f Analysis of the Poly-Phase Plate-Margin Processes of Primary Influence on Jurassic and Cretaceous Basin Development in Guyana-Demerara and Conjugate Guinea Plateau Basins

Demerara and Guinea plateaus are plate-margin conjugate structures in South America and Africa, respectively. They have experienced at least two phases of non-orthogonal extension punctuated by a phase of transpressional deformation in the Early Cretaceous prior to the opened connection between Central and Equatorial Atlantic was established. Jurassic-age rifting formed the Central Atlantic, and the Plateaus were located at the leading edge of the plate boundaries separating the incipient North American, South American, and African Plates. This rifting period is recorded by the tectonostratigraphic relationships observed at the western edge of the Plateaus. During rifting, the outer parts of the Plateaus experienced a phase of transtensional deformation that preceded the establishment of shallow-water carbonate deposition during Jurassic and Early Cretaceous times. Demerara and Guinea Plateaus became a structural accommodation zone between Central and Equatorial Atlantic opening in Early Cretaceous times. Poles of rotation necessitate transpressional deformation of both Plateaus that accommodated differential plate-margin extension with most compressional deformation accommodated by the South American plate-margin. The Albian unconformity regionally correlated on seismic lines across Demerara, Guinea and Florida is interpreted to be evidence for a contemporaneous exhumation of Jurassic and Early Cretaceous carbonate facies. Oceanic spreading has progressed, and separation of the plateaus continued with the marine connection between Central and Equatorial Atlantic established during Cenomanian times. Different parts of the Plateaus responded to the stress regimes differentially during these times.

In this work, we consider stratigraphic relationships and propose a spatially asymmetric structural evolution model that accounts for empirical seismic, gravity, and well data observations.

Recent authors postulate a volcanic origin for the Demerara and Guinea plateaus. Voluminous extrusive Jurassic magmatism from the Sierra Leone hot-spot is supported by the seismic observation of SDRs on deep seismic profiles. These SDRs are interpreted by recent authors to be of volcanic origin and associated with extrusive basaltic lava flows during the CAMP event in the Jurassic. In addition, the recovery from two sites of three dredged basalt samples of the Early Jurassic age confirms the presence of extrusive volcanism. The sites are located on the northeastern side of the Demerara Plateau, with local gradients up to 60 deg. The origin of these fault-related slopes we believe to be more consistent with high-angle strike-slip deformation. Additional evidence for rift-related magmatism of a younger age in the area comes from Demerara Plateau well FG2–1. Dating of recovered basalt samples of 125 MA and 120 MA have been linked recently to a sequence of SDRs.

The popularity of SDR interpretations from the Demerara Plateau are based on the similarity of the seismic character of deep reflectors from Pelotas basin regional 2D-seismic lines.

These seismic observations are further linked to the interpretations of SDRs on the Guinea Plateau, where existing seismic interpretations are equivocal, especially beneath the Top Jurassic reflector. Hence, we contend that the model-driven interpretations of SDR presence and significance are subject to the variable quality of the seismic data used. Could the significance of a primary SDR origin for the plateau areas be better explained by the structural solution proposed by this paper? If so, then heat-flow, maturity models, and basin models would require important modification. Recent drilling offshore Guyana has now shown mixed results. Some wells illustrate charge challenges as well as the predictability of the type of hydrocarbon that has ranged from immature heavy sulfur oil to gas condensate. The depositional environment plays a critical role in the type of organic matter that is deposited. The type of organic matter (oil or gas prone) and thermal maturity can be better constrained by basement development history in light of tectonic dynamics. Better understanding across the basin is needed for which prospects have the best volume and quality of charge to reservoirs.

Recent gravity, seismic data, well penetration records and dating of recovered material offer us an opportunity to fully integrate these data and to consider alternative models for the formation of the Demerara and Guinea marginal plateaus. We believe that the poly-phase structural origin of basement evolution better describes the primary origin and consequent subsidence patterns observed in time and space. The implication for this new structural model is critical to better explain temporal accommodation space development (subsidence patterns and driving mechanisms) for key Jurassic and Cretaceous SR and reservoir facies development, distribution, and sequence preservation.