basin. The top two grades of the Chang 7 tight oil core area cover the total area of three layers comprising 12,745 km2 (4921 mi2), with 22.51 × 108 t (157.57 × 108 bbl) of in situ resources. Furthermore, the four most promising zones have been identified.