Sequence stratigraphy and facies architecture of a mound‐shoal‐dominated dolomite reservoir in the late Ediacaran Dengying Formation, central Sichuan Basin, SW China

Abstract

The late Ediacaran Dengying Formation is one of the oldest carbonate microbial mound and shoal reservoirs in the world. This formation has recently attained widespread attention with the significant discovery of a huge volume of natural gas in the central part of the Sichuan Basin, South China. To fully understand the depositional processes and impacts on the reservoir, a comprehensive study of the sequence stratigraphy and facies architecture as well as reservoir analysis was undertaken by integrating analysis of seismic, well-logging, and core data. The D4 member consists of nine types of lithofacies representing distinguished depositional environments.These types of lithofacies can be grouped into five types of lithofacies associations, which are platform-margin shoal, mound-shoal complex, microbial mound, lagoon, and tidal flat. The platform margin was dominated by shoal and mound-shoal complexes, whereas the platform interior contained microbial mounds, lagoon, and tidal flat facies. We constructed a high-frequency sequence stratigraphy framework and identified two third-order sequences (SQ1 and SQ2) in the Dengying Formation and five fourth-order sequences (SQ2-1 to SQ2-5) in the fourth member of the Dengying Formation (the D4 member). In the D4 member, the thickness of the mound-shoal complex increases from the bottom to the top. The reservoir spaces in this Formation generally have low porosity and low permeability and include dissolution vugs and caves, and intercrystalline vugs and pores. Three types of reservoirs are identified, which are early-stage karst mound-shoal reservoirs, late-stage karst superimposed reservoirs, and other factors impacted reservoirs. Microbial mound and shoal facies are the material basis for a favourable reservoir, with the key factor for the improvement of reservoir quality being early-stage meteoric-water dissolution related to fourth-order sequence boundaries. The tectonic unconformity that was formed at the end of the Sinian Stage dominates the development and distribution of the most beneficial reservoir beds. Organic acid, hydrothermal fluid, and tectonic fracturing processes modified the reservoir beds in advance. Results from our investigation enable us to propose a model for the depositional evolution and reservoir formation in the D4 member of the Dengying Formation.

Terminal Ediacaran microbialite lithofacies associations with paleo-environmental constraints in a high-frequency sequence stratigraphic framework of Sichuan Basin, SW China

“…Similarly, the KTH values increase instantly with spikes at fourthorder sequence boundaries. On FMI images, dissolution-related Frontiers in Earth Science frontiersin.org characteristics such as random dark spots with oblique modes and the sudden mode change that indicates lithofacies transformation can be observed at these high-frequency sequence boundaries (Hu et al, 2018;Wang et al, 2021). By correlating the GR, KTH and FMI logging data with core observation and a study of recent publications, the Z 2 dn 4 of the Gaoshiti -Moxi area is divided into five fourth-order depositional sequences (Figure 4).…”

“…Based on outcrop observation and 3D seismic interpretation with well log calibration, the Dengying Formation in the Gaoshiti -Moxi area can be divided into two third-order depositional sequences: the first third-order sequence includes Z 2 dn 1 (transgressive systems tract, TST) and Z 2 dn 2 (highstand systems tract, HST); the second third-order sequence is composed of Z 2 dn 3 (TST) and Z 2 dn 4 (HST) (Liu et al, 2015;Hu et al, 2018). The third-order sequence boundaries are characterized by regional unconformities of top of Z 2 dn 2 and Z 2 dn 4 on the outcrops.…”

“…From late HST to early TST of the next fourth-order sequence, microbialite lithofacies associations are more diverse. The platform margin develops three types of lithofacies associations: thin-bedded dark-grey dolomudstone-thick-bedded spotted thrombolite Associations G, H and I is usually less than 8 m. Lithofacies Associations E and H are referred to as "platform margin moundshoal complex" and "platform interior mound-shoal complex" respectively (Hu et al, 2018;Peng et al, 2021).…”

“…In the past few years, great progresses have been made on the exploration of the Z 2 dn 4 in the Dengying Formation of the Gaoshiti-Moxi area, most of which are microbialite-related oil and gas reservoirs (Zou et al, 2014;Hu et al, 2018;Wang et al, 2021). Our high-frequency sequence stratigraphic and paleo-environmental study shows that the scale of platform margin spotted thrombolitic reefs and shoals from late fourth-order HST to next early fourth-order TST is obviously larger than any other microbialite or non-microbialite lithofacies, therefore are the best targets for oil and gas exploration.…”

Facies, Depositional Environment and Reservoir Quality of an Early Cambrian Carbonate Ramp in the Tarim Basin, NW China

“…Interpretation: The dominance of mud and the presence of scattered angular silty quartz grains with silty and argillaceous laminae point to a lower-energy depositional setting adjacent to the paleohighs [54]. The occurrence of silt-bearing argillaceous dolostone with silty and argillaceous laminae as interbedded thin layers in TFA5, TFA6 and TFA10 indicates that F5 was deposited in a low-energy, tidal flat environment [55] or restricted lagoonal environment [56].…”