Study On Chang-7 Tight Reservoir Characteristics Of Triassic Yanchang Formation In Ordos Basin

Ordos Basin is rich in tight oil resources. The tight sandstone reservoir which developed in half deep lake-deep lake area is the tipical type in Ordos Basin. But now all aspects of Chang-7 tight sandstone reservoir research are still relatively weak. On the basis of previous studies, this paper uses a variety of analytical techniques, such as conventional and casting thin section, scanning electron microscope, cathodoluminescence, X-ray diffraction, determination of porosity and permeability, conventional mercury injection and rate-controlled mercury penetration, to comprehensively study lithological characteristics, pore types, pore and throat characteristics, physical characteristics, diagenesis and diagenesis facies of Chang-7 tight sandstone reservoir. Finally this paper discusses the main control factors of reservoir performance, and evaluated the reservoir.The study area is located in the Midwest of ordos basin, and it mainly influenced by the provenances from north-northeast and west-southwest. In Chang-7 period, lake area in the basin is largest. Delta, lake and sublacustrine fan facies which includes slope-shifting turbidite fan and slip turbidite fan develop in the study area. The main subfacies are delta-front, upper fan and middle fan. The sand bodies formed by gravity flow are research emphases of tight sandstone reservoir.Both compositional maturity and textural maturity of Chang-7 sandstone reservoir are low. The main types of sandstone are lithic arkose, feldspathic litharenite and arkose, of which the roundness is subangular and the sorting is medium. The interstitial fillings of the sandstone are hydromica, carbonate minerals (ferrocalcite and ankerite), silica minerals, chlorite, etc. The grain size of sandstone is finer, and most of them are siltstone and fine sandstone.Intergranular pore and feldspar dissolved pore are main pore types of Chang-7 sandstone reservoir, and other types of pore are rare. The conventional mercury injection tests show that the samples with middle threshold-pressure and thin throat are more. The rate-controlled mercury penetration tests show that penetrating quality and throat are close related, and throat radius, effective throat volume, effective pore volume and effective throat-to-pore radius ratio are main factors affecting reservoir physical properties and permeability. Average porosity of Chang-7 sandstone reservoir is 9.56%, and average permeability is 0.19×10"3?m2. The permeability is low and the porosity is relative high. The permeability and the porosity are well positive correlated.In the long diagenetic history, the reservoir rock is densified by many kinds of diagenesis. Compaction is the most important factor of porosity decreasing.58.9% of original porosity is decrease by compaction, and 39.2% is decrease by cementation. Most sandstone belongs to stage A of middle diagenesis phase, and few belong to stage B of middle diagenesis phase. According to pore structure and petrology features, there are four kinds of diagenetic facies could be divided:chlorite membrane-residual intergranular pore facie, Feldspar dissolved pore facie, weak dissolution-micropore facie and cementing dense phase. Generally speaking, their reservoir physical properties are worse successively.Sand bodyes from underwater distributary channels of delta front, main channels of upper fan and braided channels of middle fan are main reservoir sandstones. Reservoir properties of massive sand body (Single sand layer thickness>5m) are better than layered sand body (Single sand layer thickness 2-5m). On the basis of the sedimentation, diagenesises later have good or bad effects on the physical properties of sandstone reservoir, and make things complicated. On the other hand, thick high-quality source rock is the guarantee of high-quality reservoir. Considering sedimentation, diagenesis, source rock characteristics amd reservoir physical property, this paper evaluates the reservoir, and identifies three kinds of reservoirs:high-quality reservoir, medium reservoir and poor reservoir. Reservoirs of Chang-71 and Chang-72 are obvious better than Chang-73.