Cretaceous-Cenozoic tectonostratigraphic evolution and hydrocarbon prospectivity of the Sandino forearc basin, offshore Nicaragua

Date

2021-12

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Abstract

The Central American forearc basin overlies the actively subducting Cocos plate, extends 1,730 km along the length of the Middle America trench, and is adjacent to the active Central American volcanic arc along the western edge of the Caribbean plate. The Central American forearc basin consists of three segments: 1) the 800-km-long northern segment is submarine, contains up to 10 km of sedimentary rocks, undeformed, and extends from offshore northern Guatemala to Sandino forearc basin (SFB) offshore of Nicaragua; 2) the 630-km-long SFB is submarine, is up to 8 km thick, moderately deformed along the offshore Nicaraguan margin; and 3) the 300-km-long southern segment in Costa Rica includes the onshore Tempisque and Terraba deformed forearc belts that contain up to 5 km of sedimentary rocks and were partially-inverted by shallow subduction of the Cocos Ridge. To better understand the Cretaceous-Cenozoic tectonostratigraphic evolution and hydrocarbon prospectivity of the SFB I integrate and interpret ~5,000 line-km of 2D PSTM newly acquired and reprocessed seismic data in 2015, associated AVA angle stacks, an offshore well log, velocity data, biostratigraphic data from wells, bottom hole temperature, and a compilation of previously published onshore and offshore geologic data. Chapter 1 summarizes my educational background, the data that I used for this study, and a history of how I developed this MS project on the SFB. In Chapter 2, I construct four regional cross-sections across the 630-km-long SFB to show the structural transition between the northern, less deformed late Cretaceous to Recent forearc segment in Guatemala to the onland Tempisque and Terraba basins in Costa Rica that were inverted by shallow subduction of the Cocos Ridge from the Late Miocene to Recent. The structural characteristics of the SFB includes: 1) the presence of two fold trends within the forearc basin that is constrained from deep-penetration seismic reflection data as a Late Cretaceous-Eocene forearc rift that was structurally inverted as a popup block from the Oligocene to Recent; 2) the fold axes that derform the offshore forearc basin converge near the western coast of the Nicoya Peninsula and are colinear with onland folds and thrusts; 3) fold amplitudes increase from northwest (100 m) to the southeastern offshore area (1500 m) and onto the Nicoya Peninsula (2500 m) where the forearc basin is in ; 4) depth to crystalline basement of the Sandino forearc basin shallows from northwest to southeast from its deepest area of 8 km in the Guatemalan forearc basin, to 6 km in the Nicaraguan offshore area, to 700-20 m above sea level on the Nicoya Peninsula with up to 1 km of sedimentary, to 10-500 m above sea level with 4 km of thrust-imbricated, Oligocene to Pliocene sedimentary rocks within the Terraba fold-thrust belt. Ages of unconformities along the length of the SFB show an Oligocene to recent folding event that was initially triggered by subduction of bathymetric highs and maintained subduction of the Cocos Ridge and low-angle subduction of the 150-km-wide seamount province that forms the northeast flank of the Cocos Ridge. In Chapter 3, I apply the structural information from the SFB described in Chapter 2 to elucidate the depositional history, seismic facies, patterns of shelf progradation and clastic infilling of the SFB, and in turn apply these observations to assess hydrocarbon prospectivity. The depositional history of the SFB can be subdivided into a 50-km-wide Paleocene-Eocene extensional forearc basin filled with up to 6 km of deep-marine turbidites and containing two source rock intervals. Oligocene to recent inversion deformed the normal-fault-bounded earlier basin into a folded and thrusted, inner shelf – outer shelf depositional environment (150 - 30 m water depth) with greater shortening to the southeast. The post-folding filling of the SFB was from southeast to northwest as recorded by the geometry of clinoforms at nine localities. Basin modeling predicts that Type II and III Cretaceous and Eocene source rocks expelled hydrocarbons during the Eocene to present. A summary of observed, direct hydrocarbon indicators (DHIs) support these model predictions with amplitude increases across normal fault planes and along the crests and flanks of anticlines. Additionally, amplitude versus offset analysis of near to far-angle stacks indicates significant Class II AVO response with concentrated up-dip accumulations of a generative hydrocarbon system.

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Keywords

Structure, Geology, Nicaragua, Sandino Forearc Basin, Stratigraphy, Basin Modeling, Seismic Interpretation

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