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Giant Palaeo Oilfields in the Silurian Sandstone, Tarim Basin, China: Evidence from Bitumen, Quantitative Fluorescence and Fluid Inclusions

Keyu Liu1, Xiongqi Pang2, Zhenxue Jiang2, Jun Zhang3, and Stephen Fenton1
1 CSIRO Petroleum, Bentley, WA, Australia
2 China University of Petroleum, Beijing, China

Over 25,000 square kilometres of bitumen-bearing reservoir sandstones have been mapped out in the Silurian stratigraphic succession in the Tarim Basin, western China. An integrated investigation of the bitumen-bearing sandstones from 22 exploration and production wells using fluid inclusion analysis, Quantitative Grain Fluorescence (QGF) and QGF on extracts (QGF-E) and Total Scanning Fluorescence (TSF) techniques have delineated the spatial distribution of the one-time giant palaeo oilfields. The palaeo oilfields consist of two basin-wide major reservoir units of several metres to tens of metres on average separated by an intraformational capillary barrier, the “red-bed” shale, and in places the palaeo oil columns may attain up to 70 m high. The fluid inclusion results and the quantitative fluorescence fingerprints of the inclusion oils also indicate the presence of oils from multiple sources in the study area and possible from multiple charge events.

An initial estimate for the area investigated suggests that the one-time palaeo reserve amounts to over 13 billion tons of liquid hydrocarbons equivalent. The destruction of the giant palaeo oilfields is thought to be primarily due to tectonic movement after the emplacement of the hydrocarbons. Biodegradation of the oil in relatively shallow depths has been suggested to be the major mechanism for the development of the wide-spread bitumen.