Characteristics of the Lacustrine Limestone Reservoir in the Paleogene Nadu Formation, Eastern Bose Basin, Guangxi

The carbonate reservoir in lacustrine facies, a new field in oil and gas exploration, has not been thoroughly studied yet. This paper deals in detail with the lacustrine limestone developed in the lower part of the third member of Paleogene Nadu Formation in Nakun area, eastern Baise basin, Guangxi. Based on observations of cores and measured profiles, the authors studied characteristics of lacustrine limestone in such aspects as features of petrology and sedimentary facies, types of reservoir space and pore structure and petrophysical properties. The reservoir rocks consist mainly of all kinds of grained limestone, and the grained beach in the shallow lake is the main part of the sedimentary facies. The reservoir pore space is mainly made up of moldic pores and intergranular dissolved pores, with medium or small throats and pore configuration in a single medium. The porosity is mainly in the range of 5%～25%, and the permeability is smaller than 1×10 -3 μm2 . Among all microfacieses, the beach ridge has the largest porosity and permeability. According to the distribution of porosity and permeability and their correlation, the lower limit of reservoir physical properties and the threshold value for classification of reservoir types are studied. In this paper, the reservoir rocks in this area are classified into four basic types, i.e., the low porosity and low permeability reservoir, the medium-low porosity and medium-low permeability reservoir, the medium-high porosity and medium-high permeability reservoir, and the high porosity and high permeability reservoir. The first two are well developed and distributed widely while the last two are poorly developed and only exist in the grained limestone of beach ridge and beach edge.