Three-Dimensional Seismic Paleodrainage System

Abstract

ABSTRACT A large 3-D marine reflection survey with a 25 by 50 m grid, was shot in the southeastern Jeanne d'Arc Basin, offshore Newfoundland to delineate the 400 MMbbls recoverable Terra Nova oil field. During the interpretation of the regional Kimmeridgian unconformity, it appeared that this strong erosional event was associated with widespread river discharge and delta creation. Several workstation implemented methods were used to identify, map and visualize this paleodrainage system that is approximately 150 million years old. A very clear, almost photographic image of the valley incised Jurassic paleotopography, which is buried today at depths between 2 and 3 km, was ultimately corroborated. The seismically reconstructeddepositional environment has immediate implications for the evaluation of the origin and characteristics of the Terra Nova field reservoir. A near shore, marine origin for the reservoir sandstone is postulated. The 3- D workstation mapping techniques presented and exemplified here are applicable whenever interpretation of erosional surfaces and drilling hydrocarbon stratigraphic traps are attempted. INTRODUCTION Correct spatial representation of a paleodrainage system allows the explorationist to indicate source and direction of sediment transport, describe depositional environments, identify stratigraphic traps and predict the distribution of potential reservoir facies. Traditional sub-surface mapping techniques involving interpretation of well logs combined with two dimensional seismic grids are limited by a) scarcity of control points and b) high lateral and vertical variability of the deposited fill within the paleovalleys, transposed into extreme variation of reflection character (Figure 1). Modern improvements in the use of 3-D marine surveys and seismic workstations have brought about fast manipulation of large three dimensional data sets and meaningful analysis of attributes as an exploration tool (Brown, 1989, 1992; Buchanan et al, 1988; Enachescu, 1991, 1993; Tilbury and Bush, 1991; Rijks and Jouffred, 1991). Different workstation approaches to mapping erosional surfaces have been tested on data extracted from a large marine three-dimensional seismic survey covering the Terra Nova oil field (Figures 2, 3 and also Dwyer, 1987; Wilcox et al, 1991). In the southern portion of the survey, individual 3-D migrated lines contain visible erosional events at the top of the Egret Member of the Rankin Formation and suggest that an important river system has acted during the late Kimmeridgian as a source of coarse elastics for the area (Figure 4). METHODS Several workstation-driven methods of mapping paleogeomorphic elements from a three-dimensional reflection data set can be applied.Time slice method. Application of this method depends on the regional structural setting. Horizontal Layering In the presence of flat laying reflectors, the interpretation is done on amplitude-dependent horizontal slices: A=A(x,y,ti), where i receives values over the elevation range of the erosional surface. These time sequential surfaces are approximately parallel with the general layering, but they intersect the paleovalleys and all the other non-horizontal seismic objects such as the velocity pull-up under the valleys and differential compacted layers overlying the valley fill. Significant changes in seismic amplitude values characterize the channel margins on every time slice. This method is widely used to reestablish channel geometry when erosion has sculptured a layered sequence that is conformable, a situation often encountered in passive margin, platform, foreland and cratonic b