Reservoir Space Research Of Cambrian-Ordovician Carbonate Rock In Tazhong, Keping And Bachu Area

This paper is based on the research of the Cambrian-Ordovician interval in the Tazhong, Keping and Bachu area. Combined with the analysis of structural evolution and sedimentary facies, the study carried out the observation of a large number of core samples and thin-sections to identify the different types of reservoir spaces. The results claim that the Cambrian-Ordovician carbonate reservoir spaces can be divided into the pore, cave and fracture. It still could be further divided into intragranular pores, intergranular pores, intercrystal pores, intracrystal pores, organic skeleton pores, moldic pores, vugs, caves, tectoclases, diagenetic fractures and corroded fissures. The caves and fractures are the most effective reservoir space. The fracture is the main channel for the oil percolation. Most of reservoir spaces are secondary pores while a few are primary pores.Through analyzing the petrologic features, microfacies and palaeogeography, and in combination of the distribution characteristics and variation regularity of carbon and oxygen stable isotopes in carbonate rocks, we can acquire the factors that dominate the carbonate depositional setting and diagenisis. The results show that: There is great variation ofδ¹³C andδ¹⁸O values of the carbonate reservoir in the research area, the oxygen isotope values are apparently apt to the minus values, the values ofδ¹³C are in range of the normal sea values. According to the verticalδ¹³C andδ¹⁸O variable characteristics, theδ¹³C andδ¹⁸O vertical values have the same change trend. The impact from atmospheric fresh water and the unconformity surface becomes greater, the values ofδ¹⁸O trends lower. During the Lower Ordovician, the stable carbonate shallow open platform depositional setting keeps for a long time in the region of Tazhong. Accordingly, the stable features of the rock typesδ¹³C andδ¹⁸O the values of carbonate rocks are present. However, above the surface of T₇⁴, theδ¹³C andδ¹⁸O values of the Upper and Middle Ordovician carbonate rocks are apparently becoming lower, which approves that there is exactly existing a short uplift, erosion and the following large-scale marine transgression during the Upper and Lower Ordovician. During this time, much ¹²C migrates into the sea, which makes the density of marineδ¹³C become lower. Accordingly, the carbonate rocks which deposit from this marine setting have lowerδ¹³C values.