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ABSTRACT: Tectonic Evolution and Petroleum Potential of Tarim Basin in the Northwestern China

Stephan A. Graham, Simon C. Brassell, Edmund Z. Chang

The Mesozoic-Cenozoic Tarim basin is 560,000 km2 in area and overlaps the Precambrian Tarim craton. The basement of the craton crops out in the surrounding areas of the basin and consists of Archaean to upper Proterozoic metamorphics. The sharp unconformity on top

of the basement marks the Tarim movement through which the Tarim craton was converted into a stable area.

The oldest cover strata, the Sinian (800-570 m.y.), and the lower Paleozoic are neritic-carbonate and littoral-clastic rocks, with minor tillite and volcanics in the Precambrian, thickening from the southwest to the northeast. The Caledonian movement at the end of Silurian created uplifts and depressions within the basin and caused erosion of Ordovician and Cambrian sections from top of some uplifts. The only Devonian marine facies are seen in the southwestern margin of the craton, the Tiekelike area, where no sedimentary break can be found between Silurian and Devonian. In all the other areas, the Devonian consists of clastic rocks of terrestrial or transitional facies. From Devonian to Lower Permian, there were a few cycles of marine transgression and regression. The Carboniferous a d Lower Permian are mainly marine facies. The Upper Permian is mainly lagoonal facies and is continuous upward to the terrestrial Mesozoic. The thickness of upper Paleozoic strata increases from the northeast to the southwest. The most obvious unconformity is between the Lower and Upper Permian, corresponding to the collision of Tarim craton with the Junggar microcontinent and the folding and uplift of the Tienshan and Kunlun Mts. The Triassic-Jurassic are distributed in a few separated depressions, especially along the marginal areas, where foredeeps were formed by the Variscan movement in front of the Tianshan, Kunlun, and Arjin Mts. The Late Cretaceous to Paleogene marine transgression in the southwestern part of Tarim basin was connected with the Tethys in the west. From the Neogene o the present time, the Tarim basin generally has been a subsiding area. Except for the four marginal uplifts, i.e., the Kuluktagh, Keping, Tiekelike, and Arjin, the main part of Tarim basin can be divided into the Baicheng, the Northern, the Southwestern, and the Southeastern depressions and the Northern, Bachu, the Central, and Cherchen uplifts.

There are numerous oil-generating rocks in Tarim basin. Based on comprehensive studies, it is concluded that the oil of Shacan 2 well in the Northern uplift is derived from marine algae and lower aquatic precursors and is a highly matured, migrated, and biodegraded petroleum. The oil was sourced from Ordovician and Cambrian source rocks, which are the main sources of petroleum in Tarim and are widely distributed. The Carboniferous-Lower Permian carbonaceous mudstone and carbonate of marine facies in the western Tarim are also source rocks, but of secondary importance compared with those of the lower Paleozoic. Dark mudstone and coal seams in Upper Triassic and Jurassic lacustrine-swamp facies are also important source rocks in Tarim basin. They are widely distributed in Baicheng, the outhwestern and the Central depressions and are proved to be oil sources in Yiqikelike and Kekeya oil fields and in the oil seeps of the Kashi area. Upper Cretaceous-Paleogene carbonate, clastic, and salt-gypsum assemblages could be source rocks in the Southwestern depression, but need to be studied. Miocene dark mudstones of lacustrine facies are probably source rocks in Kashi, Kuqa, Awati and Yiecheng areas.

The petroleum reserves of Tarim basin are hard to estimate because large-scale exploration has been going on for just a few years. The Chinese explorationists believe that the reserve of petroleum in the whole Tarim basin is in the range of 70 to 130 billion barrels. After recent testing of the Tazhong 1 well in the Central depression, some Chinese experts have even more optimistic estimates.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990