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More than 100 million tons of tight oil reserves have been discovered in the Xin’anbian area of the Ordos basin in recent years, of which the main reservoir is the Chang-7 Member of the Yanchang Formation. To identify the characteristics of the tight sandstone reservoir and their the development of the Chang-7 Member in the study area, casting thin sections, scanning electron microscopy (SEM), high-pressure mercury injection, cathodoluminescence and other technical methods were used to analyze the reservoir rock types and their physical properties and pore structure. Based on reservoir diagenesis research and quantitative evaluations of reservoir pores, a reservoir pore evolution mode was established. The results showed that the main rock types in the tight sandstone reservoirs in the Chang-7 Member of the Xin’anbian area were lithic feldspathic sandstone and feldspathic detritus sandstone and that the reservoir space types were mainly feldspar dissolved pores, with a few residual intergranular pores, clay intercrystalline pores, and microfractures. The reservoirs were classified as having low porosity and low permeability and extra-low porosity and extra-low permeability. The porosity and permeability of the reservoirs were between 4 ~ 10% and 0.01 ~ 1 mD, respectively, and were mainly controlled by compaction, cementation, and dissolution. Compaction and cementation reduced the porosity and permeability, while dissolution increased the porosity and permeability. The Chang-7 Member reservoir mainly experienced weak-medium compaction, medium-strong cementation, and different degrees of dissolution. The average loss of porosity due to cementation was around 25%, while the average loss due to compaction was around 13%. This research determined four key stages in the porosity evolution of the Chang-7 Member in the study area: (1) the porosity was reduced to 26% from the Triassic to the Early Jurassic period, which was mainly due to compression compaction; (2) the reservoir was seriously damaged by cementation from the Middle Jurassic to Late Jurassic period, where the porosity decreased to 5%; (3) in the Early Cretaceous period, the porosity increased to 11% due to dissolution; and (4) the porosity was reduced to 7% from the late Early Cretaceous to early Late Cretaceous period, which was mainly due to cementation. [ABSTRACT FROM AUTHOR]

The research on cementation periods of deep marine carbonate reservoirs has significance to understand multiphase fluid activities and pore evolution history. Taking the Changxing Formation of the Yuanba area in northeast Sichuan Basin as a case, the formation and distribution characteristics of the reservoir cements were investigated by microscopic observation and fluid inclusions. The results show that the reservoir cements in the Changxing Formation are mainly distributed in intragranular pores, intergranular pores, and fractures in allochemical dolomites and dolomitic biolithite limestones. The cementation of reservoirs can be divided into four periods. In the first period, the intragranular calcite cements and a few non-transformed intergranular calcite cements occurred, which give out dark light or no light under cathodoluminescence, and they had low contents of Fe, Mn, and Ba, and formed in the shallow diagenetic environment. In the second period, the dolomite cements in intergranular pores and fractures occurred, which mainly give out bright light under cathodoluminescence, and they had relatively high contents of Fe, Mn, and S, and relatively low contents of K + Na, Sr, and Ba, and high homogenization temperatures of fluid inclusion, indicating that the cements formed in the middle burial diagenetic environment in early Middle Jurassic. In the third period, the intergranular calcite cements and fracture calcite cements occurred, which mainly give out dark light or no light under cathodoluminescence, and they had high contents of K + Na, Ba, and S, and wide homogenization temperature of fluid inclusion, indicating that the cements formed in the middle-deep burial diagenetic environment in middle-late Middle Jurassic. In the fourth period, the pyrite, quartz, rutile, and strontium minerals occurred, with symbiotic relationship, and they give out no light under cathodoluminescence, and formed in the deep burial environment, perhaps, in the hydrothermal environment. [ABSTRACT FROM AUTHOR]

Pores and throats are the key parameters for determining the pore structure and fluid flow behavior in tight sandstone reservoirs. In this paper, pores and throats of the tight sandstone reservoir of the Upper Triassic Xujiahe Formation in the northern Sichuan Basin were investigated using classical petrography, scanning electron microscopy, and confocal laser scanning microscopy. In addition, a method of pore as well as throat identification and quantitatification using high-pressure mercury injection (HPMI) measurements was worked out. The results show that clay mineral-related pores are well developed in lithic sandstone and feldspathic lithic sandstone in addition to pores produced by dissolution of calcite cement and clastic grains. Intergranular pores are connected by lamellar, curved lamellar, and narrow pore throats. The pores and throats associated with intragranular dissolution and clay minerals are mostly tube shaped. The rapid-increasing and slow-increasing profiles of the mercury injection curves represent the dominance of pores and throats, respectively, allowing to differentiate pores and throats. Calculations show, in addition to fractured lithic sandstone, that has micron-sized pore–throat systems, the other lithic sandstones possess dominatly nano-sized pore–throat systems. In case of feldspathic lithic sandstones, micron-sized and nano-sized pore throat systems are equally important. In micron-sized pore throat dominated reservoirs, the pore volume is slightly larger than the throat volume, whereas for the nano-sized pore throat system, the pore volume is considerably larger than the throat volume. The key factors that are favorable for the formation of lithic sandstone pore throat systems are early calcite dissolution, lithic fragments dissolution, and early chlorite cementation. In contrast the key factors that are favorable for the formation of feldspathic lithic sandstone pore throat systems are dissolution of feldspars and lithic fragments. • A new method to distinguish pores and throats within tight sandstone reservoirs is proposed. • Lithic and feldspathic lithic sandstone reservoirs host different pore and throat types. • Pore volume is slightly larger than throat volume in the micron-size pore network. • Pore volume is considerably larger than throat volume in the nano-size pore network. • Compaction, cementation, and dissolution directly affected the development of pore throat systems. [ABSTRACT FROM AUTHOR]