Alkaline diagenesis and its effects on reservoir porosity: A case study of Upper Triassic Chang 7 Member tight sandstone in Ordos Basin, NW China

Effects of mineralogy on petrophysical properties and permeability estimation of the Upper Triassic Yanchang tight oil sandstones in Ordos Basin, Northern China

Diagenetic heterogeneity, pore throats characteristic and their effects on reservoir quality of the Upper Triassic tight sandstones of Yanchang Formation in Ordos Basin, China

Upper Triassic sandstones in the Ordos Basin, northern-central China, comprise tight oil reservoirs. Using a combination of thin sections, SEM, BSE, EDS, XRD, and fluid inclusion analyses, 24 core samples from 13 wells were collected to study the petrology, paragenesis, and diagenetic processes and implications for reservoir quality. Quartz cement usually occurs as overgrowths or euhedral quartz. Extensive dissolution and albitization of K-feldspar can be observed. Five types of carbonate cements, ferrocalcite, ankerite, dolomite, calcite, and siderite, occur during different diagenetic stages. Two main types of illite and 5 main habits of chlorite are observed in this study. Kaolinite mainly occurs as booklets and vermicular aggregates. Diagenetic illite, chlorite, biotite, mixed-layer illite/smectite (I/S), and other minor minerals are also observed. The diagenetic processes include compaction, alteration of volcanic materials and mica, clay mineral transformation, cementation (silica, aluminosilicate, and carbonate), and dissolution of feldspars and rock fragments. Compaction was a significant porosity-reducing agent, and the presence of carbonate cement exerts a dominant impact on the reduction of porosity. Quartz cement and authigenic clays are less important; however, it is worth mentioning that pore-lining clays are conducive to porosity preservation. In this study, most of the porosity variation is caused by a combination of compaction, carbonate cements, quartz cement, and authigenic clays. This study gives insights into diagenetic alterations within tight sandstones and has implications for reservoir quality prediction in similar settings.

Diagenesis of tight oil sand reservoirs: Upper Triassic tight sandstones of Yanchang Formation in Ordos Basin, China

Low-field nuclear magnetic resonance (NMR) coupled with gas–water centrifugation (GWC) was performed to investigate petrophysical characteristics and water movability of the Chang 7 tight sandstone. Microscopic observation, mineral composition analysis and grain size analysis were applied to determine lithology and pore characteristics. Then, the controls of petrophysical properties and geological factors on the water movability were discussed. The results indicate that the T2 distribution of the Chang 7 tight sandstone can be divided into three groups according to the shape, range and amplitude of T2 spectrum. The removable water volume increases with the centrifugal force, but its gradient presents a remarkable difference among the different reservoir types. The final removable water volume (Φmov) at centrifuge force of 400 psi (average value of 5.28%) is approximately half of the total pore volume (porosity). The T2 geometric mean (T2gm), fractal dimension (Df) and NMR permeability calculated from NMR data can be used to characterize the pore structure and petrophysical properties of tight sandstone. The reservoir quality index can also be estimated based on the NMR permeability models. Removable water volume under the different centrifugation conditions is positively related to porosity, permeability and T2gm and negatively related to Df. Furthermore, removable water volume under different centrifugation conditions is positively related to detrital mineral (quartz + feldspar) content and grain size but negatively related to clay content.

Zhu Haihu

Papers

Microscopic characteristics and formation mechanism of Upper Triassic Chang 7 tight oil reservoir in the southwest Ordos basin