The Recognition of Ordovician Karst Landform in Ordos Basin,China

Jianrong, Gao; Wanglin, Xu; Yanru, Guo
Ordos Department, Research institute of Petroluem Exploration and Development PetroChina, Beijing, China.

The Ordos basin is the largest oil and gas resources Basin in western China, in which Jingbian gas field is the earliest and largest marine carbonate gas field in China. The Ordovician karst weathering crust has a capacity of 55 billion cube per year in Jingbian gas field.

Recently the Industrial gas has been found in some exploratory wells of western Jingbian gas field yet. According to the geological study, it founds that the reservoir development is conducive to accumulation if the wells drilled ancient karst residual hills, and it is not reservoir if drilling flutings, because the strata is corroded. Therefore, the identification of ancient karst topography is particularly important.

Previously the ancient landform recognization through the seismic even. The seismic even "V" or "U" shape was to predict the ancient flutings. This method can identify the width of the ancient flutings on the kilometers, but it is difficult to identify the relatively small width swallow depth flutings, especially width tens of meters and depth less 30 meters. However, this ancient flutings is most widely distributed in the western Jingbian gas field, Therefore, the geology parameter and well establish the true model. With the full wave field modeling method to arrange shot point and detector to arouse and receive the corresponding seismic reflection data. and these data is processed , finally get a post-stack migration profile ,the characteristics of the seismic even respond to all kinds of flutings.If the erosion depth of flutings is 10 to 5 meters, it's seismic even phase is reversal, if the erosion depth of flutings is 30 to 10 meters, it will show the seismic even gains increases, the erosion depth of flutings is greater than 30 meters, then it is the seismic even complex wave, This method can quickly and effectively identify the distribution of the flutings. The drilling new wells had confirmed the effectiveness of this method.

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012