f Cretaceous-Cenozoic Tectonostratigraphic Evolution and Hydrocarbon Prospectivity of the Sandino Forearc Basin, Offshore Nicaragua

In map view, the Central American forearc basin extends 1730 km along the Middle America trench and the western edge of the Caribbean plate and overlies the actively subducting Cocos plate. The entire forearc basin can be divided into three, structurally and stratigraphically distinct segments: 1) the 800-km-long northern segment that is submarine, undeformed, and extends from northern Guatemala to northern Nicaragua; 2) the 630-km-long, central segment of the Sandino forearc basin (SFB) is submarine, moderately deformed, and extends along the Nicaraguan margin to northernmost Costa Rica; and 3) the 300-km-long southern segment in Costa Rica is partially inverted by shallow subduction of the Cocos Ridge and includes the onshore Tempisque and Terraba deformed forearc belts of Costa Rica.

This study uses 5,582 line-km of 2D seismic reflection data with an average line spacing of 10 km tied to 6 wells to describe the central segment of the forearc of Nicaragua and northernmost Costa Rica. The offshore study area covers the forearc area from the shelf edge of Nicaragua to the slope of the Middle America trench in the dip direction and from the Gulf of Fonseca in northern Nicaragua to the northwestern margin of the Nicoya Peninsula in northern Costa Rica in the strike direction.

Our seismic and well data define three distinctive structural domains within the SFB: 1) the northwestern outer arc high that was uplifted during the Eocene and subaerially exposed during the early Miocene that resulted in significant erosion into the Eocene and lower section; structural inversion and persistent reactivation of the forearc high throughout the Miocene resulted in syn-depositional wedging onto the northwestern forearc high and extensional faults at the crest of the structure; 2) the forearc high bounding the central basin of the SFB was active during the Late Cretaceous-late Miocene and buried from late Miocene-Present as the SFB was overfilled and began to spill its clastic sediments into the Middle America trench; thick-skinned structural inversion of earlier normal faults of the Paleogene forearc depocenter and adjacent forearc high begins in the late Miocene and subjected the forearc high and main depocenter of the SFB to its most intensive periods of folding and erosion from the Pliocene to the present-day; and 3) the southwestern part of the SFB is significantly eroded with the Cretaceous oceanic plateau and island arc basement rocks of the Nicoya and Osa Peninsulas of Costa Rica representing the exhumed basement of the inverted SFB. Using 2D seismic data, this study maps the anticlinal trace of at 12, northwest-trending, offshore fold axes which result from Miocene to Recent inversion of Paleogene normal faults. This interpretation differs from the previous interpretations of these structures as transpressional flower structures that formed during Eocene time. This partially inverted, submarine fold-thrust belt is inferred to be an incipient stage in the structural evolution of the Central American forearc basin that is seen in its more advanced stage in the topographically-elevated and inverted forearc basins of southern Costa Rica.

Within this regional structural framework, the 2D seismic and well database were used to better understand the stratigraphy and hydrocarbon prospectivity of the SFB. We mapped the Miocene-Recent shelf rollover point of eight clinoforms to understand this period of basin infilling of the SFB that led to its eventual overfilling and spillover into the Middle America trench. Generally, shelf margins of the SFB trend to the northwest and prograde seaward to the southwest towards the forearc high to transversely fill the SFB. This study identifies an orthogonal Pliocene shift in shelf margin orientation that prograde to the northwest, axially filling the SFB.

We infer this shift in infilling is a response to the uplift and erosion of the more advanced inversion area of the Nicoya Peninsula that bounds the SFB to the southeast. The majority of the older Miocene, southwest-prograding SFB clinoforms are sigmoidal and shingled indicative of a low energy depositional environment. The younger northeast-prograding Pliocene shelf margins contain oblique clinoforms. These deposits occur within the shallow Pliocene section and may form favorable reservoir intervals but are too shallow to have hydrocarbon prospectivity.

Petroleum systems modeling was applied to the northwest and southeast deeper, depocenters of the SFB and created a basin model using the Corvina-2 wellbore and three pseudowell locations. This study improves on a previous basin model by previous workers that was based on a single seismic reflection line with more limited depth penetration. We also used lower heat flow than the previous basin model of 31 mW/m2 which is based on improved constraints from vitrinite reflectance calibration, bottom hole temperature, and BSR heat flow studies.

Results from our basin modeling indicates generation, migration, and accumulation from: 1) high-quality source rocks of the Coniacian-Upper Campanian Loma Chumico Formation from the deepest basin during the period of Eocene to middle Miocene; 2) from source rocks in the Brito formation during the period of early Miocene to present; and 3) from source rocks in the Masachapa formation during the period from late Miocene to Present. A new, untested conventional play type is inferred from AVO from the comparison of near-angle and far-angle stacks along with traditional DHI identification using the full-angle stacks. Uplift of the forearc high has led to updip horizontal migration along bedding planes with trapping mechanisms along crestal extensional faults that parallel the axis of the forearc high. Other play types include structural conformance within large anticlines.