Genesis of Excellent Xiaoerbulak Microbial Carbonate Reservoir in Kalpin Area of Tarim Basin, NW China

Key words: 
• Tarim Basin /
• Xiaoerbulak Formation /
• excellent microbial carbonate reservoirs /
• microbial sedimentation /
• hydrothermal dissolution

Abstract: Microbial carbonate reservoirs are important research field of oil and gas geology, especially for oil-gas reservoirs. Regarding to the debate of excellent reservoirs of ancient and deep buried Xiaoerbulak microbial carbonate rocks in the Tarim Basin and their genesis, the Xiaoerbulak microbial carbonate rocks in the Kalpin area were investigated and studied systematically through field outcrops observation, section measurement, dense sampling, property test, thin section analysis and CT scanning in this paper. The results showed that the thrombolite with bright crystalline clotted texture was the excellent microbial carbonate reservoir of the Xiaoerbulak Formation with the max porosity about 9.35%. The proportion of porosity between 2.69% and 9.35% was 70.3%, the mean porosity was 5.07%, and there was an excellent positive linear correlation between the porosity and permeability of the rocks, suggesting the thrombolite as the excellent porous microbial carbonate reservoir of the Xiaoerbulak Formation. The excellent reservoir was composed of 6 types of reservoir spaces including intergranular pore, intercrystalline pore, intergranular dissolution pore, intercrystalline dissolution pore, extensive dissolution pore and dissolution vuge. The bright crystalline clotted texture formed in the high energy primary sedimentary environments of the upper subtidal zone provided the precondition for the thrombolite to form excellent microbial carbonate reservoir. Hydrothermal dissolution forming abundant large aperture fabric non-selective extensive dissolution pores and dissolution vuges were vital diagenesis process during the formation process of excellent reservoir, largely developing the porosity of thrombolite. This study can not only deepen the understanding of the type and genesis of the excellent Xiaoerbulak microbial carbonate reservoirs, but also provide sedimentary accordance for the effective selection of fair target in the future exploration.