The characteristics and diagenetics of ultra-deep, fractured and tight sandstone reservoirs in Kuqa foreland, Tarim basin，Northwest China

The cretaceous ultra-deep reservoirs in Kuqa depression of Tarim basin are over 6000 m deep, which have the coefficient of formation fluid pressure greater than 1.6 and the formation temperature higher than 150 centigrade. In... [ view full abstract ]

The cretaceous ultra-deep reservoirs in Kuqa depression of Tarim basin are over 6000 m deep, which have the coefficient of formation fluid pressure greater than 1.6 and the formation temperature higher than 150 centigrade. In these ultra-deep reservoirs, netted-vertical fractures are developed at the density of 3~12 items/m, with average matrix porosity and permeability of 3.8% and 0.128×103μm2, respectively. This set of ultra-deep reservoirs is presently the core area of in Kuqa depression for increasing reserve and production of natural gas. To elucidate the formation mechanism and exploration significance of ultra-deep reservoirs in Kuqa depression, experimental analysis was carried out through a combination of acoustic emission, confocal microscopy, field emission scanning electron microscopy, electron microprobe, and laser carbon and oxygen isotope techniques. Studies indicated that the ultra-deep reservoirs experienced two types of typical superposition effect of diagenesis: 1) strong structure extrusion - strong dissolution - moderate buried compaction - moderate cementation in the Keshen zone associated with the development of fracture-dissolved pore reservoirs; and 2) strong structure extrusion - strong dissolution - moderate dissolution - moderate buried compaction - moderate cementation in the Dabei zone associated with the development of fracture - intergranular pore reservoirs. the effective reservoir spaces are consisting of structural fracture, micron pore and nanoscale pore throat. The dominating radius of matrix pore is 2～100μm, that of matrix throat is 10～500nm and the main area of fracture opening degree is 100～300μm. Effective reservoirs were formed mainly through the following mechanisms: In the early-mid stage, long-term shallow burial preserved pores; in the mid-late stage, internal suspended top structure of gypsum-salt rocks restrained vertical compaction; in the late stage, lateral tectonic extrusion formed the seam-net system; and multi-staged dissolution continuously increased pores. The effective reservoirs were controlled by lithofacies, tectonic extrusion and supergene dissolution. The favourable reservoir under 7000m distributes into a contiguous band with thickness generally from 80m to 200m while reservoir under 8000m still can be effective.Reservoir densification and hydrocarbon filling concurrently occurred and lateral superposition continuously developed, providing favorable conditions for the formation of continuous petroliferous zones, local hydrocarbon accumulation with high abundance, and natural gas reserves on the scale of thousand-billion square meters.