Abstract
The Tazhong area of the Tarim Basin contains abundant oil and gas resources in Ordovician carbonate rocks, especially in the karst pores and caves of the Yingshan Formation. Research has indicated that the Yingshan Formation underwent a 7–11 Ma exposure during the middle Caledonian Period, resulting in large-scale karst pores and caves. However, the continental freshwater karst model cannot adequately explain the origin and distribution of karst pores and caves. In order to develop a more accurate karst model to guide petroleum exploration in the region, we analyzed the karst morphology, cave development statistics, and paleokarst environments. Karst reservoir characteristics were analyzed on the basis of the following analysis: (1) Karst morphological analyses based on core description and formation micro-imager (FMI) log analyses. The results showed that alveolar-like and Swiss cheese-like solution pores, spongy dissolution zones, pit cenotes, and small continuous karst caves developed in the Yingshan Formation. (2) The statistical analysis of pore and cave characteristics indicated that most of the karst pores and caves developed within 50 m below the unconformity where the average height of these features ranged from 0.1 to 3.0 m and their widths ranged up to 100 m. These pores and caves were commonly filled with gravel, clay, and calcite. Horizontal well and seismic attribute analysis indicated that these pores and caves were distributed over a large area. In plain view, the karst pore-cave system is comprised of cross-linked anastomosing networks of horizontal cave passages. And (3) Cathode luminescence and electron microprobe analyses suggested that clay filling within karst caves was freshwater related, while calcite filling was of seawater origin. Cements within solution pores showed three phases of luminescence, suggesting an alternating freshwater and seawater environment. Based on these characteristics, the karsts of the Yingshan Formation in the Tazhong area are interpreted to be similar to the eogenetic karsts in the Yucatan Peninsula of modern Mexico. Accordingly, this study indicates that the pore-caves of the Yingshan Formation can be subdivided into three sections. Further, the development and filling of these pore-cave sections are interpreted to have formed by eogenetic mixed-water karstification during three phases of relatively stable sea level in a coastal margin environment.
Highlights
The study of karst systems has long focused primarily on fracture-caves and geomorphology dominated by continental freshwater karsts [1,2]
This study identified paleokarst phenomena, carried out development statistics on the solution pore-cave system, and analyzed fillings in pores-caves in order to identify the characteristics and environment of the Yingshan Formation paleokarst
About 90 wells were drilled in the Yingshan Formation on the Northern Slope of the Tazhong Area, where 15 wells were cored and 25 wells underwent image logging
Summary
The study of karst systems has long focused primarily on fracture-caves and geomorphology dominated by continental freshwater karsts [1,2]. Studies of paleokarst-type oil and gas reservoirs used the continental freshwater karst model as a reference [3,4,5,6,7,8]. This, in turn, resulted in the formation of typical Guilin-type karst peak-cluster landforms and cavern systems due to the long-term reception of meteoric freshwater karst [12,13]. This model has successfully guided the exploration of carbonate karst fracture-cavern reservoirs in the Lunnan and Tahe oil fields [14,15].
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