A well-preserved 250 million-year-old oil accumulation in the Tarim Basin, western China: Implications for hydrocarbon exploration in old and deep basins

Abstract

A giant oilfield (YM-2) with an estimated reserve of close to one billion bbl was recently discovered in an Ordovician carbonate reservoir at a burial depth of 5800–6200 m in the northern Tarim Basin, western China. Biomarker and isotope geochemistry of the hydrocarbons indicate that the oil was derived from Ordovician marine source rocks at early to peak oil generation. Authigenic illite (K–Ar) dating, fluid inclusion analysis, fluid inclusion PVTx and thermal history modeling indicate that the accumulation is of primary in origin, and the original charge occurred in the Permian during the Late Hercynian Orogenic Stage, approximately 290–250 million years ago. The physiochemical compositions of the hydrocarbons and formation water remained largely unchanged since the initial accumulation. The excellent preservation of such an old accumulation at such a great depth is due to continuous burial of the YM-2 structure since the Triassic, a thick effective seal, and a relatively low geothermal gradient with a current reservoir temperature of 127–130 °C. This finding suggests that under suitable conditions old petroleum accumulations can be well-preserved, and some old and deep basins may be prospective frontiers for future exploration.