Quality, hydrocarbon generation, and expulsion of the Eocene Enping Formation source rocks in the Wenchang Depression, western Pearl River Mouth Basin, South China Sea

Jinheng Li,Guangjie Xie,Ziyi Wang,Dongxia Chen,Fuwei Wang,Wenhao Liao,Lu Chang,Xuebin Shi

doi:10.1177/0144598720915583

Abstract

Recently, increasing numbers of oil and gas reservoirs have been discovered in the Wenchang Depression, western Pearl River Mouth Basin, South China Sea, revealing prospects for hydrocarbon exploration. The Enping Formation (E3e) is a key target layer for the development of source rocks. However, previous work has only focused on lacustrine swamp source rocks of E3e in the Wenchang A Sag, without a systematic study of shallow lacustrine source rocks. In this study, the quality of E3e shallow lacustrine source rocks is reevaluated, and the hydrocarbon generation and expulsion characteristics are analyzed using relevant geological data and constructing a conceptual model. The results show that the E3e2 source rocks have greater thickness (50–600 m) and similar organic matter abundance (0.5–2.5%) compared with the E3e1 source rocks (50–500 m and 0.5–2.5%). On the whole, the E3e source rocks were deposited in the continental environment and are dominated by Type II and Type III kerogen. Meanwhile, the E3e source rocks of the Wenchang A Sag are in the stage of mature to over mature, while those of the Wenchang B Sag are in the stage of low mature. Vertically, the hydrocarbon generation potential of the E3e2 source rocks is greater than E3e1. Also, the cumulative hydrocarbon production of steep slope in the Wenchang A Sag is larger than that in the Wenchang B Sag. In addition, the corresponding vitrinite reflectances of hydrocarbon expulsion threshold and peak are 0.72 and 0.96%, respectively. Horizontally, four hydrocarbon generation and expulsion centers were mainly concentrated in different subsags of the Wenchang A and B Sags for E3e. The maximum values of hydrocarbon generation and expulsion intensity for E3e1 are 1500 × 104 t/km2 and 1000 × 104 t/km2, respectively, while those for E3e2 are 1800 × 104 t/km2 and 1200 × 104 t/km2, respectively, with the expulsion efficiency of 75%.

Highlights

The Pearl River Mouth Basin is a Cenozoic hydrocarbon-rich basin in the northern part of South China Sea (Gong and Li, 1997; You et al, 2018)
Some geologists have suggested that only a small area of lacustrine swamp source rock from E3e is distributed in the northern part of the Wenchang A Sag, while the central and southern parts are dominated by shallow lacustrine source rocks (Cheng et al, 2013a)
The total organic carbon (TOC) content and the rock pyrolysis hydrocarbon potential (Pg 1⁄4 S1 þ S2), are indexes commonly used for evaluating the abundance of organic matter

Summary

Introduction

The Pearl River Mouth Basin is a Cenozoic hydrocarbon-rich basin in the northern part of South China Sea (Gong and Li, 1997; You et al, 2018). In this basin, the Wenchang Depression is an important part of the Zhu III Superdepression. Other studies have shown that lacustrine swamp source rocks of E3e provide oil and gas for the formation of reservoirs in the Wenchang A Sag (Huang et al, 2007). The characteristics of crude oil drilled in the Wenchang 10–8 structure recently are quite different from those of E2w, implying the possibility of generating hydrocarbons of E3e in the Wenchang A Sag (Lu et al, 2016)

Methods

Results

Conclusion