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.
Highlights
Karst is a kind of topography that is formed by the dissolution of soluble rocks, such as limestone, dolomite, and gypsum [1,2,3,4]
The oil was not generated in such depth; it was mainly generated in the “oil window” in 322–327 Ma, which belonged to the Middle Caledonian, with the buried depth of 2218–2453 m [59]
Due to the strong heterogeneity of the paleokarst reservoirs, the drilling success rates of different blocks in the oilfield are quite different. This distribution of oil and gas is related by many factors, such as the formation and evolution of the Akekule Arch, the characteristics of the stress field, the degree of paleokarst development, hydrocarbon accumulation conditions, and preservation conditions. It is very important for petroleum geologists to accurately understand the characteristics of the paleokarst system in the Tahe Oilfield
Summary
Karst is a kind of topography that is formed by the dissolution of soluble rocks, such as limestone, dolomite, and gypsum [1,2,3,4]. It can develop a series of underground drainage systems with sinkholes and caves. The dissolution of soluble rocks results in formation of large karst caves that formed in the vicinity of unconformity, which is called a paleokarst system [10]. Paleokarst is the general term for ancient karst features that have been fossilized
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