Characteristics, chronology and controls on natural fractures in tight sandstone reservoirs of the Triassic Chang 6 Member, Yanchi area, Ordos Basin, China

The Triassic Chang 6 Member is a significant oil layer in the Ordos Basin, north-west China, and is considered to be a tight sandstone reservoir. Natural fractures have significantly exerted enormous influence on the reservoir quality of the sandstone. This study employs similar outcrops, cores, imaging logs, thin sections, scanning electron microscopy (SEM), fluid inclusions and stable isotope analyses to investigate the characteristics, chronology and controlling factors impacting fracture development in the tight reservoir. There are four recognized types of natural fractures in the Chang 6 Member: 1) tectonic shear, 2) extension, 3) bedding-parallel, and 4) micro. Tectonic shear fracture plays a predominant role in the reservoir modification, with two sets of crosscutting strikes (E-W and ENE-WSW), high-dip angle, middle aperture and fillings (i.e. calcite, ankerite and quartz). The others are not the main fractures. Among which, the extension are characterized by unstable occurrence, rough surface; while the bedding-parallel have an extremely narrow aperture, filling with mud; and the micros probably developed in the stress field with a mix of Mode I-Mode II. In detail, the E-W oriented shear fractures occur with fills, but without in the ENE-WSW oriented ones. According to C-O isotope and fluid inclusion microthermometry, the shear fracture filling of calcite is characterized by 18 O (δ 18 O PDB varying from -21.30‰ to -18.30‰, mean = -20.00‰, and corresponding to the O isotope calculated temperature from 62.1℃ to 85.3℃ with an average of 74.4℃), 13 C (δ 13 C PDB varying between -5.70‰ and -3.10‰, mean = -4.10‰) and lower homogenization temperatures (Th’s) ranging from 71.2℃ to 92.7℃, mean = 80.3℃. The fracture filling of ankerite is abundant in 16 O (δ 18 O PDB varying from -25.60‰ to -17.50‰, mean = -21.80‰, and corresponding with the O isotope calculated temperature from 82.1℃ to 121.5℃ with an average of 103.2℃), 12 C (δ 13 C PDB varying between -7.10‰ and -5.30‰, mean = -6.06‰) and higher Th’s ranging from 100.6℃ to 112.1℃, mean = 105.1℃. For the quartz fracture filling, Th’s mainly sits between 120.5℃ and 129.8℃, mean = 122.2℃. Combining with mineralization and burial-thermal history, the Chang 6 natural fractures are thought to develop during three periods: 1) the Yanshanian Ⅰ episode during the end of the Jurassic to the Early Cretaceous, 2) the Yanshanian Ⅱ episode in the middle of the Cretaceous, and 3) the Himalayan episode during the end of the Cretaceous to the Paleogene. Historically, it was documented the fractures developed during only two periods (i.e. the end of the Jurassic and the end of Cretaceous to the Paleogene) in the past decades, but changed with the advances in this chronology analysis. When it comes to fracture density, it’s constricted by sandstone thickness, which increases first and then decreases with the increasing net-gross-ratio (NGR). Mineralogically, fracture density correlates positively with rigid component contents (i.e. quartz, feldspar, dolomite and calcite cements), but negatively with plastic composition (i.e. rock fragment, clay, and mica). Porosity, to some extent, restricts fractures in the way of strain accommodation by coordination and adjustment between the pore system and its filling fluids. This study provides insights into when natural fractures occur and how sedimentary factors and modal compositions influence fracture density of tight sandstones.