The depositional evolution, reservoir characteristics, and controlling factors of microbial carbonates of Dengying Formation in upper Neoprotozoic, Sichuan Basin, Southwest China

Jinmin Song,Luba Jansa,Di Yang,Shugen Liu,Wei Sun,Hanlin Peng,Tong Lin,Ping Luo,Hairuo Qing,Zhiwu Li

doi:10.1177/0144598717743995

Abstract

The Dengying Formation of Neoprotozoic age deposited in north Sichuan Basin, China, is dominated by dolomitic strata containing microbial carbonates. Thirteen cyanobacteria forms, one oncolite and two stromatolitic structures have been identified. Different microfacies may be related to different microbe forms or assemblages as well as depositional environments. Potential hydrocarbon reservoirs in microbial carbonates are of low porosity and permeability. Microbialites develop in the members Z2 dn1, Z2 dn2, and Z2 dn4. The member Z2 dn1 and Z2 dn2 lying in the lower part, dominated by thrombolitic and spongiostromata dolostone, with three reservoir intervals of overall 190 m thick. Laminite and stromatolitic dolostone are the most important in member Z2 dn4, with three reservoir intervals of 119 m thick. Microbial carbonate reservoirs in members Z2 dn1 and Z2 dn2 were effected by two stages of fresh water dissolution, three stages of burial dissolution, and one stage of hydrocarbon invasion. But one stage of fresh water dissolution, two stages of burial dissolution, and three stages of hydrocarbon invasion modified the reservoirs of member Z2 dn4. The dominant factors for microbial reservoirs were microbial textures and development of Mianyang-Changning intracratonic sag.

Highlights

Microbes are represented in geologic records since early Archean (Lowe, 1980)
The Praesolenopora is tube-like in shape, with micritic walls and dolosparite filled intra skeletal voids (Figure 2(d)), which is similar in the early Neoproterozoic, northwestern Canada (Batten et al, 2004)
The Renaclis-resembling is showing chambers and dendritic growth structure, forming very porous framework with fibrous isopachous dolomite microspar rims around the colonies (Figure 2(g)), which is reported in Neoproterozoic of western and southern Australia (Lemon, 2000; Wallace et al, 2015)

Summary

Introduction

Microbes are represented in geologic records since early Archean (Lowe, 1980). They are the principal constituents of microbial carbonates. The Yangba outcrop section and borehole TX1, located in northern Sichuan Basin (Figure 1(a)), are examined to determine the microbes, depositional evolution and reservoir characteristics of microbial carbonates in Dengying Formation. The Neoproterozoic strata are comprised of Guanyinya and Dengying Formations (Figure 1(c)) The former is mostly yellowish gray, thin to medium bedded, fine to coarse grained feldspar quartz sandstone, with conglomerate at base. It is disconformably overlain by Dengying Formation, which is subdivided into four members according to their lithology structures. An initial carbonate platform has been formed in north Yangtze Block during early Neoproterozoic (Yu et al, 2011) It is overlain by marginal-marine siliciclastic sequence of Guanyinya Formation. Late Neoproterozoic unconformity, depositional environment suddenly changed into a deep shelf, with black shale well developed (Yu et al, 2011)

Materials and methods

Results and discussion

Conclusions