Study On The Carboniferous Carbonate Property And Its Controlling Factors In Eastern RANGNAUO Of The Pinlihai Basin

The Nanor area at the eastern margin of the Pre-Caspian Basin was studied, with lower Carboniferous carbonate rocks as the target. Besides core observation, drilling and geologic well log analysis, an approach integrating sedimentology, reservoir geology, paleokarst geology and statistics as well as techniques like micropolariscope, cathodelumine- scence, electron microprobe, cast thin section analysis and carbon-oxygen isotope were used to study the reservoir characteristics and distribution features of the target carbonates.Sedimentary facies analysis reveals that, the KT-II developed open platform deposition whereas the KT-I saw the coexistence of open platform and restricted platform deposition. Four microfacies, i.e., banks, inter- bank low-lands, lagoons and tidal channels are recognized through optical identification and core observation.Carbon-oxygen isotope analysis shows that, in the primary pores in the Carboniferous carbonates, the fillings are poor in iron and manganese, with a positiveδ¹³C and an appreciable negativeδ¹⁸O, which suggests the fillings formed in a near surface to shallow burial environment before organic maturation. By contrast, a negativeδ¹³C in the iron- and man- ganese-rich fillings in the secondary pores indicates participation of organic decomposed CO2 in the crystallization, and a negativeδ¹⁸O demon- strates a higher temperature. Generally, the fillings in the secondary pore spaces formed in deep burial.Mainly, secondary pores, vugs and fissures compose reservoir spaces in the studied Carboniferous carbonates. These vug-fissure reservoirs developed under the control of sedimentary microfacies and diagenesis, especially burial dissolution, and consequently exhibit heterogeneity in the vertical profile. Microfacies and rock type constituted the main factors that made vugs and fissures possible, while diagenesis chiefly improved reservoir character. Fissure statistics attests the diagenetic origin of the fissures but not tectonic.There are two diagenetic evolution models in the study area, i.e., model I, seafloorâ†'shallow burialâ†'mediate and deep burial, and model II, seafloorâ†'meteoric freshwater and mixed waterâ†'shallow burialâ†'mediate and deep burial. Pore spaces evolved through dynamic equilibrium between the constructive and destructive diagenesis during rock forma- tion.Evaluation maps of classified main oil-bearing reservoirs in KT-II were drawn and favorable reservoir zones predicted. Pores resulted from burial dissolution were readily preserved due to their late formation in time. The most favorable for reservoir development is shoal facies, follo- wed by tidal microfacies, and lagoon the least.