Search

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 meteoric diagenetic environment has multiphase characteristic, but the scales of the reservoirs in different meteoric diagenetic environments have been rarely considered. On the basis of geological and well-logging analyses, three distinct phases of the meteoric diagenetic process were observed in the Ordovician of the Jingxi area in Ordos Basin, China. The phreatic zone formed in the first phase is partially preserved, whereas those formed in the second and third phases are completely retained. Strata in the western (close to the Central Uplift Belt) and eastern parts underwent bedding-parallel meteoric dissolution and cross-layer meteoric dissolution, respectively. The distributions of the dissolution formation thicknesses in two successive phases of the meteoric diagenetic process exhibit complementarity at the same positions in the plane. The favorable reservoirs at the top of the Ordovician are mainly distributed in the western parts. The second phase of the meteoric diagenetic process is most conducive to the formation of carbonate reservoirs. By dividing and comparing the multiphases of the meteoric diagenetic environment of carbonates, the favorable karst reservoirs can be predicted in each phase. [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]