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Characteristics and Main Controlling Factors of Karst Caves in Tarim Basin


The Middle Caledonian led to extensive lateral compression and vertical uplift in Tarim Basin, and Yijianfang and Yingshan formation of Ordovician were exposed three times, resulting in vast karst and the forming of different types of paleocaves. Morphology and structure of 174 karst caves are studied in detail and categorized in Tarim outcrops, and as analogy, karst caves of Tarim Oilfield buried deep in Ordovician formation are analyzed using seismic attribute and seismic inversion data, and the main controlling factors of different types of karst caves were studied. The research shows that: karst caves can be divided into four types according to their morphology and structure, which are tubular and branching type, karst caverns group type, plate profile type and single isolated type. Different types have different characteristics and controlling factors. 1) Controlled by underground rivers, the tubular and branching type of karst caves are long slender pipes with circular shape in cross section, and many branches connected each other. The biggest one found in Sanjianfang outcrop is more than 100m long and has 7 branches with more than 10m length. This type of karst caves were found in Hahahatang Oilfield as well with linear property in seismic amplitude gradient. 2) The karst caverns type is usually comprised of many caves with different scale and shapes, and they connected with each other, these caves distribute in several layers which are controlled by different level of water table. Karst cavern type caves show high impedance in seismic inversion body and disorder reflection in conventional seismic section, and this type of caves are the main reservoir space of Halahatang Oilfield and Tahe Oilfied in Tarim basin. 3) The single isolated type of caves are usually small scale, distribute scatterly in the plane and vertically in geomorphic lower part in outcrops, their shapes can be funnel-shaped, inverted funnel-shaped, plate profile or combined shape, controlled by different factors (faults, fractures, or corrosion along layers). Some larger caves of single isolated type shows ball chain reflections, while most single caves with less than 50m width cannot be identified due to low resolution of seismic data. 4) Controlled by regional corrosion along layers, caves of the plate profile type show property of great width along layers and relatively small height, and mostly distribute on geomorphic lower part of the side of ravine along layers.