Geology of the Tarim Basin with special emphasis on petroleum deposits, Xinjiang Uygur Zizhiqu, Northwest China

Abstract

This report presents general information on the geology of the Tarim basin with special emphasis on petroleum deposits. The basin covers about 563,000 km2 with a basin-fill of sediments amounting to 2,301,000 km3 . It is generally confined within lat 37°10 f -42°00 T N. and long 75°00 T -93°00 T E. Tarim basin evolved on a craton and acquired its individuality relative to the Tian Shan geosyncline on the north and the Kunlun geosyncline on the south during the Late Carboniferous Variscan orogeny. The basin reached full development, including the marginal foredeeps both to the north and to the south, in the Neogene owing to the Himalayan orogeny. The Archean continental nucleus, trending northeasterly, occupied most of the southern part of the basin. This continental nucleus came into contact with an extensively faulted east-west magmatic arc of basic to ultrabasic plutonic rocks on the north. The nucleus was enlarged in early Late Proterozoic time to the present Tarim continental block by Proterozoic accretion onto both the north and south margins (Wang and Qiao, 1984). Subsequently, deposition of platform marine sedimentary cover on the nucleus occurred from the Sinian to Upper Paleozoic Permian. The processes of sedimentation were associated closely with geotectonic movements, especially in relation to the deeply seated basement faults within the Tarim block. Principal structural units in the present-day Tarim basin are the Kuqa Foredeep, the Northern Tarim Uplift, the Eastern Tarim Depression, the Central Uplift, the Southwestern Depression, the Kalpin Uplift, and the Southeastern Faulted Blocks. Current petroleum exploration is concentrated in the Northern Tarim Uplift, the Southwestern Depression, the Central Uplift, and the Kuqa Foredeep. The Eastern Tarim Depression is particularly favorable for future petroleum exploration. Potential petroliferous source rocks in the basin are the marine asphaltene carbonate rocks, shale, and mudstone of the Early Cambrian, Early Ordovician, Early Silurian, Early Devonian, Carboniferous, Late Cretaceous, and Paleogene ages; continental lacustrine mudstone, shale, and oil shale of the Late Permian and Triassic ages; and transitional marine and continental mudstone of the Miocene age. The reservoir rocks are chiefly sandstone, conglomerate, and fractured, cavernous, algal-matted and bioherm-reef limestone and dolomite, of which the carbonate rocks are present chiefly in sedimentary sequences of Late Sinian, Cambrian, Ordovician, Carboniferous, and Early Permian age. Dominant and potential types of traps in the basin are anticlinal fold, faulted anticline, stratigraphic unconformity and onlap as well as overlap, and bioherm-reef complexes. Cap rocks are mainly shale, gypsum beds, mudstone, and, locally, rock salt. It is believed that the petroleum potential in the Tarim basin is substantial. The potential for coal resources is unknown. Triassic and Jurassic coal beds are mined at Kuqa and Luntai in the Kuqa Foredeep; Jurassic coal is mined locally at Yutian in the southeastern part of the Southwestern Depression.