China's Tarim Basin is considered to be the last large onshore basin where giant oil and gas deposits remain to be discovered and developed. In 1977, the discovery well, Ke No. 1, located in what is now the Kekeya oil and gas field on the southwest margin of the basin, produced oil and gas at high flow rates. The initial rates were 1600 m³ (10,000 bbl) of oil and 270,000 m³ (9.5 mmcf) of gas per day. In the northern part of the basin, on the Shaya uplift, another discovery well, the Shacan No. 2, produced oil and gas from Ordovician dolostone at initial rates of 1000 m³ (6290 bbl) of oil and 2 million m³ (70 mmcf) of gas per day in 1984. Recently there have been still other discoveries of substantial volumes of oil and gas in va ious formations. The ages of the producing zones vary over a wide range--Cretaceous, Jurassic, Triassic, Carboniferous, and Ordovician. Especially encouraging was the Central No. 1 discovery well in the Central No. 1 structure, which yielded 576 m³ (3620 bbl) of oil and 340,000 m³ (12 mmcf) of gas per day in 1989. All these discoveries indicate that Tarim Basin is promising.

The Tarim Basin, with an area of 560,000 km², was a huge platform during the Paleozoic. Later, in the Mesozoic-Cenozoic, the platform subsided and became a basin. There were two major cycles of marine transgression and regression that resulted in thick sediments containing two marine facies favorable for oil generation--one in the Sinian-Devonian, and the second, after a hiatus, in the Carboniferous-Permian. In addition, the Triassic-Jurassic section contains good continental facies source rocks.

In two oil fields of the Tarim Basin--the Yiqikelike field in the Kuche depression and the Kelatuo field in the Southwest depression--the crude oils were derived primarily from Jurassic and Triassic sediments and have geochemical characteristics of the Jurassic continental coal measure strata. However, some

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production of Miocene oil and gas of the Kekeya field might be derived from Carboniferous-Permian source rocks. Most of the oil and gas in the four fields/blocks on the Shaya uplift was generated from Paleozoic marine source rocks, but part of it is from Triassic-Jurassic continental source rocks, resulting in an oil and gas accumulation from cosources. Separate oil and gas accumulations from Paleozoic marine facies and Mesozoic continental facies also have been found there. The oil and gas of the Central No. 1 structure may have been derived from Paleozoic source rocks.

In the Tarim Basin, the areas considered to have optimal conditions for giant oil and gas fields are the Shaya uplift, the Central uplift, and the southwest margin of the Southwest depression.