Genetic Relation Between Volcanic Activity and High-Quality Source Rocks of the Wenchang Formation in the Zhu 1 Depression: New Ideas on Source-Rock Evaluation in the Pearl River Mouth Basin of the South China Sea

Abstract

The results of the present oil and gas exploration show that the Zhu I Depression mainly developed two sets of lacustrine facies source rocks: the Eocene Wenchang Formation and the Enping Formation. This paper analyzes in depth the depositional setting of the two sets of source rocks in the Zhu I Depression. Starting with the genetic mechanism of source rocks, the paper explores the depositional setting which is favorable for the development of high-quality source rocks. Through this approach, we obtained new ideas on the evaluation of source rocks in the Pearl River Mouth Basin of the South China Sea. This paper describes the provenance analysis of two sets of source rocks from the Wenchang and Enping Formations in the Zhu I Depression. By using drilling, logging, and core data in parallel, we find that there is a good relation between the spatiotemporal coupling of the volcanic ash and tephra that commonly occurs in the Wenchang Formation stratum and the high-quality source rocks of the Wenchang Formation. Our geochemical analyses reveal that the ash-rich high-quality source rocks of the Wenchang Formation are rich in Fe by 12% more than those of the Enping Formation. Iron plays an important role in the growth and propagation of planktonic algae. Other elements that can cause water eutrophication show a similar distribution. These results demonstrate that volcanic activity brought a large amount of Fe into the lake basin via volcanic ash and tephra, caused eutrophication of water, and led to marine algal blooms and vigorous primary productivity. This, in turn, provided high-quality source rocks in the Wenchang Formation with abundant organic matter. This paper establishes the genetic relation between the high-quality source rocks, the synchronous or previous volcanic activity, and the relevant eutrophication of the water system. The volcanic activity provided the material basis for the development high-quality source rocks. Studying volcanic activity and detecting volcanic source content is an important means for evaluating the source rocks when there are only a few wells, such as in the Pearl River Mouth Basin.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015