Zhang Ronghu
Hangzhou Institute of Geology, China
Title: The formation mechanism and exploration significance of ultra-deep, lowporosity and tight sandstone reservoirs in Kuqa depression, Tarim basin
Biography
Biography: Zhang Ronghu
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 130°C. 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 × 103 μm2, 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. Results 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. 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, with the depth over 8000 m and a thickness generally between 80 and 200 m. Reservoir densification and hydrocarbon filling concurrently occurred and lateral superposition continuously developed, providing favorable conditions for the formation of continuous petroliferous zones, local hydrocarbon