- Copyright © 2002 Society of Exploration Geophysicists
In Yinggehai Basin, an important area for offshore gas exploration in China, rapid filling and deposition caused undercompaction of a massive layer of mudstone. Several diapiric anticlinal structures formed in the center of this basin under the effect of deep hot fluids, making Yinggehai Basin a favorite area for exploration of oil and gas. Estimated gas reserves in the shallow areas are about 200 billion m3.
However, these anticlinal structures on P-wave seismic profiles were unclear and became known as “fuzzy zones.” Interpretation of the fuzzy zones has changed several times during the exploration of the area. During the early exploration period, these fuzzy zones were believed diapir structures because of the rapid deposition and high temperature/high pressure in Yinggehai Basin. This began to be doubted when reprocessing of the data made some events apparent. After the discovery of the LD22-1 gas field in 1996, an evaluation well through the shallow fuzzy region on the high part of the structure indicated that the fuzzy zone in the shallow structure was not mudstone but a sandstone reservoir with high permeability. Thus, some experts believed that there were no diapirs in Yinggehai Basin.
Obviously, conventional P-wave data cannot solve the long-standing geologic questions posed by the often-encountered fuzzy zones and lithological false bright spots. Therefore, in 1998, China National Offshore Oil Corporation (CNOOC) and Geco-Prakla acquired a 132-km offshore 2D multicomponent seismic survey. A set of processing algorithms for offshore multicomponent seismic data, the first in China, was developed by CNOOC in collaboration with Tongji University and China Geology University (Wuhan). The results from this 2D multicomponent data set proved vastly superior to the conventional method. In 2001, the 4C processing system was used …