Formation mechanisms of secondary hydrocarbon pools in the Triassic reservoirs in the northern Tarim Basin

Abstract

The northern Tarim Basin (Tabei) is a rich composite hydrocarbon accumulation region with multi-layers of oil and gas reservoirs in the Ordovician, Carboniferous and Triassic strata. In the Triassic, the Tabei region was dominated by non-marine sedimentation that developed deltaic sandstone reservoirs with favourable petrophysical properties for hydrocarbon accumulations. Hydrocarbons in the Triassic hydrocarbon pools are dominated by oil and condensate gas. Analysis of the hydrocarbons, in combination with well testing and drilling data, reveals that oils in the Triassic hydrocarbon pools are commonly biodegraded and originated mainly from the middle-upper Ordovician source rocks. The predominant dry gases in the reservoirs are cracked from oils derived from the Cambrian source rocks. The origin of hydrocarbons in the Triassic pools is similar to those in the underlying reservoirs. Fluid inclusions and K–Ar dating confirm that the hydrocarbons were accumulated during the Himalayan orogenic movement. Because the middle–upper Ordovician source rocks already experienced peak hydrocarbon generation in the late Hercynian orogeny (late Permian), the Triassic hydrocarbon pools are from secondary adjustment of earlier oil and gas pools from the underlying Ordovician and Carboniferous reservoirs. Due to the rapid subsidence of the Kuqa foreland basin to the north of the Tabei region since the beginning of the Himalayan orogenic movement, the Triassic reservoir strata was changed from south-dipping to north-dipping, resulting in the formation of plunging anticlines and hydrocarbon migration toward the south. At the same time, fault reactivation occurred in the area, causing the hydrocarbons from the deeper reservoirs to migrate into the Triassic structural traps through faults and unconformities, and thus formed the widespread secondary hydrocarbon pools. The structure inversion during the Triassic and the formation of nose structures since the Himalayan orogeny (250 ∼ 290 Ma, Permian) controlled the secondary accumulation in the Triassic reservoirs. The reformation process of the hydrocarbon pools continued until the late Himalayan orogeny when the gas was charged and developed condensate pools in the eastern area.