2019 AAPG Annual Convention and Exhibition:

Datapages, Inc.Print this page

Gravity-Driven Deep-Water Fold-and-Thrust Belts Along the Continental Passive Margin of East Africa and Their Control on Petroleum Play

Abstract

Passive-margin deep-water fold-and-thrust belts (DWFTBs) are geological structures that have been the focus of many studies in the recent decades, since many oil and gas fields have been discovered in traps associated with DWFTBs. Advance in seismic processing and acquisition of new data sets have revealed two DWFTBs in the offshore Rovuma Basinand (SE Tanzania and NE Mozambique) the offshore Lamu Basin (NE Tanzania and SE Kenya), both of which originated over an shale detachment due to a mainly gravity-spreading mechanism. Recently, a prolific natural gas fairway has been proved to exist in the offshore Rovuma Basin, trapped in a system of gravity-driven, extensional-compressional deep-water fold-and thrust system. However, the petroleum play of the DWFTB in the offshore Lamu Basin receives less attention. In this study, we propose a possible model for the petroleum play in the DWFTB in the offshore Lamu Basin based on an integrated analysis of seismic, geochemical and geology data. Our results indicate that the DWFTB in the offshore Lamu Basin originates over an lower Cretaceous shale detachment and extends for more than 450 kilometers along the continental passive margin, the scale of which is much larger than that in the offshore Rovuma Basin that originats over an Eocene shale detachment and extends for about 120 kilometers. High-quality seismic data, available in depth domains, allows us quantify the evolution of thrust anticlines exhibiting growth using the area-depth-strain method in the DWFTBs in the continental passive margin of East Africa, which indicates that the DWFTB in the offshore Lamu Basin became active as early as the late Cretaceous period to the early Miocene period with a constant sedimentation rate, while the DWFTB in the offshore Rovuma Basin became active during the Oligocene period to the early Miocene period. Organic geochemistry and basin modeling show that the widespread lower Jurassic mudstone is the major source rock in the DWFTBs, and the peaks of oil generation and gas generation are the Cretaceous period and the Cenozoic period, respectively. Thus, we suggest that oil generated from the source rocks migrated along faults as conduits and accumulated into the Cretaceous traps during late Cretaceous period, and gas migrated and accumulated into the Cenozoic traps during the Cenozoic period. Conversely, oil of the DWFTB in the offshore Rovuma Basin is insufficient due to the absence of the oil migration conduits and traps, since the DWFTB had been initially formed from the Oligocene period instead of the period of the peak of oil generation.