Characteristics and main controlling factors of carbonate reservoirs of Middle-Lower Ordovician, Shunnan area, Tarim Basin

Abstract

Natural gas breakthroughs have been made in many wells recently in the Shunnan in the southern part of the Shuntoguole low uplift in the Tarim Basin. With the advancement of exploration, the understanding of reservoir development is one of the key issues affecting exploration evaluation. Based on the research of sedimentary facies and tectonic evolution, core description, thin section analysis, physical property analysis, comprehensive logging and reservoir prediction were used to study the carbonate reservoirs in the Middle-Lower Ordovician in the study area. The reservoir spaces include pores and fractures controlled by fault and hydrothermal fluid activities, and also intercrystalline pores and intercrystalline solution pores in dolomite reservoirs, micropores in limestone reservoirs. There are three types of reservoirs, including fracture-cavity type controlled by faults, fracture-cavity type controlled by hydrothermal fluid activities, and fractures-pores. The fractures-pores mainly distribute in limestones in the Yijianfang Formation and the upper Yingshan Formation and in dolomites in the Lower Yingshan Formation. They have poor physical properties, and are featured by extra-low porosity and permeability. The fracture-cavity types controlled by faults and hydrothermal fluid activities mainly distribute along the NE strike-slip fault zone. Strike-slip fractures and associated fractures developed in multiple stages are the main controlling factors for large-scale effective reservoirs. They formed fractured-vuggy reservoirs. Meanwhile, they connected pores and micropores, intercrystalline pores, and intercrystalline dissolution pores, forming connected effective reservoirs, which was also a prerequisite for hydrothermal fluid activities.

Keywords

• reservoir feature
• carbonate rock
• middle-lower ordovician
• shunnan area
• tarim basin