The Study on How to Divide System Tracts Using Variety of Geological Data at Different Scales
Relative to the marine strata, the division of the continental sequence system tracts is difficult because of its complexity. Different scholars advocate different division schemes, and we are difficult to certain the interface position of system tracts. Combined with the division of the system tracts of the second Member of Dongying Formation in Bozhong sag of Bohai area, this paper puts forward the method using variety of geological data at different scales to distinguish between lowstand system tracts, transgressive system tracts and highstand system tracts.
First of all, we do research on the paleontological information of the wells in large scale: In the lowstand system tracts, the relative content of sporopollen of coastal swamp is dominant; In the transgressive system tracts, the sporopollen of pinaceae is dominant, because at this time only the sporopollen of pinaceae with air sac could fly to the well areas; In the highstand system tracts, the algae is dominant in the upper of the well, while in the lower the sporopollen of coastal swamp is dominant. So, through the analysis of these three types of paleontology, we could determine the approximate depth range of different system tracts.
Then, using the seismic data, we make relative lake-level change curve and do analysis on seismic response characteristics: In the lowstand system tracts, a steady upward trend is obvious in the lake-level changes, seismic shows the reflection characteristics of wedge progradation; In the transgressive system tracts, the lake-level rises quickly to the maximum flooding surface, and imbricate progradation in seismic; In the highstand system tracts, a steady downward trend is obvious in the lake-level changes, seismic shows the reflection characteristics of “S” progradation or skew progradation. Therefore, we can further identify the location of the interface position of different system tracts.
Finally, based on the frame structures set up above , we do study on the characteristics of lithology, electric property and the changes of TOC in wells to accurately divide system tracts in small scale.
The results of actual sand distribution of different system tracts also confirm the correctness of this method. This method is also effective on other area in Bohai and plays a very active role in petroleum exploration, particularly for the exploration of subtle traps.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California