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Tectono-sedimentary Keys to Understanding the Functionality of Petroleum Systems in Rift Basins: an Example from the Cameros Basin (N Spain)

Abstract

The most successful petroleum systems generally developed in rift settings. The sedimentary infill of rift basins makes them suitable to contain source, reservoir and seal rocks, and their high burial rates favor source rock maturation. This work summarizes a multidisciplinary approach to determine the tectono-sedimentary factors underlying development, evolution and success of the petroleum system elements in a rift basin from N Spain. The Lower Cretaceous Cameros Basin is an inverted intraplate extensional basin within the Mesozoic Iberian Rift System, mainly filled with continental and coastal deposits. Different evidence has proved that a petroleum system was activated during its evolution. The combination of detailed studies on sedimentology, tectonics, diagenesis, organic geochemistry and paleontology of the Cameros Basin infill has led to the distinction of general tectono-sedimentary styles, which have implications on the petroleum system analysis of the basin: Style 1, localized subsidence: sedimentation took place in small and isolated half-grabens controlled by individual extensional faults and mainly filled with alluvial-lacustrine deposits. Subsidence was intermittent and different in each half-graben, producing strongly variable thickness and facies distribution. This style did not favor the development of any of the elements of the Cameros petroleum system. Style 2, generalized subsidence: sedimentation took place in a broader area of the basin. Style 2a: rapid subsidence, combined with periods of high sea level, was compensated with high sedimentation rates, causing the development of shallow environments (saline water bodies, coastal wetlands, freshwater lakes) which accumulated organic matter-rich sediments. Despite their shallowness, preservation of the organic matter was often possible due to high sedimentation rates and/or water stratification, providing potential source rocks. Style 2b: relatively slow subsidence together with periods of lower sea level favored the development of broad fluvial systems mainly composed of well-connected sandstone bodies, which were potential reservoir rocks. This work shows that understanding both the general tectono-sedimentary setting and the sedimentological peculiarities of each sedimentary stage of the basin infill is crucial to establish the elements of a petroleum system, as well as their appropriateness to constitute successful source rocks and/or reservoirs.