Characteristics and karstification of the Ordovician carbonate reservoir, Halahatang area, northern Tarim Basin

Favorable conditions for hydrocarbon accumulation existed in Halahatang area of central Tabei, which recently made it a breakthrough area for hydrocarbon exploration in northern Tarim Basin. The main targets explored are karstified Ordovician carbonate reservoirs. Based on the sedimentary and tectonic evolution of this area, integrated data of drilling wells, boreholes and seismic analysis are used to determine the sedimentary and diagenetic characteristics of the reservoir sections. The main stages and the corresponding models of karstification, and the controlling factors of each model, are proposed. The results show that the major reservoir sections are the Middle Ordovician Yijianfang Formation and the first member of the Lower-Middle Ordovician Yingshan Formation. Nearly layered cave or caverns and fractures are main reserving spaces. These spaces were formed mainly by karstification after the deposition of the Yijianfang Formation and by the burial hill karstification and bedded karstification during the hypergenesis before the Silurian sedimentation. The first stage of karstification was generally developed with a relatively small scale. However, the formed vugs, caverns and fractures are the passageway of water, which is the basis of the later karstification. The second stage of karstification (pre-Silurian) was largely controlled by the palaeogeomorphology and could be separated into four regions: region-I to region-IV from north to south. The region-I in the north was characterized by the direct exposure of the Yijianfang Formation and the development of the burial hill karstification. The Yijianfang Formation was covered by the Upper Ordovician Tumxuk, Lianglitag and Sangtamu formations with different thicknesses in the region-II. And the karstification was controlled by the down-cutting rivers and fractures and the subsurface water fed through the exposed Yijianfang Formation from region-I. In the region-III and region-IV of southern Halahatang, bedded karstification, karstification along fractures and by mixed water of meteric from north and marine from south were developed. After these two major stages of karstification, nearly layered karstic caverns, vugs, breccias and fractures were formed with different shapes and strong heterogeneity in the Yijianfang Formation and the first member of the Yingshan Formation.