Microbial carbonate sequence architecture and depositional environments of Member IV of the Late Ediacaran Dengying Formation, Gaoshiti–Moxi area, Sichuan Basin, Southwest China

Abstract

The recent discovery of the Anyue gas field, with huge reserves, has great potential for ultradeep gas production in carbonate reservoirs. Previous exploration and exploitation of Member IV of the Dengying Formation in the Gaoshiti–Moxi area deposited during the Late Ediacaran have provided useful insight into palaeo‐gas reservoirs, and geological significance of the Precambrian microbial carbonate strata. In this study, the depositional sequence architecture has been established and typical environments from Member IV of the Dengying Formation in the Gaoshiti–Moxi area have been reconstructed through detailed sedimentological analysis of cores, petrophysical data, well‐logging data and seismic interpretation. One third‐order sequence was identified, which is subdivided into seven fourth‐order sequences and 18–19 fifth‐order sequences. The third‐order sequence is bounded by denudation surfaces or onlap surfaces, and composed of transgressive system tracts and highstand system tracts. The fourth‐order sequences are bounded by instantaneously exposed punctuated surfaces or lithology and lithofacies transition surfaces, featured by specific lithological cycles, such as ‘lower microcrystalline dolomite, middle intraclastic dolomite and sparry thrombolite, and upper intraclastic dolomite’, ‘middle–lower sparry thrombolite and upper microcrystalline dolomite and sparry thrombolite’, ‘lower micritic thrombolite, middle micritic dolomite, and upper sparry thrombolite and laminites’, and ‘lower microcrystalline dolomite, middle intraclastic dolomite and argillaceous or micritic dolomite, and upper intraclastic dolomite’. The sequence characteristics reveal that periodic high‐frequency sea‐level change and palaeogeomorphology are controlling factors affecting sequence development, which lay a significant foundation for reservoir development. During the depositional period, a tidal flat on a restricted carbonate platform is well‐developed, allowing the identification of 16 microfacies, 13 microfacies associations, and 6 seismic facies. Inter‐lower tidal flats are widely developed with band‐shaped mound–bank complex, microbial mound, and shoal on the platform margin. On the inner platform, patchy microbial mounds, mound–flat combinations, and inter‐shoal/microbial mounds are developed in the Gaoshiti area, whereas patchy shoals and shoal–flat combinations are developed in the Moxi area. During the early depositional period of Member IV of the Dengying Formation, the microbial mounds and shoals were developed on the platform margin. The dolomitic flat and inter‐shoal/microbial mounds are mainly developed on the inner platform with little microbial mound and shoal deposition. During the late depositional period of Member IV of the Dengying Formation, large‐scale microbial mounds and shoals are sustainably and stably developed on the platform margin; enlarged dolomitic flat, patchy microbial mounds, and patchy shoals are predominantly developed in the platform interior with inter‐shoal/microbial mounds dying out.

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

“…Characterized by equal spacing sampling, continuous data series and high vertical resolution, well logging data such as GR (gamma ray) curves, KTH (potassium and thorium) curves and FMI (Fullbore Microscan Imager) images are applied as effective tools to classify fourth-order depositional sequences (Peng et al, 2021). The GR values are generally low (below 30 API) and display a box-shaped mode with unobvious jags; however, at lithofacies transitional surfaces, their values increase sharply and transfer to a shallowing-upward trend of fingeror finger-joint-box-shaped curves (Wang et al, 2021). Similarly, the KTH values increase instantly with spikes at fourthorder sequence boundaries.…”

“…The Gaoshiti-Moxi area is also an important natural gas producing area. According to reservoir property differences, Z 2 dn 4 is divided into a lower sub-member and an upper sub-member (Wang et al, 2021). All the samples of the study were collected from the upper submember of Z 2 dn 4 in a series of wells.…”

“…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. On the 3D seismic profile, these two third-order sequences are bounded by denudation surfaces or onlap surfaces (Wang et al, 2021).…”

The Advances and Challenges of the Ediacaran Fractured Reservoir Development in the Central Sichuan Basin, China

“…The Lower Ediacaran Doushantuo Formation deposited thick sandstones and interbedded mudstones and progressively turned to dolomites in an intracratonic shelf environment. The Upper Ediacaran Dengying Formation composed of two sets of thin shale-thick dolomite assemblages in the central carbonate platform (Figure 1) [12][13][14]. In the end of the Ediacaran, the central Sichuan Basin had a regional uplift and subsequent unconformity karstification [15].…”

“…In the Anyue Gasfield, the late Ediacaran mound-shoal bodies widely developed along the platform margin and in the interior platform [13,14]. The reservoir mainly occurred in the high-energy mound-shoal dolomite, which is generally thought to be the major controlling factor on the carbonate reservoir [12][13][14][15]. It is assumed that the arid paleoclimate was favorable to develop the primary matrix pore-type reservoir in the mound-shoal bodies [15].…”