关键词: 台内凹陷, 开江—梁平海槽, 弧后扩张, 卡拉沙依组, 塔西南斜坡

Abstract: Slope belts are primary structural units in petroliferous basin, and key sites for oil and gas accumulation. The southwestern slope of Tarim Basin, occupying nearly one-fifth of its total area, has been a non-priority area for exploration and research. By comparing the Carboniferous-Permian tectonic setting, hydrocarbon geological conditions and exploration status in the slope areas of the four major basins in central and western China, it is pointed out that hydrocarbons in the slope area are mainly accumulated within the source rocks and in the contemporaneous lithologic reservoirs. Late-stage structures along the slope margin, such as piedmont thrust belts, are not the primary hydrocarbon accumulation zone. Regional tectonic and paleogeographic studies indicate that during the Carboniferous to Early Permian, the southwestern margin of Tarim block was a west-dipping continental margin basin, which developed a mixed sedimentary system alternately dominated by clastic rocks and carbonate rocks. The southwestern slope of Tarim Basin was largely dominated by the restricted platform and tidal flat environments. The continuous extensional rifting since the Early Carboniferous, such as the back-arc extension caused by the northward subduction of the Paleo-Tethys Ocean, as well as mantle plume activity and large-scale basalt eruption in the Early Permian, led to the development of the intra-platform sag analogous to the Kaijiang- Liangping trough on the southwestern slope of Tarim Basin. Existing research data demonstrate a predominant east-west trending structure in the intra-platform sag, and the Carboniferous to Lower Permian reservoirs exhibit the maximum sedimentary thickness exceeding 2 500 m, with an area of more than 20 000 km². High-quality source rocks in sag are developed in the Lower Carboniferous Kalashayi Formation, with a distribution area and thickness comparable to those of the Upper Permian Longtan and Dalong formations within the Kaijiang-Liangping trough in the northern slope of Sichuan Basin. The Carboniferous to Lower Permian intra-platform sag is a realistic petroleum system with two types of hydrocarbon accumulation, i.e., in-source unconventional and extra-source conventional hydrocarbon accumulations, suggesting another important target area for deep marine oil and gas exploration in Tarim Basin.

Key words: i ntra-platform sag, Kaijiang-Liangping trough, back-arc extension, Kalashayi Formation, southwestern slope of Tarim Basin