Petrology Characteristics of the Lower Cambrian Microbial Reefs and Their Significances for Hydrocarbon Exploration in Tarim Basin, China
Jinmin Song¹, Ping Luo², Shugen Liu¹, Shisheng Yang², Di Yang¹, Chuanmin Zhou², Pengwei
Li², and Xiufen Zhai²
¹State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu, China
²Research Institute of Petroleum Exploration and Development (RIPED), Beijing, China
Microbial reefs were found in the Lower Cambrian strata in Sugaitebulak area in Aksu, Northwest of Tarim Basin. Based on the outcrop section measurement and thin section identification, the petrology characteristics and hydrocarbon significances of the microbial reefs have been achieved. The Microbial reefs are massive about 60 to 100 meters thick and 3 to 4 kilometers long.
They are built by cyanobacteria. But they are strongly modified by dolomitization progress. They are composed of silt-like dolomite with remnant microbial structure. The microbial dendrolite, thrombolite and stromatolite have been found. The Epiphyton growing in-situ can be identified in dendrolite. Five rock types can be identified in the reef, which are grain, lamellar, microbial, cataclastic and crystalline dolomite. They consist of the two stages of reef growth progradation from SSW to NNE. The first reef progradation is made up of deep gray thin-bedded dolomites with stromatactoid cavities. It is about 30 to 40 meters thick. They are mainly pelletal, fine grained and microbial lamellar dolomite. The second reef progradation is composed of light gray massive reef dolomites. It is about 60 to 80 meters thick. There develop three petrologic structures from bottom to top. The dolorudite and doloarenite in the lower part constitute the reef base. The thrombolite, Epiphyton dendrolite and pelletal dolomite in the upper part serve as the reef core. At the top, light gray medium to thin bedded stromatolite, oncolite and grain dolomites develop as the reef cap. According to the microbial sediments and morphology, the reef progradation indicates a progress of gradually shallower and stronger sea water environment upward.
The dendrolite and thrombolite dolomites could be good hydrocarbon reservoirs. They are of medium porosity, lower to medium permeability. They are lens-like, about 80 meters thick in reef core and 20 meters thick in lateral, extending 3 kilometers away. Dissolved vugs and pores are developed intensively and irregularly both outcrop and microscope observation. But their development is very heterogeneitic. The bigger and more connective dissolved pores are developed where microbial structures exist. The surface porosity is about 5% to 8%. Under mercury injection experiments, the porosity ranges from 2 to 6 percent, with 4.32 percent on average. The permeability is generally 0.02 to 0.1 mD, with 4.66 mD averaged.
Given the favorable source, reservoir to cap rock assemblage of the lower Cambrian in Tarim basin, the microbial reef could be potential targets for hydrocarbon exploration.
AAPG Search and Discovery Article #90175©2013 AAPG Hedberg Conference, Beijing, China, April 21-24, 2013