关键词: 阳江凹陷, 古湖泊环境, 湖盆充填类型, 优质烃源岩, 发育模式

Abstract: In Pearl River Mouth Basin, Yangjiang sag is the one that has achieved an important breakthrough in oil-gas exploration in recent years, but the formation conditions of high-quality source rocks in the sag are still unclear. Comprehensively using drilling organic and inorganic geochemistry and seismic data, the paper analyzes the characteristics of source rocks of Eocene Wenchang Formation in Yangjiang sag, Pearl River Mouth Basin, explores the paleolacustrine environment and lake-basin filling type in the study area, and establishes the development pattern of high-quality source rocks. The findings shows that:(1) Wenchang Formation in Yangjiang sag develops thick layer semi-deep lacustrine source rocks with high organic matter abundance, dominated by the type I-II₁ organic matters. The lower Member of Wenchang Formation has strong lateral heterogeneity and an average TOC of 2.2%-3.5%. The upper Member of Wenchang Formation has strong homogeneity and an average TOC of 2.3%-2.6%, which is mainly composed of organic matters originated from phytoplankton. (2) In Yangjiang sag, the coupling of small- and medium-sized provenance inputs and rifting activities has created an accommodation space for the development of semi-deep lacustrine facies, warm and humid climate conditions are the prerequisite for the blooming of aquatic phytoplankton, volcanic eruptions can improve the initial productivity locally in the lake basin, and the strongly reducing water environment is conducive to the preservation of organic matters. (3) Based on the coupling effects of different rifting activities, sedimentary input, paleoclimate, and paleoenvironment, three lake-basin filling types are classified. In particular, the balanced filling type develops a fluctuating deep lacustrine lithofacies combination of thick layer mudstone and thin layer sandstone in the center sag, the lake-basin underfill type is characterized by a whole basin covered with thick layer homogeneous mudstone, and the lake-basin overfill type basically does not develop semi-deep lacustrine source rocks. (4) The balanced lake-basin filling type has a high-quality source rock development pattern controlled by both productivity and preservation conditions. The lake has high initial productivity and is generally in a medium reducing environment, thus forming good heterogeneous source rocks with high organic matter abundance. The lake-basin underfill type has a high-quality source rock development pattern mainly controlled by preservation conditions, of which the lake has medium productivity but strong reducibility of water body, thus forming homogeneous source rocks with medium organic matter abundance. The comprehensive analysis shows that the superimposed high-quality source rocks of balanced lake-basin filling type and lake-basin underfill type are developed in the Wenchang Formation of Enping 21 subsag, Enping 20 subsag and Enping 27 subsag, which has high hydrocarbon generation potential. These findings have a certain guiding significance for the prediction of source rocks in Yangjiang sag and sags in other new areas in Pearl River Mouth Basin.

Key words: Yangjiang sag, paleolacustrine environment, lake-basin filling type, high-quality source rock, development pattern