Eogenetic Karst Control of Carbonate Reservoirs during a Transient Exposure: A Case Study of the Ordovician Yingshan Formation in the Northern Slope of the Tazhong Uplift, Tarim Basin, China

Abstract

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.

Examples of Paleokarst in Mesozoic Carbonate Formations in the Carpathian Foreland Area

“…The best-known examples of hydrocarbon fields closely associated with karst reservoirs come from China. Reservoirs of this type have been identified there within a few large petroleum basins, including the Tarim Basin [6,7,30,[39][40][41][42][43], the Ordos Basin [44,45] or the Sichuan Basin [4,46], among others. The issue of identifying zones of paleokarst development in carbonate formations based on seismic and borehole data has been addressed by various authors, e.g., [8,10,12,47,48].…”

Review of Laboratory Scale Models of Karst Aquifers: Approaches, Similitude, and Requirements

“…The applicability of new technologies, such as light transmission (Tidwell and Glass 1994), X‐rays (Montemagno and Pyrak‐Nolte 1999; Cnudde and Boone 2013), computed tomography scans (Wildenschild et al 2002; Krotkiewski et al 2011; Vanghi et al 2015; Dan et al 2018), nuclear magnetic resonance (NMR) imaging (Florea et al 2009; Florea et al 2008), and combinations of innovative logging tools and digital optical imaging (Cunningham et al 2009; Cunningham and Sukop 2011; K. Cunningham et al 2008) to image flow and solute behavior in LSMs, is also important for improved understanding of these processes. For example, the direction and magnitude of flow through LSMs could be visualized through NMR imaging of heavy water (D 2 O) as a tracer (Florea et al 2008).…”

Inclusion Characteristics of the Fracture-Cave Calcite of Ordovician Yingshan Formation and Its Indication to the Formation of Paleokarst Reservoir in the Northern Slope of Tarim Basin Center Area, China

“…Its effective reservoir space consists of mainly karst holes, caves, and fractures (Feng et al, 2014). These fractures and caves are distributed within 300 m below the unconformity surface and often filled with calcite, clay, and organic matter (Dan et al, 2018). The large karst caves are considered the main reservoir space and are mostly filled with clay, limestone breccias, and calcite (Dan et al, 2018).…”

“…These fractures and caves are distributed within 300 m below the unconformity surface and often filled with calcite, clay, and organic matter (Dan et al, 2018). The large karst caves are considered the main reservoir space and are mostly filled with clay, limestone breccias, and calcite (Dan et al, 2018). A total of 107 karst caves (with a height of more than 0.3 m) have been discovered in 90 wells in the study area, among which 62 karst caves are filled, accounting for 57.94% (Dan et al, 2018).…”

“…In recent years, scholars have conducted indepth studies on the characteristics of fracture-cavity karst morphology, distribution law, and inclusion geochemistry (carbon and oxygen isotopes; major, trace, and rare earth elements); eogenetic karstification was proven to be the origin of the karst in this area. Larger caves were more likely to develop in mixed-water karst zones controlled by sea level (Dan et al, 2015;Chen et al, 2016;Zhang et al, 2016;Dan et al, 2018;Dan et al, 2019;Dan et al, 2021). Nonetheless, the above mentioned studies did not provide direct evidence of karst fluid salinity, temperature, etc.…”