Integrated Reservoir Characterization and Distribution of Carbonate Reef- Flat Complexes in Genetic Units of Different Phases of the Ordovician Carbonates in Tarim Basin, Western China

Abstract

Abstract The reservoir characterization and prediction of carbonate reef-flat complexes often have great difficulties because of the complexity of rock-fabrics and the rapid changes of carbonate depositional facies. This paper focuses on a carbonate reservoir in Tarim Basin, China. The dynamic behaviors between adjacent wells vary considerably although they belong to the same depositional facies, which indicates that the reservoir has strong heterogeneity. This paper proposes a new set of carbonate reservoir characterization in genetic units by integrating the geological, petrophysical and geophysical data. According to core-log interpretations, rock fabrics, pore types, reservoir properties, secondary porosity indexes have been acquired. The depositional cycles and key interfaces in stratigraphy sequence framework are determined. The relationships between the rock-fabrics, depositional cycles, reservoir properties, palaeo-weathering surfaces, responses of logs and seismic attributes have been studied, and reservoir genetic-unit models are constructed, which reveals that carbonate genetic units in different phases have specific rock-fabric associations and reservoir properties. The genetic units in different phases are delineated comprehensively in log and seismic data, and different reef-flat distributions are constructed. Finally, the seismic attribute relating to the secondary porosity indexes have been extracted in genetic-unit framework and it reveals the distribution of favorable reservoirs of carbonate reef-flat complexes. This paper reveals that different phases of genetic units of carbonate reef-flat complexes, as containing specific rock-fabric associations, crucial interfaces and fracture-vug development, thus key factors, control the distribution of favorable reservoirs. Phase I genetic unit, formed in relative sea-level rise, mostly with fine rock-fabric association of micrite calcarenite and micrite algae-bound limestones, are filling on the depressions, grooves or the flat area of carbonate platform, and its reservoir properties are very poor. Phase II genetic unit, with the tendency of upward- coarsening fabric association and top exposure surface, is overlaid on highlands along platform margin. Phase III genetic unit, composed of fracture-vug developed sparry gravel-bearing reef-skeletal limestones and sparry gravel- bearing calcarenites of reef-flat complexes, is almost distributed in the platform margin and overlaid on tops and nearby of previous genetic units, which means that, shallow water is suitable to reef-building organism mass-production, but frequent exposures and weathering have made favorable reservoirs developed. Drillings have proved that almost all high-production wells are within the area of phase II and III genetic units. Furthermore, the distribution and complicated internal structure of favorable reservoirs have been revealed by log-calibrated seismic attribute extraction in the framework of phase II and III genetic units. This paper illustrates an integrated case study, which helps geologists and reservoir engineers to support the development of Central Tarim Basin. In addition, it offers a methodology for the research of other similar oilfields.