Three-Dimensional Geophysical Characterization of Deeply Buried Paleokarst System in the Tahe Oilfield, Tarim Basin, China

Abstract

Paleokarst reservoirs are the major type of the Ordovician carbonate reservoirs in the Tahe Oilfield. Due to the strong heterogeneity in distribution, it is a real challenge to detect the spatial distribution of paleokarst reservoirs, especially those deeply buried more than 5500 m in the Tahe area. Based on the abundant core samples, this paper first described the structure of paleocaves drilled by well. Second, after time–depth conversions, the results from drilled wells were tied to three-dimensional (3D) seismic datasets, and then the threshold of host rocks and caves in wave impedance were identified. Third, the seismic-scale mapping and visualization of the paleokarst reservoirs were achieved by tracing the distribution of paleocaves. This approach was applied in the well T403 area, and the structure of the paleokarst, especially the runoff zone, was interpreted. 3D structure and spatial distribution of the paleokarst system was demonstrated by plane, vertical, and 3D models. Additionally, according to the hydrology genetic relationships, the paleocaves in the runoff zone were divided into sinkholes, main channel, and branch channel. The approach of a 3D geophysical characterization of a deeply buried paleokarst system can be applicable to Tahe and other similar paleokarst oilfields, which will guide hydrocarbon exploration in paleokarst reservoirs.

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].…”

“…The world's best-known examples of oil and gas fields associated with a buried paleokrast reservoir have been found in China. In several petroleum basins (such as Ta-rim, Ordos and Sichuan), hydrocarbon accumulations have been discovered in carbonate formations with strongly developed karst features [4][5][6][7][8]. Similar types of fields are also exploited in other countries around the world, including Canada, the US, Mexico, Venezuela, Saudi Arabia, Iraq, Russia and Kazakhstan, and also in the North Sea and the Barents Sea [9][10][11][12][13][14][15][16][17].…”

Hydrothermal Dolomite Paleokarst Reservoir Development in Wolonghe Gasfield, Sichuan Basin, Revealed by Seismic Characterization

“…The logging curve projected at a well point is a lithological indicator curve. The relatively high value is the development of dolomite, a good location [39]. Figure 14a shows the development of dolomite reservoirs from drilling data (well W93) in the target zone, further indicating that this area has a high value (Figure 14a); well W69 did not encounter dolomites in the target zone and the lithology indicator profile shows that the area is white, indicating that no dolomite developed (Figure 14b).…”

Hydraulic Behavior of Karst Aquifers

“…The article by Zhang et al [10] is a good example of a survey of deep caves and buried paleokarst systems. They combined the analyses of core samples, 3D seismic recognition, and computer modeling to identify cave distributions in the Tarim Basin, China.…”