Abstract The Guyana Basin formed during the Jurassic opening of the North Atlantic. The basin margins vary in tectonic origin and include the passive extensional volcanic margin of the Demerara Plateau in Suriname, an oblique extensional margin inboard at the Guyana–Suriname border, a transform margin parallel to the shelf in NW Guyana, and an ocean–ocean margin to the NE, which morphed from transform to oblique extension. Plate reconstructions suggest rifting and early seafloor spreading began with NNW/SSE extension ( c. 190–160 Ma) but relative plate motion later changed to NW/SE. The fraction of magmatic basin floor decreases westwards and the transition from continental to oceanic crust narrows from 200 km in Suriname to less than 50 km in Guyana. The geometry and position of the onshore Takutu Graben suggest it formed a failed arm of a Jurassic triple junction that likely captured the Berbice river during post-rift subsidence and funneled sediment into the Guyana Basin. Berriasian to Aptian shortening caused crustal-scale folds and thrusts in the NE margin of the basin along with minor inversions of basin margin and basin-segmenting faults. Stratigraphically trapped Liza trend hydrocarbon discoveries are located outboard of inverted basement faults, suggesting a link between transform margin structure and their formation.

Abstract Multiple geologic elements combine in the presalt section of deep-water Santos Basin, forming a quite unique exploration play: prolific and mature source rocks are present, synrift structures include huge intrabasinal highs, and the overlying evaporite seal extends throughout most of the area. Significant uncertainties related to reservoir presence and deliverability, which still persist as the key risks on this emerging presalt play, have been progressively reduced throughout a continued drilling campaign, which started in 2006. The most prominent and extensive intrabasinal high in the region is the Outer High of Santos Basin, a regional basement structure that forms a 12,000 km 2 (4633 mi 2 ) four-way closure at the Aptian level. The geologic historyof the Outer High involves multiepisodic uplift and erosion of a series of rift fault-block shoulders during the Barremian. At that time, regional uplift resulted from a failed sea-floor spreading process that emplaced proto-oceanic crust in the southern Santos Basin. Concurrently, magmatic underplating is postulated as the mechanism responsible for locally thickening the crust and isostatically holding the Outer High as a present-day positive feature above its surroundings. Because of the extreme extension of the continental crust in Santos Basin, zones of deep crustal, or even upper mantle exhumation, are also expected near the transition from continental to oceanic crust. Before continental breakup, the Outer High was roughly located 200 km(124 mi) away from both the African and Brazilian hinge lines. This distal setting, coupled with a positive relief, limited siliciclastic input from the margins. The presence of a long-lived paleohigh, in such a clastic-starved environment, favored the development of a broad carbonate platform, during the Lower Aptian. Tectonically controlled water-level fluctuations affected the evolving platform, serving an important function on reservoir facies development. The Outer High has been the core region of a deep-water presalt exploration outbreak, after a pioneering drilling campaign that started around the middle of this decade. The hydrocarbon potential of this vast frontier area is yet to be fully unraveled.

Abstract Reservoirs at Pony-Knotty Head Field consist of stacked, middle Miocene (Serravallian) turbidites deposited as high-frequency low-stand successions within an increasingly ponded basin. Depositional elements include: (1) high to moderate permeability channel axes, channel margins, channelized lobes, and amalgamated lobes; and (2) those having low-permeability, such as marginal to distal lobes, levee-overbank debrites, slumped mudsheterolithics, and pelagic/hemipelagic muds. Fluid pressure data demonstrate that the Pony-Knotty Head Field is segmented into pressure compartments at multiple scales. Although the field is a low-dip, faulted, four-way turtle structure, interpreted faults are neither long enough nor have sufficient throw to segment reservoirs into observed pressure cells. Analyses of individual reservoir units indicate that variations in fluid potential are often greater vertically within wells than laterally between wells. This pattern indicates that at least some segmentation at this scale is due to low-dip stratigraphic barriers between depositional elements rather than to steeply dipping barriers, such as faults. At the field scale, both fluid pressures and depositional elements change vertically. Excess pressure was used to help define compartments at Pony-Knotty Head Field. (“Excess pressure” is the difference between pressure measured in a well and pressure calculated using a datum with an expected fluid gradient.) The deepest reservoirs have the lowest excess pressures. They are dominated by laterally continuous, unconfined depositional elements that bled excess pressure laterally. Progressively shallower reservoirs have progressively higher excess pressures in progressively more confined depositional elements. Between reservoirs of different depths and ages, stratigraphic complexity increased with time as increasing structural confinement of the depocenter above mobile salt drove stratigraphic evolution from a lobe-dominated system to a channelized lobe and levee-channel complex system. We propose that compartmentalization at this scale results directly from stratigraphic responses to the structural evolution of depocenters.

Abstract In the sub-Alpine chains of Haut Provence, SE France, a very well-exposed Mesozoic sequence showing rapid thickness and facies changes associated with Jurassic and Cretaceous extension on the margin of the Ligurian Tethys has been deformed by ‘Alpine’ compression which occurred from the Late Cretaceous to the Pliocene. Although the geology has been very well known for decades, aspects of the structure remain enigmatic and cannot be explained by either Mesozoic extension or Alpine shortening alone. We infer that some deformation resulted from salt tectonics. A completely overturned, highly condensed Jurassic section near Barles village resembles the elevated roof of a Triassic salt body in a deep-marine basin. This carapace became overturned as a flap in the Middle Jurassic when salt broke out at the seafloor and overran the inverted flap as an allochthonous extrusion, comparable to those in the deepwater Gulf of Mexico or Angola. Later, Alpine compression exploited the weakness of the salt sheet as the Digne Thrust moved over the inverted flap. Although the flap is in the footwall of the thrust, evidence of soft-sediment deformation and other anomalous structures within the flap suggest that it did not originate as an overturned footwall syncline.

Abstract Three-dimensional (3D) poststack depth migration of a North Sea survey both improved the subsurface image and enhanced the structural interpretability of the 3D data volume. Prior poststack time migration of the 3D volume had produced unsatisfactory results in the vicinity of a graben that runs north-south through the center of the survey area. The complex structural geometry of the graben made building an accurate velocity model the most critical component of the depth migration. We constrained and revised the velocity model by iterating structural analysis, kinematic restoration, and seismic interpretation with successive intermediate migrations. In addition to constraining both the present-day structural geometry and the resulting velocity field, the structural analysis revealed the structural and geometric evolution of the graben. The resulting depth-migrated images are superior to existing time-migrated images. Depth migration increased the resolution, focus, and clarity of seismic reflectors throughout the volume. Over-migration “smiles” in the time-migrated data are minimized. Most importantly, subsalt faults are crisper and subsalt structure is better defined. Structures imaged by the depth-migrated data more accurately reflect true geologic structures and improve interpretation accuracy and reliability.