Defining the northeastern boundary of the supergiant Maracaibo foreland basin, Venezuela

Abstract

Differences in styles of structure and sedimentation and the distribution of hydrocarbons across the 140-km-long and 1-km-wide Burro Negro fault zone (BNFZ) constrain its origin as a deeply-rooted, right-lateral strike-slip fault that formed a major Paleogene tectonic and paleogeographic boundary along the northeastern edge of the supergiant Maracaibo basin. Complex subsurface structures and the lack of high quality seismic data and deep wells have made it difficult for previous workers to determine whether the BNFZ truncates the northeastern extension of the giant Eocene oil reservoirs in the Lake Maracaibo area or whether these reservoirs extend northeast of the BNFZ. In Chapter 2, I integrate 2848 km of two dimensional (2D) seismic reflection data, 29 wells, gravity and magnetic data, and maps of outcrop geology to provide an improved structural interpretation of the BNFZ that can be used to assist future exploration programs in the region. Across the 1-km-wide BNFZ, the structural style seen on seismic lines changes abruptly from Eocene transtensional deformation in the Outer Maracaibo foreland basin to convergent deformation of the deepwater Inner Maracaibo foreland basin northeast of the BNFZ. I determined a right-lateral strike-slip fault character for the BNFZ that formed as a result of at least 7.2 km of Eocene right-lateral offset. Stratigraphic correlations using wells and seismic data spanning the BNFZ support the idea that giant Eocene oil reservoirs of the Lake Maracaibo area may continue towards the northeast into a complexly deformed Inner Maracaibo foreland basin consisting of Eocene deep-water sedimentary rocks. Late Eocene growth strata composed of deepmarine clastic rocks of the Agua Negra Group were deposited in piggyback basins in the Inner basin and accompanied Eocene thrusting and folding of the Lara nappes to the southeast In Chapter 3, I present a subsurface geological interpretation of the subsurface of the northern edge of the Maracaibo foreland basin using 988 km of seismic data, 17 wells and 14,700 km² of gravity and aeromagnetic data in the North Ambrosio area. Mapping of key surfaces in the basin that include the Eocene unconformity show that the primary structures in the subsurface are five, north to northeast-striking, right-lateral strike-slip faults of Eocene age (Icotea, East Urdaneta, West Urdaneta, San Ramón and La Concepción). On seismic data, these faults exhibit profiles typical of "positive flower zones", where the strike-slip fault converges from a 2 to 4-km-wide fault zone at the surface to a narrow zone at the top of basement. The higher levels of the faults are composed of en echelon anticlines that form excellent structural traps for oil and gas. The 180 km² Santa Rita pull-apart basin of Eocene age was identified at a left-step between the Icotea and East Urdaneta faults. Well logs show that Eocene sand bars were deposited in a tide-dominated deltaic system similar to that described by previous workers in areas to the south. I used temperature measurements from one well to calculate the depth of the oil window in the North Ambrosio area from 2.0 to 3.5 km and the gas window from 3.5 to 4.5 km. I also estimated the depth range of the "Golden Zone", or depth interval where oil and gas is most likely to be preserved. The predicted Golden Zone corresponds well with reported depths of production and shows in the North Ambrosio area. Gas potential for the North Ambrosio area could be determined form seismic and well data within a small area ~1800 km² representing about 3.6% of the area of the Maracaibo foreland basin. The interaction of northeast-striking faults, such as the Icotea, East Urdaneta, West Urdaneta, and La Concepción fault zones controlled sedimentation during the Eocene foreland phase of the Maracaibo basin. Elongate Eocene sand bodies deposited in a tide-dominated deltaic system represent the main target for further exploration in the North Ambrosio area. The petroleum system is characterized by a strong foreland subsidence during Paleocene-Eocene times that buried Late Cretaceous source rocks of the La Luna Formation to depths of 4000 m below the surface. The source rocks reached gas generation window during two separate periods: middle-late Eocene and Miocene-Pliocene