Diagenetic characteristics and reservoir quality in tight gas sandstones: A case study of the Shanxi Formation in the north‐eastern Ordos Basin, China

Abstract

Reservoir quality is critical in tight gas exploration. A systematic study of the petrological, petrophysical, and diagenetic characteristics of the Shanxi Formation sandstones in the north‐eastern Ordos Basin (China) was undertaken to characterize the pore system of these tight sandstones and recognize the distribution of reservoirs favourable for hydrocarbon accumulation. The results suggest that the Shanxi Formation sandstone has low compositional and moderate textural maturity, with its porosity ranging from 0.7% to 11.8% (average 5.97%) and permeability ranging from 0.001 to 2.77 mD (average 0.36 mD). In addition, thin section and SEM observations suggest that the sandstone is dominated by secondary dissolution pores, primary intergranular pores as well as microfractures, and has undergone compaction, cementation, and dissolution. Diagenetic minerals, such as carbonate cements, authigenic quartz, clay minerals, and dissolved feldspar, are identified. Fluid inclusions are observed in healed microfractures of quartz grains and in quartz overgrowths, while the homogenization temperatures of fluid inclusions in healed microfractures and in quartz overgrowth are in the ranges of 92.8–179.1°C (average 134.8°C) and 104.7–169.1°C (average 145.7°C), respectively. Authigenic kaolinite is sourced from the process of K‐feldspar dissolution, authigenic illite is sourced from transformation of smectite and kaolinite, while sources for quartz cements include mineral alteration and pressure dissolution. Selective dissolution of K‐feldspar in the presence of carbonate minerals is observed in the Shanxi Formation tight sandstones due to the different equilibrium constants of carbonate minerals and K‐feldspar leaching, resulting in dissolution pores associated with K‐feldspar. Reservoir quality in the Shanxi Formation tight sandstones is greatly influenced by diagenesis, as compaction and cementation are responsible for the loss of porosity while dissolution accounts for the enhancement of secondary porosity.

Molecular Insights into Soaking in Hybrid N2–CO2 Huff-n-Puff: A Case Study of a Single Quartz Nanopore-Hydrocarbon System

“…According to data from the National Institute Standards and Technology (NIST), the corresponding gas densities of the initial structure of three soaking systems under reservoir conditions are 0.30 g/cm 3 (N 2 soaking system), 0.82 g/cm 3 (CO 2 soaking system), and 0.415 g/cm 3 (hybrid N 2 –CO 2 soaking system). Tight and ultratight shale oil reservoir matrices are characterized by heterogeneous rock compositions comprising both organic and inorganic matter. , Among the various inorganic minerals, quartz is reported to be one of the most common inorganic minerals in which numerous nanopores are well-developed. , In addition, quartz is highly brittle, which means that shale is prone to inherent and induced cracks under the action of external forces and is ultimately conducive to gas intrusion into the matrix and shale oil recovery. , In this work, the general soaking law of gas HnP in the shale reservoir is reflected based on the characteristics and conditions of shale oil reservoirs of the Lucaogou Formation in the Jimsar Sag, Xinjiang oilfield, China . Furthermore, previous mineral component analysis of shale core samples from the Jimsar reservoir showed that the mineral composition of the Lucaogou Formation shale matrix is complex and diverse, but quartz has the highest relative content and plays an important role. , Thus, a single mineral, quartz, is used in this work to represent the shale reservoir matrix.…”

“…45,46 Among the various inorganic minerals, quartz is reported to be one of the most common inorganic minerals in which numerous nanopores are well-developed. 47,48 In addition, quartz is highly brittle, which means that shale is prone to inherent and induced cracks under the action of external forces and is ultimately conducive to gas intrusion into the matrix and shale oil recovery. 49,50 In this work, the general soaking law of gas HnP in the shale reservoir is reflected based on the characteristics and conditions of shale oil reservoirs of the Lucaogou Formation in the Jimsar Sag, Xinjiang oilfield, China.…”

Diagenesis Evolution and Pore Types in Tight Sandstone of Shanxi Formation Reservoir in Hangjinqi Area, Ordos Basin, Northern China

“…The burial depth was 1500-2200 m, and the paleotemperature ranged from 70 • C to 90 • C (Figure 8). The Shanxi Formation is a coal measure stratum, H + is produced under oxidation conditions [58], promoting the occurrence of dissolution [52,56,59]. By observing the homogenization temperature of inclusions in quartz cement, a large number of inclusions were formed in this period and oil and gas began to appear (Figure 9).…”

“…Figure 9. Diagenetic evolution sequence and pore evolution of sandstone of the Shanxi Formation in the HangJinqi area (Curve data of burial history from Xinhe Shao, 2018[58]. Gas filling data from Zhou Jinsong, 2020[59]).…”