Basin evolution, configuration styles, and hydrocarbon accumulation of the South Atlantic conjugate margins

Zhixin Wen,Chengpeng Song,Zhengjun He,Zhaoming Wang,Shu Jiang

doi:10.1177/0144598719840751

Abstract

The basins of the South Atlantic passive margins are filled with early rifting stage lacustrine sediments (Barremian, 129–125 Ma), transitional lacustrine and marine sediments (Aptian, 125–113 Ma), and drift stage marine sediments since early Cretaceous (Albian, 113 Ma). The South Atlantic margins can be divided into three segments by the Rio Grande Fracture Zone and the Ascension Fracture Zone according to variations in the basin evolution history and configuration style. The lacustrine shale and marine shale source rocks are developed in the rift stage and drift (post-rift) stage in the South Atlantic passive margins, respectively. The southern segment of the margins is dominated by the lacustrine sedimentary filling in the rifted stage overlain by a thin marine sag system as a regional seal, where the hydrocarbons are mainly accumulated in the structural-stratigraphic lacustrine reservoirs formed in the rift stage. The middle segment developed salty rift-sag-type basins with rift and sag systems and with salt deposited in the transitional intercontinental rift stage, where the lacustrine shale in the lower part of the rifted lacustrine sequence and the marine shale in the lower part of the sag sequence formed in the marine post-rift stage are high-quality source rocks. This segment in the middle is mainly dominated by pre-salt lacustrine carbonate and post-salt marine turbidite plays. The northern segment is characterized by sag-type basins with a narrowly and locally distributed rifted lacustrine system and its overlying widely distributed thick marine sag systems. Gravity-flow (mostly turbidite) marine sandstones as good reservoirs were extensively developed in the sag stage due to the narrow shelf and steep slope. The post-rift marine shales in the lower part of the sag sequence are the main source rocks in the northern segment and the hydrocarbons generated from these source rocks directly migrated to and accumulated in the deep marine turbidite sandstones in the same sag sequence formed in the drift stage. From southern segment to northern segment, source rocks and hydrocarbon accumulations tend to occur in the stratigraphically higher formations. The hydrocarbon accumulations in the southern segment are mainly distributed in the rifted lacustrine sequence while that in the northern segment primarily occur in the post-rift marine sequence.

Highlights

Passive continental margin basins have been confirmed to have the most abundant oil and gas resources, 304 out of 877 giant oil and gas fields with 2P recoverable reserves of greater than 500 MMboe in the world are located in passive continental margin tectonic setting (Mann et al, 2003)
25 passive continental margin basins (Figure 1) have been explored, 80% basins are in deepwater setting; currently, a total of 111 giant oil and gas fields with recoverable reserve of 255,878 MMboe have been discovered along the South Atlantic Margins (Figure 1), accounting for more than 70% of the total discovered oil and gas equivalents in the passive continental margins all over the world
This paper systematically studies the formation and evolution of the prototype basins and their paleogeography in the conjugated passive continental margin basins along the coasts of the South Atlantic based on the plate tectonic evolution and paleogeographic reconstruction, basin filling styles in each segment are characterized using updated regional geology data, seismic reflection data, recent drilling data, and new oil and gas discoveries

Summary

Introduction

Passive continental margin basins have been confirmed to have the most abundant oil and gas resources, 304 out of 877 giant oil and gas fields with 2P recoverable reserves of greater than 500 MMboe in the world are located in passive continental margin tectonic setting (Mann et al, 2003). The unique structure of the sag-type passive continental margin basins in the northern segment (Figure 5) is characterized by the following features: (1) the rift system is small comparing to the post-rift system and only occurs in limited basins, such as the Saira Basin, which is because the early rift was a narrow strike-slip pull-apart rift basin controlled by steep faults; (2) the sag system is thick (>5000 m) and the gravity-flow sand bodies are well developed since the sediments can be transported down from the narrow shelf and steep slope.

Results

Conclusion