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Petroleum System of Carboniferous-Lower Permian in Western Tarim Basin, China

As early as 1986, when the first round of petroleum resources evaluation in the Tarim Basin was made, it was recognized that two sets of marine source rocks of Cambrian-Ordovician (C-O) and Carboniferous-Lower Permian (C-P1) were developed within the basin. However, since the campaign of Tarim exploration in 1989, some medium-large scale oil-gas fields were discovered in the craton area, with oil/gas coming mainly from C-O but the contribution of C-P1 source rocks not clearly established.

Based on oil/gas source correlation of two oil-gas fields, rock pyrolysis analysis of about 500 samples from eight field sections and core and cuttings of eight wells, this paper presents a recognition of C-P1 petroleum system in the western Tarim Basin.

The oil/gas source correlation results of Ake gas field, Kekeya condensate gas field in the Western Tarim Basin confirmed that its oil-gas came from the C-P₁ source rocks. High quality C-P₁ source rocks were found in some outcrop sections from southern Tianshan piedmont to Kunlun piedmont. By sampling densely in the developed segments of C-P₁ potential source rocks of eight wells in the northwest Tarim Basin, analysis data showed that Carboniferous source rocks have definite hydrocarbon generation capacity with wider distribution, but Lower Permian source rocks have relatively limited distribution. However, test results of wells and sections indicated that the distribution of source rocks in different formations/members showed lateral zonation and vertical intersecting superposition, with possible multiple centers of hydrocarbon generation.

Through analyzing gas reservoirs of Ake and Kekeya gas fields and outcrops of C-P₁ oil seepage, the authors have found that C-P₁ petroleum system comprises of two sets of source-reservoir-cap assemblages. The former constitutes a normal-type source-reservoir-cap assemblage. The latter is self generating, self reservoiring and self sealing. Local structures were developed in the Late Himalayan in the study area. At that time, C-P₁ source rocks were at the stage of over-mature or mature respectively, so all structures were formed at or after the peak of gas generation, while faults formed in this period acted as the pathway between source rocks and reservoir beds. In a word, source rocks were well developed in Western Tarim Basin. And the evolution of organic matter well matching with later formed structures could make a prospective petroleum system in the Tarim Basin.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California