The Origin of Ultra-deep Fractured Tight Sandstone Reservoir and the Model of diagenetic Evolution in Kuqa Depression, NW China

Abstract

Abstract

The early Cretaceous bashijike sequence of Kuqa Depression is typical ultra-deep fractured tight sandstones, and it is disconformably overlain by over 8000 meters of clastic red beds in Cretaceous and Tertiary sequence. This paper studies this reservoir's formation mechanism and reveals the diagenesis-physical property evolutionary law. And this study is based on special experimental analyses(acoustic emission method paleostress test, laser scanning confocal microscopy, scanning electron microscopy, micro-cements carbon and oxygen isotopes, the core Micro-CT, constant pressure injection Hg and electronic probe) in order to evaluate and forecast this kind of reservior.

The results show that the effective reservoir spaces are consisting of nanometer pore throat, structural fracture and micron pore. The average matrix helium porosity and permeability of 246 samples are 3.68% and 0.08mD respectively. The dominating diameter of matrix pore is 1μm ∼200μm, the matrix throat diameter is 7nm∼1000nm, and the main area of fracture opening degree is 30-150μm. The unusual growth of fracture provides considerable possibility for the tight sandstones' high production.

The reservoir is now in the middle diagenetic phase A. Main diagenesis types include compaction, cementaration, dissolution, cataclasis and hydrocarbon emplacement. Sediment facies is the base of the reservoir development. And the intense compaction is the radical contribution to the pore beravement in early phase. Dissolution, cataclasis, and hydrocarbon emplacement have constructive work on physical property. Through undergone peak tectonic compression, the ultra-deep reservoir has the characteristics of rapid densifying in later period, and it has four diagenetic effects: sandstone composition and diagenesis intensity will control its porosity, lower quartz, weaker diagenesis and lower porosity; The fractures depend on maximal stress and tectonic style; with formation fluid pressure increasing, high pressure can protect reservoir pore; Activity between formation water and rock can strengthen late fracture cemented filling. Based on the above analysis, this paper summarizes the ultra-deep fractured tight sandstone reservoir geological model: strong tectonic compression-middle burial compaction-middle cementation-strong dissolution reservoir.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016