Tectonics and Sedimentation in Extensional Rifts

Abstract

Extensional rifts and their overlying sag basins host prolific hydrocarbon provinces in many parts of the world. This chapter reviews the development of rifts and the controls of tectonics on sedimentation patterns and hydrocarbon prospectivity. Rift tectonics exert the most important control on sedimentation and trap formation, and subsidence rate controls the geometry and facies of the rift fill. Rifting also controls the heat flow and burial history that determine the source rock maturation. Many rifts commence with evenly distributed small extensional faults when they are commonly characterized by closed drainage and continental sedimentation with localized lacustrine facies. As continental rifts develop, source rocks commonly accumulate in deep lakes, especially when the rifting is rapid, and organic shales are commonly located in the bottom third of the rift. Fault displacements increase and faults grow laterally to produce linked normal fault arrays. Marine deposits commonly replace the early rift continental deposition (e.g., North Sea) as the rift propagates to reach the world ocean system. Extremely prolific source rocks may be produced during rapid rifting in the marine phase, especially if half-graben depocenters are starved and oceanographic circulation is poor (e.g., Kimmeridge Clay Formation of northwestern Europe). As rifting wanes the rift fills, and fluvial sedimentation predominates, or in a passive margin, the basins enter into an extensional sag and/or postrift thermal sag phases when shallow-marine to deep-marine sediments infill and bury the former half grabens as sedimentation catches up and exceeds basin subsidence. Recent studies in western Australia, Brazil, and west Africa indicate that thick (unfaulted) sag basins can develop very rapidly above rifted continental crust. The subsidence is too rapid to be produced by normal thermal conductive cooling and is believed to be caused by stretching of the lower/middle crust with no observable faulting (depth-dependent stretching; Driscoll and Karner, 1998). The stretching process can produce a broad passive margin (100–500 km [62–311 mi]) where the continental crust is only 5 km (3.1 mi) thick ( = 7). The sag basin in southern Brazil reaches about 1 to 2 km (0.6–1.2 mi) in thickness and drapes over the earlier rifted blocks to produce some of the largest oil fields discovered in the last 30 yr (e.g., Lula with 20 to 30 billion bbl of oil in place). These fields are trapped in lacustrine algal-bacterial carbonates that pinch out onto the fault block crests. Many rift basins remain unexplored in remote or inaccessible areas, and new future hydrocarbon provinces are anticipated in the African, Southeast Asian, Arctic, and Atlantic margin rift basins.