XIE, DEYI, Institute of Petroleum Geology, Beijing, China

ABSTRACT: Geothermal Characteristics of Northern Tarim Basin in NW China

Based on a large amount of collected data, such as geotemperature, vitrinite reflectance, fluid inclusion, and so on, this study is focused on three subjects:

1. The characteristics of the present geothermal gradient are shown in contour maps and cross plots and explanations have been made for their possible origins. The present thermal gradients are very different in various structural units. Depressions generally have lower gradient, while uplifts and huge gas fields accumulating in Cenozoic have higher gradient. High gradient anomalies on the gas fields may be generated by an upward migration and accumulation of the heat flow from depth. The results demonstrate that the present gradient in northern Tarim basin is low, and increases obviously from the lower to the upper levels in the vertical section, due to different thermal conductivities of the rocks and depositional rates of the sediments.

2. Estimates have been made for the palaeo-geothermal gradient in several significant periods by using the crust thickness method in relation to the major source rocks.The low present and palaeo-geothermal gradients in Tarim basin result in having a long period of hydrocarbon generation. Oil was generated from the Cambrian source rock in the major depressions from Late Ordovician to Silurian with the peak generation in Middle-Late Silurian, and gas was generated before Cretaceous oil generation in the Ordovician source rock started in Silurian and ended at the end of Tertiary, and gas generation continued up to the present time. The Triassic-Jurassic source rock is not possible to generate oil until Quaternary.

3. The specific relationship between the Mesozoic low vitrinite reflectance and high geotemperature is attributed to the very fast deposition of the overlying Cenozoic, which causes the great burial depth and short effective heating time of the Mesozoic.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.