Ordovician deep dolomite reservoirs in the intracratonic Ordos Basin, China: Depositional model and Diagenetic evolution

Abstract

Recent natural gas discoveries indicate that non-karstification-dominated reservoirs exist in the intracratonic Ordos Basin. This study examines the sedimentological and geochemical characteristics needed to clarify the depositional model and diagenetic evolution process of this newly discovered reservoir type. The depositional environment of the dolomite reservoir can be characterized as a tidal flat that grew from the Central Paleo-uplift to the eastern depression by cyclic progradation on an epeiric platform. A tidal flat sequence can extend laterally as a progradational wedge in each cycle of sea level fluctuation. The sheet-shaped peritidal shoal facies associations patched on the wedge represent potential dolomite reservoirs and can be recognized by the presence of doloarenite that has been altered into a vaguely relict grained-texture by diagenesis. Although continuing destructive diagenesis has led to reservoir densification, burial dolomitization and burial dissolution with facies selectivity have tended to occur in peritidal shoal facies associations, thus improving the quality of the dolomite reservoirs. These models provide new insights for targeting deep dolomite hydrocarbon reservoirs in intracratonic basins.

Dolomitization of the Ordovician subsalt Majiagou Formation in the central Ordos Basin, China: fluid origins and dolomites evolution

“…In addition, intense burial recrystallization can be observed in the Md2 dolomite, as shown by the relict textures of grains and overgrowths of dolomite crystals. Previous studies have suggested that the burial fluid, comprising concentrated seawater sealed in pores of contemporaneous rocks from the intracratonic depression, has had a strong influence on the petrological features of the porous Md2 dolomite (Chen et al 2018;Fu et al 2019). This highlights the influence of multiple diagenetic fluids on the formation of the Md2 dolomite.…”

“…Due to the abundant gas resources in the Ordos Basin, particularly the subsalt Majiagou Formation reservoir with proven natural gas reserves of > 200 × 10 9 m 3 (He et al 2009), numerous studies have been conducted on the Ordovician Majiagou Formation dolomites. A number of models have been proposed, including penecontemporaneous dolomitization (Han and Xin 1995;He et al 2014), burial dolomitization (Su et al 2011;Chen et al 2018;Fu et al 2019), mixed-water dolomitization (Zhao et al 2005), and localized hydrothermal dolomitization (Wang et al 2009;Huang et al 2011). These studies provide important basis for understanding the origins of dolomites in the Majiagou Formation.…”

Special Issue: Advances of petroleum exploration and geology research in West China's petroliferous basins

“…Based on sedimentology and geochemistry data, Chen et al (2018) constrain diagenetic processes and establish a depositional model for dolomite reservoirs of the Middle Ordovician Majiagou Formation in the Ordos Basin. The reservoir was deposited on a semi-restricted platform on a tidal flat, with the peritidal shoal facies distributed along the shoreward side of the central paleo-uplift.…”

Geochemistry characteristics and dolomitization mechanism of the Upper Cambrian dolomite, eastern Ordos Basin, China

“…In the newly discovered oil fields from 2000 to 2014, the main producing formations were deep carbonate, accounting for 58.6%, and these often contained larger recoverable reserves and a higher output (Davies & Smith, ; Rahimpour‐Bonab, Esrafili‐Dizaji, & Tavakoli, ; Z. Wang, Su, Zhu, Han, & Wang, ; J. Zhang, Hu, & Luo, ; Zou et al, ). In particular, these types of carbonate reservoirs are made almost completely of dolomite from the Lower Palaeozoic and Triassic formations (A. Q. Chen et al, ; W. Z. Zhao et al, ). In addition, about 80% of the carbonate reservoirs are dolomite reservoirs in North America (Zengler, Dunham, & Ethington, ).…”

“…In particular, these types of carbonate reservoirs are made almost completely of dolomite from the Lower Palaeozoic and Triassic formations (A. Q. Chen et al, 2018;W. Z. Zhao et al, 2014).…”