Oil and Gas Accumulations in the Ordovician Carbonates in the Aiding Block, Tarim Basin, China

You Wu^1,2, Honghan Chen^1,2,3, Yong Feng⁴, Taizhu Huang⁵, and Lu Yun^5
1Key Laboratory of Tectonics and Petroleum Resources (China University of Geosciences), Ministry of Education, Wuhan, China
²Department of Petroleum Geology, China University of Geosciences, Wuhan, China
³Institute of Geology & Geophysics, Chinese Academy of Sciences, Lanzhou, China
⁴College of Geosciences, Yangtze University, Jingzhou, China
⁵Research Institute of Petroleum Exploration & Production, Northwest Oilfield Company, SINOPEC, Urumqi, China

Tarim Basin is a huge multicycle superimposed basin composed of Paleozoic cratonic basin and Meso-Cenozoic foreland basin, northwestern China. The Aiding block is located in the northwestern Akekule uplift, where the Ordovician carbonate reservoir is so far the largest productive intervals of oil and gas.

In this study, the tectonic evolution of the Akekule uplift and 20 fluid inclusion core samples from 6 wells in Aiding block have been analyzed to unravel the relationship between tectonic evolution of the Akekule uplift and the complex oil and gas charging history. The restoration of the paleo-morphologies and fracture interpretation from 3D seismic data indicates that the structural layer of the Ordovician reservoirs in Aiding block had undergone four major tectonic movements from Caledonian to Himalaya periods. The first was in the Middle-Late Caledonian, the Aiding block formed the nose shaped high which was at the top of the Akekule uplift; the second was in the Early Hercynian, the structural crest of Akekule uplift changed from the northwest to the northeast, and the Aiding block was still at the high part. In the Late Hercynian, the structural crest of the Akekule uplift transferred to Yuqi block so that the Aiding block was turned into a slope of the uplift. From the Indosinian to the Himalayan period, the Akekule uplift evolved into a huge plunging uplift, in which the Aiding block became a depression.

The fluid inclusion analysis results indicate that two events of oil charging in the Ordovician reservoirs occurred during the Middle-Late Caledonian to the Early Hercynian (433-426 Ma) and Late Himalaya (9-5 Ma), respectively. But also the evidence from the fluid inclusion confirmed that the moderate mature, mature and high mature oils charged in the Ordovician reservoirs to mix into the normally reservoired oils in the first charging event, which were corresponding to the structural crest formation of the Aiding block in the Middle- Late Caledonian. In the intermittent periods of oil charging, the normally reservoired oils underwent biodegradation and became heavy reservoired oils due to tectonic uplift. Even though some high mature or overmature oil and gas charged in the second event, but this charging content was so weak that the heavy reservoired oils have not been changed obviously. It is, therefore, suggested that even now the Aiding block is located at the depression of the Akekule uplift, but some considerable and heavy oils could be expected in this block.

AAPG Search and Discovery Article #90175©2013 AAPG Hedberg Conference, Beijing, China, April 21-24, 2013