Characteristics and genetic models of Lower Ordovician carbonate reservoirs in southwest Tarim Basin, NW China

Lower Ordovician carbonate successions in the Taxinan area are intensively cemented and filled. Matrix porosity and permeability are low in the Lower Ordovician carbonate reservoirs of the Taxinan area, and reservoir space is composed of karst vugs and fractures. The reservoir space is dominated by four types of karst vug, including vesicular vugs, enlarged dissolved vugs along fractures, tabular vugs and isolated vugs, and three types of fractures, including medium-high-angle fractures, low-angle and horizontal fractures as well as irregular fracture networks. The quality and scale of reservoirs in different zones of the Taxinan area are obviously different, and the best reservoir rock is developed in the eastern part of the Taxinan area. These karstic reservoirs can be divided into three types according to the reservoir formation mechanism, including buried-hill vug-fracture reservoirs, vug reservoirs below unconformity surfaces and deep bedding underflow karst vug reservoirs. Different types of reservoir developed in significantly different successions and areas and are controlled by different factors. Buried-hill vug-fracture reservoirs are mainly developed in the Yingshan Formation and distributed in the fault-uplift zones of the eastern part of the Taxinan area. Their formation is controlled by paleogeomorphology and tectonic movement. Vug reservoirs beneath unconformity surfaces are developed in the Yingshan Formation and distributed in the gentle zone in the central-western Taxinan area. Their formation is controlled by the scale and erosion intensity of the unconformity. Deep bedding underflow karst vug reservoirs are mainly developed in the Penglaiba Formation and distributed in the fault-sag zones between the NE-trending fault-uplift zones in the eastern part of the Taxinan area. Their formation is controlled by paleogeomorphology, rifting and unconformity. Guided by this geological model of reservoir development, combined with drilling, well logging and seismic data, the distribution of favourable high-quality reservoirs in the Taxinan area is predicted. The results showed that the flanks of the fault-uplift and fault-sag zones in the eastern part of the Taxinan area and the haystack hills and karst highlands in the central-western parts of the Taxinan area are favourable reservoir exploration areas. Correlation section of Lower Ordovician reservoir in different zones of the Taxinan area.1. Reservoir petrological, space, petrophysical and devel