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Granath, James (1Granath & Assoc. Consulting Geology, Highlands Ranch, Colorado, United States) | Rango, Rolf (2Takamaka Energy Limited, Singapore) | Emmet, Pete (3Brazos River Services, Spring, Texas, United States) | Ford, Colin (2Takamaka Energy Limited, Singapore) | Lambert, Robert (2Takamaka Energy Limited, Singapore) | Kasli, Michael (2Takamaka Energy Limited, Singapore)
ABSTRACT We have reprocessed, re-imaged, and interpreted 10000+ km of legacy 2D seismic data in the Seychelles, particularly in the western part of the Plateau. Seychelles data have been difficult to image, particularly for the Mesozoic section: volcanics are a major attenuator of low frequency signal, and a hard water bottom contributes to signal problems. Enhanced low frequency techniques were applied to improve the signal fidelity in the 4 to 20 Hz range, and to remove spectral notches of shallow geologic origin. These efforts have allowed a reasonable view of the structure of the Plateau to a depth equivalent to about 3.5 sec TWT, and permit a comparison of areas atop the Plateau to the south coast where the three 1980's Amoco wells were drilled. It is clear that the main Plateau area of the Seychelles (excluding the outlying territories) is comprised of several separate basins, each with similar Karoo, Cretaceous, and Cenozoic sections that relate to the East African and West Indian conjugate margins, but the basins each have nuanced tectono-stratigraphic histories. The previously recognized Correira Basin in the SE and the East and West South Coast Basins face the African conjugate margin; other unimaged ones complete the periphery of the Plateau. The interior of the Plateau is dominated by the Silhouette Basin to the west of the main islands and the Mahé Basin to the east. The co astal basins have harsh tectono-thermal histories comparable to other continental margins around the world; they are typically characterized by stretching, subsidence and breakaway from their respective conjugate margins. In contrast the interior basins are comparable to ‘failed’ rift systems such as the North Sea or the Gulf of Suez. The South Coastal Basins, for example, tend to be more extended which complicated interpretation of the Amoco wells, but they have significant upside, as exemplified by the Beau Vallon structure. The interior basins, on the other hand, have typically simpler structure: the Silhouette Basin contains a system of NW-trending linked normal faults that could easily harbor North Sea-sized hydrocarbon traps with a variety of rift-related reservoir possibilities. Bright, reflective, hard volcanic horizons are less common than usually presumed, but most of the basins may contain considerable pyroclastic material in parts of the section. All of the basins appear to be predominantly oil prone, with considerable upside prospectivity.
The Guinea margin, situated within the Equatorial Atlantic represents the final point of separation between Africa and South America during Triassic to Cretaceous rifting to form the North and South Atlantic. Despite being in such a tectonically interesting region, relatively little data have been published about the Guinean continental margin. Consequently, prior plate reconstructions within the Equatorial Atlantic lack sufficient detail to provide a fully reasonable explanation for the complex rift structure observed within new 2-D and 3-D seismic datasets. New observations drawn from the seismic data, and local gravity and magnetic data, permit development of a new paleo-reconstruction model across the Guinea Plateau. Furthermore, using magnetic reversals, fracture zones have been extended farther towards the continental margin. This has provided further accuracy and constraint of plate motions, and suggests a greater north-south extensional component is required during initial rifting. These revised plate motions and their timings have provided information on fault kinematics that are observed within the 3-D seismic data, facilitating a more accurate basin development framework. The creation of this more-detailed Equatorial Atlantic plate reconstruction not only aids in better understanding of rift evolution, but presents opportunities for increased insight into how global oceanic circulation patterns and climate change are affected by tectonic activity.
Abstract Horizontal wells with hydraulic fractures in tight oil reservoirs show producing gas-oil ratio (GOR) behavior that is very different from conventional, higher-permeability reservoirs. This paper explains the reasons for the observed behavior using reservoir simulation, with field examples from the STACK and SCOOP plays of the Anadarko Basin in central Oklahoma. A framework for interpreting observed GOR behavior in tight black-oil reservoirs is based on the following stages in a well's history. Some stages may not be visible due to degree of undersaturation, flowing bottomhole pressure schedule, finite-conductivity fractures, and duration of the transient flow period. Early GOR constant at the initial solution gas-oil ratio (Rsi) while bottomhole flowing pressure is above the bubble point; A rise in GOR as bottomhole flowing pressure declines below the bubble point; The transient GOR "plateau", which is characteristic of transient linear flow; A continuous rise in GOR during boundary-dominated flow. Fundamental differences between linear and radial flow, which cause the dependence of GOR on flowing bottomhole pressure, are explained using simulation. During transient linear flow, the GOR response to changes in flowing bottomhole pressure is independent of permeability for infinite-conductivity fractures, but not for finite-conductivity fractures. Several practical observations are made. Knowing Rsi and the transient GOR plateau level in an area can help you interpret where a well is in its GOR history. Rate transient analysis (RTA) diagnostic plots are altered by rising GOR, and sometimes show an early unit slope. During boundary-dominated flow, GOR is more a function of cumulative production than of time; wells with closer fracture spacing have a faster GOR rise with time, but also recover oil more quickly. If compound linear flow develops, GOR can decline late in the well life. The Meramec and Woodford formations in STACK can be history-matched without invoking a suppressed bubble-point due to pore-proximity effects. The critical gas saturation in the Meramec appears to be in the range of zero to 5%. Technical contributions include: a framework for interpreting GOR behavior over well life; the effect of changing bottomhole flowing pressure on GOR; the effect of fracture spacing, conductivity, and half-length on GOR; and the effect of GOR on RTA diagnostic plots.