Abstract:

Microbial carbonates are an important part of Sinian and Cambrian formations in China. They are a significant reservoir type, and it is of great significance for carbonate propecting to understand their genesis and reservoir forming mechanisms. However, the formation mechanism of the Cambrian microbial carbonate reservoir in the Tarim Basin is still controversial with various explanations proposed, such as simultaneous and penecontemporaneous dissolution, burial hydrothermal dissolution or multiple fluid interactions, and so on. In this study, we investigated the Xiaoerbulake Formation of the Shiairike and Xiaoerbulake sections in the northwest margin of the Tarim Basin. We analyzed the effect of fresh water on the formation of microbial carbonate reservoir in the early diagenetic period through means of outcrop observation, thin section identification, cathode luminescence, carbon/oxygen isotope analysis and others. The results show that (1) four microbialite facies can be identified in the Xiaoerbulake Formation: stromatolite dolostone, thrombolite dolostone, foam spongy dolostone and microbial-related dolograinstones. The pore system in microbial carbonate rock is complex and the pore types in microbial mounds include intergranular pores, dissolution pores, various scale dissolution vugs/caves and fractures, among which the dissolved pores and vugs/caves are the main pore types. (2) Short-term subaerial exposure and penecontemporaneous dissolution of carbonates by atmospheric fresh water predominantly contributed to pore formation as evidenced by three observations. Firstly, there are some small-scale tracks formed by dissolution on the surface of microbial buildups. Secondly, meniscus cement and unconformity are found in slices, indicating short-term exposure. Meniscus calcite cement is a sign of vadose freshwater diagenesis; and unconformity means a partial absence of fibrous cementing materials between grains and drusy cement that generally formed in burial environment. Thirdly, luminescence of the cement is mottled medium bright, distinctive from that of cement formed in meteoric or burial environment as bright and orange-red or dark red rings in general. In addition, affected by atmospheric fresh water, carbon and oxygen isotopes in the outcrops are slightly lower than in seawater. (3) The microbial carbonate reservoir is heterogeneous and its distribution was controlled by the sedimentary architecture of microbial mounds and sea level fluctuations. It is clear that thrombolite dolostone, foam spongy dolostone and cyanobacteria related dolo-grainstone are developed successively from bottom to top, constituting a complete sedimentary assemblage of mound base, core and cap, respectively. The porosity of the mound core (average 5.47%) is much favorable than that of mound cap (average 3.51%) and mound base (average 2.01%). Difference in rock types in microbial mounds is the main cause of disparity in reservoir quality.

Key words: microbial carbonate, contemporaneous-penecontemporaneous dissolution, reservoir architecture, Cambrian, Tarim Basin