Accumulation and Reformation of Silurian Reservoir in the Northern Tarim Basin

Abstract

Abstract:Silurian sandstone in Tarim Basin has good reservoir properties and active oil and gas shows, especially thick widely‐distributed bituminous sandstone. Currently, the Silurian was found containing both bitumen and conventional reservoirs, with petroleum originating from terrestrial and marine source rocks. The diversity of their distribution was the result of “three sources, three stages” accumulation and adjustment processes. “Three sources” refers to two sets of marine rocks in Cambrian and Middle‐Upper Ordovician, and a set of terrestrial rock formed in Triassic in the Kuqa depression. “Three stages” represents three stages of accumulation, adjustment and reformation occurring in Late Caledonian, Late Hercynian and Late Himalayan, respectively. The study suggests that the Silurian bitumen is remnants of oil generated from Cambrian and Ordovician source rocks and accumulated in the sandstone reservoir during Late Caledonian‐Early Hercynian and Late Hercynian stages, and then damaged by the subsequent two stages of tectonic uplift movements in Early Hercynian and Pre‐Triassic. The authors presumed that the primary paleo‐reservoirs formed during these two stages might be preserved in the Silurian in the southern deep part of the Tabei area. Except for the Yingmaili area where the Triassic terrestrial oil was from the Kuqa Depression during Late Himalayan Stage, all movable oil reservoirs originated from marine sources. They were secondary accumulations from underlying Ordovician after structure reverse during the Yanshan‐Himalayan stage. Oil/gas shows mixed‐source characteristics, and was mainly from Middle‐Upper Ordovician. The complexity and diversity of the Silurian marine primary properties were just defined by these three stages of oil‐gas charging and tectonic movements in the Tabei area.